Protection circuit

Abstract

A protection circuit for protecting a target component from an in-rush current resulting from the initiation of an electrical connection between a DC source and the target component during an initialisation phase comprises a pre-charging circuit comprising a secondary connection path providing a first electrical connection between the input terminal and the output terminal during the initialisation phase; a main connection path for providing a second electrical connection between the input terminal and the output terminal during a steady-state phase. The main connection path comprises a main switching element configured to disable the second electrical connection when the main switching element is in an initialisation configuration and enable the second electrical connection when the main switching element is in a steady-state configuration during the steady-state phase. The secondary connection path comprises a current limiting element to regulate the in-rush current during the initialisation phase and, during the initialisation phase, the pre-charging circuit reduces a potential difference across the main switching element to a predetermined threshold and then transition from the initialisation phase to the steady-state phase by switching the main switching element from the initialisation configuration to the steady-state configuration.

Description

Field of the Invention The present invention relates to a protection circuit for protecting a target component from an in-rush current, and to a system comprising the protection circuit and the target component. Background Electronic components typically have an inherent input capacitance which causes the electronic component to initially have a low input resistance upon the initiation of a connection between a power source and the electronic component. This initially low resistance results in a surge of high current being drawn from the power source by the electronic component which is known as an in-rush current. In-rush currents can cause damage to electronic components and can be particularly problematic in applications where the power source includes a battery, such as a Li-ion battery, which is capable of providing a high current very quickly. Therefore it is necessary to limit the in-rush current upon the initiation of an electrical connection between a power source and a target component. Some existing circuits include protection circuits with soft-start functions in which a resistor is used to limit the in-rush current. In high-voltage direct current (DC) applications where a target component requires a high DC input voltage (for example, in applications having a DC voltage of at least 50V) existing soft-start circuits typically include relays for enabling and disabling DC power being provided to the target component. For example, this can be seen in charging circuits for EVs (electric vehicles). In these existing solutions, the relays must close a connection between a high voltage DC power source and the remainder of the circuit which is at a much lower voltage. This large potential difference across the relay can cause welding or arcing of the relay contacts during the opening and closing of the relay and so a relay with a very high voltage rating must be used for such applications. These relays may include gas filled chambers surrounding the contacts to reduce arcing. Others may have mechanical breaks to physically separate the contacts during turn-off to prevent or overcome welding. These relays can be very large, noisy (both mechanically and electrically), and very expensive. Moreover, the energising coils of such high power relays in these existing protection circuits for high power applications can consume a lot of power. Additionally, the relays used in these existing solutions must be specified specifically for the voltages involved in the particular application and required by the target component. Therefore, existing high power protection circuits are not scalable for different applications without inconvenient redesign of the circuit and replacement of the components used. Accordingly, there is a desire for a more portable protection circuit for providing power to a target component which is more adaptable, less bulky, and consumes less power than existing solutions. The present invention has been devised in light of the above considerations. Summary of the Invention Broadly the present invention relates to a protection circuit for providing power from a DC source to a target component (e.g. electrical load) which is configured to, initially, provide the power from the DC source to the target component through a current limiting element in order to limit an in-rush current. The protection circuit is then configured to bypass the current limiting element by actuating a switch. Importantly, the protection circuit is configured to wait until each side of the switch is charged to a target voltage before actuating the switch so that there is only a small potential difference (e.g. zero volts) provided across the switch before it transitions. Such a transition, wherein a switch is transitioned with a small or zero potential difference across it, and zero or near zero current may be referred to as a class E transition which enables a lower power switch to be used in the protection circuit than in existing solutions which require higher power, and more complex switches. Accordingly, in a first aspect of the present invention there is provided: a protection circuit for protecting a target component from an in-rush current resulting from the initiation of an electrical connection between a DC source and the target component during an initialisation phase, the protection circuit comprising: an input terminal for electrically connecting the protection circuit to the DC source; an output terminal for electrically connecting the protection circuit to the target component; a pre-charging circuit comprising a secondary connection path for providing a first electrical connection between the input terminal and the output terminal during the initialisation phase; and a main connection path for providing a second electrical connection between the input terminal and the output terminal during a steady-state phase. The main connection path comprises a main switching element which is configured to selectively disable the second electrical connection in the initialisation phase when the main switching element is in an initialisation configuration, and enable the second electrical connection in the steady-state phase when the main switching element is in a steady-state configuration, and the secondary connection path comprises a current limiting element configured to regulate the in-rush current during the initialisation phase. During the initialisation phase, the pre-charging circuit is configured to reduce a potential difference across the main switching element (e.g. in an open state) to or below a predetermined threshold and then transition from the initialisation phase to the steady-state phase by switching the main switching element from the initialisation configuration to the steady-state configuration. In this way, the protection circuit provides a soft start function so that the target component, and the protection circuit itself, is protected from an in-rush current at start up. Advantageously, by reducing the potential difference across the main switching element to the predetermined voltage, before the main switching element is transitioned between states, reduces the current through the switching element during the transition and so reduces a likelihood of the main switching element from being damaged during the transition. Therefore, since voltage across the main switching element when it is transitioned has been reduced to the predetermined voltage, the main switching element may have a voltage rating which is lower than an input voltage provided by the DC source. Accordingly, a smaller, less complex, and more power efficient component may be used as the main switching element making the protection circuit more portable and suitable for battery-powered applications. Moreover, transitioning the main switching element in this way produces less electrical noise than existing solutions. This low noise switching solution can reduce a likelihood of undesirable spectral splatter (also known as switch noise) wherein spurious emissions that result from an abrupt change in a transmitted signal can cause noise at a range of frequencies. Therefore, the target component may be subject to less noise compared to existing solutions. Additionally, the protection circuit of the first aspect is scalable for operating at different voltages without needing to replace the main switching element with a higher or lower rated component because the voltage across the main switching element is reduced to the predetermined voltage before the main switching element is transitioned. Therefore, the protection circuit is more adaptable for different applications and DC sources than existing solutions. When the protection circuit is connected to a DC source and a target component, the protection circuit may be for providing power from the DC source to the target component via the main connection path and / or via the secondary connection path. Additionally, by initially regulating the in-rush current using the current limiting element, the protection circuit provides a soft-start functionality. Therefore, the protection circuit may also be referred to herein as a power supply module or a soft-start circuit. The target component may be an electronic component or load which is configured to receive power from the DC source. In some examples, the target component may receive additional signal inputs from additional signal sources. For example, the target component may be an audio amplifier which is configured to receive and amplily audio signals. The target component may be a high-power target component which is configured to receive a target power from the protection circuit, during the steady-state phase of operation, of at least 1 kW, more preferably at least 1500kW, or at least 2kW. The target component may be configured to receive a target voltage from the DC source, via the protection circuit, of at least 50V, more preferably at least 100V, more preferably at least 150V. The DC source, which may be one or more batteries such as a Li-Ion battery, may be configured to provide the target power and / or the target voltage required by the target component. An “electrical connection” as used herein may refer to a conductive path for enabling for current to flow between two points. A “switching element” as used herein may refer to a component having an open state, in which the switching element is configured to create an open circuit between two sides of the switching element, and a closed state, in which the switching element creates a closed (e.g. short) circuit between the two sides of the switching element. When a switching element is located in a connection path the switching element may be described as enabling / completing or disabling / terminating the electrical connection provided by that connection path depending on if the switching element is in the closed state or in the open state respectively. The skilled person would understand that a switching element may be arranged with respect to a connection path (e.g., in parallel to the connection path) so that, when the switching element is in the open state it may instead enable / complete the electrical connection provided by that connection path, and when the switch is in the closed state it may instead disable / terminate the electrical connection provided by that connection path. The initiation of the electrical connection between the DC source and the target component (when they are connected to the protection circuit) may be referred to as a turn-on time. For example, the initiation which results in the in-rush current, may be caused by any event which completes an electrical connection between the DC source and the target component through the protection circuit. For example, the initiation may be caused by the actuation of a main power switch on the protection circuit, the connection of the DC source to the input terminal, or the connection of the target component to the output terminal. The input terminal may be any suitable contact, connector, or header, etc for providing the electrical connection between the DC source and the protection circuit. The input terminal may include a negative contact and a positive contact for connecting to negative and positive contacts of the DC source respectively. In some examples, the protection circuit may comprise a plurality of input terminals for electrically connecting the protection circuit to a respective plurality of DC sources (for example, a plurality of battery cells arranged in parallel or in series). The output terminal may be any suitable contact, connector, or header, etc for providing the electrical connection between the target component and the protection circuit. The output terminal may include a negative contact and a positive contact for connecting to negative and positive power inputs of the target component respectively. The initialisation phase may refer to a turn-on phase of operation of the protection circuit wherein the first electrical connection between the input terminal and the output terminal, via the secondary connection path, is enabled and DC power is provided from the input terminal to the output terminal via the secondary connection path. The configuration of the protection circuit at turn-on and during the initialisation phase may be referred to as an initialisation state of the protection circuit wherein current may flow from the input terminal to the output terminal through only the secondary connection path where it is regulated by the current limiting element. When the protection circuit is in the initialisation state, the main switching element may be operated in the initialisation configuration so that the second electrical connection provided by the main connection path is disabled. The steady-state phase may refer to a normal operating phase of the protection circuit where the DC power is provided from the input terminal to the output terminal via the main connection path, and, optionally, also via the secondary connection path. The term steady-state may also refer to the current draw by the target component which has settled to a steady-state after the in-rush current has subsided. The transition from the initialisation phase to the steady-state phase, by switching the main switching element from the initialisation configuration to the steady-state configuration, may cause the main connection path to be completed thereby bypassing the current limiting element. Since the main connection path does not comprise the current limiting element, the main connection path may be referred to as a high-current path. Accordingly, the secondary connection path may be referred to as a low-current path. The main switching element may comprise one or more transistors (such as Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET)) or one or more relays for enabling and disabling the second electrical connection provided by the main connection path. In some examples, the main switching element may comprise a plurality of transistors electrically connected to each other in parallel. In this way the current carrying capability of the transistors can be increased to meet the requirements of the target component without needing to increase the current carrying capabilities of the individual transistors. The initialization configuration of the main switching element may refer to an open state of the main switching element. When it is in the open state, the main switching element may create an open circuit thereby terminating (e.g. disabling or interrupting) the main connection path. The steady-state configuration of the main switching element may refer to a closed state of the main switching element. When it is in the closed state the main switching element may provide a closed circuit (between two sides of the main switching element) thereby completing (e.g. enabling) the main connection path. The main switching element may be configured to switch from the initialization configuration to the steady-state configuration upon the application of an active switching voltage (or an energising current) to a control terminal of the main switching element. When the main switching element comprises one or more transistors, the control terminal may be a gate of the transistors. When the main switching element comprises one or more relays, the control terminal may be an energising coil of the relays. The main switching element may be configured to be in the initialization configuration in the absence of the active switching voltage. Thus the main switching element may be a “normally open” switching element which is configured to be in the initialisation configuration upon turn on of the protection circuit. In this way, the inrush current may be prevented from flowing along the main connection path until the transition from the initialisation phase to the steady-state phase. The regulation of the in-rush current may refer to the control or reduction of current flowing through the secondary connection path compared to if no regulation were applied. The current limiting element may be configured to provide active or passive regulation wherein the in-rush current is reduced by a predetermined amount, or by a predetermined proportion. In some examples, the current limiting element may be configured to actively limit the current flowing through the secondary connection path to a predetermined current. The current limiting element may be a resistor or any other resistive element which is capable of regulating a DC current. The current limiting element may be configured to limit the in-rush current to an acceptable current which is within a current specification of the target component (and other components in the protection circuit). In particular, the current limiting element may be an impulse resistor which is configured to regulate the energy generated by the in-rush current. For example, the impulse resistor may have an energy rating of at least 20J, more preferably at least 50J. Configuring the energy requirements, as opposed to the power requirements of the current limiting element can usefully provide impulse regulation for short-term current impulses such as when the in-rush current surges very quickly. This is particularly advantageous when the DC source is a battery, such as a Li-ion battery, which can provide a high current very quickly. The main switching element may comprise an input side electrically connected to the input terminal for receiving an input voltage, and an output side electrically connected to the output terminal. The potential difference across the open main switching element may therefore refer to a voltage difference between the input side and the output side. The potential difference across the open main switching element upon turn on of the protection circuit may therefore be equal to the input voltage from the DC source. The precharging circuit may be configured to reduce the potential difference across the open main switching element, during the initialisation phase, by charging the output side of the main switching element through the current limiting element until the voltage at the output side reaches the input voltage. The predetermined voltage threshold may be dependent on a voltage rating of the main switching element. For example, the pre-charging circuit may be configured to close the main switching element only when the potential difference across it is less than the voltage rating of the main switching element. Accordingly, the pre-charging circuit may be configured switch the main switching element from the initialisation configuration to the steady-state configuration when the potential difference across the open main switching element is equal to or less than the predetermined switching voltage. Preferably, the potential difference across the main switching element may be reduced to substantially zero (zero or near zero) Volts before it is switched by the pre-charging circuit. That is, in an embodiment, the predetermined threshold is substantially zero (zero or near zero) Volts. In this example, the voltage on either side of the switching element may be brought to a same voltage (e.g., wherein the output voltage at the output terminal is substantially equal to the input voltage at the input terminal) so that the potential difference across the main switching element is reduced to zero or near zero, before the pre-charging circuit switches the main switching element from the initialisation configuration to the steady-state configuration. At the point of switching, there may be zero (or near zero) Volts across the main switching element so as to cause a class E transition and / or a current flow of zero (or near zero) Amps across the main switching element during the transition. That is, in an embodiment, the predetermined threshold is set so as to cause a class E transition of the main switching element when the main switching element transitions from the initialisation configuration to the steady-state configuration. When the main switching element includes one or more relays, reducing the potential difference across the main switching element before it is transitioned in this way can reduce the chance of arcing or welding of the relay contacts during the relay transition. When the main switching element includes one or more transistors, reducing the potential difference across the main switching element before it transitions, enables lower voltage transistors to be used than if this were not the case. Accordingly, the one or more transistors may be physically smaller, cheaper, and have less complex thermal requirements than higher power transistors which may need bulky heatsinks and are inefficient. The protection circuit may further comprise a main power switch. The main power switch may be configured to selectively initiate and terminate the electrical connection between the DC source and the target component provided by the first and / or second electrical connections. The main power switch may be configured to selectively complete and interrupt the main connection path and / or the secondary connection path. Thus, the main power switch may be switchable between an open state, in which an open circuit is provided by the main power switch (thereby interrupting the electrical connection provided by the main connection path and / or the secondary connection path), and a closed state in which the open circuit is completed (thereby completing the electrical connection provided by the main connection path and / or the secondary connection path). The enabling of the first electrical connection between the input terminal and the output terminal, e.g., via the secondary connection path, may be referred to as a “turn-on time” of the protection circuit. The main power switch may be located in the secondary connection path wherein it is configured to selectively enable and disable the first electrical connection between the input and output terminals. By providing the main power switch in the secondary connection path, the main power switch may be subject to a lower current (as regulated by the current limiting element) than that if the main power switch were positioned, e.g., between the input terminal and the main and secondary connection paths. In each of the above examples, the main power switch may be a mechanical switch (e.g., a panel mounted switch) for e.g., providing a short or open circuit between the audio input and output terminals. In other examples, the main power switch may be a switching element in the secondary connection path. For example, the main power switch may be a transistor (e.g., a MOSFET) wherein a gate control circuit is configured to increase and decrease a gate voltage of the transistor in order to enable and disable the electrical connection between the DC source and the target. The gate control circuit may be hardware or software controlled. For example, when the protection circuit comprising one or more protection switching elements (discussed in detail below) the one or more protection switching elements may comprise one or more transistors connected to a gate control circuit and configured to act as a main power switch (e.g., under hardware or software control). The pre-charging circuit may be configured to transition the protection circuit from the initialisation phase to the steady-state phase by switching the main switching element from the initialisation configuration to the steady-state configuration after a predetermined time period has elapsed. For example, the precharging circuit may be configured to close the main switching element after the predetermined time period has elapsed. The predetermined time period may begin upon the initiation of the electrical connection between the DC source and the target component through the protection circuit. When the protection circuit comprises a main power switch, the predetermined time period may be from a turn-on time corresponding to the actuation of the main power switch wherein the first electrical connection between the input terminal and the output terminal is enabled. The predetermined time period may be at least a charging time of the target component as dependent on a resistance of the current limiting element. That is, the charging time may be dependent on an RC time constant of the current limiting element and the input capacitance of the target component. The predetermined time period may be at least 200ms, more preferably at least 500ms. The pre-charging circuit may comprise a delay circuit configured to generate a delayed switching signal after the predetermined time delay has elapsed. The delayed switching signal may be configured to switch the main switching element from the initialisation configuration to the steady-state configuration, thereby completing the main connection path between in the input and output terminals. The generation of the delayed switching signal may comprise transitioning an output of the delay circuit from a first voltage to a second voltage after the predetermined time period has elapsed, the output of the delay circuit being the delayed switching signal which is provided to the main switching element. The first voltage (e.g., 0V), when applied to the main switching element, may be configured to hold the main switching element in the initialisation configuration (e.g., the open state). The second voltage (e.g., 12V) may be a switching voltage configured to transition and hold the main switching element in the steadystate configuration (e.g., the closed state). The delay circuit may be configured to receive an output voltage from the output terminal and generate the delayed switching signal based on the output voltage after the predetermined time period has elapsed. That is, delay circuit may be configured to begin monitoring the predetermined time period upon detecting that the output voltage has exceeded a predetermined threshold. The delay circuit may comprise a voltage regulator (such as an isolated DC / DC converter) having an associated start time corresponding to the predetermined time delay. The voltage regulator may be configured to generate the delayed switching signal by regulating the output voltage at the output terminal to a switching voltage (e.g., 12V), wherein the switching voltage is configured to switch the main switching element from the initialisation configuration to the steady-state configuration. In this way, the main switching element may be actuated by a lower switching voltage than the potentially very high input and output voltages. Additionally, by using a voltage regulator in this way, the delay time may be applied using fewer components since there is no need for additional timing circuitry. The protection circuit may comprise one or more protection switching elements which are configured to terminate the provision of DC power from the input terminal to the output terminal (via the first and / or second electrical connections) upon detection of a fault condition. The one or more protection switching elements may be configured to terminate the provision of DC power from the input terminal to the output terminal by disabling the first and second electrical connections provided by the main connection path and the secondary connection path upon detection of the fault condition. For example, the one or more protection switching elements may be configured to selectively create an open circuit (e.g., by opening a switch) between the input terminal and the output terminal. The protection circuit can therefore implement safety shut-off for protecting the DC source, the target component, and the protection circuit from damage caused by a fault condition as well as protect users from potential danger caused by electrical faults. The one or more protection switching elements may be configured to disable the provision of DC power from the input terminal to the output terminal for at least a predetermined fault lock-out time. Therefore, the electrical connections cannot be re-enabled immediately thus providing a time for the detected fault condition to potentially resolve (e.g., allowing components to cool down) and preventing the circuit from being continuously switched off and on which could cause further circuit damage. Each of the one or more protection switching elements may comprise a transistor, such as a MOSFET, or a relay. The transistors or the relays may be operable in a closed, conducting state and in an open, insulating state. The one or more protection switching elements may be configured to be held in the closed state by a hold current as discussed in more detail below. One of the one or more protection switching elements may be located in the secondary connection path, wherein the one or more switching elements are configured to terminate the first electrical connection provided by the secondary connection path upon detection of the fault condition, for example, by creating an open circuit in the secondary connection path. Additionally, the one or more protection switching elements may be configured to terminate the second electrical connection provided by the main connection path, for example, by causing the main switching element to transition from the steady-state configuration to the initialisation configuration. By providing the one or more protection switching elements in the secondary connection path, they may be subject to a lower current than if they were provided in the main connection path or in series with the main connection path and the secondary connection path. The one or more protection switching elements may, therefore, be referred to as “shut-off switching elements”, “pre-charge switching elements”, or “safety switches”. The fault condition may include a thermal fault wherein a temperature is determined to have exceeded a predetermined threshold temperature or where a temperature is determined to have exceeded a predetermined threshold temperature for a predetermined period of time. Further examples of fault conditions may include the detection of a short circuit, an open-circuit, or a sensed current or voltage which exceeds an expected current or voltage. The protection circuit may comprise one or more fault sensors configured to detect the fault condition. For example, the one or more fault sensors may comprise one or more thermal sensors configured to monitor a respective temperature. For example, the one or more thermal sensors may comprise a bimetallic or reed sensor. The one or more fault sensors may be plurality of fault sensors configured to detect respective fault conditions. In these examples, the one or more protection switching elements may be configured to terminate the provision of the DC power from the input terminal to the output terminal upon detection of any of the respective fault conditions. When the fault sensors comprise a plurality of thermal sensors, each of the thermal sensors may be configured to monitor a respective temperature of a different component or area of the protection circuit, such as the DC source or the current limiting element. When there is a plurality of fault sensors, the plurality of fault sensors may be connected to each other in series. The one or more sensors may be configured to create an open circuit upon detection of a respective fault condition thereby disabling the provision of DC power from the input terminal to the output terminal. For example, as described below, the one or more fault sensors may be configured to disable a hold current provided to the one or more protection switching elements upon detection of the fault condition by creating an open circuit. The one or more protection switching elements may be operable in an enabling (closed) state wherein the provision of DC power from the input to the output terminal is enabled, and in a disabling (open) state wherein the provision of DC power from the input to the output terminal is disabled. The one or more protection switching elements may be configured to be held in the closed state by a hold current. The hold current may be provided from the input terminal to the one or more protection switching elements via a hold current connection (e.g. a hold current circuit). A resistance of the hold current connection may be configured to provide a required hold current of the one or more protection switching elements from the input terminal to the one or more protection switching elements. The one or more fault sensors may be configured to create an open circuit in the hold current connection upon detection of a fault condition thereby transitioning the one or more protection switching elements to the open state and terminating the provision of DC power from the input terminal to the output terminal. As mentioned above, the one or more protection switching elements may each comprise a relay. Each relay may be operable in a disabling state, in which the first and / or second electrical connections between in the input and output terminal are disabled, and an enabling state, in which the first and / or second electrical connections between in the input and output terminal are enabled. Each relay may be configured to switch from the disabling to the enabling state upon application of an energising current. The relay may be in the open state when it is in the disabling state and in a closed state when it is in the enabling state. An energising current may be provided from the input terminal to the one or more protection switching elements via an energising connection (e.g. an energising circuit). The energising connection may be connected between the input terminal and the one or more protection switching elements in parallel with the hold current connection. Therefore, the hold current connection may be a first parallel connection branch of a current divider, a second parallel connection branch of the current divider being configured to provide an energising current to the one or more protection switching elements as discussed below. The energising connection may comprise an energising capacitor configured to provide the energising current to the one or more protection switching elements upon initiation of an electrical connection between the input terminal and the energising capacitor. For example, the electrical connection between the input terminal and the energising capacitor may be enabled by the actuation of the main power switch. Advantageously, by providing separate energising and hold current connections, a higher energising current may be provided to close the protection switching elements than the hold current which is used to hold them closed, thereby reducing an overall power consumption of the protection circuit. As mentioned above, the one or more protection switching elements may be configured to disable the provision of DC power from the input terminal to the output terminal for at least a predetermined fault lock-out time. The predetermined fault lock-out time may be implemented by a discharge time of the energising capacitor after the provision of DC power is disabled (e.g., when a fault condition is detected). That is, the energising capacitor may be configured to discharge upon the transition of the one or more protection switching elements from the enabling state to the disabling state. A discharge time of the energising capacitor may be at least the predetermined fault lock-out time. The energising connection may further comprise a discharge resistor in parallel with the energising capacitor. A time constant of the discharge resistor and the energising capacitor may be configured to be at least the predetermined fault lock-out time for which the energising capacitor is configured to discharge. Therefore, the one or more protection switching elements may be prevented from being reenabled for at least the predetermined lock-out time. By implementing the lock-out in this way, the safety lock out can be achieved using passive analogue components without requiring complex digital timers or active components. Additionally, the lock-out can persist even if power is removed from the circuit, for example, if the main power switch turned off and on again during the predetermined lock-out time. The discharge resistor may have a larger resistance than the resistance of the hold current connection. The discharge resistor may have a resistance configured to limit a current from the input terminal to less than the hold current. Therefore, when the hold current connection is interrupted, e.g., by the one or more fault sensors, the current provided by the energising connection may be less than the hold current, thus causing the one or more protection switching elements to transition from the enabling state to the disabling state and triggering the safety lock-out. The predetermined fault lock-out time may be at least 3 seconds, more preferably at least 5 seconds, more preferably between 6 and 8 seconds. The one or more protection switching elements may be a plurality of protection switching elements. Each of the protection switching element may be configured to enable and disable a different electrical connection provided in the protection circuit. As mentioned above, the secondary connection path may comprise a first protection switching element of the one or more protection switching elements. The first protection switching element, when closed (in the enabling state), may be configured to complete the secondary connection path, and when open (in the disabling state), being configured to terminate the secondary connection path. The first protection switching element may be configured to close upon the application of a switching voltage and / or an energising current. The first protection switching element may be configured to open upon the detection of the fault condition. When the protection circuit comprises a main power switch, the main power switch may be for selectively providing the switching voltage and / or energising current from the input terminal to the first protection switching element thereby initiating the electrical connection between input terminal and the output terminal via the secondary connection path. The first protection switching element may be configured to be open (in the disabling state) if the respective switching voltage is not provided. Therefore, the protection switching element may be a “normally open” switch which is configured to be open (in the disabling state) until the protection circuit is turned on, for example, by actuation of the main power switch. A second protection switching element of the one or more protection switching elements may be configured to selectively enable and disable the second electrical connection provided by the main connection path between the input and output terminals. The second protection switching element may be configured to selectively enable and disable the second electrical connection by selectively enabling and disabling an electrical connection between the delay circuit and the main switching element. Therefore, the second protection switching element may be configured to terminate the provision of the delayed switching signal from the delay circuit to the main switching element upon detection of the fault condition. For example, the second protection switching element may be configured to open upon detection of the fault condition. Therefore, the main switching element may be configured to terminate the main connection path upon detection of the fault condition. The protection circuit may be a split rail circuit having negative and positive rails. In these examples, the protection circuit may comprise an additional protection switching element of the one or more switching elements which is configured to enable and disable an electrical connection provided by a negative main connection path for electrically connecting a negative rail of the input terminal to a negative rail of the output terminal during the steady state phase. The each of the protection switching element may comprise a relay or a transistor, e.g., a MOSFET. When the protection switching elements comprise relays, each of the relays may be included in a common relay component. Therefore, each of the protection switching elements may be switchable by a common relay coil. The common relay coil may be configured to close each of the relays upon application of the energising current and hold the relays closed upon application of a hold current which is lower than the energising current as discussed above. The protection circuit may comprise one or more power saving circuits electrically connected in series to a control terminal of the main switching element, and / or a control terminal of the one or more protection switching elements. Each power saving circuit may comprise an energising capacitor and a hold current resistor connected in parallel to each other. The energising capacitor may be for providing an energising current to the respective control terminal upon initiation of a current being provided to the power saving circuit. The hold current resistor may be for providing a hold current to the control terminal after the energising capacitor has charged. The hold current resistor may be configured to provide a hold current to the respective control terminal which is a lower current than the energising current. Therefore, the power required to operate the switching elements is reduced. The power saving circuits are particularly useful when the switching elements comprise relays which require a higher energising current to initially switch the relay and a lower hold current to maintain the current state of the relay. The present inventors have found that protection circuits having such power saving circuits can reduce the overall power consumption of the switching elements by up to 75%. The energising capacitor and the hold current resistor of a power saving circuit connected to the one or more protection switches may also be the energising capacitor of the energising connection and the hold current resistance of the hold current connection discussed above in relation to the detection of fault conditions. In some examples, the main switching element may be connected to a suppression circuit comprising a flyback diode which is configured to reduce kick-back induction upon termination of the electrical connection between the DC source and the electrical load (when the protection circuit is turned off). The use of a flyback diode is particularly useful when the main switching element comprises a transistor which may be damaged by such kick-back induction. The use of a suppression circuit enables additional filtering, which may include reactive components such as inductors and chokes, to be implemented in the protection circuit without damaging the main switching elements upon circuit shut-off. Such filtering is particularly useful when the target component is sensitive to noise, for example, in audio applications wherein the target component may be e.g., an audio amplifier. In each of the above examples, the protection circuit may be a split-rail protection circuit. In these examples, the main connection path may be a first main connection path for electrically connecting a positive rail of the input terminal to a positive rail of the output terminal, and the secondary connection path may be a first secondary connection path for electrically connecting the positive rail of the input terminal to a positive rail of the output terminal. Additionally, the protection circuit may comprise a second main connection path for electrically connecting a negative rail of the input terminal to a negative rail of the output terminal. The second main connection path may comprise a second main switching element which is operable in an initialisation configuration and in a steady-state configuration as described above for the first main switching element. The pre-charging circuit may comprise a second secondary connection path for electrically connecting the negative input terminal and the negative output terminal, the second secondary connection path may comprise a second current limiting element. As described above for a single-ended protection circuit, the pre-charging circuit may be configured to reduce a potential difference across the second main switching element to a predetermined threshold and then transition from the initialisation phase to the steady-state phase by switching the second main switching element from the initialisation configuration to the steady-state configuration. The pre-charging circuit may be configuration to switch the second main switching element to the steadystate configuration after a predetermined time period thereby bypassing the second current limiting element. The predetermined time period may be the predetermined time period implemented by the delay circuit described above. When the protection circuit is a split rail circuit the input voltage provided by the input terminal may be considered as the potential difference between the negative and positive rails of the input terminal. As mentioned above, the input voltage may be at least 50V, more preferably at least 100V more preferably at least 150V. Such voltages can cause damage and high power dissipation which is especially problematic for portable systems, for example, where the DC power source is a battery. Therefore, the protection circuit is particularly effective for such high power circuits owing, at least, to the reduction of the potential difference across the main switching element before it is switched, and the use of the power saving circuits discussed above. The output terminal may be configured to provide at least the input voltage to the target component. The input voltage may therefore be a target voltage of the target component. An input capacitance of the target component may be between 1mF and 20mF. For example, when the target component is a high power audio amplifier it may have large reservoir capacitors resulting in this input capacitance. A target voltage requirement of the target component may be at least 50V, at least 100V, or at least 150V. The target component may be, for example, an audio amplifier. In particular, the audio amplifier may be a high power audio amplifier requiring a target DC power during steady state operation of at least 200 W, more preferably 300W during the steady-state phase. The audio amplifier may be configured to receive a peak DC power of at least 1 kW into 2 Ohms, more preferably at least 2kW. In some examples, the target component may comprise two audio amplifiers configured to receive a total peak DC power of at least 2kW, more preferably at least 4kW. In a second aspect of the present invention there is provided a system comprising the protection circuit of the first aspect. The system may further comprise a DC source electrically connected to the input terminal of the protection circuit. In further examples, the system may comprise the target component electrically connected to the output terminal of the protection circuit. In a third aspect of the present invention, there is provided an audio system comprising the protection circuit of the first aspect, and a target component electrically connected to the output terminal of the protection circuit, wherein the target component is an audio amplifier. The audio system may further comprise a DC source, such as a battery, solar panel, etc. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Fig 1. shows a simplified diagram of a protection circuit according to aspects of the present invention; Fig 2. shows another simplified diagram of a protection circuit including a protection switching element; Fig 3. shows a schematic diagram of a protection circuit according to aspects of the present invention; Fig 4. shows a schematic diagram of another protection circuit according to aspects of the present invention; and Fig 5. shows a system comprising a protection circuit according to aspects of the present invention. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Fig. 1 shows a simplified diagram of a protection circuit 1 for providing DC power from a DC source to a target component and for protecting the target component from an in-rush current. The protection circuit 1 comprises an input terminal 4 for electrically connecting the protection circuit 1 to a DC source (not shown) such as a battery or a solar panel, an output terminal 6 for electrically connecting the protection circuit 1 to a target component (not shown), a main connection path 20 for electrically connecting the input terminal 4 to the output terminal 6 during a steady-state phase of operation, and a secondary connection path 10 for electrically connecting the input terminal 4 to the output terminal 6 during an initialisation phase of operation. The input terminal 4 comprises a negative contact and a positive contact for connecting to positive and negative rails of the DC source. Likewise, the output terminal 6 comprises a negative contact and a positive contact for connecting to positive and negative inputs of the target component. In this example, the negative contacts are used as a reference for electrical ground of the protection circuit 1. However, in other examples, the negative contacts may be connected to a negative rail of the protection circuit 1, as shown in the example of Fig. 3. The output terminal 6 is for connecting the protection circuit 1 to a power input of the target component in order to provide DC power to the target component. The target component may comprise additional inputs for receiving signals. Therefore, the protection circuit 1 may also be referred to as a power supply module for the target component. The secondary connection path 10 forms part of a pre-charging circuit which is configured to provide current from the input terminal 4 to the output terminal 6 of the protection circuit 1 during an initialisation phase when an electrical connection between the DC source and the target component is first enabled. The secondary connection path 4 comprises a current limiting element 12 for regulating an in-rush current resulting from the initialisation of the electrical connection between the DC source and the target component during the initialisation phase. The current flowing through the secondary connection path 10 is therefore a pre-charge current for charging an inherent input capacitance of the target component to a target voltage. The main connection path 20 comprises a main switching element 22 for selectively enabling and disabling the flow of current from the input terminal 4 to the output terminal 6 along the main connection path 20. The main switching element 22 is operable in an open state and a closed state. When the main switching element 22 is in the open state (which may also be referred to as a disabling or initialisation configuration), the electrical connection provided by the main connection path 20 is terminated by the main switching element 22 so that current cannot flow though the main connection path 20. When the main switching element 22 is in the closed state (which may also be referred to as an enabling or steadystate configuration), the electrical connection provided by the main connection path 20 is enabled and current is able to flow from the input terminal 4 to the output terminal 6 via the main connection path 20. The main switching element 22 comprises a control terminal which is configured to receive a switching voltage for operating the main switching element 22 in the open or the closed state. As discussed below in relation to Figs. 3 and 4, the main switching element 22 may be a transistor, in which case the control terminal is a gate of the transistor, or the main switching element 22 may be a relay, in which case the control terminal an energising coil of the relay. The main switching element 22 is configured to be “normally open” so that, in the absence of an active switching voltage being applied to the control terminal, the main switching element 22 is in the open state. Therefore, when the protection circuit is initially turned on, the main switching element 22 is configured to be in the open state and the main connection path 20 is terminated thus preventing the flow of current through the main connection path 22. Rather, the current must flowthrough the secondary connection path 10 where it is regulated by the current limiting element 12 during the initialisation operating phase. The protection circuit may therefore be referred to as a soft-start circuit since the target component is protected from an in-rush current during start-up. The protection circuit 1 also comprises a main power switch 14 for selectively completing and terminating the electrical connection between the input terminal 4 and the output terminal 6 via the main 10 and / or secondary 20 connection paths. In this example, the main power switch 14 is provided in the secondary connection path 10 for selectively terminating or completing the secondary connection path 10. Therefore, the protection circuit 1 may be “turned on” by actuating the main power switch 14 to enable current to flow from the input terminal 4 to the output terminal 6 via the secondary connection path 10. When the input terminal 4 is connection to a DC source and the output terminal 6 is connected to a target component, the actuation of the main power switch 14 causes the initial in-rush current to flow from the DC source to the target component. At turn on, during this initialisation phase, current is only permitted to flow from the input terminal 4 to the output terminal 6 through the secondary connection path 10. The presence of the current limiting element 12 causes the input capacitance of the target component to charge at a slower rate than if the in-rush current where not limited. While the input capacitance is charging an output voltage at the output terminal 6 will rise until it reaches the input voltage at the input terminal 4 owing to the increasingly higher input resistance presented by the input capacitance of the target component. The current limiting element 12 may be a current limiting resistor. In particular, the current limiting element 12 is an impulse resistor which has an energy rating configured to limit current impulses in the incoming power which is drawn from the DC source. Owing to the short timescales involved in a current surge due to in-rush current, particularly when the DC source is a battery such as a Li-ion battery, the regulation of the in-rush current may be viewed as an ‘energy’ problem rather than a ‘power’ problem. Therefore, the current limiting resistor 12 is configured to have an impulse energy rating of at least an expected impulse energy required to charge the input capacitance of the target component. For example, when the input capacitance of the target component is C=10mF and an input voltage from the DC source is V=84V, then the energy required from the DC source to charge the input capacitance is given by TCV2 = 35.28J. If the in-rush current were not limited, then this energy would be provided to the target component very quickly, as an impulse. Therefore, in this example, the impulse resistor has an impulse energy rating of at least 35.28J. During the initialisation phase, the pre-charging circuit is configured to reduce a potential difference across the open main switching element 22 to a predetermined threshold by pre-charging an output side of the main switching element 22 to the same voltage as the input side. In the ideal case for the example shown, both sides of the main switching element 22 may be increased to the input voltage provided to the input terminal 4. Therefore, the potential difference across the main switching element 22, in the ideal case, is reduced to zero. In reality, inherent resistances in the protection circuit 1 and the input resistance of the target component after the pre-charge being less then infinite, may result in a very small potential difference across the main switching element 22. After the potential difference across the main switching element 22 has been reduced to the predetermined threshold, the pre-charging circuit is configured to transition from the initialisation phase to the steady-state phase by switching the main switching element 22 from the initialisation configuration to the steady-state configuration. That is, in this example, the pre-charging circuit is configured to close the main switching element 22 and complete the main connection path 20. This causes the current limiting element 12 to be bypassed and enables a steady-state current to flow from the input terminal 4 to the output terminal 6 via the main connection path 20. Note, in this context, the steady-state current is intended to refer to the current draw by the target component, after the input capacitance of the target component has been charged, after the in-rush current has subsided. The steady-state current may vary depending on the current requirements of the target component but without the sudden impulse caused by the in-rush current. The pre-charging circuit comprises a delay circuit 16 which is configured to generate a switching signal for transitioning the main switching element 22 from the open to the closed state after a predetermined time period from the turn-on time has elapsed. The delay circuit 22 is configured to begin waiting for the predetermined time period when the output voltage at the output terminal 6 has increased to a threshold voltage, thus triggering a time delay counter in the delay circuit 16. The predetermined time period is configured to be at least a predicted time for the potential difference across the main switching element 22 to reduce to the predetermined threshold. This may be determined by the charge time of the target component which may be determined from the RC time constant of the current limiting resistor 12 (R) and the input capacitance (C) of the target component. Preferably the predetermined time period is configured to be longer than the RC charge time. For example, in the system of Fig. 3 discussed below, the target component has a charge time of ~0.1s, and the delay circuit provides a predetermined time period of 1s before closing the main switching element 122. Fig. 2 shows a simplified diagram of another protection circuit 2 according to aspects of the present invention. The protection circuit 2 comprises an input terminal 4, a main connection path 20 comprising a main switching element 22, a secondary connection path 10 comprising a current limiting resistor 12, an output terminal 6, and a delay circuit 12 as described above in for Fig. 1. However, in this example, a protection switching element 30 is provided in the secondary connection path 10 which is configured to selectively terminate and complete the secondary connection path 10 depending on a switching signal provided to a control terminal of the protection switching element 30. The protection switching element 30 is configured to be in an open state in the absence of an active switching signal. Therefore the protection switching element 30 is a “normally off’ switching element which is therefore in the open state until the protection circuit 2 is turned on and the active switching signal is provided to the control terminal. In this example, the main power switch 14 is provided between the input terminal 4 and the control terminal of the protection switching element 30. Therefore, when the main power switch 14 is in a closed “ON” state, the input voltage is provided to the control terminal of the protection switching element 30 thereby enabling current to flow through the secondary connection path, and, after the predetermined time delay, through the main connection path. Alternatively, when the main power switch is in an open “OFF” state, the control terminal of the protection switching element 30 no longer receives an active switching signal from the input terminal and the secondary connection path 10 is terminated. Although not critical, the placement of the main power switch 14 is optimised in its current position on the control terminal branch of the circuit. If it were to be placed, for example, in series immediately after the DC power source, then it would need to handle a main working current and power being drawn by the target component. By placing the main power switch 14 on the control terminal branch, it can be a low power switch and need only handle the input voltage but not the full current draw. In this example, the protection circuit 2 comprises a second protection switching element 32 for enabling and disabling an electrical connection between the delay circuit 16 and the main switching element 22. The control terminals of each protection switching element 30, 32 are connected together. Therefore, when the first protection switching element 30 in the secondary connection path 10 is transitioned to an open “OFF” state, for example by the protection circuit 2 being turned off by the main power switch 14, the second protection switching element 32 is also moved to an open “OFF” state which, in turn causes the main switching element 22 to switch to an open state and terminate the main connection path 20. Therefore, when the protection circuit is turned “off’, e.g., using the main power switch 14, all of the electrical connections between the input 4 and output 6 terminals are disabled. The first 30 and / or second 32 protection switching elements may be transistors, such as MOSFETs, or relays as discussed above for the main switching element 22. Since the protection switching elements 30, 32 in this example are provided in low current paths (i.e., the first protection switching element 30 is provided in the secondary connection path 20 in which the current is limited by the current limiting resistor 12 and the second protection switching element 32 is in a control line without a large current draw), a relay or a MOSFET may be used for the protection switching elements 30, 32 which has a lower current rating than, for example, if the protection switching elements 30, 32 where provided directly in the main connection path 10 or between the DC source and the main / secondary connection paths 10, 20. Finally, the protection circuit 2 of Fig. 2 also comprises a fault sensor 34 for detecting a faut condition. The fault sensor 34 is configured to cause the protection switching elements 30, 32 to transition from the closed “ON” states to the open “OFF” states upon detection of a fault condition, thereby terminating the provision of DC power from the input terminal 4 to the output terminal 6. For example, the fault sensor 34 may be configured to detect a thermal fault, a short circuit, an open circuit, a humidity, or unexpected voltage / current levels, etc. The fault sensor 34 is configured to generate a open circuit between the input terminal 4 and the control terminals of the protection switching elements 30, 32 upon the detection of a fault. The open circuit prevents the active switching voltage from being provided to the protection switching elements 30, 32 causing them to revert to the open “OFF” state. In some examples, a plurality of fault sensors 24 may be provided in series for detecting respective fault conditions as described below in relation to Fig. 3. Fig. 3 shows a schematic diagram of another protection circuit 100 according to aspects of the present invention. In this example, the protection circuit 100 is for providing DC power from a battery to an audio amplifier. Additionally, In this example, the protection circuit 100 is a split rail circuit having a negative rail and a positive rail. The protection circuit 100 comprises two input terminals 104 for electrically connecting the protection circuit 100 to a first battery and a second battery. In this example, each battery provides 84V. Since, the first and second batteries are connected in series, the total input voltage is 168V i.e., -84V to +84V. Immediately adjacent to the input terminals 104 are fuses 106 for protecting the downstream components from an electrical fault. Respective fuses 106 are provided in the positive and the negative rails. Next the protection circuit 100 comprises a positive main connection path 120a for the positive rail, and a negative main connection path 120b for the negative rail. The positive main connection path 120a comprises a main relay 122a and the negative main connection path also comprises a main relay 122b. The main relays 122a, 120b are configured to act as the main switching elements 22 discussed above in relation to Fig.1 and Fig. 2. Thus, when no power is provided to the protection circuit 100, the main relays 122a, 122b are configured to default to an open “OFF” state which terminates the main connection paths 120a, 120b. The main connection paths 120a, 120b connect the input terminals 104 to a plurality of output terminals 106 which are for connecting the protection circuit 100 to the target component, which in this example is an audio amplifier. Each output terminal 106 comprises a positive contact which is connected to the positive main connection path 120a, a ground contact which is connected to electrical ground, and a negative contact which is connected to the negative main connection path 120b. Additionally, the protection circuit 100 comprises a positive secondary connection path 112a between the input terminals 104 and the output terminals 106 for the positive rail and a negative secondary connection path 110b between the input terminals 104 and the output terminals 106 for the negative rail. The positive secondary connection path 110a comprises a first current limiting resistor 112a for limiting an in-rush current to the positive contacts of the output terminals 106 and the negative secondary connection path 110b comprises a second current limiting resistor 112b for limiting an in-rush current to the negative contacts of the output terminals 106. The positive 112a and negative 112b secondary connection paths are each routed from the input terminals 104 through a protection relay 130 which is configured to enable (i.e., complete) the secondary connection paths 112a, 112b upon turn-on of the protection circuit 100. The protection circuit 100 comprises a main power switch 114 connected to a control terminal of the protection relay 130 which operated as discussed above for Fig.2. In this example, the main power switch 114 is shown as a terminal for connecting to a physical switch provided on a front panel of an audio system. When the main power switch 114 is switched to a closed “ON” state the protection relay 130 receives an energising current from the input terminals 104 which energises a relay coil which causes the switches in the relay to transition and complete the positive and negative connection paths. The protection relay 130 comprises four sets of switchable contacts in a common relay package. The common relay package comprises a single control terminal for energising a common relay coil 139 which is configured to switch each set of switchable contacts simultaneously. In this example, the switchable contacts are switchable between an open state, where either side of a connection across the contacts is floating, and a closed state wherein the connection across the contacts is completed. Each switchable set of contacts may be considered as a protection switching element for enabling and disabling the provision of DC power from the input terminals 104 to the output terminals as discussed above for Fig. 2. Accordingly, when the batteries are connected to the input terminals 104, a target component is connected to the output terminals, and the main power switch 114 is switched to the “ON” state, the protection relay 130 completes the secondary connection paths 110a, 110b. Current begins to flow through the secondary connection paths 110a, 110b and the input capacitance of the target component is charged through the current limiting elements 112a, 112b causing an output voltage at the output terminals 106 to increase to the input voltage provided by the batteries (which in this example is + / -84V. The output voltage across the positive and negative output terminals 106 is provided to an isolated DC / DC voltage regulator 116 via a filtering 118 arrangement comprising inductive and capacitive filtering as well as a common mode choke. The regulator 116 is configured to regulate the output voltage to a second voltage for providing to other components 150 in the system and to the main relays 122a, 122b. In this example, the voltage regulator 116 is configured to regulate the output voltage to 12V. Importantly, the voltage regulator 116 is configured to implement a start-up time before generating the second voltage and so also acts as a delay circuit. The start-up time implemented by the voltage regulator 116 is the predetermined time period for ensuring that the input capacitance of the target component has fully charged before the current limiting resistors 112a, 112b are bypassed. Therefore, once the output voltage at the output terminals 106 has increased to a minimum required input voltage of the voltage regulator 116, the voltage regulator 116 waits for the predetermined time period before increasing an output of the regulator 116 from 0V to the second voltage (e.g., 12V). In this example, the voltage regulator 116 (AE15B-EW-S12) is configured to implement a 1 second start time. In this example, input capacitors of the target component take 0.1-0.2s to charge up to 84V. Therefore, the 1s delay provided by the voltage regulator 116 allows ample time for the input capacitance to charge fully before powering the main relay coils 124a, 124b. The output of the voltage regulator 116 is connected to each of the main relays 122a, 122b as a delayed switching signal. Therefore, the main relays 122a, 122b are configured to close after the predetermined time period has elapsed from the turn-on time. An input side of the main relays 122a, 122b are each connected to the input terminals 104 and an output side of the main relays 122a, 122b are each connected to the output terminals 106. Therefore, the secondary connection paths 110a, 110b are configured to pre-charge the output sides of the main relays 122a, 122b to the input voltage through the current limiting elements 112a, 112b. This reduces the potential difference across the main relays 122a, 122b to substantially zero so that there is only a small voltage (substantially zero) between the relay contacts when they are brought in what is known as a class E transition. This means there is a reduced risk of welding the main relay contacts during turn-on or arcing during turn-off. Therefore, a low voltage relay may be used as the main relays 122a, 122b in this high voltage circuit without being damaged by the input voltage. Additionally, the (12V) output of the regulator 116 is provided to other regulators and circuit boards 150 for use in other parts of the audio system. Notably, the 12V delayed switching signal from the voltage regulator 116 is provided to the main relays 122a, 122b via (one of the sets of switchable contacts in) the protection relay 130. Therefore, when the main power switch 114 is transitioned to an open “OFF” state, the protection relay 130 transitions quickly from a closed “ON” state to an open “OFF” state which terminates the connection between the voltage regulator 116 and the main relays 122a, 122b causing the main relays 122a, 122b to transition to an open state and terminate the main connection paths 120a, 120b. As discussed above for Fig. 2, the control terminal of the protection relay 130 in Fig. 3 is connected to a plurality of fault sensors 132 such that the protection relay 130 is configured to disable the provision of power from the input terminals 104 to the output terminals 106 upon the detection of a fault. The fault sensors 132 are shown as terminals for connecting to physical fault sensors 132 which are located in appropriate locations for sensing the respective faults. In this example, each of the fault sensors 132 is a thermal sensor, such as a bimetallic or reed sensor, which is configured to sense a respective temperature. If a respective temperature has exceeded a predetermined threshold, then a fault condition is detected. For example, thermal sensors may be provided for measuring a temperature of each battery and additional thermal sensors may be provided for measuring respective temperatures of the firstand second current limiting elements 112a, 112b. Since the current limiting elements 112a, 112b are only expected to regulate a high current for a small amount of time during the initialisation phase, they are not expected to get very hot. Therefore, if a temperature of one or both of the current limiting elements 112a, 112b is determined to be higher than a predetermined threshold, this may be indicative of a fault in the circuit. Each fault sensor 132 is configured to create an open circuit when a respective fault condition is detected. The open circuit causes each of the sets of switchable contacts in the protection relay 130 to transition to an open state thereby terminating the main 120a, 120b and secondary 110a, 120b connection paths and “shutting off’ the protection circuit 100. In this example, the fault sensors 132 are located in a hold current connection branch between the main power switch 114 and a control terminal of the protection relay 130. The hold current connection branch is configured to provide a hold current to the protection relay 130 which is sufficient to hold the protection relay 130 in the closed “ON” state. The hold current is determined by a hold current resistor 134 provided in the hold current connection branch which is configured to convert the input voltage from the input terminals 104 to a hold current sufficient to hold the protection relay 130 in the closed state. Therefore, the creation of an open circuit by one of the fault sensors 132 in the hold current connection branch causes the relay coil 139 of the protection relay 130 to de-energize and the protection relay 130 to transition to the open “OFF” state. The hold current connection branch forms a first branch of a current divider between the control terminal of the protection relay 130 and the main power switch 114. The hold current connection branch is routed through one of the sets of switchable contacts in the protection relay 130. Therefore, the hold current connection branch is not completed until an energising current is provided to the protection relay 130 causing the switchable contacts to close. A second, energising branch is provided in parallel with the hold current connection branch, between the main power switch 114 and the control terminal. The second energising branch is for providing the energising current to initially “turn on” the protection relay 130. The energising branch comprises an energising capacitor 138 for energising the protection relay 130 at turn on. Typically, a relay, such as the protection relay 130 of Fig.3, requires an energising current to energise the relay coil 139 and transition the relay from one state to the other. However, since the relay coil 139 is magnetic, after the transition only a hold current, which is lower than the energising current, is required to hold the relay in the transitioned state. Therefore, the hold current resistor 134 is configured to provide a hold current which is lower than a required energising current of the protection relay 130. The energising capacitor 138 is configured to provide the energising current at start up when the main power switch 114 is first actuated and transitioned to the closed “ON" state. The operation of the high voltage, low current protection relay 130 is such that the energising capacitor 138 acts like a short circuit on initial turn on until the voltage across it has stabilised. This provides a burst of energy to the protection relay coil 139 which energises it. This causes the protection relay 130 to transition from the normally-open “OFF” state to the closed “ON” state and connect the hold current connection branch to the control terminal of the protection relay 130 for providing the hold current. Once the voltage has stabilised after a short time, the energising capacitor 138 acts like an open circuit and no longer provides current to the protection relay 130. Since the energising capacitor 138 no longer provides the energising current after start-up, when one of the fault sensors 132 detects a fault condition, thereby breaking the hold current connection, an insufficient current is provided to the protection relay coil 130 and the protection relay 130 reverts to the open “OFF” state. Additionally, the protection circuit 100 is configured to disable the provision of power from the input terminals 104 to the output terminals 106 after a fault condition is detected for a predetermined lock out time. Therefore, even if the main power switch 114 is cycled off and on again, the protection circuit 100 is prevented from operating. In the example of Fig 3, the predetermined lock out time is implemented by the energising capacitor 138 and a discharge resistor 136 provided in parallel with the energising capacitor 138. When the protection relay 130 transitions from the closed state to the open state, the relay coil 139 cannot be re-energized by the energising current until the energising capacitor 138 has discharged (to allow the short burst to energise the relay coil 139 again). Therefore, to implement the predetermined lock out time, the discharge resistor is configured to discharge the energising capacitor 138 at a discharge rate determined by an RC time constant of the energising capacitor 138 and the discharge resistor 136. The RC time constant is configured to be at least the predetermined lock out time. For example, in Fig. 3, the predetermined lock out time is between 6-8 seconds. Notably, the discharge resistor 136 has a resistance value which is must larger than the hold current resistor 134. Therefore, when the hold current resistor 134 is disconnected from the control terminal or the protection relay 130 by the fault sensors 132, the discharge resistor 136 is unable to maintain the hold current and the protection relay 130 is transitioned to the open “OFF” state as described above. If the fault sensors 132 have not returned to their normally-closed state when the main power switch 114 is actuated again (for example if one of the sensed temperatures is too high) then the protection circuit 100 will still not turn back on because the discharge resistor 136 cannot supply a sufficient hold current for the relay coil 139, and the hold current resistor 134 is still not in circuit (owing to the open circuit created by the fault sensors 132). In addition to operating the fault shut-off functionality, the provision of the hold current resistor 134 in parallel with the energising capacitor 138 for operating the protection relay 130 also forms a power saving circuit. Since, the hold current resistor 134 is configured to provide a hold current which is smaller than the energising current, and power is proportional to the square of current according to P=I2R, the power saving circuit comprising the energising capacitor 138 and the current hold resistor 134 results in an overall reduction the power consumption of the protection relay 130. Additionally, power saving circuits are also provided in series with the energising coils 124a, 124b of the main relays 122a, 122b. The power saving circuits are configured to reduce the power consumption of the main relays by regulating a current provided to the relays. Each power saving circuit may comprise an energising capacitor 138 for providing an energising current in parallel with a hold current resistor 134 for providing a hold current. The present inventors have found that the presence of the power saving circuits enables an overall coil power saving of 75%. Fig. 4 shows a portion of a protection circuit 200 according to the present invention in which the main switching element and the protection switching element are implemented using MOSFETs U1, U2. Fig. 4 shows a simplified version of the protection circuit 200 where only the positive channel is shown. However, the teaching of Fig. 4 may be applied to the protection circuit 100 of Fig. 3 wherein the main relays coils and / or the protection relay of Fig. 3 may be replaced by one or more MOSFETs U1, U2 as shown in Fig. 4. The protection circuit 200 comprises an input terminal CON1 for connecting the protection circuit 200 to a DC power source, which in this example is a battery. In Fig. 4 the target component is represented by a capacitor C1 having a capacitance of 10mF which results in an inrush current upon connection of the DC source to the target component C1. As in Fig. 3, a fuse F1 is provided between the input terminal and the remainder of the protection circuit 200. As described above for the previous examples, the protection circuit 200 comprises a main connection path between the input terminal CON1 and the target component (represented by capacitor C1). The main connection path comprises a first MOSFET U1 for enabling and disabling current to flow via the main connection path. The first MOSFET U1 is configured to be operated by a delay circuit (not shown) which provides a delayed switching voltage to a gate of the first MOSFET U1. The protection circuit 200 also comprises a secondary connection path between the input terminal and the target component (represented by capacitor C1). The secondary connection path comprises a current limiting resistor R1 for limiting an inrush current and a protection switching element implemented by a second MOSFET U2. The current limiting resistor R1 is configured to operate in the same way as the current limiting resistors of Fig. 3. The second MOSFET U2 is configured to receive a switching signal to a gate of the MOSFET U2. The switching signal may be enabled and disables by fault sensors as described for Fig. 3. Finally, in contrast to the previous examples, the protection circuit 200 does not include a separate main power switch. Instead the protection switching element U2 is configured to act as a main power switch for enabling and disabling the current flow through the secondary connection path. The gate of protection switching element U2 is connected to a gate control circuit (not shown) for increasing and decreasing a gate voltage. The gate voltage may be hardware or software controlled. The MOSFET U2 won't conduct until the gate voltage reaches a threshold voltage and so acts as a main power switch. In the absence of a fault condition being detected, the second MOSFET U2 is configured by the gate control circuit to switch to a conducting state, when the protection circuit is turned “ON” using the hardware or software control, thereby enabling current to flow through the secondary connection path via the current limiting resistor and begin charging the input capacitance C1 of the target component. As described above for the previous examples, the delay circuit (not shown) is configured to switch the MOSFET U1 from an initialisation configuration (in which the main connection path is disabled) to a steady-state configuration (in which the main connection path is enabled) after both sides of the MOSFET U1 are charged to the input voltage from the DC power source. Therefore, the first MOSEFT U1 may be a low voltage first MOSEFT U1, albeit the first MOSFET U1 must be able to conduct the main operating current required by the target component. The main switching element U1 of the main connection path may include a plurality of MOSFETs U1 connected to each other in parallel in order to reduce the current through each MOSFET. This results in a reduces an RDS-on (drain-source on-resistance) of each MOSFET which in-turn reduces the overall power consumption of the MOSFETs. In contrast, the second MOSFET U2 must be configured to receive a high voltage from the input terminal CON1 upon start up. However, a current rating of the second MOSFET U2 may be lower than the current requirement of the first MOSFETs U1 since the current thought the secondary connection path is limited by the current limiting resistor R1. In contrast to the relays of Fig. 3, the MOSFETs U1, U2 of the protection circuit 200 in Fig. 4 are solid state devices which therefore have an extended longevity compared to relays since they do not include moving parts resulting in increased reliability. This also results in less electrical and mechanical noise being generated when the MOSFETs U1, U2 are switching. Additionally, the MOSFETs U1, U2 may be smaller than the relays enabling the protection circuit 200 to be smaller overall and more portable. In some examples, the protection circuit 200 may comprise multiple stages to accommodate even higher voltages and powers. For example, an initial pre-charge may be implemented as described in the present examples to being charging the input capacitance C1 and increase the output voltage, then an intermediate stage (not shown) may be enabled to further increase the output voltage towards the input voltage, and then a final stage (comprising the main connection path) may be enabled for providing power to the target component during normal, steady-state operation. Fig 4. shows a system 300 comprising a protection circuit 310 according to aspects of the present invention. In this example, the system 300 is an audio amplifier system 300 comprising the protection circuit 310 for providing power from a DC power source in the form of two batteries 306a, 306b to a high power audio amplifier 312. Such a system 300 may be for a portable outdoor sound system for used in e.g., a large outdoor event like a festival. This audio amplifier system 300 comprises a split-rail high voltage battery system with peripheral control circuitry to provide significant audio output power without the need for continuous mains input. The amount of energy storage within the system means that peaks in the input audio programme do not demand peaks from the input power source and so the power demand during steady state operation is smoothed over the whole audio programme, on average. In summary, the audio amplifier system 300 comprises charging inputs 302a, 302b for connecting the system 300 to a power source (such as the mains or a generator) for providing charging power, a charging input circuit 304 for charging the batteries 306a, 306b using the charging power, battery indicators 308a, 308b for indicting when the batteries 306a, 306b are charging, the protection circuit 310 as described above for providing power from the batteries to an audio amplifier 312, a balanced XLR audio input circuit 316, audio inputs 318a, 318b for receiving audio signals, the audio amplifier 312 for amplifying the audio signals, loudspeaker shut-off circuits 320a, 320b, and output connectors 322a, 322b for providing the amplified audio signals to one or more loudspeakers. The charging input circuit 304 is configured to provide soft-start charging for the batteries 306a, 306b to prevent spectral splatting when connecting the charging power source to the batteries with the additional benefit of protecting the batteries 306a, 306b from in-rush currents. A reverse protection diode is used to prevent the battery voltage (and power) from being available on a front panel of the audio system 300. Additionally, DC filtering is included in the charging input circuit 204 to remove noise from the charger DC output. Next, once the batteries 306a, 306b have charged, or are charging, the audio amplifier 312 may be enabled by actuating the main power switch of the protection circuit 310. The protection circuit 310 then enables the provision of power from the batteries 306a, 306b to the audio amplifier 312 while protecting the audio amplifier 312 from an in-rush current and implementing fault detection as discussed above. The rest of the system 300 includes the loudspeaker shut-off circuits 320a, 320b, which are configured prevent audio splatting during turn off, and the balanced XLR audio input circuit 316 which takes in a balanced audio signal and converts it to single-ended for the audio amplifier 312. The audio amplifier system 300 of Fig. 4 is therefore more portable and consumes less power than existing audio amplifier systems. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed 5 herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%. 10

Claims

1. A protection circuit for protecting a target component from an in-rush current resulting from the initiation of an electrical connection between a direct current, DC, source and the target component through the protection circuit during an initialisation phase, the protection circuit comprising:an input terminal for electrically connecting the protection circuit to the DC source; and an output terminal for electrically connecting the protection circuit to the target component;a pre-charging circuit comprising a secondary connection path for providing a first electrical connection between the input terminal and the output terminal during the initialisation phase; anda main connection path for providing a second electrical connection between the input terminal and the output terminal during a steady-state phase,wherein the main connection path comprises a main switching element which is configured to selectively disable the second electrical connection during the initialisation phase when the main switching element is in an initialisation configuration, and enable the second electrical connection during the steady-state phase when the main switching element is in a steady-state configuration,wherein the secondary connection path comprises a current limiting element configured to regulate the in-rush current during the initialisation phase; andwherein, during the initialisation phase, the pre-charging circuit is configured to reduce a potential difference across the main switching element to or below a predetermined threshold and then transition from the initialisation phase to the steady-state phase by switching the main switching element from the initialisation configuration to the steady-state configuration.

2. The protection circuit of claim 1 further comprising a main power switch configured to selectively initiate and terminate the electrical connection between the DC source and the target component provided by the first and / or second electrical connections.

3. The protection circuit according to any preceding claim wherein the pre-charge circuit is configured to transition from the initialisation phase to the steady-state phase by switching the main switching element to the steady-state configuration after a predetermined time period has elapsed.

4. The protection circuit of claim 3 wherein the pre-charging circuit comprises a delay circuit configured to receive an output voltage from the output terminal, wherein the delay circuit is configured to generate a delayed switching signal based on the output voltage after the predetermined time delay, wherein the delayed switching signal is configured to switch the main switching element from the initialisation configuration to the steady-state configuration.

5. The protection circuit of claim 4 wherein the delay circuit comprises a voltage regulator having an associated start time corresponding to the predetermined time delay.

6. The protection circuit of claim 5 wherein the voltage regulator is configured to generate the delayed switching signal by regulating an output voltage at the output terminal to a switching voltage, wherein the switching voltage is configured to switch the main switching element from the initialisation configuration to the steady-state configuration.

7. The protection circuit of any preceding claim further comprising one or more protection switching elements which are configured to terminate the provision of DC power from the input terminal to the output terminal upon detection of a fault condition.

8. The protection circuit of claim 7 wherein the protection circuit comprises one or more fault sensors configured to detect the fault condition.

9. The protection circuit of claim 8 wherein the one or more fault sensors comprise one or more thermal sensors configured to monitor a temperature, wherein the fault condition is detected when the temperature is determined to be higher than a predetermined threshold temperature.

10. The protection circuit of claim 8 or 9 wherein the one or more fault sensors are a plurality of fault sensors,wherein the plurality of fault sensors are connected to each other in series and each fault sensor is configured to create an open circuit upon detection of a respective fault condition thereby disabling the provision of DC power from the input to the output terminal.

11. The protection circuit according to any one of claims 8 to 10 wherein the one or more protection switching elements are configured to be held in a closed state by a hold current wherein the provision of DC power from the input terminal to the output terminal is enabled,wherein the hold current is provided from the input terminal to the one or more protection switching elements via a hold current connection,wherein the one or more fault sensors are configured to create an open circuit in the hold current connection upon detection of a respective fault condition thereby transitioning the one or more protection switching elements to an open state and terminating the provision of DC power from the input terminal to the output terminal.

12. The protection circuit of claim 11 wherein the one or more protection switching elements each comprise a relay, wherein the relays are configured to switch from disabling configuration, in which the first and / or second electrical connections between in the input and output terminal are disabled, to an enabling configuration, in which the first and / or second electrical connections between in the input and output terminal are enabled, upon application of an energising current.

13. The protection circuit of claim 12 wherein the energising current is provided from the input terminal to the one or more protection switching elements via an energising connection provided in parallel with the hold current connection,wherein the energising connection comprises an energising capacitor configured to provide the energising current to the one or more protection switching elements upon initiation of an electrical connection between the input terminal and the energising capacitor.

14. The protection circuit according to any one of claims 7 to 13 wherein the one or more protection switching elements are configured to disable the provision of DC power from the input terminal to the output terminal for at least a predetermined fault lock-out time.

15. The protection circuit of 14, as dependent on claim 13, wherein the energising connection further comprises a discharge resistor in parallel with the energising capacitor, wherein a time constant of the discharge resistor and the energising capacitor is configured to be at least the predetermined fault lock-out time.

16. The protection circuit of any one of claims 14 or 15 wherein the predetermined fault lock-out time is at least 5 seconds.

17. The protection circuit according to any one of claims 7 to 16 wherein the secondary connection path comprises a first protection switching element of the one or more protection switching elements, the first protection switching element, when closed, being configured to complete the secondary connection path, and when open, being configured to break the secondary connection path.

18. The protection circuit according to any one of claims 7 to 17, wherein a second protection switching element of the one or more protection switching elements is configured to selectively enable and disable the second electrical connection provided by the main connection path between the input and output terminals.

19. The protection circuit of claim 18, as dependent on any one of claims 4 to 8, wherein the second protection switching element is configured to selectively enable and disable the second electrical connection by selectively enabling and disabling an electrical connection between the delay circuit and the main switching element.

20. The protection circuit according to any preceding claim wherein the main switching element, and / or, the one or more protection switching elements if included, comprise relays;wherein the protection circuit further comprises one or more power saving circuits electrically connected in series to a control terminal of the main switching element and / or the one or more protection switching elements;wherein each power saving circuit comprises an energising capacitor and a resistor connected in parallel to each other.

21. The protection circuit according to any one of claims 1 to 20 wherein the main switching element comprises a transistor.

22. The protection circuit according to claim 21 wherein the main switching element comprises a plurality of transistors electrically connected to each other in parallel.

23. The protection circuit according to any preceding claim wherein the protection circuit is a split-rail protection circuit,wherein the main connection path is a first main connection path for electrically connecting a positive rail of the input terminal to a positive rail of the output terminal, and the secondary connection path is a first secondary connection path for electrically connecting the positive rail of the input terminal to the positive rail of the output terminal,wherein the protection circuit comprises a second main connection path for electrically connecting a negative rail of the input terminal to a negative rail of the output terminal, the second main connection path comprising a second main switching element which is operable in an initialisation configuration and a steady-state configuration,wherein the pre-charging circuit comprises a second secondary connection path for electrically connecting the negative input terminal and the negative output terminal, the second secondary connection path comprising a second current limiting element;wherein the pre-charging circuit is further configured to reduce a potential difference across the second main switching element to the predetermined threshold and then transition from the initialisation phase to the steady-state phase by switching the second main switching element to the steady-state configuration.

24. An audio system comprising the protection circuit of any preceding claim, and a target component electrically connected to the output terminal, wherein the target component is an audio amplifier.

25. The audio system of claim 24 further comprising a DC source electrically connected to the input terminal, wherein the DC source comprises a battery.