Charge pump

Abstract

A charge pump 3 comprises a switch 5 connected between an energy storage device and an electrical bus, particularly for delivering power to an electric motor of a vehicle. A gate driver 32 is configur

Description

TECHNICAL FIELD The present disclosure relates to a charge pump. Aspects of the invention relate to a charge pump, an electrical supply system for a vehicle, a vehicle incorporating such electrical supply system and to a method of controlling a charge pump. BACKGROUND Electric vehicles and hybrid electric vehicles may utilise a charge pump to control a switch used to connect a 48V (for example) energy storage device (battery) to a 48V (for example) electrical bus used to provide electrical energy to the propulsion system and other systems of the vehicle. More specifically, the charge pump includes a gate driver for applying a voltage to a gate of the switch, and a switched capacitor stage which serves to multiply an input voltage of the charge pump to a level capable of activating a gate of the switch. It is important that the gate driver voltage is retained within a specific band or range, since if it is too low the switch may not open fully (which may cause electrical problems), and if it is too high the switch may be damaged. In general terms, a charge pump is a form of DC-to-DC converter that uses switching and capacitors to raise or lower voltage. In the present case the charge pump is used to increase voltage. Charge pumps are well known electrical components and will not be described in detail herein. The switching within the charge pump when used in this way is conventionally controlled with a pulse width modulated (PWM) signal of fixed frequency and fixed duty cycle. The fixed frequency and duty cycle are set so that there is always enough energy provided to fully turn on the isolation switch when asserted at the gate of the isolation switch. The PWM signal is a square wave. Conventionally, a Zener diode is used to clamp the output voltage of the gate driver to an appropriate level as a protection mechanism. A problem with this is that energy is dissipated as heat by this clamping, which is wasteful. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a charge pump, an electrical supply system for a vehicle, a vehicle incorporating such electrical supply system, and to a method of controlling a charge pump, as claimed in the appended claims. According to an aspect of the invention, there is provided a charge pump, comprising: a switch connected between an energy storage device and an electrical bus for delivering power to an electric motor of a vehicle; a gate driver configured to apply a voltage to a gate of the switch; voltage measurement circuitry configured to measure the voltage output of the gate driver; and a controller configured to: control the gate driver using a pulse width modulated input signal; detect a change in the measured voltage; and modify the duty cycle of the pulse width modulated input signal in dependence on the detected change. The controller may for example be responsive to detecting that the measured voltage has risen above a first voltage threshold to reduce the duty cycle of the pulse width modulated input signal. The initial duty cycle in this case may be (relatively) optimal for charging, while the reduced duty cycle either prevents, inhibits or slows the voltage from going out of range when it reaches the threshold. This has efficiency benefits (less power dissipated by the Zener diode) as well as robustness benefits (less heating, and giving the system longer to react to a fault). The controller may comprise: at least one electronic processor having an electrical input for receiving the measured voltage; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to modify the duty cycle of the pulse width modulated input signal in dependence on a change in the measured voltage. The duty cycle may be adjusted in a variable manner, for example in proportion to a rate or magnitude of change measured by the voltage measurement circuit. Alternatively, the duty cycle may be reduced by switching from a first duty cycle to a second duty cycle lower than the first duty cycle. The second duty cycle may be much lower than the first duty cycle, with the first duty cycle providing relatively optimal (and fast) charging, and the second duty cycle effectively providing a trickle charge, or temporarily pausing charging until the output voltage drops to an acceptable level. Subsequently to the transition to the second duty cycle (or more generally, to the decrease in duty cycle), the controller may be responsive to detecting that the measured voltage has fallen below a second threshold voltage to increase the duty cycle of the pulse width modulated input signal. For example, the duty cycle may be increased by switching from the second duty cycle back to the first duty cycle. This enables reestablishment of the first, higher (and more efficient for charging) duty cycle in the case that the voltage measured by the measurement circuity drops to the second threshold level. The second threshold voltage may be much lower than the first threshold voltage, to reduce the likelihood of rapid flip-flopping between the two duty cycles. The second threshold voltage may be significantly above a gate voltage at which the switch is fully open. A clamping diode, for example a Zener diode, may be provided which is configured to clamp the voltage applied to the gate of the switch to prevent it from going above a maximum voltage rated for the switch. In principle the use of a variable duty cycle charge pump as described herein could be used without a clamping diode (which is essential for robustness reasons in more conventional arrangements), but a clamping diode may still be provided as an additional backup. A breakdown voltage of the clamping diode may be approximately the same or higher than the first threshold voltage. This would mean that the controller intervenes to reduce the voltage at or before the point at which the clamping diode would start to act, thereby reducing the power dissipated by the clamping diode. While the switch is open, the first duty cycle may be applied by the controller irrespective of the voltage applied to the gate of the switch. The first duty cycle may be approximately 50% (for example between 45% and 55%, or between 49% and 51%). This provides (relatively) optimal efficiency within the switched capacitor stage of the charge pump. In this event, the second duty cycle is less than 50%, and may be significantly less than 50%. For example, the second duty cycle may be between 5% and 30%, and may be approximately 5%, or even 0%. The second duty cycle may therefore be considered as a trickle charge mode of operation, or in the case of 0% a non-charging mode. The controller may be responsive to measured output voltage above a target voltage to reduce the duty cycle, and be responsive to measured output voltage below the target voltage to increase the duty cycle. The controller may be configured to open the switch to isolate the energy storage device from the bus in response to the detection of a fault. The fault could be the failure or faulty operation of an electrical component, including in the charge pump, and potentially including the clamping diode. Various different formulations of voltage measurement and control circuitry may be used. For example, the voltage measurement and control circuitry may comprise or use a Schmitt trigger, or a PID (proportionalintegral-derivative) controller. These elements are well known to the person skilled in the art. According to another aspect of the invention, there is provided an electrical supply system comprising the charge pump described above, the energy storage device and the electrical bus. According to another aspect of the invention, there is provided a vehicle comprising the electrical supply system and / or charge pump as described above. According to another aspect of the invention, there is provided a control method for a charge pump, the method comprising: selectively opening and closing a switch connected between an energy storage device and an electrical bus for delivering power to an electric motor of a vehicle; controlling a gate driver, which applies a voltage to a gate of the switch, using a pulse width modulated input signal; measuring the voltage output of the gate driver; and responsive to detecting that the measured voltage has changed, modifying the duty cycle of the pulse width modulated input signal in dependence on the detected change. The detected change may be a detection that the measured voltage has risen above a first voltage threshold, and the modification of the duty cycle may be a reduction in the duty cycle of the pulse width modulated input signal. The method may further comprise, responsive to detecting that the measured voltage has fallen below a second threshold voltage, increasing the duty cycle of the pulse width modulated input signal. In summary of one implementation, as a default the duty cycle of the PWM pulse to the charge pump circuit, to boost the voltage to be applied to the isolation switch gate, is 50%. At the start (when the switch is open and the charge pump is initialising) a 50% duty cycle is adequate to build up the charge pump voltage, however subsequently this 50% duty cycle may tend to generate a higher charge pump output voltage than necessary to close the effect closing of the switch. To avoid damaging the isolation switch, the output voltage has conventionally been clamped by the clamping (Zener) diode, resulting in an increase in diode temperature / heat generation, and consequently a loss of overall efficiency. While in some implementations of the present technique the clamping diode is retained as a backup, by modifying the PWM duty cycle it is possible to avoid (or at least reduce) excess gate drive voltage when the isolation switch is closed. According to another aspect of the invention, there is provided a charge pump to apply a voltage to a gate of a switch that controls the connection of an electricity power source and a load, the charge pump comprising a gate driver configured to operate the gate of the switch, voltage measurement circuitry configured to measure the voltage output of the gate driver and a controller configured to control the gate driver using a pulse width modulated input signal detect a change in the measured voltage and modify the duty cycle of the pulse width modulated input signal in dependence on the detected change. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a schematic representation of a vehicle having an energy storage device, electrical bus and charge pump driven isolation switch; Figure 2 shows a charge pump according to an embodiment of the present invention; and Figure 3 shows a schematic flow diagram of the control method for the charge pump. DETAILED DESCRIPTION An electric or hybrid vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figure 1. The vehicle 1 comprises an electrical supply system which in turn comprises an energy storage device (battery) 2 which is connectible to a 48V bus 4. The bus 4 is able to deliver electrical power to the propulsion system of the vehicle, and to other vehicle systems (not shown). A smaller 12V battery (not shown) is also provided, separately from the battery 2, which is not intended to be used for propulsion, but which can power other vehicle systems. A charge pump 3 (considered here to include an isolation switch 5 for selectively connecting the 48V bus 4 to the battery 2) is disposed between the battery 2 and the 48V bus 4. To electrically connect the battery 2 to the bus 4, a gate of the switch 5 must be supplied with a sufficiently high control voltage. The charge pump 3 achieves this using a switched capacitor stage, as will be described subsequently. The switched capacitor stage generates a desired control voltage from a supply voltage provided by the 12V battery. Referring to Figure 2, the charge pump 3 is shown in more detail. In particular, the charge pump 3 can be seen to comprise the switch 5, a switched capacitor gate driver 32, voltage measurement circuitry 34 and a controller 36. The switched capacitor gate driver 32 uses voltage doubler (or equivalent) circuitry to boost an input voltage from the 12V battery (or other low voltage auxiliary supply) to a level required to activate a gate 5a of the switch 5, to turn the switch 5 on fully (to close it and connect the battery 2 to the bus 4). This is achieved using a collection of switched capacitors controlled by a PWM input, operating at a specified duty cycle (described subsequently) and frequency (for example of 20kHz). Voltage change does not happen instantaneously, but builds up over a (relatively short, from the perspective of the driver) period of time as the capacitors are repeatedly switched under PWM control. The charge pump 3 also comprises a clamping diode 37 at the output of the switched capacitor gate driver 32 which serves to clamp (limit) the output voltage being provided to the gate 5a of the switch 5 to a desired level (preferably 16.8V in the present implementation). Finally, the charge pump 3 comprises a control switch 38 which selectively connects the output of the switched capacitor gate driver 32 and clamping diode 37 to the gate 5a. When the control switch 38 is open, no voltage is applied to the gate 5a, irrespective of the level of output voltage generated by the switched capacitor gate driver 32, while when the switch 38 is closed, the voltage generated by the switched capacitor gate driver 32 (and if necessary clamped by the clamping diode 37) is applied to the gate 5a. The purpose of the control switch 38 is to make it possible to assert the output voltage to the gate 5a only when it has reached a level at which it can turn on the isolation switch 5 fully. The control switch is also able to immediately turn off the isolation switch 5 in the event of a fault by disconnecting the gate 5a from the output voltage of the charge pump 3. The controller 36 (which may be implemented wholly in hardware or by using a combination of hardware and software) comprises several functional components, including a feedback control part 36a, an out-of-bounds voltage detection part 36b and a deactivation part 36c. The feedback control part 36a controls the duty cycle of the PWM input signal in dependence on the output voltage of the gate driver 32, as measured by the measurement circuitry 34. The generation of the output voltage based on the PWM signal, the measurement of the output voltage, and the adjustment of the duty cycle of the PWM signal based on the measured voltage define a feedback loop which is used to maintain the output voltage at a desired value, or within a desired range, by dynamically adjusting the duty cycle of the PWM input signal. In one implementation, the feedback loop is responsive to the output voltage increasing above a target value to lower the duty cycle of the PWM signal until the output voltage is back at the target value, and is responsive to the output voltage decreasing below the target value to raise the duty cycle of the PWM signal until the output voltage is back at the target value. The respective increases and decreases of the duty cycle may be fixed (that is, to a fixed lower or higher duty cycle), or variable, such as being increased in proportion to the magnitude of the difference between the measured voltage and the target voltage. One suitable device for this implementation is a PID (proportional-integral-derivative) controller, which is able to smoothly follow a target value by finely adjusting the duty cycle of the input PWM signal. In another implementation, the feedback loop serves to maintain the output voltage within a desired range. In this case, while the output voltage as measured by the measurement circuitry 34 remains below a predetermined (first) threshold (of 16.8V, in the present implementation) the PWM duty cycle is set by the feedback control part 36a to be at a first level, of preferably 50%, which provides maximum charging speed and electrical efficiency. However, when the output voltage is measured as increasing to above the predetermined first threshold, the feedback control part 36 modifies or adjusts the PWM duty cycle to be at a lower level of (substantially) less than 50%, and preferably drops it to a trickle charge duty cycle of around 5%, or even a value of 0%. One suitable device for this implementation is a Schmitt Trigger controller, which flip flops between 0% and 50%, 0% when the high voltage threshold is reached and 50% when the low threshold is reached. From the above it will be appreciated that the controller dynamically adjusts the duty cycle to keep the voltage steady, or within range. In principle, it will be adjusted for a steady state so the energy pumped into the system is the same as the energy lost whilst keeping a fixed average voltage. It is for this reason that the Zener diode will not be loaded, resulting in improved efficiency. If the output voltage subsequently drops below a (second) predetermined threshold, which is either the same, or preferably lower (for example 15.5V) than the first predetermined threshold, then the higher duty cycle may be reinstated. Optionally, this reversion to the original (first) duty cycle may be based not only on the output voltage dropping below the predetermined threshold, but also on it being in this state for at least a predetermined period of time (for example 1 second). In contrast, the transition from the first duty cycle to the second duty cycle may be instantaneous. It does not require the output voltage to be above the first threshold for any specific minimum duration. The out of bounds voltage detection part 36b detects when the output voltage has reached an unacceptably high level (out of bounds), and if so triggers the deactivation part 36c to switch off the PWM input signal to stop the gate driver 32 from applying a level of voltage which might damage the switch 5. In one implementation the out of bounds voltage is 18V. Switching off the PWM input signal will automatically result in the output voltage dropping and the switch 5 transitioning to a disconnected state. The unacceptable level (of 18V for example) used to trigger deactivation is higher than the first predetermined threshold (of 16.8V for example) used to adjust the PWM duty cycle. In practice, by adjusting the duty cycle to a very low level (of say 5%), any output voltage upward drift from the first predetermined threshold due to a fault in the circuitry will be much slower than would be the case if the switched capacitor gate driver 32 continued to be driven using a 50% duty cycle. This improves robustness since the controller 36, and in particular the out of bounds voltage detection part 36b is given longer to react to a voltage uplift (for example by deactivating the PWM signal entirely) before the gate voltage reaches levels which might damage the switch 5. Referring to Figure 3, a control method for operating the charge pump 3, and more generally for connecting the battery 2 to the bus 4, is shown. The method starts at a step S1, for example when the vehicle 1 is first switched on. At a step S2 a built-in check is carried out before the battery 2 is electrically connected to the bus 4. The built-in check comprises at least a step S2a of switching on a PWM control signal to a switched capacitor stage of the charge pump 3, and a step S2b of checking the voltage level being output from the switched capacitor stage. The output voltage is required to be high enough that when it is applied to the gate 5a, the switch 5 will be turned fully on, and since it will take the charge pump 3 a non-zero time period to reach a state, the built-in check S2 cannot complete until this level has been reached. At a step S3 the build in check is completed (when the voltage is at the required level), and the process moves onto a step S4 where the control switch 38 is closed such that the output voltage is applied to the gate 5a of the isolation switch 5, thereby turning on the isolation switch 5 to connect the energy storage device 2 to the bus 4. The switch 5 is switched on by the application of the voltage to its gate 5a. At a step S5 the output voltage, as provided to the gate 5a, is continuously monitored, with the duty cycle of the charge pump being adjusted if necessary depending on the output voltage level of the charge pump, and with the gate driver 32 being deactivated entirely (using the switch 38) in the event of a fault being detected (for example when the voltage at the gate goes out of bounds). At a step S6 the process stops, generally when the vehicle is deactivated. It will be appreciated that the energy storage device and the electrical bus may be referred to as an electricity power source. It will be appreciated that the electric motor of a vehicle may be referred to as a load, and that any suitable load may be connected to electricity power source by way of the switch. It will be appreciated that the duty cycle adjustment as a function of voltage only takes place while the switch 38 (and thus the isolation switch 5) is closed. While the switches 38 and 5 are open (for example during initialisation and start-up), a fixed duty cycle is used. While in the described example a single isolation switch 5 is indicated, in practice multiple such switches may be provided in series with each other between the battery 2 and the bus 4. In this case the output voltage of the charge pump is simultaneously used to drive the gates of all such switches. Each of these switches is only able to handle a gate voltage of a certain level (for example the 18V out of bounds level indicated above, or a voltage a little above this), and so the control methodology using variation of PWM duty cycles is beneficial in ensuring that the gate voltage for all of these switches remains high enough to activate the gate, but low enough so as not to damage the switches, while maximising efficiency (in other words, minimising heat dissipation through the clamping diode). It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. For example, all of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of 5 such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic 10 series of equivalent or similar features. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or 15 process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

1. A charge pump, comprising:a switch connected between an energy storage device and an electrical bus for delivering power to an electric motor of a vehicle;a gate driver configured to apply a voltage to a gate of the switch;voltage measurement circuitry configured to measure the voltage output of the gate driver; anda controller configured to:control the gate driver using a pulse width modulated input signal;detect a change in the measured voltage; andmodify the duty cycle of the pulse width modulated input signal in dependence on the detected change.

2. The charge pump of claim 1, wherein the controller is responsive to detecting that the measured voltage has risen above a first voltage threshold to reduce the duty cycle of the pulse width modulated input signal.

3. The charge pump of claim 2, wherein the duty cycle is reduced by switching from a first duty cycle to a second duty cycle lower than the first duty cycle.

4. The charge pump of any preceding claim, wherein the controller is responsive to detecting that the measured voltage has fallen below a second threshold voltage to increase the duty cycle of the pulse width modulated input signal.

5. The charge pump of claim 3 and claim 4, wherein the duty cycle is increased by switching from the second duty cycle to the first duty cycle.

6. The charge pump of claim 4 or claim 5, wherein the second threshold voltage is lower than the first threshold voltage.

7. The charge pump of any preceding claim, comprising a clamping diode configured to clamp the voltage applied to the gate of the switch.

8. The charge pump of claim 7, wherein a breakdown voltage of the clamping diode is approximately the same or higher than the predetermined threshold value.

9. The charge pump of any preceding claim, wherein while the switch is open, the first duty cycle is applied by the controller irrespective of the voltage applied to the gate of the switch.

10. The charge pump of any preceding claim, wherein the controller is responsive to an increase in measured output voltage above a target voltage to reduce the duty cycle, and is responsive in a measured output voltage below the target voltage to increase the duty cycle11. The charge pump of any preceding claim, wherein the controller is configured to open the switch in response to the detection of a fault.

12. An electrical supply system comprising the charge pump of any preceding claim, the energy storage device and the electrical bus.

13. A vehicle comprising the electrical supply system according to claim 12.

14. A control method for a charge pump, the method comprising:selectively opening and closing a switch connected between an energy storage device and an electrical bus for delivering power to an electric motor of a vehicle;controlling a gate driver, which applies a voltage to a gate of the switch, using a pulse width modulated input signal;measuring the voltage output of the gate driver; andresponsive to detecting that the measured voltage has changed, modifying the duty cycle of the pulse width modulated input signal in dependence on the detected change.

15. The control method of claim 14, wherein the detected change is a detection that the measured voltage has risen above a first voltage threshold, and the modification of the duty cycle is a reduction in the duty cycle of the pulse width modulated input signal.

16. A charge pump to apply a voltage to a gate of a switch that controls the connection of an electricity power source and a load, the charge pump comprising:a gate driver configured to operate the gate of the switch;voltage measurement circuitry configured to measure the voltage output of the gate driver; anda controller configured to:control the gate driver using a pulse width modulated input signal;detect a change in the measured voltage; andmodify the duty cycle of the pulse width modulated input signal in dependence on the detected change.

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