Power supply circuit and meter device

Abstract

There is provided a power supply circuit 100, comprising: a power converter 120 having a first input node 122 and a first output node 124, wherein the power converter is operable to receive an input voltage at the first input node 122 and to generate an output voltage at the first output node 124 which is at a higher voltage level than the input voltage; a battery 110 having a second output node 112, wherein the second output node 112 is electrically coupled to the first input node 122; an output terminal 160; a P-channel transistor 130 comprising a first electrode, a second electrode and a gate electrode, wherein the first electrode is electrically coupled to the second output node 112, and the second electrode is electrically coupled to the output terminal 160; a first unidirectional conducting device 150 electrically connected between the first output node 124 and the output terminal 160, wherein the first unidirectional conducting device 150 comprises a first anode and a first cathode, and wherein the first anode is electrically coupled to the first output node, and the first cathode is electrically coupled to the output terminal; and a second unidirectional conducting device 170 electrically connected between the first output node 124 and the gate electrode of the P-channel transistor 130, wherein the second unidirectional conducting device 170 comprises a second anode and a second cathode, and wherein the second anode is electrically coupled to the first output node 124, and the second cathode is electrically coupled to the gate electrode.

Description

[0001] Power Supply Circuit and Meter Device

[0002] Technical Field

[0003] This disclosure relates to a power supply circuit and a meter device including the same. More particularly, but not exclusively, this disclosure relates to a power supply circuit that includes a battery with an OR-ing P-channel transistor, and a feedback output network which prevents back-driving the battery through the OR-ing P-channel transistor.

[0004] Background

[0005] Figure 1 schematically illustrates a known power supply circuit 100’ which supplies power to a load at its output terminal 160’. The output terminal 160’ is driven by OR-ing the outputs of a battery 110’ and a power converter 120’. The power converter 120’ converts an output voltage of the battery 110’ to a different, higher, voltage level. The OR-ing function is achieved by a P-channel metal-oxide-semiconductor field-effect transistor (P-MOSFET) 130’ connected between an output of the battery 110’ and the output terminal 160’, and by a diode 150’ connected between an output of the power converter 120’ and the output terminal 160’. The gate of the P-MOSFET 130’ is driven by a control circuit 140’ which may be powered by the battery 110’. The power supply circuit 100’ is suitable for maintaining a constant and stable output voltage over battery input voltage variations and over temperature changes.

[0006] The P-MOSFET 130’ is used instead of a regular diode to minimize the battery voltage drop. This is desirable when the power converter 120’ is turned OFF and the battery current flows through the P-MOSFET 130’, which has very low DC resistance. While the power supply circuit 100’ has many advantages, it can sometimes present a back- drive current when the battery voltage is low enough. More specifically, when the output of the power converter 120’ minus the forward voltage drop of the diode 150’ becomes higher than a gate voltage of the P-MOSFET 130’ by a threshold voltage Vcs(th) of the P-MOSFET 130’, the P-MOSFET 130’ is turned ON instead of OFF. Consequently, a large back-drive current flows into an anode of the battery 110'. In the event that the battery 110’ is a primary cell lithium battery, back-driving the battery essentially amounts to charging the battery. This poses grave danger as a primary cell lithium battery is not constructed to be charged unlike a secondary battery. If this takes place, gas is formed within the cell of the battery 110’, causing its internal pressure to rise. This may result in venting, or most seriously, in explosion of the battery 110’.

[0007] Known solutions to this problem include (i) preventing the battery voltage from getting low enough so as to trigger a current back-drive condition, and (ii) using a P-MOSFET with a higher Ves h) which is not overcome by the output voltage of the power converter 120’ at the source of the P-MOSFET. Solution (i) has the disadvantage of wasting usable battery energy. While Solution (ii) works, it is not ideal since a P-MOSFET with a higher will present a higher channel DC resistance when the battery voltage decreases and therefore negating the purpose of using a P-MOSFET instead of a diode for the OR-ing function. Additionally, a P-MOSFET with high Vcstth; cannot be perfectly controlled in production and the Vcs(th) will also vary with temperature.

[0008] It is an object of the present invention, among others, to provide an improved power supply circuit which solves the problem of back-driving the battery.

[0009] According to a first aspect of the present disclosure, there is provided a power supply circuit, comprising: a power converter having a first input node and a first output node, wherein the power converter is operable to receive an input voltage at the first input node and to generate an output voltage at the first output node which is at a higher voltage level than the input voltage; a battery having a second output node, wherein the second output node is electrically coupled to the first input node; an output terminal; a P-channel transistor comprising a first electrode, a second electrode and a gate electrode, wherein the first electrode is electrically coupled to the second output node, and the second electrode is electrically coupled to the output terminal; a first unidirectional conducting device electrically connected between the first output node and the output terminal, wherein the first unidirectional conducting device comprises a first anode and a first cathode, and wherein the first anode is electrically coupled to the first output node, and the first cathode is electrically coupled to the output terminal; and a second unidirectional conducting device electrically connected between the first output node and the gate electrode of the P-channel transistor, wherein the second unidirectional conducting device comprises a second anode and a second cathode, and wherein the second anode is electrically coupled to the first output node, and the second cathode is electrically coupled to the gate electrode.

[0010] Advantageously, the second unidirectional conducting device forms an output feedback network which establishes an electrical connection between the output voltage of the power converter at the first output node and the gate electrode of the P-channel transistor. With the second unidirectional conducting device, the voltage at the gate electrode of the P-channel transistor can be controlled to be slightly less or equal to the voltage at the output terminal, such that a threshold voltage of the P-channel transistor is not overcome and the P-channel transistor stays off. As a result, there is no back- drive current flowing into the battery through the P-channel transistor, regardless of the voltage level of the battery. The use of the second unidirectional conducting device therefore provides a low cost and rather simple solution that prevents possibly dangerous back-drive current into the battery, and also lengthens the usable life of the battery by allowing the output voltage of the battery to reach very low levels.

[0011] It would be understood that the first / second unidirectional conducting device allows current flow if a voltage level at the first / second anode is higher than a voltage level at the first / second cathode (i.e., when forward-biased), and blocks current flow if a voltage level at the first / second anode is lower than a voltage level at the first / second cathode (i.e., when reverse-biased).

[0012] It would further be understood that the unidirectional conducting device conducts current primarily in one direction and thus has asymmetric conductance. During normal operating conditions (excluding reverse breakdown), the unidirectional conducting device has low resistance in one direction and high resistance in the other.

[0013] The term “electrically coupled” includes that one or more intervening element(s) adapted for signal transmission may exist between the electrically coupled elements.

[0014] It would also be understood that the gate electrode controls a current flow between the first and second electrodes. A forward voltage drop of the first unidirectional conducting device may be greater than or equal to a forward voltage drop of the second unidirectional conducting device.

[0015] A type of the first unidirectional conducting device may be the same as a type of the second unidirectional conducting device.

[0016] In other words, the first and second unidirectional conducting devices may be made of the same material and / or may have the same structure. Alternatively, the first and second unidirectional conducting devices may be different types of electronic devices.

[0017] At least one of the first and second unidirectional conducting devices may comprise a diode.

[0018] At least one of the first and second unidirectional conducting devices may comprise a Schottky diode.

[0019] At least one of the first and second unidirectional conducting devices may comprise a circuit configured to operate like a diode.

[0020] The power supply circuit may further comprise a third unidirectional conducting device electrically connected between the first and second electrodes of the P-channel transistor. The third unidirectional conducting device may comprise a third anode and a third cathode. The third anode may be electrically coupled to the second output node, and the third cathode may be electrically coupled to the output terminal.

[0021] The third unidirectional conducting device may comprise a diode.

[0022] The P-channel transistor may be a P-channel metal-oxide-semiconductor field-effect transistor.

[0023] The power converter may comprise a DC-to-DC converter.

[0024] The power converter may comprise a switched-mode DC-to-DC converter. The switched-mode DC-to-DC converter may be a switched-mode DC-to-DC boost converter or a switched-mode DC-to-DC buck-boost converter.

[0025] The power supply circuit may further comprise a control circuit configured to control a voltage at the gate electrode of the P-channel transistor. The control circuit may be powered by the battery.

[0026] The control circuit may be further configured to generate an enabling signal which enables an operation of the power converter.

[0027] The power supply circuit may further comprise: a resistor electrically connected in series with the second unidirectional conducting device between the first output node and the gate electrode of the P-channel transistor.

[0028] The resistor may be configured to limit a current flowing through the control circuit.

[0029] The battery may comprise a primary cell lithium battery.

[0030] The battery may comprise at least one of a Lithium Thionyl Chloride battery and a Lithium Manganese Dioxide battery.

[0031] According to a second aspect of the present disclosure, there is provided a meter device comprising a power supply circuit according to the first aspect of the present disclosure.

[0032] The meter device may be configured to measure a usage amount of at least one of gas and water.

[0033] The meter device may further comprise: a wireless communication unit which is powered by the power supply circuit.

[0034] In other words, a voltage input of the wireless communication unit is electrically coupled to the output terminal of the power supply circuit.

[0035] The wireless communication unit may comprise a modem. According to a third aspect of the present disclosure, there is provided a method of assembling a power supply circuit which comprises a power converter, a battery, an output terminal, a P-channel transistor, and first and second unidirectional conducting devices, wherein: the power converter has a first input node and a first output node, and is operable to receive an input voltage at the first input node and to generate an output voltage at the first output node which is at a higher voltage level than the input voltage; the battery has a second output node; and the P-channel transistor comprises a first electrode, a second electrode and a gate electrode, the method comprising: electrically coupling the first input node of the power converter to the second output node of the battery; electrically connecting the first unidirectional conducting device between the first output node of the power converter and the output terminal, wherein the first unidirectional conducting device comprises a first anode and a first cathode, and electrically connecting the first unidirectional conducting device comprises electrically coupling the first anode to the first output node and electrically coupling the first cathode to the output terminal; electrically coupling the first electrode of the P- channel transistor to the second output node, and electrically coupling the second electrode of the P-channel transistor to the output terminal; and electrically connecting the second unidirectional conducting device between the first output node and the gate electrode of the P-channel transistor, wherein the second unidirectional conducting device comprises a second anode and a second cathode, and wherein electrically connecting the second unidirectional conducting device comprises electrically coupling the second anode to the first output node, and electrically coupling the second cathode to the gate electrode.

[0036] It would also be understood that the terms “first”, “second” and “third” are simply used in the present disclosure to label the relevant elements (“unidirectional conducting device”, “output node” etc.) for the ease of description, and do not imply any limitations to the sequence, location or the total number of the relevant elements.

[0037] Where appropriate any of the optional features described above in relation to one of the aspects of the disclosure may be applied to another one of the aspects of the disclosure.

[0038] Brief Description of the Drawings In order that the disclosure may be more fully understood, a number of embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings, in which:

[0039] Figure 1 is a schematic block diagram of a known power supply circuit;

[0040] Figure 2 is a schematic block diagram of a power supply circuit according to an aspect of the present disclosure;

[0041] Figure 3 is a schematic circuit diagram of a power supply circuit according to an aspect of the present disclosure;

[0042] Figure 4 schematically illustrates simulation waveforms of the power supply circuit of Figure 3;

[0043] Figure 5 is a schematic circuit diagram of a comparison power supply circuit which does not include a feedback output network of Figure 2 or 3;

[0044] Figure 6 schematically illustrates simulation waveforms of the power supply circuit of Figure 5;

[0045] Figure 7 is a schematic block diagram of a meter device according to an aspect of the present disclosure;

[0046] Figure 8 schematically illustrates processing steps of a method of assembling a power supply circuit, according to an aspect of the present disclosure.

[0047] In the figures, like parts are denoted by like reference numerals.

[0048] It will be appreciated that the drawings are for illustration purposes only and are not drawn to scale.

[0049] Detailed Description of the Preferred Embodiments Figure 2 schematically illustrates a power supply circuit 100 according to an aspect of the present disclosure. The power supply circuit 100 is for supplying power to an external load (not shown in Figure 2), and includes an output terminal 160 for electrically coupling to the load.

[0050] The power supply circuit 100 includes a power converter 120 having a first input node 122 and a first output node 124, and a battery 110 having a second output node 112. The second output node 112 is electrically connected to the first input node 122, such that the input voltage received by the power converter 120 at the first input node 122 is substantially the same as the output voltage of the battery 110. The power converter 120 generates an output voltage at the first output node 124 which is at a higher voltage level than the input voltage received at the first input node 122. In an example, the power converter 120 is a DC-to-DC converter, such as, a switched-mode DC-to-DC converter. More specifically, the power converter 120 may be a switched-mode DC-to- DC boost converter or a switched-mode DC-to-DC buck-boost converter. It would be appreciated that the power converter 120 may take any suitable form. For simplicity, Figure 2 does not show the GND node, and it will be understood that the nodes 112, 122, 124 and the output terminal 160 are non-GND (e.g., positive) voltage nodes.

[0051] The power supply circuit 100 further includes a P-channel transistor 130 having a source electrode, a drain electrode and a gate electrode. In an example, the P-channel transistor 130 is a P-MOSFET. It would however be appreciated that other types of P- channel transistors can also be used, such as, insulated-gate field-effect transistor (IGFET) or metal-insulator-semiconductor FET (MISFET). Figure 2 shows that the drain electrode of the P-channel transistor 130 is electrically connected to the second output node 112 of the battery 110, and that the source electrode of the P-channel transistor 130 is electrically connected to the output terminal 160. It would be understood that the P-channel transistor 130 may be structurally symmetric such that the source and drain electrodes are determined by voltages applied thereon and cannot be distinguished from one another by structure. The source and drain electrodes may be respectively referred to as “first electrode” and “second electrode”, or vice versa. A voltage difference between the gate electrode and the source electrode controls an ON / OFF status of the P-channel transistor 130. Therefore, it can be said that a voltage applied to the gate electrode controls a current flow between the first and second electrodes. A control circuit 140 is used to drive the gate electrode. The control circuit 140 is powered by the battery 110. In particular, a voltage input of the control circuit 140 is electrically connected to the second output node 112 of the battery 110. The voltage at the gate electrode may be equal to or slightly lower than an output voltage of the battery 110 at the second output node 112.

[0052] A diode 150 is electrically connected between the first output node 124 and the output terminal 160. In particular, the anode of the diode 150 is electrically connected to the first output node 124, and the cathode of the diode 150 is electrically connected to the output terminal 160. In an example, the diode 150 is a Schottky diode which has a lower forward voltage drop (e.g., 0.35V) than that of a P-N junction based diode (e.g., 0.7V). The “diode” used in the present disclosure may be referred to as a “unidirectional conducting device” which allows current flow (i.e., is forward-biased) if a voltage level at the anode is higher than a voltage level at the cathode, and blocks current flow (i.e., is reverse-biased) if a voltage level at the anode is lower than a voltage level at the cathode. The unidirectional conducting nature of the diode 150 prevents the battery voltage from back driving the output of the power converter 120. It would be appreciated that the diode 150 may be replaced by other types of unidirectional conducting devices, which include an integrated ideal diode (e.g., LM66100 manufactured by Texas Instruments®), or a MOSFET connected in diode fashion by shorting its gate and drain, etc. An integrated ideal diode is a circuit which operates like a diode. Generally, any device which can be controlled to have a low resistance in one direction and a high resistance in the other direction during normal operating conditions (excluding reverse breakdown) may be considered as a unidirectional conducting device and may be used to replace the diode 150. The anode and cathode of such a unidirectional conducting device can be readily determined based upon the voltage polarity along the low-resistance direction.

[0053] The P-channel transistor 130 and the diode 150 collectively form an OR-ing circuit, by OR-ing the outputs of the battery 110 and the power converter 120 to the output terminal 160. The control circuit 140 may also be referred to as an OR-ing controller. When the power converter 120 is off, the P-channel transistor 130 stays ON, and accordingly a voltage at the output terminal 160 is equal to an output voltage of the battery 110 at the second output node 112 minus an ON-state voltage drop across the source and drain of the P-channel transistor 130. When the power converter 120 is on, the power converter 120 generates a higher output voltage than the output voltage of the battery 110, and the voltage at the output terminal 160 is equal to an output voltage of the power converter 120 at the first output node 124 minus a forward voltage drop across the diode 150.

[0054] The power supply circuit 100 further comprises an output feedback network 175 to prevent a back-drive current flowing into the battery 110 when the power converter 120 is on and the battery voltage is low. With reference to Figure 2, the output feedback network 175 includes a resistor 180 and a diode 170 which are electrically connected in series with one another. The output feedback network 175 samples the output voltage of the power converter 120 at the first output node 124, and ties the output voltage to a node 182 between the control circuit 140 and the gate of the P-channel transistor 130. The anode of the diode 170 is electrically connected to the first output node 124 of the power converter 120, and the cathode of the diode 170 is electrically coupled to the node 182 via the resistor 180. It would of course be understood that the relative positions of the diode 170 and the resistor 180 may be swapped such that the cathode of the diode 170 is directly connected to the node 182 and the resistor 180 is directly connected to the first output node 124.

[0055] The voltage at the gate electrode of the P-channel transistor 130 (i.e., the voltage at the node 182) is determined by the output voltage of the power converter 120, the output feedback network 175 and the control circuit 140. For example, the control circuit 140 may include a current sinking path from the gate electrode to ground, and because the gate of the P-channel transistor 130 is insulated, a current flowing through the current sinking path would be substantially the same as the current flowing through the output feedback network 175. Therefore the voltage at the gate electrode would be equal to the output voltage of the power converter 120 minus the forward voltage drop of the diode 170 and the voltage drop across the resistor 180. In other words, a total resistance of the current sinking path of the control circuit 140 and the resistor 180 form a voltage divider, and affect the voltage at the node 182. In this sense, the node 182 may be considered as a “voltage summing node”. A purpose of the resistor 180 is to limit a maximum sinking current along the current sinking path of the control circuit 140. Therefore, the particular value of the resistor 180 may be selected suitably based upon the current sinking capability of the control circuit 140. In some applications, the resistor 180 may be omitted entirely from the output feedback network 175. The output feedback network 175 therefore controls the voltage at the gate electrode of the P-channel transistor 130 to be slightly less or equal to the voltage at the output terminal 160 (hence the voltage at the source electrode of the P-channel transistor 130). Hence, a voltage difference between the source and gate electrodes of the P- channel transistor 130 would not be sufficient to overcome a threshold voltage VGs(th; of the P-channel transistor 130, and the P-channel transistor 130 stays off. As a result, there is no back-drive current flowing into the battery 110, regardless of the voltage level of the battery 1 10. The use of the output feedback network 175 provides a low cost and rather simple solution that prevents possibly dangerous back-drive current into the battery 110. In addition, the output feedback network 175 lengthens the usable life of the battery 1 10 by allowing the output voltage of the battery 110 to reach very low levels, thereby maximizing the use of the battery capacity.

[0056] Similar to the diode 150, the diode 170 may be replaced by other types of unidirectional conducting devices. For the ease of description, the diode 150 may be referred to as a “first unidirectional conducting device” while the diode 170 may be referred to as a “second unidirectional conducting device”. Preferably, a forward voltage drop of the first unidirectional conducting device 150 is greater than or equal to a forward voltage drop of the second unidirectional conducting device 170. A greater forward voltage drop of the first unidirectional conducting device 150 means that the voltage at the source of the P-channel transistor 130 would be lower. Thus, there is a lower chance that a voltage difference between the source and gate of the P-channel transistor 130 can overcome the threshold voltage VGsrth). The first and second unidirectional conducting devices 150, 170 may be of the same type of devices (e.g., both being Schottky diodes), and may be of different types (e.g., the device 150 being a P-N junction based diode while the device 170 being a Schottky diode or an integrated ideal diode). In the event that the first and second unidirectional conducting devices 150, 170 are diodes, the forward voltage drop may also be referred to as a forward threshold voltage or a knee voltage.

[0057] While it is not shown in Figure 2, the power supply circuit 100 may include a further circuit which generates an enabling signal to enable an operation of the power converter 120. Alternatively, the control circuit 140 may generate the enabling signal. In the power supply circuit 100, the battery 110 may be a primary cell lithium battery, such as, a Lithium Thionyl Chloride primary battery, or a Lithium Manganese Dioxide primary battery. Back-driving a primary cell lithium battery may damage the battery and also pose serious safety risks. The use of the output feedback network 175 with the primary cell lithium battery significantly reduces the risks and also lengthens the usable battery life.

[0058] The power supply circuit 100 further includes a diode 190 electrically connected in parallel with the P-channel transistor 130. In particular, the anode of the diode 190 is electrically connected to the second output node 112 and the drain of the P-channel transistor 130, and the cathode of the diode 190 is electrically connected to the output terminal 160 and the source of the P-channel transistor 130. The diode 190 is useful during the initial powering-on stage of the power supply circuit 100, as it allows the power supply circuit 100 to quickly output a stable voltage at the output terminal 160 (which is equal to the output voltage of the battery 110 minus the forward voltage drop of the diode 190), before the control circuit 140 switches on the P-channel transistor 130 and / or before the power converter 120 is enabled or enters a stable working mode. This early voltage output by the power supply circuit 100 through the diode 190 may be useful for certain loads which require a quick response time of the power supply circuit 100. Once the control circuit 140 switches on the P-channel transistor 130, the P- channel transistor 130 pulls up the voltage level at the output terminal 160, thereby causing the diode 190 to turn off. The diode 190 may be suitably selected based upon a magnitude of an initial current flowing from the battery 110 through the diode 190 to the output terminal 160. The current rating of the diode 190 is preferably higher than the magnitude of the initial current. In an example, the diode 190 is a Schottky diode. Similar to the diodes 150, 170, the diode 190 may be replaced by other types of unidirectional conducting devices. For the ease of description, the diode 190 may be referred to as a “third unidirectional conducting device”, The third unidirectional conducting device 190 may be of the same type as the first and second unidirectional conducting devices 150, 170, or may be of a different type. It would be appreciated that the diode 190 is optional and may be omitted.

[0059] Figure 3 shows a schematic circuit diagram which implements the power supply circuit 100 of Figure 2. The corresponding relationship between the circuit diagram of Figure 3 and the block diagram of Figure 2 is shown by Figure 3. In the circuit diagram of Figure 3, the battery 110 has an output voltage of 2.7 Volts (represented by a voltage source labelled as ‘V_BATT_2V7’). The battery 110 may output a higher voltage (e.g., 3.6V) at the early stage of its life. The circuit diagram of Figure 3 is to simulate the almost-drained condition of the battery 110, at which back- driving current is more likely to occur. A P-MOSFET T2 (e.g., part number NTR4101 P manufactured by On Semiconductor®) is used to implement the P-channel transistor 130. A Schottky diode SD2 (e.g., part number MBR160 manufactured by On Semiconductor®) is used to implement the diode 190. An output terminal ‘Vout’ corresponds to the output terminal 160 of Figure 2. A resistor R3 (10kQ) and a capacitor C5 (2.2pF) represent a load 200 powered by the output terminal ‘Vout’. A buck-boost DC-to-DC converter U1 (part number TPS63082 manufactured by Texas Instruments®) is used to implement the power converter 120, and generates an output voltage at around 3.8 Volts. A Schottky diode SD4 is used to implement the diode 150. A Schottky diode SD1 and a resistor R4 (501 Q) are used to implement the output feedback network 175. Both the Schottky diodes SD1 and SD4 are based upon part number MBR160 manufactured by On Semiconductor®. The gate of the P-MOSFET T2 is driven by the battery voltage via a microcontroller’s general-purpose input / output (GPIO). Considering that the output by the GPIO would be slightly lower than the battery voltage, a voltage source labelled as ‘V_EN_2V5’ is used to simulate the output by the GPIO and has an output voltage of 2.5 Volts. In Figure 3, the control circuit 140 includes the voltage source ‘V_EN_2V5’, resistors R8, R9, R1 , a capacitor C1 and a diode D2. The resistor R1 and the capacitor C1 form a low pass filter which controls a rise time of the gate voltage while the diode D2 allows quick discharge of the gate capacitance. In this example, the control circuit 140 not only drives the gate of the P- MOSFET T2, but also sends an enabling signal to the buck-boost converter U2.

[0060] Figure 5 shows a comparison schematic circuit diagram which does not have the output feedback network 175 (i.e., the Schottky diode SD1 and the resistor R4) but is identical to that of Figure 3 otherwise.

[0061] Figure 4 shows the SPICE simulation results of the circuit diagram of Figure 3. Figure 6 shows the SPICE simulation results of the circuit diagram of Figure 5. In both cases, the simulation time is 5.5 ms, and the current meter AM1 measures the current flows from the P-MOSFET T2 to the battery. With reference to Figure 6, at time point t1 (around 2.15ms), a back-drive current with a negative magnitude of a few hundred milliamps is captured by the current meter AM1 . This is because at that moment the output of the buck-boost converter U1 has reached a voltage level that surpasses the gate drive voltage (2.5V) of the P-MOSFET T2 by the threshold voltage of the PMOSFET T2 plus the forward voltage drop of the Schottky diode SD4. In contrast, within the simulation results of Figure 4, the back- drive current as measured by the current meter AM1 is practically zero at the same time point t1 . This is because the feedback output network (R4 and SD1 ) prevents the turning-on of the P-MOSFET T2.

[0062] It would be appreciated that within the block diagram of Figure 2 and the circuit diagram of Figure 3, a direct electrical connection of two components may be modified by adding a further component (e.g., a resistor or the like) which is suitable for signal transmission between the two components such that the two components are electrically coupled by the further component.

[0063] Figure 7 shows a meter device 1000 which includes the power supply circuit 100, and a wireless communication unit 200 which receives power from the output terminal 160 of the power supply circuit 100. The wireless communication unit 200 may comprise a modem. The meter device 1000 may be used to measure a usage amount of at least one of gas and water. The gas may include methane and / or carbon monoxide, although other types of gases are possible. The wireless communication unit 200 may wirelessly transmit data indicative of the usage amount to a recipient device remote from the meter device 1000. Other components of the meter device 1000 are not shown by Figure 7 for simplicity. The feedback output network 175 maximizes the use of the available battery capacity and therefore maximizes the battery life of the meter device 1000 (e.g., to at least 15-20 years) by allowing any P-channel transistor to be used regardless of its threshold voltage Vcstth). It would be appreciated that the power supply circuit 100 may be used with any type of battery-powered devices including but not limited to the meter device 1000.

[0064] Figure 8 schematically illustrates processing steps of a method of assembling a power supply circuit (e.g., the power supply circuit 100). The power supply circuit comprises a power converter (e.g., the power converter 120), a battery (e.g., the battery 110), an output terminal (e.g., the output terminal 160), a P-channel transistor (e.g., the transistor 130), and first and second unidirectional conducting devices (e.g., the diodes 150, 170). The power converter has a first input node (i.e. , the first input node 122) and a first output node (the first output node 124), and is operable to receive an input voltage at the first input node and to generate an output voltage at the first output node which is at a higher voltage level than the input voltage. The battery has a second output node (e.g., the second output node 112). The P-channel transistor comprise a first electrode (e.g., the drain electrode), a second electrode (e.g., the source electrode) and a gate electrode.

[0065] At step S1 , the first input node (e.g. the node 122) of the power converter is electrically coupled to the second output node (e.g., the node 112) of the battery.

[0066] At step S2, the first unidirectional conducting device (e.g., the diode 150) is electrically connected between the first output node (e.g., the node 124) of the power converter and the output terminal (e.g., the terminal 160). The first unidirectional conducting device comprises a first anode and a first cathode. Electrically connecting the first unidirectional conducting device comprises electrically coupling the first anode to the first output node and electrically coupling the first cathode to the output terminal.

[0067] At step S3, the first electrode of the P-channel transistor is electrically coupled to the second output node (e.g., the node 112), and the second electrode of the P-channel transistor is electrically coupled to the output terminal (e.g., the terminal 160).

[0068] At step S4, the second unidirectional conducting device (e.g., the diode 170) is electrically connected between the first output node (e.g., the node 124) and the gate electrode of the P-channel transistor. The second unidirectional conducting device comprises a second anode and a second cathode. Electrically connecting the second unidirectional conducting device comprises electrically coupling the second anode to the first output node (e.g., the node 124), and electrically coupling the second cathode to the gate electrode of the P-channel transistor.

[0069] It would be appreciated that the steps may be performed in a temporal order that is different from the order of description. The terms “having”, “containing”, “including”, “comprising” and the like are open and the terms indicate the presence of stated structures, elements or features but not preclude the presence of additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise.

[0070] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the disclosure, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS:1 . A power supply circuit, comprising: a power converter having a first input node and a first output node, wherein the power converter is operable to receive an input voltage at the first input node and to generate an output voltage at the first output node which is at a higher voltage level than the input voltage; a battery having a second output node, wherein the second output node is electrically coupled to the first input node; an output terminal; a P-channel transistor comprising a first electrode, a second electrode and a gate electrode, wherein the first electrode is electrically coupled to the second output node, and the second electrode is electrically coupled to the output terminal; a first unidirectional conducting device electrically connected between the first output node and the output terminal, wherein the first unidirectional conducting device comprises a first anode and a first cathode, and wherein the first anode is electrically coupled to the first output node, and the first cathode is electrically coupled to the output terminal; and a second unidirectional conducting device electrically connected between the first output node and the gate electrode of the P-channel transistor, wherein the second unidirectional conducting device comprises a second anode and a second cathode, and wherein the second anode is electrically coupled to the first output node, and the second cathode is electrically coupled to the gate electrode.

2. A power supply circuit according to claim 1 , wherein a forward voltage drop of the first unidirectional conducting device is greater than or equal to a forward voltage drop of the second unidirectional conducting device.

3. A power supply circuit according to claim 1 or 2, wherein a type of the first unidirectional conducting device is the same as a type of the second unidirectional conducting device.

4. A power supply circuit according to any preceding claim, wherein at least one of the first and second unidirectional conducting devices comprises a diode.

5. A power supply circuit according to claim 4, wherein at least one of the first and second unidirectional conducting devices comprises a Schottky diode.

6. A power supply circuit according to any preceding claim, wherein at least one of the first and second unidirectional conducting devices comprises a circuit configured to operate like a diode.

7. A power supply circuit according to any preceding claim, further comprising a third unidirectional conducting device electrically connected between the first and second electrodes of the P-channel transistor, wherein the third unidirectional conducting device comprises a third anode and a third cathode, and the third anode is electrically coupled to the second output node, and the third cathode is electrically coupled to the output terminal.

8. A power supply circuit according to claim 7, wherein the third unidirectional conducting device comprises a diode.

9. A power supply circuit according to any preceding claim, wherein the P-channel transistor is a P-channel metal-oxide-semiconductor field-effect transistor.

10. A power supply circuit according to any preceding claim, wherein the power converter comprises a DC-to-DC converter.

11. A power supply circuit according to any preceding claim, wherein the power converter comprises a switched-mode DC-to-DC converter.

12. A power supply circuit according to claim 11 , wherein the switched-mode DC- to-DC converter is a switched-mode DC-to-DC boost converter or a switched-mode DC-to-DC buck-boost converter.

13. A power supply circuit according to any preceding claim, further comprising a control circuit configured to control a voltage at the gate electrode of the P-channel transistor, wherein the control circuit is powered by the battery.

14. A power supply circuit according to claim 13, wherein the control circuit is further configured to generate an enabling signal which enables an operation of the power converter.

15. A power supply circuit according to any preceding claim, further comprising: a resistor electrically connected in series with the second unidirectional conducting device between the first output node and the gate electrode of the P-channel transistor.

16. A power supply circuit according to claim 15 as dependent from claim 14, wherein the resistor is configured to limit a current flowing through the control circuit.

17. A power supply circuit according to any preceding claim, wherein the battery comprises a primary cell lithium battery.

18. A power supply circuit according to any preceding claim, wherein the battery comprises at least one of a Lithium Thionyl Chloride battery and a Lithium Manganese Dioxide battery.

19. A meter device comprising a power supply circuit according to any preceding claim.

20. A meter device according to claim 19, wherein the meter device is configured to measure a usage amount of at least one of gas and water.

21. A meter device according to claim 19 or 20, further comprising: a wireless communication unit which is powered by the power supply circuit.

22. A meter device according to claim 21 , wherein the wireless communication unit comprises a modem.

23. A method of assembling a power supply circuit which comprises a power converter, a battery, an output terminal, a P-channel transistor, and first and second unidirectional conducting devices, wherein: the power converter has a first input node and a first output node, and is operable to receive an input voltage at the first input node and to generate an output voltage at the first output node which is at a highervoltage level than the input voltage; the battery has a second output node; and the P- channel transistor comprises a first electrode, a second electrode and a gate electrode, the method comprising: electrically coupling the first input node of the power converter to the second output node of the battery; electrically connecting the first unidirectional conducting device between the first output node of the power converter and the output terminal, wherein the first unidirectional conducting device comprises a first anode and a first cathode, and electrically connecting the first unidirectional conducting device comprises electrically coupling the first anode to the first output node and electrically coupling the first cathode to the output terminal; electrically coupling the first electrode of the P-channel transistor to the second output node, and electrically coupling the second electrode of the P-channel transistor to the output terminal; and electrically connecting the second unidirectional conducting device between the first output node and the gate electrode of the P-channel transistor, wherein the second unidirectional conducting device comprises a second anode and a second cathode, and wherein electrically connecting the second unidirectional conducting device comprises electrically coupling the second anode to the first output node, and electrically coupling the second cathode to the gate electrode.

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