Protective circuit

Abstract

Provided is a protective circuit capable of stably detecting an overvoltage of an input terminal even when a current outputted from an output terminal becomes large.　The protective circuit comprises an input terminal that receives DC power, an output terminal that outputs DC power, a first FET that is connected to the input terminal, a reverse connection detection circuit that controls the gate voltage of the first FET, a second FET that is connected to the output terminal, and an overvoltage detection circuit that controls the gate voltage of the second FET. The overvoltage detection circuit includes: a first switch element that is provided between a connection point of the first FET and the second FET and the gate of the second FET; and a reference voltage adjustment circuit that controls the first switch element. The reference voltage adjustment circuit decreases a voltage for turning on the first switch element as the current inputted from the input terminal to the first FET increases.

Description

Protection circuit

[0001] The present disclosure relates to a protection circuit.

[0002] Conventionally, there is a protection circuit including a transistor whose emitter is connected to an input terminal, a first FET element whose gate is connected to the collector of the transistor and whose drain is connected to the input terminal, a second FET element whose gate is connected to the collector of the transistor and whose drain is connected to an output terminal, and an error detection unit that is connected to the base of the transistor and compares the divided voltage of the voltage at the output terminal with a reference voltage. When the divided voltage of the voltage at the output terminal is higher than the reference voltage, the first FET element reduces the drain voltage based on the increase in the gate voltage caused by the increase in the collector current of the transistor due to the current supplied from the error detection unit. When a reverse voltage is applied between the input terminal and the output terminal, the first FET element and the second FET element are in a non-conducting state (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2008-278619

[0004] In the conventional protection circuit, when an overvoltage occurs at the input terminal, the second FET element turns off. However, when an overvoltage occurs at the input terminal in a state where the current output from the output terminal is a large current, there is a problem that the voltage value of the input terminal when the second FET element is turned off fluctuates due to an increase in the voltage drop in the first FET element.

[0005] Therefore, an object is to provide a protection circuit that can stably detect an overvoltage at the input terminal even when the current output from the output terminal is a large current.

[0006] The protection circuit of the embodiment of the present disclosure includes an input terminal into which DC power is input, an output terminal that outputs DC power, a first FET connected to the input terminal, a reverse connection detection circuit that controls the gate voltage of the first FET, a second FET connected to the output terminal, and an overvoltage detection circuit that controls the gate voltage of the second FET, wherein the overvoltage detection circuit has a first switch element provided between the connection point of the first FET and the second FET and the gate of the second FET, and a reference voltage adjustment circuit that controls the first switch element, wherein the reference voltage adjustment circuit lowers the voltage that turns on the first switch element as the current input from the input terminal to the first FET increases.

[0007] This provides a protection circuit that can reliably detect overvoltage at the input terminal even when the current output from the output terminal becomes large.

[0008] This figure shows an example of the configuration of the protection circuit 100 of the embodiment. This figure shows an example of the positional relationship between the PTC thermistor R7 and the FET 1. This figure shows an example of the configuration of a modified protection circuit 100M of the embodiment.

[0009] The following describes embodiments to which the protection circuit of this disclosure is applied.

[0010] <Embodiment> Figure 1 shows an example of the configuration of the protection circuit 100 of the embodiment. The protection circuit 100 includes an input terminal 101, an output terminal 102, a transmission line 103, FETs (Field Effective Transistors) M1 and FTEM2, a reverse current detection circuit 110, and an overvoltage detection circuit 120. FTEM1 is an example of a first FET, and FTEM2 is an example of a second FET. GND (ground) is an example of a reference potential point and is shown as a triangle symbol in Figure 1.

[0011] The protection circuit 100 is, for example, a circuit that protects a mobile printer from reverse current, overvoltage, etc., when charging the battery built into the mobile printer. In this case, for example, the input terminal 101 is a USB (Universal Serial Bus) connector, and the input terminal 101 is detachably connected to the USB connector of an external device such as a PC (Personal Computer). The output terminal 102 is connected to the USB connector of the mobile printer. The mobile printer's battery is charged by receiving DC power from the PC via the protection circuit 100.

[0012] The protection circuit 100 protects the printer by preventing reverse current flow from the input terminal 101, by having the reverse current detection circuit 110 turn off (non-conductive) the FTEM 1.

[0013] Furthermore, the protection circuit 100 protects the printer by having the overvoltage detection circuit 120 turn off (non-conducting) the FETM2 if an overvoltage occurs where the voltage value input to the protection circuit 100 from the input terminal 101 is higher than a predetermined value. In particular, the protection circuit 100 can detect the overvoltage at the input terminal 101 with the same threshold in both cases: when a large current is not generated where the current voltage value output to the mobile printer from the output terminal 102 is higher than a predetermined value, and when a large current is generated where the current voltage value output to the mobile printer from the output terminal 102 is higher than a predetermined value.

[0014] This is just one example of how the protection circuit 100 can be used, and it can be used in forms other than those described above. However, here we will describe, as an example, the case in which the protection circuit 100 is used to charge the battery of a mobile printer.

[0015] <Configuration of the protection circuit 100> <Input terminal 101> The input terminal 101 is an input terminal to which DC power is input, and within the protection circuit 100, it is connected to the drain of the FETM1.

[0016] <Output Terminal 102> Output terminal 102 is an output terminal that outputs DC power and is connected to the drain of the FETM2 within the protection circuit 100.

[0017] <FETM1> FETM1 is, for example, a P-channel type MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), and has a drain connected to the input terminal 101, a source connected to the source of FETM2, and a gate connected to the connection point of resistors R1 and R2. FETM1 has a parasitic diode connected in parallel between the drain and source.

[0018] <FETM2> FETM2 is, for example, a P-channel MOSFET, and has a drain connected to the output terminal 102, a source connected to the source of FETM1, and a gate connected to the connection point of resistors R3 and R4. FETM2 has a parasitic diode connected in parallel between the drain and source.

[0019] The line 103 between the source of FET 1 and the source of FET 2 is connected to the cathode of Zener diode D1, one end (upper terminal) of resistor R1, one end (upper terminal) of PTC thermistor R7, one end (upper terminal) of resistor R5, the emitter of switch element Q1, and one end (upper terminal) of resistor R3.

[0020] <Reverse Current Detection Circuit 110> The reverse current detection circuit 110 includes a resistor R1, a resistor R2, and a Zener diode D1. Resistor R1 is an example of a first resistor, resistor R2 is an example of a second resistor, and Zener diode D1 is an example of a first Zener diode.

[0021] The reverse current detection circuit 110 includes a Zener diode D1 having a cathode connected to the source of the FET M1, a resistor R1 connected in parallel with the Zener diode D1, and a resistor R2 connected between the anode of the Zener diode D1 and GND.

[0022] The reverse current detection circuit 110 is a circuit that controls the FTEM 1 to turn off when a reverse current occurs, where current flows out from the input terminal 101 to the outside. When a reverse current occurs, the reverse current detection circuit 110 switches the FTEM 1 to off (non-conductive state). Details of the operation of the reverse current detection circuit 110 will be described later.

[0023] <Overvoltage detection circuit 120> The overvoltage detection circuit 120 includes a switch element Q1, a resistor R3, a resistor R4, and a reference voltage adjustment circuit 125 that controls the switch element Q1. The switch element Q1 is an example of a first switch element, the resistor R3 is an example of a third resistor, and the resistor R4 is an example of a fourth resistor.

[0024] The switching element Q1 is a PNP type transistor and has an emitter connected to the source of FETM2, a collector connected to the gate of FETM2, and a base connected to the connection point of resistors R5 and R6.

[0025] Resistor R3 has one end (upper terminal) connected to the emitter of switch element Q1 and the other end (lower terminal) connected to the collector of switch element Q1. Resistor R4 has one end (upper terminal) connected to the collector of switch element Q1 and the other end (lower terminal) connected to GND.

[0026] <Reference Voltage Adjustment Circuit 125> The reference voltage adjustment circuit 125 includes resistors R5 and R6, a Zener diode D2, and a PTC (Positive Temperature Coefficient) thermistor R7. Resistor R5 is an example of a fifth resistor, and resistor R6 is an example of a sixth resistor. Zener diode D2 is an example of a second Zener diode. The reference voltage adjustment circuit 125 lowers the voltage that turns on the switch element Q1 as the current input from the input terminal 101 to FETM1 increases.

[0027] Resistor R5 has one end (upper terminal) connected to the emitter of switch element Q1 and the other end (lower terminal) connected to the base of switch element Q1. Resistor R6 has one end (upper terminal) connected to the base of switch element Q1 and the other end (lower terminal) connected to the cathode of Zener diode D2. Zener diode D2 has a cathode connected to the other end (lower terminal) of resistor R6 and an anode connected to GND.

[0028] The PTC thermistor R7 is a variable resistor with a positive temperature characteristic and is connected in parallel with resistor R5. The PTC thermistor R7 is located around the FETM1 and its resistance increases when the current flowing through the FETM1 increases, utilizing the heat generated by the FETM1. The FETM1 is provided to counteract the effect of the increased drain-source voltage drop of the FETM1, which occurs when the current flowing through the FETM1 increases, on the operation of the protection circuit 100. Here, the positional relationship between the PTC thermistor R7 and the FETM1 will be explained in detail using Figure 2.

[0029] The wiring board 10 shown in Figure 2 is the wiring board on which each component of the protection circuit 100 is mounted. Figure 2 shows only the wiring board 10, the FETM1, and the PTC thermistor R7, and omits the other components.

[0030] The wiring board 10 has a first surface 11 and a second surface 12. For example, the FETM1 is mounted on the first surface 11, and the PTC thermistor R7 is mounted on the second surface 12. The second surface 12 is the surface opposite to the first surface 11.

[0031] As an example, the PTC thermistor R7 is located on the second surface 12 in a position that overlaps with the FTEM 1 in a plan view. This is to increase the resistance of the PTC thermistor R7 by utilizing the heat generated by the increased current of the FTEM 1. In order to transfer the heat generated by the FTEM 1 to the PTC thermistor R7, the PTC thermistor R7 is located on the back side (opposite side) of the position where the FTEM 1 is mounted on the wiring board 10.

[0032] Note that the position of the PTC thermistor R7 shown is just one example; the PTC thermistor R7 may be located in a position that does not overlap with the FTEM 1 in a plan view, or on the same surface as the FTEM 1 (first surface 11 or second surface 12). The PTC thermistor R7 only needs to be able to offset the effect of the increased voltage drop in the FTEM 1 with the increased resistance value due to the heat generated by the FTEM 1. The PTC thermistor R7 only needs to be placed close enough to the FTEM 1 that its resistance value increases in this way due to the heat generated by the FTEM 1. This is what it means for the PTC thermistor R7 to be located around the FTEM 1.

[0033] <Operation of Reverse Current Detection Circuit 110> The FET 1 turns on (conducts) when the source voltage is higher than the gate voltage. In the normal state where no reverse current occurs at the input terminal 101, the source voltage is divided by resistors R1 and R2 and applied to the gate, so it turns on. When reverse current occurs at the input terminal 101, a forward current flows through the Zener diode D1, causing the Zener diode D1 to conduct, and the gate voltage of the FET 1 becomes close to the source voltage, turning it off (non-conducting).

[0034] In this manner, if a reverse current flows at the input terminal 101, the reverse current detection circuit 110 turns off the FETM1. As a result, the printer is protected by the protection circuit 100.

[0035] <Drive control of FETM2 by overvoltage detection circuit 120> Overvoltage refers to a voltage input to input terminal 101 that is higher than a predetermined value. When overvoltage occurs, the overvoltage detection circuit 120 turns off FETM2 to protect the printer.

[0036] FTEM2 turns on (conducts) when the source voltage is higher than the gate voltage. In normal conditions where no overvoltage occurs, the source voltage of FTEM2 is divided by resistors R3 and R4 and applied to the gate, so FTEM2 turns on.

[0037] <When an overvoltage occurs> When an overvoltage occurs, the Zener diode D2 breaks down and conducts, and the switching element Q1 conducts, causing the gate voltage of the FETM2 to become close to the source voltage, and the FETM2 turns off (non-conducting). In this way, when an overvoltage occurs at the input terminal 101, the overvoltage detection circuit 120 turns off the FETM2. As a result, the printer is protected by the protection circuit 100.

[0038] <When a large current occurs> A large current refers to a situation where the current output from the output terminal 102 to the printer exceeds a predetermined value. A large current occurs when the printer's power consumption becomes excessive.

[0039] At high currents, the current flowing between the drain and source of FTEM1 increases, which increases the amount of heat generated by FTEM1. This causes the temperature of PTC thermistor R7 to rise, and the resistance of PTC thermistor R7 increases. When the resistance of PTC thermistor R7 increases, the current flowing through Zener diode D2 decreases, causing the Zener voltage of Zener diode D2 to drop. At high currents, if no overvoltage occurs, Zener diode D2 remains in the off state, and FTEM2 remains in the on state. Note that as the current flowing between the drain and source of FTEM1 increases, the voltage drop between the drain and source of FTEM1 also increases, but if no overvoltage occurs, FTEM2 remains in the on state, so no problem occurs.

[0040] <When both overvoltage and high current occur> When both overvoltage and high current occur, the current flowing between the drain and source of FET TM 1 increases, which increases the amount of heat generated by FET TM 1. This causes the temperature of PTC thermistor R7 to rise, and the resistance of PTC thermistor R7 increases. When the resistance of PTC thermistor R7 increases, the current flowing through Zener diode D2 decreases, causing the Zener voltage of Zener diode D2 to drop.

[0041] At this time, the large current increases the voltage drop between the drain and source of FTEM1, causing the input voltage of the overvoltage detection circuit 120 to decrease compared to when no large current is present. However, as the resistance of the PTC thermistor R7 increases, the Zener voltage of the Zener diode D2 decreases, causing the Zener diode D2 to break down and conduct, just as when an overvoltage occurs alone. When the Zener diode D2 breaks down and conducts, the switch element Q1 turns on, the gate voltage of FTEM2 becomes close to the source voltage, and FTEM2 turns off (non-conducting). In other words, when both an overvoltage and a large current are present, it is possible to deliberately turn off FTEM2 by setting the output voltage to the printer to a low voltage (a voltage lower by the amount of the voltage drop in FTEM1 and FTEM2).

[0042] Therefore, even if both overvoltage and high current occur, the overvoltage detection circuit 120 can switch off the FETM2 when the voltage value at the input terminal 101 rises to a voltage value equivalent to that of a case where overvoltage occurs alone without high current. Thus, the overvoltage standard at the input terminal 101 can be kept constant whether high current is occurring or not.

[0043] <Modified Example> Figure 3 shows an example of the configuration of a modified protection circuit 100M of the embodiment. Protection circuit 100M differs from protection circuit 100 shown in Figure 1 in that it includes an overvoltage detection circuit 120M instead of the overvoltage detection circuit 120, and it includes a control unit 130 and a current sensor 131. Here, we will explain the differences in detail. Also, components similar to those in protection circuit 100 shown in Figure 1 are denoted by the same reference numerals, and their descriptions are omitted.

[0044] The overvoltage detection circuit 120 includes a switching element Q1, a resistor R3, a resistor R4, and a reference voltage adjustment circuit 125M. The reference voltage adjustment circuit 125M includes a resistor R7 connected in parallel with the resistor R5, a switching element Q2 provided between the resistor R5 and GND, and a resistor R8 connected between the emitter and the base of the switching element Q2, and controls the base voltage of the switching element Q1. Note that the resistors R5 and R7 may be a single resistor.

[0045] The switching element Q2 is a PNP bipolar transistor and has an emitter connected to the resistors R5 and R7 and a collector connected to GND.

[0046] The control unit 130 is implemented by a computer including a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), etc. An MCU (Micro Controller Unit) can be used as the control unit 130. The current sensor 131 is provided at the output terminal 102 and detects the current value of the current flowing through the output terminal 102. The current sensor 131 outputs a signal representing the detected current value to the control unit 130.

[0047] The control unit 130 controls the switching element Q2 such that the current flowing through the switching element Q2 decreases as the current detected by the current sensor 131 increases. More specifically, the control unit 130 increases the voltage at the base of the switching element Q2 as the current detected by the current sensor 131 increases.

[0048] More specifically, when the current detected by the current sensor 131 is smaller than a predetermined threshold value, the control unit 130 outputs a signal of L (Low) level to the base of the switching element Q2, thereby turning on the switching element Q2. As a result, the cathode of the Zener diode D2 is connected to GND via the switching element Q2, so that the voltage value of the cathode of the Zener diode D2 becomes low. When the voltage value of the cathode of the Zener diode D2 becomes low, the voltage drop across the resistors R5 and R7 increases, and the voltage value of the line 103 when the Zener diode D2 breaks down becomes high. Thereby, the protection circuit 100M can perform an operation of turning off the FET M2 in response to an overvoltage, similarly to when no large current is generated in the protection circuit 100 shown in FIG. 1.

[0049] Further, when the current detected by the current sensor 131 is larger than a predetermined threshold value (when a large current is flowing), the control unit 130 outputs a signal of H (High) level to the base of the switching element Q2, thereby turning off the switching element Q2. As a result, the cathode of the Zener diode D2 is connected to the line 103 via the resistor R6 and the resistors R5 and R7, so that the voltage value of the cathode of the Zener diode D2 becomes high. When the voltage value of the cathode of the Zener diode D2 becomes high, the voltage drop across the resistors R5 and R7 decreases, and the voltage value of the line 103 when the Zener diode D2 breaks down becomes low. Thereby, the protection circuit 100M can perform an operation of turning off the FET M2 in response to an overvoltage, similarly to when a large current is flowing in the protection circuit 100 shown in FIG. 1.

[0050] As described above, similar to the protection circuit 100 shown in FIG. 1, when the voltage value of the input terminal 101 rises to the same voltage value as when an overvoltage occurs alone without a large current flowing even when both an overvoltage and a large current occur, the protection circuit 100M can switch off the FET M2 by the overvoltage detection circuit 120. Therefore, the reference for the overvoltage at the input terminal 101 can be made constant whether or not a large current is flowing.

[0051] <Effects> The protection circuit 100 includes an input terminal 101 into which DC power is input, an output terminal 102 that outputs DC power, an FTEM 1 connected to the input terminal 101, a reverse current detection circuit 110 that controls the gate voltage of the FTEM 1, an FTEM 2 connected to the output terminal 102, and an overvoltage detection circuit 120 that controls the gate voltage of the FTEM 2. The overvoltage detection circuit 120 has a switch element Q1 provided between the connection point of the FTEM 1 and the FTEM 2 and the gate of the FTEM 2, and a reference voltage adjustment circuit 125 that controls the switch element Q1. The reference voltage adjustment circuit 125 lowers the voltage that turns on the switch element Q1 as the current input from the input terminal 101 to the FTEM 1 increases.

[0052] Therefore, even if the current output from the output terminal 102 becomes large, a protection circuit capable of reliably detecting overvoltage at the input terminal 101 can be provided.

[0053] Furthermore, FTEM1 is a P-channel MOSFET with a drain connected to the input terminal 101, and FTEM2 is a P-channel MOSFET with a drain connected to the output terminal 102, and the sources of FTEM1 and FTEM2 may be connected. By using P-channel MOSFETs, which allow for easy gate control, as FTEM1 and FTEM2, the system can be operated with a single power supply.

[0054] Furthermore, the reverse current detection circuit 110 may include a Zener diode D1 having a cathode connected to the source of the FETM1, a resistor R1 connected in parallel with the Zener diode D1, and a resistor R2 connected between the anode of the Zener diode D1 and GND. A simple circuit can prevent reverse current flow at the input terminal 101.

[0055] Furthermore, the switch element Q1 is a PNP type transistor having an emitter connected to the source of the FETM2 and a collector connected to the gate of the FETM2. The overvoltage detection circuit 120 has a resistor R3 having one end connected to the emitter of the switch element Q1 and the other end connected to the collector of the switch element Q1, and a resistor R4 having one end connected to the collector of the switch element Q1 and the other end connected to GND. The reference voltage adjustment circuit 125 may have a resistor R5 having one end connected to the emitter of the switch element Q1 and the other end connected to the base of the switch element Q1, a resistor R6 having one end connected to the base of the switch element Q1, and a Zener diode D2 having a cathode connected to the other end of resistor R6 and an anode connected to GND. Overvoltage can be prevented with a simple circuit.

[0056] Furthermore, the reference voltage adjustment circuit 125 has a PTC thermistor R7 connected in parallel with the resistor R5, and the PTC thermistor R7 may be provided around the FETM1. By increasing the resistance value of the PTC thermistor R7 using the heat generated by the increase in current flowing through the FETM1, the effect of the increased voltage drop across the FETM1 can be offset.

[0057] The wiring board 10 further includes a first surface 11 on which the FTEM 1 is mounted, and the PTC thermistor R7 may be provided at a position that overlaps with the FTEM 1 in a plan view of the second surface 12 of the wiring board 10. By more efficiently utilizing the heat generated due to the increase in current flowing through the FTEM 1 and increasing the resistance value of the PTC thermistor R7, the effect of the increased voltage drop across the FTEM 1 can be more efficiently offset.

[0058] Furthermore, the reference voltage adjustment circuit 125M includes a switch element Q2 provided between resistor R5 and GND, and the switch element Q2 is a bipolar transistor. The circuit may also further include a control unit 130 that controls the switch element Q2 so that the current flowing through the switch element Q2 decreases as the current flowing through the output terminal 102 increases. Even in cases where the correlation between the amount of heat generated by the FETM1 and the temperature of the FETM1 is low due to large temperature fluctuations or other reasons, the circuit can stably detect overvoltage and turn off the FETM2 regardless of the current value flowing through the output terminal 102.

[0059] Furthermore, the switch element Q2 is a PNP bipolar transistor having an emitter connected to resistor R5 and a collector connected to GND, the reference voltage adjustment circuit 125M has a resistor R8 connected between the emitter and base of the switch element Q2, and the control unit 130 may increase the voltage of the base of the switch element Q2 as the current flowing through the output terminal 102 increases. Even in cases where the correlation between the amount of heat generated by the FETM1 and the temperature of the FETM1 is low due to large temperature fluctuations or other reasons, an overvoltage can be stably detected and the FETM2 turned off with a simple circuit, regardless of the current value of the current flowing through the output terminal 102.

[0060] Furthermore, the input terminal 101 is a USB connector, and DC power may be input from an external device while connected to the USB connector of the external device. When DC power is input from an external device, a reverse current detection circuit 110 is required, so a protection circuit 100 that corresponds to the supply of DC power from an external device can be provided.

[0061] Furthermore, a mobile printer with a built-in battery may be connected to the output terminal 102. In applications where an external circuit with significant power fluctuations, such as a mobile printer, is connected, even when the voltage drop across FTEM1 fluctuates greatly, the overvoltage can be stably detected and FTEM2 turned off, regardless of the current value flowing through the output terminal 102.

[0062] While exemplary embodiments of protection circuits in this disclosure have been described above, this disclosure is not limited to the specifically disclosed embodiments, and various modifications and changes are possible without departing from the scope of the claims.

[0063] This international application claims priority based on Japanese Patent Application No. 2024-216559, filed on 11 December 2024, the entire contents of which are incorporated herein by reference.

[0064] 10 Wiring board 11 First surface 12 Second surface 100, 100M Protection circuit 101 Input terminal 102 Output terminal 103 Transmission line 110 Reverse current detection circuit 120, 120M Overvoltage detection circuit 125, 125M Reference voltage adjustment circuit 130 Control unit 131 Current sensor D1 Zener diode (example of first Zener diode) D2 Zener diode (example of second Zener diode) M1 FET (example of first FET) M2 FET (example of second FET) Q1 Switch element (example of first switch element) Q2 Switch element (example of second switch element)

Claims

1. A protection circuit comprising: an input terminal into which DC power is input; an output terminal for outputting DC power; a first FET connected to the input terminal; a reverse connection detection circuit for controlling the gate voltage of the first FET; a second FET connected to the output terminal; and an overvoltage detection circuit for controlling the gate voltage of the second FET, wherein the overvoltage detection circuit comprises: a first switch element provided between the connection point of the first FET and the second FET and the gate of the second FET; and a reference voltage adjustment circuit for controlling the first switch element, wherein the reference voltage adjustment circuit is a protection circuit that lowers the voltage that turns on the first switch element as the current input to the first FET from the input terminal increases.

2. The protection circuit according to claim 1, wherein the first FET is a P-channel MOSFET and has a drain connected to the input terminal, and the second FET is a P-channel MOSFET and has a drain connected to the output terminal, and the source of the first FET and the source of the second FET are connected.

3. The protection circuit according to claim 2, wherein the reverse connection detection circuit comprises a first Zener diode having a cathode connected to the source of the first FET, a first resistor connected in parallel with the first Zener diode, and a second resistor connected between the anode of the first Zener diode and a reference potential point.

4. The protection circuit according to claim 3, wherein the first switching element is a PNP type transistor having an emitter connected to the source of the second FET and a collector connected to the gate of the second FET, the overvoltage detection circuit has a third resistor having one end connected to the emitter of the first switching element and the other end connected to the collector of the first switching element, and a fourth resistor having one end connected to the collector of the first switching element and the other end connected to a reference potential point, and the reference voltage adjustment circuit has a fifth resistor having one end connected to the emitter of the first switching element and the other end connected to the base of the first switching element, a sixth resistor having one end connected to the base of the first switching element, and a second Zener diode having a cathode connected to the other end of the sixth resistor and an anode connected to a reference potential point.

5. The protection circuit according to claim 4, wherein the reference voltage adjustment circuit has a PTC thermistor connected in parallel with the fifth resistor, and the PTC thermistor is provided around the first FET.

6. The protection circuit according to claim 5, further comprising a wiring board having a first surface on which the first FET is mounted, wherein the PTC thermistor is provided at a position overlapping with the first FET in a plan view of the second surface of the wiring board.

7. The protection circuit according to claim 4, wherein the reference voltage adjustment circuit comprises a second switch element provided between the fifth resistor and a reference potential point, the second switch element being a bipolar transistor, and further comprises a control unit that controls the second switch element such that the current flowing through the second switch element decreases as the current flowing through the output terminal increases.

8. The protection circuit according to claim 7, wherein the second switching element is a PNP type bipolar transistor having an emitter connected to the fifth resistor and a collector connected to a reference potential point, the reference voltage adjustment circuit has an eighth resistor connected between the emitter and the base of the second switching element, and the control unit increases the voltage of the base of the second switching element as the current flowing through the output terminal increases.

9. The protection circuit according to any one of claims 1 to 8, wherein the input terminal is a USB connector, and the DC power is input from the external device while the input terminal is connected to the USB connector of the external device.

10. The protection circuit according to claim 9, wherein a mobile printer with a built-in battery is connected to the output terminal.

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