Isolated power supply

Abstract

This invention provides an isolated power supply unit that reduces the number of parts, thereby enabling miniaturization of the device and lowering costs. [Solution] An isolated power supply device comprising a transformer equipped with an auxiliary winding and a power supply control semiconductor device that generates on / off control signals for a switching element connected to the primary winding, comprising a first rectifier and smoothing circuit that rectifies and smooths the voltage induced at one terminal of the auxiliary winding and a second rectifier and smoothing circuit that rectifies and smooths the voltage induced at the center tap of the auxiliary winding, wherein the power supply control semiconductor device comprises first and second external power terminals to which the voltages generated by the first and second rectifier and smoothing circuits are input, a linear regulator that steps down the input voltage of the first external power terminal, and an internal circuit that generates on / off control signals for the switching element, wherein the internal circuit is operated by the voltage of the second external power terminal when the voltage of the second external power terminal is high, and by the voltage of the linear regulator when the voltage of the second external power terminal is low.

Description

Technical Field

[0001] The present invention relates to an isolated power supply device including a semiconductor device for power supply control that controls a switching element connected in series with a primary winding of a transformer for voltage conversion.

Background Art

[0002] Conventionally, as one type of switching power supply device, a transistor (including transistors on GaN or SiC substrates in addition to silicon substrates) as a switching element for intermittently flowing a current through a primary winding of a transformer and a control circuit (IC) for controlling on and off of the switching element are provided. A switching power supply device (isolated DC-DC converter) rectifies a current induced in a secondary winding by flowing a current through the primary winding with a diode, smoothes it with a capacitor, and outputs it.

[0003] In addition, in a DC power supply device, there is an isolated AC-DC converter (AC adapter) that rectifies an AC power supply with a diode-bridge circuit and steps down a DC voltage rectified by the circuit to a DC voltage of a desired potential with the above switching power supply device (isolated DC-DC converter). As a control IC (semiconductor device for power supply control) on the primary side of the AC adapter, a PWM control / pseudo-resonant control type IC is known.

[0004] By the way, regarding recent power supply devices, particularly AC adapters, there is a demand for high output and high power density for miniaturization. For miniaturization, a method of increasing the switching frequency to reduce the size of the transformer is effective. However, when the switching frequency is increased with a semiconductor device for power supply control of the PWM control / pseudo-resonant control type, the power loss increases and the efficiency decreases. In addition, for efficiency improvement, there is also a semiconductor device for power supply control that adopts a zero voltage switching (ZVS) control method in which an active clamp circuit having an energy regeneration function is provided on the primary winding side of the transformer and switching is performed at the timing when the voltage is zero. An invention related to a switching power supply device provided with an active clamp circuit is described in, for example, Patent Document 1.

[0005] Conventionally, a method for supplying power voltage to the control IC on the primary side of an AC adapter is known to be a method of supplying the voltage induced in the auxiliary winding of a transformer after rectification and smoothing, and such a method is also employed in the power supply device described in Patent Document 1. Furthermore, because AC adapters compatible with the USB-PD standard have a wide output voltage range of 3.3V to 20V, some AC adapters have a primary-side control IC that rectifies and smooths the voltage induced in the auxiliary winding, and then steps down the resulting voltage using a regulator built into the control IC to generate the internal operating voltage. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-8846 [Overview of the project] [Problems that the invention aims to solve]

[0007] AC adapters compatible with the USB-PD standard have a wide output voltage range of 3.3V to 21V. Therefore, if a primary-side control IC with an internal step-down regulator employs an active clamp control method and uses a device that integrates the control circuit and drive circuit into a single package, the power consumption becomes very high, increasing power loss in the device and potentially exceeding the allowable limit. Therefore, it is conceivable to rectify and smooth the voltage induced in the auxiliary winding, and then step down or step up the voltage using a regulator located outside the IC before supplying it to the control IC. However, such a method has the drawback of increasing the number of components, making the device larger, and increasing costs. In particular, USB-PD adapters are currently being made more powerful and smaller.

[0008] This invention was made in view of the above-mentioned problems, and its objective is to reduce the number of components in an isolated power supply device with a wide output voltage range or multiple outputs, thereby enabling miniaturization of the device and reduction of costs. Another objective of the present invention is to configure an isolated power supply device with a wide output voltage range or multiple outputs, thereby reducing the power consumption of a primary-side power control semiconductor device (control IC) that operates using the voltage induced in the auxiliary winding as the power supply voltage, suppressing heat generation, and improving power efficiency. [Means for solving the problem]

[0009] To achieve the above objective, the present invention An isolated power supply device comprising a voltage conversion transformer with an auxiliary winding having a center tap, a first switching element connected in series with the primary winding of the transformer, and a power supply control semiconductor device for controlling the first switching element, The aforementioned power control semiconductor device is A first external terminal that outputs an on / off drive signal for the first switching element, A second external terminal to which a feedback signal from the secondary side of the transformer is input, A first external power supply terminal to which the voltage induced at one terminal of the auxiliary winding is input, A second external power supply terminal to which the voltage induced at the center tap of the auxiliary winding is input, A linear regulator that steps down the voltage input to the first external power terminal, The circuit comprises an internal circuit that generates an on / off control signal for the first switching element, The aforementioned internal circuit is If the voltage input to the second external power terminal is higher than the output voltage of the linear regulator, the device is operated by the voltage input to the second external power terminal. If the voltage input to the second external power supply terminal is lower than the output voltage of the linear regulator, the device is configured to operate using the voltage generated by the linear regulator.

[0010] With the above configuration, the primary-side power control semiconductor device (control IC) constituting the isolated power supply unit incorporates a regulator that generates the power supply voltage for the said semiconductor device. Therefore, even when the output voltage range of the power supply unit is wide or it has multiple outputs, there is no need to provide an external regulator, which reduces the number of components and allows for miniaturization and cost reduction of the power supply unit. Furthermore, since the regulator that generates the power supply voltage for the power control semiconductor device is configured as a linear regulator, noise generation can be suppressed.

[0011] Furthermore, by using a transformer equipped with an auxiliary winding with a center tap, the internal circuit is configured such that if the voltage at the second external power terminal (VDD) is higher than the output voltage of the linear regulator (i.e., the secondary output voltage is higher), it operates using the voltage supplied from the center tap at the second external power terminal (VDD), and if it is lower, it operates using the voltage obtained by stepping down the voltage at the first external power terminal (AUXR) with the linear regulator. As a result, the regulator's losses can be reduced by preventing it from operating in a way that would result in large losses and heat generation due to stepping down high voltages. Consequently, compared to using an auxiliary winding without a center tap, the power consumption of the primary-side control IC with the built-in regulator can be reduced, heat generation can be suppressed, and power efficiency can be improved. [Effects of the Invention]

[0012] According to the present invention, in isolated power supply devices with a wide output voltage range or multiple outputs, there is no need to provide a regulator composed of external components, thus reducing the number of components, enabling miniaturization of the device, and lowering costs. Furthermore, it has the effect of reducing the power consumption of the primary side power control semiconductor device (control IC) of the isolated power supply device, which operates with the voltage induced in the auxiliary winding, thereby suppressing heat generation and improving power efficiency. [Brief explanation of the drawing]

[0013] [Figure 1]This is a circuit configuration diagram showing an embodiment of an AC-DC converter to which an isolated power supply device according to the present invention is applied. [Figure 2] This is a circuit configuration diagram showing a configuration example of a semiconductor device for power supply control that constitutes the AC-DC converter of FIG. 1. [Figure 3] This is a characteristic diagram showing the change of the voltage of the external power supply terminal VDD of the semiconductor device for power supply control of FIG. 2 with respect to the secondary-side output voltage. [Figure 4] This is a circuit diagram showing a specific example of a built-in regulator in a control chip. [Figure 5] This is a characteristic diagram showing the input voltage-output voltage characteristics of the linear regulator of FIG. 4.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of an AC-DC converter to which an isolated power supply device according to the present invention is applied. The AC-DC converter 10 of this embodiment is a flyback converter, and includes an AC power line filter circuit (AC filter) 12 for noise removal composed of an X capacitor C0 and a common mode choke coil L1 connected between a pair of AC terminals to which an AC voltage (AC) from an AC power supply 11 is input, a diode bridge circuit 13 for rectifying the AC voltage from the AC power supply 11, a smoothing capacitor C1, and a transformer 14 for voltage conversion having a primary winding Np, a secondary winding Ns, and an auxiliary winding Nb.

[0015] Further, the AC-DC converter 10 of this embodiment includes a switching transistor SW1 connected in series with the primary winding Np of the transformer 14, and a semiconductor device for power supply control (hereinafter referred to as a power supply control device) 15 for driving the switching transistor SW1 on and off. Furthermore, the AC-DC converter 10 of this embodiment includes an active clamp circuit 16 having a switching transistor SW2 and a capacitor C2 in parallel connection between the low-side terminal of the primary winding Np of the transformer 14 and the diode bridge circuit 13. The power control device 15 is provided with an active clamp control function for controlling the transistor SW2 of the active clamp circuit 16. Also, a discharge resistor R2 for discharging the charge of the capacitor C2 is connected in parallel with the capacitor C2.

[0016] Also, in the AC-DC converter 10 of this embodiment, a transformer 14 having an auxiliary winding Nb with a center tap is used. A rectifying and smoothing circuit 17A formed by connecting a rectifying diode D11, a resistor R11, and a smoothing capacitor C11 in series is provided between the high-side terminal of the auxiliary winding Nb and the ground point. The voltage rectified and smoothed by the rectifying and smoothing circuit 17A is applied to the auxiliary power supply terminal AUXR of the power control device 15. By using a transformer in which a voltage, for example, four times the secondary-side output voltage is induced in the auxiliary winding Nb, when the secondary-side output voltage has a fluctuation range of 3V to 20V, a voltage of 12V to 80V can be input to the auxiliary power supply terminal AUXR.

[0017] Furthermore, a rectifying and smoothing circuit 17B formed by connecting a rectifying diode D12, a resistor R12, and a smoothing capacitor C12 in series is provided between the center tap of the auxiliary winding Nb and the ground point. The voltage rectified and smoothed by the rectifying and smoothing circuit 17B is applied to the external power supply terminal VDD of the power control device 15. Furthermore, the power control device 15 is provided with an external terminal VBUS to which a voltage obtained by dividing the voltage between the terminals of the auxiliary winding Nb with resistors R4 and R5 is input, and a high-voltage input start terminal HV to which a voltage from the AC power supply 11 at the time of power-on is input as a starting voltage through diodes D1, D2, and resistor R1.

[0018] In this embodiment, the switching transistor SW1 and the active clamping transistor SW2, which are connected in series with the primary winding Np, are configured as discrete components using N-channel MOSFETs (insulated-gate field-effect transistors). Furthermore, to reduce losses due to drive current, it is desirable to use GaN transistors with small gate charge amounts for SW1 and SW2. Furthermore, the power control device 15 is provided with an external output terminal LGATE that outputs a signal to drive the gate terminal of transistor SW1, an external output terminal HGATE that outputs a signal to drive the gate terminal of transistor SW2, and a current detection terminal CS which is an external terminal to which a voltage converted from current by a sense resistor R3 that detects the drain current of transistor SW1 is input. Furthermore, the power control device 15 is provided with an external terminal FB to which a phototransistor PT is connected, which receives a light feedback signal from the secondary side of the transformer 14.

[0019] On the other hand, the secondary side of the transformer 14 includes a synchronous rectifier MOS transistor TR1 connected in series with the secondary winding Ns, a secondary side control IC 20 that uses the voltage generated on the secondary side as the power supply voltage, detects the source and drain voltages of the synchronous rectifier MOS transistor TR1 and generates on / off control signals for the transistor TR1, and a smoothing capacitor C3 connected between the output terminals to stabilize the output voltage.

[0020] Furthermore, although not particularly limited, the AC-DC converter of this embodiment is configured to be applicable to a system to which a USB (Universal Serial Bus) standard power supply device 30 is connected as a load circuit. The power supply device 30 comprises a plug 31 having four terminals VBUS, PSUB, CC1, and CC2 as defined by the USB standard, a USB-PD (Power Delivery) compliant controller 32 connected to terminals PSUB, CC1, and CC2 of the plug 31, and a transistor TR2 connected between the secondary winding Ns and terminal VBUS. Transistor TR2 is controlled according to information received by the controller 32 via terminals CC1 and CC2 of the plug 31, and outputs a voltage compliant with the USB-PD standard to the VBUS terminal.

[0021] Furthermore, a feedback photodiode PD is connected between the VBUS terminal of the secondary winding Ns and the USB-PD controller 32, and the controller 32 is configured to supply a current to the photodiode PD that corresponds to the level of the secondary output voltage. Specifically, the USB-PD controller 32 incorporates a shunt regulator to which a voltage obtained by dividing the secondary output voltage is applied, and the shunt regulator is configured to supply a current to the photodiode PD that is proportional to the level of the secondary output voltage. The photodiode PD and the primary-side phototransistor PT constitute a photocoupler as an isolated signal transmission means. Light emitted from the secondary-side photodiode PD is received by the primary-side phototransistor PT, generating a feedback signal corresponding to the light intensity, which is input to the external terminal FB. The primary-side power control device 15 generates a signal to turn off the switching transistor SW1, which is connected in series with the primary-side winding Np, according to this feedback signal and the voltage of the external terminal (current detection terminal) CS.

[0022] The AC-DC converter 10 of this embodiment is characterized in that, as a power supply control device 15, it is configured as a package device in which an IC chip (driver chip) that incorporates a driver for driving the gate terminals of the switching transistors SW1 and SW2, and an IC chip (control chip) that generates a control signal to be input to the driver chip are mounted on a thin plate made of, for example, a CU alloy material and enclosed in a single package, and that the control chip supplies an enable signal EN to the driver chip to control the operating state of the driver.

[0023] Figure 2 shows an embodiment of the power control device 15 having the features described above. As shown in Figure 2, the power control device 15 of this embodiment includes a control chip 51 and a driver chip 52. Of these, the control chip 51 includes an internal regulator 511 that generates the internal power supply voltage Vdd of the device using the voltage of the external power supply terminal AUXR of the device 15, and an internal circuit 512 that generates a signal ON / OFF1 indicating the on / off timing of the switching transistor SW1 and a signal ON / OFF2 indicating the on / off timing of the transistor SW2 of the active clamp circuit 16, and outputs them to the driver chip 52. This internal circuit 512 operates using the internal power supply voltage Vdd generated by the internal regulator 511 or the voltage from the rectifier smoothing circuit 17B.

[0024] The built-in regulator 511 within the control chip 51 is composed of a linear regulator. By using a linear regulator, the generation of noise associated with the internal power supply voltage can be suppressed. In addition, a capacitor C13 is connected to the VDD terminal to raise the external power supply terminal VDD to the operating start voltage (switching start) by flowing current from the HV terminal when the power is turned on. Furthermore, in the power control device 15 of this embodiment, although not particularly limited, the internal circuit 512 is configured to have a function of performing ZVS (zero voltage switching) control, which turns on the switching transistor SW1 based on the drain voltage of SW1 (voltage at the external terminal HGND).

[0025] Furthermore, the internal circuit 512 monitors, for example, the ON / OFF1 and ON / OFF2 on / off timing signals of switching transistors SW1 and SW2 that the internal circuit 512 outputs to the driver chip 52. When it determines that the output voltage on the secondary side has dropped and that both SW1 and SW2 are not switching, the internal circuit 512 determines that there is no need to operate the driver, and therefore negates the enable signal EN output to the driver chip 52, transitioning to a standby state and stopping the operation of some circuits within the driver chip 52. The state in which switching transistors SW1 and SW2 cease to switch occurs when the output power decreases and the feedback signal from the secondary side (voltage at the FB terminal) drops, causing the on / off timing signals ON / OFF1 and ON / OFF2 of SW1 and SW2 to stop changing. Therefore, the internal circuit 512 may monitor the voltage at the FB terminal to determine the standby state and negate the enable signal EN. In this embodiment, the standby state is the period during burst mode operation in which the signals ON / OFF1 and ON / OFF2 do not change.

[0026] Meanwhile, the driver chip 52 includes a MOS transistor M1 for raising the gate voltage of switching transistor SW2 to a high level, a MOS transistor M2 for lowering the gate voltage of SW2 to a low level, a MOS transistor M3 for raising the gate voltage of switching transistor SW1 to a high level, and a MOS transistor M4 for lowering the gate voltage of SW1 to a low level. Furthermore, the driver chip 52 includes an internal circuit 52A that generates control signals for transistors M1 and M2 based on an ON / OFF timing signal ON / OFF2 input from the control chip 51, and an internal circuit 52B that generates control signals for transistors M3 and M4 based on an ON / OFF timing signal ON / OFF1 input from the control chip 51.

[0027] In the power control device 15 of this embodiment, the totem-pole output stage consisting of the internal circuit 52B on the low side of the driver chip 52 and M3, M4 is operated by the internal power supply voltage Vdd generated by the built-in regulator 511 or the voltage generated by the rectifier-smoothing circuit 17B and applied to the external power supply terminal VDD. Specifically, as shown in Figure 3, when the output voltage of the power supply is lower than a certain voltage value C, the internal circuit 512 of the control chip 51, the internal circuit 52B on the low side of the driver chip 52, and the totem pole output stages (M3, M4) operate with a voltage Vdd obtained by stepping down the voltage Vrsa generated by the rectifier-smoothing circuit 17A (Figure 1) and supplied to the auxiliary power terminal AUXR by the built-in regulator 511. When the output voltage is higher than the voltage value C, they operate with a voltage Vrsb supplied from the rectifier-smoothing circuit 17B (Figure 1) connected to the center tap of the auxiliary winding Nb. In Figure 3, Va' is the average voltage of the induced voltage Va of the auxiliary winding Nb (Figure 4), and the voltage Vrsa at terminal AUXR is lower than Va' by the forward voltage Vf of diode D11.

[0028] Furthermore, the built-in regulator 511 outputs the input voltage as is when the voltage Vrsa supplied from the rectifier-smoothing circuit 17A is lower than the set output voltage Vsv, and when the voltage Vrsa is higher than the voltage Vsv, it steps down the input voltage Vrsa to Vsv and outputs it. As described above, in this embodiment, when the output voltage is high, the internal circuit 52B of the driver chip 52 and the totem-pole output stages (M3, M4) are configured to operate with the voltage Vrsb from the external rectifier-smoothing circuit 17B (Figure 1) of the device 15. This reduces the power consumption of the built-in regulator 511 compared to when the built-in regulator 511 generates the voltage necessary to operate the internal circuit 52B and the totem-pole output stages (M3, M4) even at high output voltages.

[0029] Lowering the output voltage of regulator 511 reduces the switching voltage C, thereby reducing losses and heat generation. However, if the output voltage of regulator 511 is too low, there are design issues such as the relationship with the UVLO (Undervoltage Lockout) voltage within the IC and the operating point, so it cannot be lowered too much. For example, in the case of a USB-PD adapter (output: 3.3V~21V), it is conceivable to set the output voltage of regulator 511 to around 10V and the switching voltage C to around 11V. The ratio of the auxiliary winding Nb to the winding at the center tap position is set so that the switching voltage C can take the above values.

[0030] Figure 4 shows a specific example of the built-in regulator 511. The built-in regulator 511 in this embodiment is a linear regulator and, as shown in Figure 4, consists of an N-channel MOS transistor MT connected between the auxiliary power terminal AUXR and the external power terminal VDD of the power control device 15, a resistor R13 connected between the auxiliary power terminal AUXR and the gate terminal of transistor MT, and a Zener diode DZ for constant voltage connected in reverse between the gate terminal of transistor MT and the ground point GND. Figure 5 shows the input / output voltage characteristics of the built-in regulator 511 having the above configuration.

[0031] In the regulator configuration shown in Figure 4, if the N-channel MOS transistor MT is designed so that the gate-source voltage Vgs is approximately 5V when a certain current is supplied, and the voltage Vrsa at the auxiliary power terminal AUXR is, for example, 14.3V when the USB-PD adapter outputs 3.3V, then the voltage at the external power terminal VDD will be 14.3V - 5V = 9.3V, which is lower than the set voltage Vsv (for example, 10V) of the regulator 511. When the voltage at the auxiliary power terminal AUXR falls below 9.3V, the regulator 511 stops operating. Also, when the voltage at the external power terminal VDD rises above the set voltage Vsv of the regulator, the transistor MT turns off and the regulator stops operating. Furthermore, the built-in regulator 511 is not limited to the circuit configuration shown in Figure 4; any linear regulator having the characteristics shown in Figure 5 is acceptable, such as one using a bipolar transistor instead of a MOS transistor.

[0032] On the other hand, the totem-pole output stage, consisting of the internal circuit 52A on the high-side of the driver chip 52 and M1, M2, is configured to operate using the voltage boosted by the external diode D14 and capacitor C14 and supplied to the external power supply terminal HVDD as the power supply voltage. This allows the switching transistor SW2 to be turned on by applying a voltage higher than the source voltage of SW2 to the gate terminal of SW2. Furthermore, the anode terminal of diode D14 is connected to the output node of the rectifier-smoothing circuit 17B, and the cathode terminal is connected to the connection node N1 (the drain terminal of SW1) between SW2 and SW1 via capacitor C14. Node N1 is connected to the external terminal HGND of the power control device 15. Therefore, when SW1 is turned on, charge is accumulated in capacitor C14 via diode D14, and when SW1 is turned off, the charge accumulated in capacitor C14 pushes up the potential of the HVDD terminal, driving SW2. In other words, a voltage based on the potential of the HGND terminal is applied to the HVDD terminal.

[0033] As described above, in the AC-DC converter (isolated power supply) of this embodiment, a transformer 14 equipped with an auxiliary winding having a center tap is used, and a linear regulator that steps down the voltage obtained by rectifying and smoothing the voltage induced throughout the auxiliary winding is provided in the power control device 15. Furthermore, the device is configured to supply the voltage obtained by rectifying and smoothing the voltage taken from the center tap, or the output of the linear regulator, as the power supply voltage to the internal circuit of the power control device.Therefore, in power supply devices with multiple outputs or a wide range of secondary output voltages, such as USB-PD adapters, power consumption can be reduced compared to cases where a transformer equipped with an auxiliary winding without a center tap is used to generate the power supply voltage supplied to the internal circuit.

[0034] Furthermore, since a linear regulator, rather than a switching power supply, is used as the circuit that generates the power supply voltage for the internal circuit of the IC constituting the primary-side power control device 15, noise generation can be suppressed. In addition, because the linear regulator is provided within the primary-side power control device 15, there is no need to configure the regulator using an external component of the IC, which has the advantage of reducing the number of components in the power supply unit.

[0035] Although the present inventors' inventions have been described in detail based on embodiments, the present invention is not limited to the above embodiments. For example, in the above embodiments, a control chip and a driver chip were enclosed in a single package to form a package device, but the present invention can also be applied to a primary-side power control IC that constitutes a multi-output isolated power supply device in which the control unit and driver unit are formed as a semiconductor integrated circuit on a single semiconductor chip, and the power consumption of the power control IC can be reduced by configuring the control unit to supply the enable signal EN to the driver unit.

[0036] Furthermore, although the above embodiment described the case where the present invention is applied to an AC-DC converter using an active clamp type primary-side power supply control IC, the present invention can also be applied to an AC-DC converter using a PWM control type primary-side power supply control IC. Furthermore, although the above embodiment described the case in which the present invention is applied to an AC-DC converter, the present invention can also be applied to a DC-DC converter in which the diode bridge circuit 13 is omitted. [Explanation of Symbols]

[0037] 11…AC power supply, 12…AC power filter, 13…Diode bridge circuit, 14…Transformer, 15…Power supply semiconductor device (power control device), 16…Active clamp circuit, 17A,17B…Rectifier and smoothing circuit, 51…Control chip, 511…Built-in regulator, 512…Internal circuitry, 52…Driver chip, 52A,52B…Internal circuitry

Claims

1. An isolated power supply device comprising: a voltage conversion transformer with an auxiliary winding having a center tap; a first switching element connected in series with the primary winding of the transformer; and a power supply control semiconductor device for controlling the first switching element, The aforementioned power control semiconductor device is A first external terminal that outputs an on / off drive signal for the first switching element, A second external terminal to which a feedback signal from the secondary side of the transformer is input, A first external power supply terminal to which the voltage induced at one terminal of the auxiliary winding is input, A second external power supply terminal to which the voltage induced at the center tap of the auxiliary winding is input, A linear regulator that steps down the voltage input to the first external power terminal, The system comprises an internal circuit that generates an on / off control signal for the first switching element, The aforementioned internal circuit is If the voltage input to the second external power terminal is higher than the output voltage of the linear regulator, the device is operated by the voltage input to the second external power terminal. An isolated power supply device characterized in that, if the voltage input to the second external power supply terminal is lower than the output voltage of the linear regulator, it is configured to operate using the voltage generated by the linear regulator.

2. A first rectifier and smoothing circuit that rectifies and smooths the voltage induced at one terminal of the auxiliary winding, The circuit comprises a second rectifier and smoothing circuit that rectifies and smooths the voltage induced at the center tap of the auxiliary winding, The voltage rectified and smoothed by the first rectifier-smoothing circuit is input to the first external power supply terminal. The isolated power supply device according to claim 1, characterized in that the voltage rectified and smoothed by the second rectifier and smoothing circuit is input to the second external power supply terminal.

3. An active clamp circuit connected between the terminals of the primary winding of the transformer, The second switching element constituting the active clamp circuit, A third external terminal that outputs an on / off drive signal for the second switching element, A fourth external terminal to which the voltage at the connection point between the first switching element and the second switching element is applied, A voltage boosting means that boosts the voltage input to the second external power terminal with reference to the voltage of the fourth external terminal, The system further comprises a current-voltage conversion element connected in series with the first switching element, The aforementioned power control semiconductor device is A third external power supply terminal to which the voltage boosted by the aforementioned boosting means is input, The system comprises a drive circuit section that generates and outputs on / off drive signals for the first switching element and on / off drive signals for the second switching element, The internal circuit is configured to generate and supply on / off control signals for the first switching element and the second switching element to the drive circuit section. The aforementioned drive circuit section is A first drive circuit that outputs an on / off drive signal for the first switching element, and a first signal generation circuit that generates a control signal for the first drive circuit based on the on / off control signal for the first switching element from the internal circuit, The system comprises a second drive circuit that outputs an on / off drive signal for the second switching element, and a second signal generation circuit that generates a control signal for the second drive circuit based on the on / off control signal for the second switching element from the internal circuit, The first drive circuit and the first signal generation circuit operate by the voltage generated by the linear regulator or the voltage input to the second external power supply terminal. The isolated power supply device according to claim 2, characterized in that the second drive circuit and the second signal generation circuit are configured to operate by a voltage input to the third external power supply terminal.

4. The isolated power supply according to any one of claims 1 to 3, characterized in that the secondary side of the transformer is equipped with a transistor connected between one terminal of the secondary winding and the secondary output terminal, and a USB-PD control circuit for controlling the transistor, and is configured to output a voltage compliant with the USB-PD standard.

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