SWITCHING POWER SUPPLY AND METHOD OF SHORT-CIRCUIT-TO-GROUND DETECTION THEREFOR

• US 20180278045A1
• Filed: 03/23/2018
• Published: 09/27/2018
• Est. Priority Date: 03/27/2017
• Status: Active Grant

• Current Assignee: ROHM Co., Ltd.
• Original Assignee: ROHM Co., Ltd.
• Inventors: WATANABE, Yukihiro
• Granted Patent: US 10,374,413 B2
• Time in Patent Office: Days
• CPC Class Codes: G01R 19/16538 in AC or DC supplies G01R19/16519 and G01R19/16528 take precedence
• CPC Class Codes: G01R 31/52 Testing for short-circuits, leakage current or ground faults
• CPC Class Codes: H02H 1/0007 concerning the detecting means in general G01R or other subclasses of G01; reed switches H01H71/2445
• CPC Class Codes: H02H 7/1213 for DC-DC converters
• CPC Class Codes: H02M 1/08 Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
• CPC Class Codes: H02M 1/32 Means for protecting converters other than automatic disconnection
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters
• CPC Class Codes: H02M 3/157 with digital control
• CPC Class Codes: H02M 3/1588 comprising at least one synchronous rectifier element H02M3/1582, H02M3/1584 take precedence
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A switching power supply detects a short circuit to ground, for example, by comparing a soft-start voltage Vss with a feedback voltage Vfb while securing an offset voltage ΔV. The switching power supply includes, for example, a short-circuit-to-ground detection circuit which detects a short circuit to ground at a monitoring target terminal. When a short circuit to ground occurs for the first time, the output of the short-circuit-to-ground detection circuit is masked for a first period. When a short circuit to ground occurs for the second or any succeeding time, the output of the short-circuit-to-ground detection circuit is masked for a second period shorter than the first period.

Claims

1. A switching power supply comprising:

a switching unit arranged to turn on and off an output transistor and a synchronous rectification semiconductor element to drive an inductor so that an output voltage is generated from an input voltage;

an error amplifier arranged to generate an error voltage corresponding to a difference between a predetermined first reference voltage and the output voltage or a feedback voltage corresponding to the output voltage;

a soft-start circuit arranged to generate a soft-start voltage;

a pulse width modulation (PWM) comparator arranged to compare the error voltage with the soft-start voltage so as to generate a pulse width modulation signal;

a driving circuit arranged to receive an input of the pulse width modulation signal so as to generate a drive signal for the switching unit; and

an offset comparator arranged to compare the soft-start voltage with the feedback voltage, with a predetermined offset voltage, wherein

when Vss≥

Vfb+Δ

V holds, where Vss represents the soft-start voltage, Vfb represents the feedback voltage, and Δ

V represents the offset voltage, then the output transistor is turned off via the driving circuit based on an offset detection signal output from the offset comparator.

2. The switching power supply according to claim 1, further comprising:

an short-circuit-to-ground detection circuit arranged to compare the feedback voltage with a second reference voltage so as to output a first short-circuit-to-ground output signal; and

an AND circuit arranged to perform an AND operation between the offset detection signal and the first short-circuit-to-ground output signal, wherein

the output transistor is turned off via the driving circuit by a short protection control signal output from the AND circuit.

3. The switching power supply according to claim 1, further comprising:

a short-circuit-to-ground detection circuit arranged to compare the feedback voltage with a second reference voltage so as to output a first short-circuit-to-ground output signal;

a short-circuit-to-ground detection unit arranged to detect the feedback voltage so as to output a short-circuit-to-ground detection signal after the soft-start voltage reaches the first reference voltage;

a protection signal selection circuit arranged to select one of the short-circuit-to-ground detection signal and the offset detection signal so as to output a second short-circuit-to-ground output signal; and

an AND circuit having a first input terminal and a second input terminal applied with the first short-circuit-to-ground output signal and the second short-circuit-to-ground output signal, respectively, wherein

the output transistor is turned off via the driving circuit by a short protection control signal output from the AND circuit.

4. The switching power supply according to claim 1, wherein the offset voltage is set to be lower than the first reference voltage.

5. The switching power supply according to claim 4, wherein Δ

V>

Vmax−

Vref1 holds, where Δ

V represents the offset voltage, Vmax represents a maximum value of the soft-start voltage, and Vref1 represents the first reference voltage.

6. The switching power supply according to claim 1, wherein an input stage of the offset comparator includes a differential amplifier, an offset resistor is connected to one of differential pair transistors of the differential amplifier, and the offset voltage is set by the offset resistor.

7. The switching power supply according to claim 6, wherein each of the differential pair transistors is constituted of a MOS transistor or a bipolar transistor.

8. The switching power supply according to claim 7, wherein the differential pair transistors are constituted of a first MOS transistor and a second MOS transistor, the source of the first MOS transistor is connected to a first offset resistor, and the source of the second MOS transistor is connected to a second offset resistor having a resistance value different from that of the first offset resistor.

9. The switching power supply according to claim 7, wherein the differential pair transistors are constituted of a first MOS transistor and a second MOS transistor, and a gate-source threshold voltage of the first MOS transistor is set to be different from that of the second MOS transistor.

10. The switching power supply according to claim 7, wherein the differential pair transistors are constituted of a first bipolar transistor and a second bipolar transistor, the emitter of the first bipolar transistor is connected to a first offset resistor, and the emitter of the second bipolar transistor is connected to a second offset resistor having a resistance value different from that of the first offset resistor.

11. The switching power supply according to claim 7, wherein the differential pair transistors are constituted of a first bipolar transistor and a second bipolar transistor, and base-emitter forward voltages of the first bipolar transistor and the second bipolar transistor are set to be different from each other.

12. The switching power supply according to claim 8, wherein one of the first offset resistor and the second offset resistor has a resistance value of substantially 0Ω

.

13. The switching power supply according to claim 1, wherein a first input terminal and a second input terminal as noninverting input terminals and a third input terminal as an inverting input terminal are prepared on an input side of the error amplifier, the first input terminal is applied with the soft-start voltage, the second input terminal is applied with the first reference voltage, and the third input terminal is applied with the feedback voltage.

14. The switching power supply according to claim 2, wherein the second reference voltage is set to a potential lower than the first reference voltage.

15. The switching power supply according to claim 3, wherein the protection signal selection circuit includes a two-input latch circuit supplied with two signals including the short-circuit-to-ground detection signal and an enable signal for raising the soft-start voltage, and a selector arranged to select and output one of the short-circuit-to-ground detection signal and the offset detection signal according to an output of the two-input latch circuit.

16. The switching power supply according to claim 15, wherein the two-input latch circuit is constituted of a NAND circuit or a NOR circuit.

17. The switching power supply according to claim 1, wherein short-circuit-to-ground detection is masked until the offset detection signal is generated.

18. The switching power supply according to claim 3, wherein short-circuit-to-ground detection is masked until the second short-circuit-to-ground output signal is generated.

19. A method for detecting a short circuit to ground using the switching power supply according to claim 3, the method comprising:

masking short-circuit-to-ground detection during a short-circuit-to-ground detection time period tx1 until the short-circuit-to-ground detection signal is generated when a short circuit to ground occurs for the first time after an enable signal is input; and

masking short-circuit-to-ground detection during a short-circuit-to-ground detection time period tx2 until the offset detection signal is generated when a short circuit to ground occurs for the second time after the first short circuit to ground is cancelled in a state where input of the enable signal is continued.

20. The method for detecting a short circuit to ground according to claim 19, wherein a relationship of tx2≤

tx1 is satisfied between the short-circuit-to-ground detection time period tx2 and the short-circuit-to-ground detection time period tx1.

21. A switching power supply comprising a short-circuit-to-ground detection circuit arranged to perform short-circuit-to-ground detection of a monitoring target terminal, wherein an output of the short-circuit-to-ground detection circuit is masked during a first period when a short circuit to ground occurs for the first time, and the output of the short-circuit-to-ground detection circuit is masked during a second period shorter than the first period when a short circuit to ground occurs for the second or any succeeding time.