X-cap. discharge method for flyback converter

Patent metadata

Field	Value
Publication number	US-9837917-B1
Application number	US-201615220026-A
Country	US
Kind code	B1
Filing date	Jul 26, 2016
Priority date	Jul 26, 2016
Publication date	Dec 5, 2017
Grant date	Dec 5, 2017

Abstract

In accordance with a preferred embodiment of the present invention, a method of operating a switch-mode power supply includes: receiving a dropout detection signal from a dropout detection circuit coupled to an input of the switch-mode power supply; and discharging an input capacitor coupled to the input of the switch-mode power supply via a switching transistor having a first load path coupled to the input capacitor through an inductive element.

First claim

What is claimed is:

1. A method of operating a switch-mode power supply, the method comprising:
receiving a dropout detection signal from a dropout detection circuit coupled to an input of the switch-mode power supply;
detecting, with the dropout detection circuit, a dropout of an AC input voltage at the input of the switch-mode power supply; and
discharging an input capacitor coupled to the input of the switch-mode power supply via a switching transistor having a first load path coupled to the input capacitor through an inductive element based on the received dropout detection signal by activating the switching transistor with a gate drive circuit coupled to a control node of the switching transistor.

2. The method of claim 1 , wherein the dropout detection circuit is coupled to the input capacitor through a first resistor in series with a diode.

3. The method of claim 1 , wherein discharging the input capacitor comprises turning on the switching transistor.

4. The method of claim 3 , further comprising turning on a first transistor coupled in series with the switching transistor when receiving the dropout detection signal.

5. The method of claim 4 , wherein:
the turning on of the switching transistor comprises fully turning on the switching transistor, and
the method further comprises limiting a current flowing through the first transistor to a predetermined value.

6. The method of claim 4 , wherein the inductive element comprises a first winding of a transformer, and the method further comprises:
during start-up,
keeping the first transistor off, and
turning on the switching transistor; and

during normal operation,
turning on the first transistor.

7. The method of claim 6 , further comprising:
during start-up, allowing a current flowing through the switching transistor to charge a capacitor coupled to a first input of the switch-mode power supply; and
during normal operation, allowing a current flowing through a third winding of the transformer to charge the capacitor coupled to the first input of the switch-mode power supply.

8. The method of claim 6 , wherein turning on the switching transistor during start-up comprises charging a gate of the switching transistor through a first resistor.

9. The method of claim 6 , wherein the first transistor comprises a normally-off transistor, and the keeping the first transistor off comprises charging a gate of the first transistor to a turn-off voltage.

10. The method of claim 1 , further comprising limiting a current flowing through the first load path to a predetermined value after receiving the dropout detection signal.

11. The method of claim 1 , further comprising:
monitoring a first voltage at a terminal of the input capacitor; and
generating the dropout detection signal based on the monitored first voltage.

12. The method of claim 1 , further comprising generating the dropout detection signal from the dropout detection circuit, wherein the generating the dropout detection signal comprises:
receiving a first peak voltage based on an input voltage between inputs of the switch-mode power supply;
receiving a second peak voltage based on the input voltage, wherein the second peak voltage happens in an AC cycle after the first peak voltage;
receiving a third peak voltage based on the input voltage, wherein the third peak voltage happens in an AC cycle after the second peak voltage; and
generating the dropout detection signal when the third peak voltage is smaller than the second peak voltage, and the second peak voltage is smaller than the first peak voltage.

13. The method of claim 1 , further comprising generating the dropout detection signal from the dropout detection circuit, wherein the generating the dropout detection signal comprises:
receiving a first peak voltage based on an input voltage between inputs of the switch-mode power supply;
receiving a second peak voltage based on the input voltage, wherein the second peak voltage happens in an AC cycle after the first peak voltage;
receiving a third peak voltage based on the input voltage, wherein the third peak voltage happens in an AC cycle after the second peak voltage; and
generating the dropout detection signal when the third peak voltage is smaller than the second peak voltage multiplied a factor k, and the second peak voltage is smaller than the first peak voltage multiplied by the factor k, or when the first peak voltage is smaller than a predetermined value.

14. The method of claim 13 , wherein the factor k is a number between 0.9 and 0.98.

15. The method of claim 1 , further comprising generating the dropout detection signal from the dropout detection circuit, wherein the generating the dropout detection signal comprises:
receiving a first peak voltage based on an input voltage between inputs of the switch-mode power supply;
receiving a second peak voltage based on the input voltage, wherein the second peak voltage happens in an AC cycle after the first peak voltage;
receiving a third peak voltage based on the input voltage, wherein the third peak voltage happens in an AC cycle after the second peak voltage;
receiving a fourth peak voltage based on the input voltage, wherein the fourth peak voltage happens in an AC cycle after the third peak voltage; and
generating the dropout detection signal when the fourth peak voltage is smaller than the third peak voltage multiplied a factor k, the third peak voltage is smaller than the second peak voltage multiplied a factor k, and the second peak voltage is smaller than the first peak voltage multiplied by the factor k.

16. The method of claim 1 , wherein the inductive element comprises a first winding of a transformer, the method further comprising:
detecting an overvoltage condition in a second circuit path coupled to a second winding of the transformer;
turning off a synchronous rectifier transistor coupled to the second winding of the transformer after detecting the overvoltage condition; and
turning on a second transistor coupled to the second circuit path after detecting an overvoltage condition.

17. The method of claim 1 , further comprising turning on and off the switching transistor.

18. A circuit comprising:
a first gate drive circuit configured to provide a first signal to a switching transistor having a first load path coupled to an input capacitor of the circuit through an inductive element;
a discharge condition detector circuit configured to
receive a first peak voltage based on a first voltage between inputs terminals of the input capacitor,
receive a second peak voltage based on the first voltage, wherein the second peak voltage happens in an AC cycle after the first peak voltage,
receive a third peak voltage based on the first voltage, wherein the third peak voltage happens in an AC cycle after the second peak voltage, and
generate a dropout detection signal when the third peak voltage is smaller than the second peak voltage multiplied a factor k, and the second peak voltage is smaller than the first peak voltage multiplied by the factor k, or when the first peak voltage is smaller than a predetermined value; and

a discharge controller coupled to the first gate drive circuit, the discharge controller configured to
receive the dropout detection signal from the discharge condition detector circuit, and
turn on the switching transistor with the first gate drive circuit after receiving the dropout detection signal.

19. The circuit of claim 18 , wherein the discharge condition detector circuit is configured to mo

Assignees

• Infineon Technologies Austria Ag

Inventors

• Furtner Wolfgang
• Gong Xiaowu

Classifications

• H02M1/44

  Circuits or arrangements for compensating for electromagnetic interference in converters or inverters · CPC title

• H02M3/33523 Primary

  with galvanic isolation between input and output of both the power stage and the feedback loop · CPC title

• H02M1/322

  Means for rapidly discharging a capacitor of the converter for protecting electrical components or for preventing electrical shock · CPC title