Current limiting technique for a switching power converter

• US 20030020442A1
• Filed: 06/21/2002
• Published: 01/30/2003
• Est. Priority Date: 06/21/2001
• Status: Active Grant

• Current Assignee: Champion Microelectronic Corporation
• Original Assignee: Champion Microelectronic Corporation
• Inventors: Hwang, Jeffrey H.
• Granted Patent: US 6,674,272 B2
• Time in Patent Office: Days
• US Class Current: 323/288
• CPC Class Codes: H02M 1/007 Plural converter units in cascade push-pull DC/DC converters with pre-regulator H02M3/3374; DC-AC converters following a DC-DC stage including a high frequency transformer H02M7/4807; DC-AC converters following a DC-DC conversion stage generating periodically varying voltages H02M7/4826
• CPC Class Codes: H02M 1/0845 digitally controlled (or with digital control)
• CPC Class Codes: H02M 1/092 the control signals being transmitted optically
• 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 1/4225 using a non-isolated boost converter
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop
• 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
• CPC Class Codes: Y02P 80/10 Efficient use of energy, e.g. using compressed air or pressurized fluid as energy carrier
• CPC Class Codes: Y10S 323/901 Starting circuits
• CPC Class Codes: Y10S 323/908 Inrush current limiters

Abstract

An improved technique for limiting current in a switching power converter. The switching power converter includes a soft-start circuit which slowly increases a switching duty cycle upon power-up. Once the converter is operating normally, the duty cycle is controlled to regulate the output voltage. In the event an excessive output current is detected, soft-start circuit is controlled to reduce the switching duty cycle. More particularly, a soft-start capacitor may be discharged during portions of a clock period used to control switching.

Claims

1. A switching power converter for providing a load with an output current and voltage, comprising:

a switch for transferring energy from a source to the load by opening and closing the switch according to a duty cycle; and

a duty cycle limiter coupled to the switch wherein the duty cycle limiter is triggered when the output current exceeds a predetermined level, the duty cycle limiter including a capacitor having a charge that is gradually changed in response to the output current exceeding the predetermined level and wherein the duty cycle is related to a voltage on the capacitor.

2. The power converter according to claim 1, wherein a time constant for changing the charge on the capacitor is such that a voltage across the capacitor is changed by approximately one-third for each switching cycle that the duty cycle limiter is triggered.

3. The power converter according to claim 1, further comprising a sensor for sensing the output current.

4. The power converter according to claim 3, wherein the sensor is resistive.

5. The power converter according to claim 3, wherein the sensor is inductive.

6. The power converter according to claim 3, further comprising a comparator for comparing the output current to the predetermined level wherein the charge on the capacitor is changed according to an output of the comparator.

7. The power converter according to claim 1, wherein the capacitor is charged by constant current source and discharged in response to the duty cycle limiter being triggered.

8. The power converter according to claim 1, further comprising a comparator for controlling the duty cycle of the switch by comparing a feedback signal that is representative of the output voltage to a periodic ramp signal.

9. The power converter according to claim 8, wherein when the voltage across the capacitor goes beyond a limit set by the feedback signal in response to changing the charge on the capacitor, the voltage across the capacitor controls the dutycycle.

10. The power converter according to claim 8, wherein the periodic ramp signal is representative of the output current.

11. The power converter according to claim 10, wherein the duty cycle limiter is triggered when the periodic ramp signal exceeds a predetermined level.

12. The power converter according to claim 1, wherein the duty cycle limiter provides a soft-start in which the switching duty cycle is gradually increased upon power-up.

13. A switching power converter for providing a load with an output current and voltage comprising a switch for transferring energy from a source to the load by opening and closing the switch according to a duty cycle wherein when the output current reaches an excessive level, the duty cycle is limited to an amount that is related to a degree to which the output current is excessive.

14. The power converter according to claim 13, further comprising a duty cycle limiter that is triggered when the output current exceeds a predetermined level.

15. The power converter according to claim 14, wherein the duty cycle limiter comprises a capacitor in which a charge on the capacitor is gradually changed when the duty cycle limiter is triggered and wherein the duty cycle is related to a voltage on the capacitor.

16. The power converter according to claim 14, wherein the duty cycle limiter comprises a counter that initiated to count when the duty cycle limiter is triggered and wherein the duty cycle is related to a count of the counter.

17. A switching power converter for providing a load with an output current and voltage, comprising

a switch for transferring energy from a source to the load by opening and closing the switch according to a duty cycle;

and

a duty cycle limiter coupled to the switch wherein the duty cycle limiter is triggered when the output current exceeds a predetermined level, the duty cycle limiter including a counter and wherein the duty cycle is related to a count of the counter.

18. The power converter according to claim 17, wherein a count of counter is gradually changed in response to the duty cycle limiter being triggered.

19. The power converter according to claim 18, further comprising a sensor for sensing the output current.

20. The power converter according to claim 19, wherein the sensor is resistive.

21. The power converter according to claim 19, wherein the sensor is inductive.

22. The power converter according to claim 19, further comprising a comparator for comparing the output current to the predetermined level wherein the counter is initiated to count according to an output of the comparator.

23. The power converter according to claim 18, further comprising a comparator for controlling the duty cycle of the switch by comparing a feedback signal that is representative of the output voltage to a periodic ramp signal.

24. The power converter according to claim 23, further comprising an analog to digital converter for converting the count to a voltage and wherein when the voltage at the digital to analog converter goes beyond a limit set by the feedback signal, the voltage at the digital to analog converter controls the duty cycle.

25. The power converter according to claim 23, wherein the periodic ramp signal is representative of the output current.

26. The power converter according to claim 25, wherein the duty cycle limiter is triggered when the periodic ramp signal exceeds a predetermined level.

27. The power converter according to claim 17, wherein the duty cycle limiter provides a soft-start in which the switching duty cycle is gradually increased upon power-up.