Soft-start apparatus for power supplies

• US 20070057649A1
• Filed: 09/13/2005
• Published: 03/15/2007
• Est. Priority Date: 09/13/2005
• Status: Active Grant

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: System General Corporation (ON Semiconductor Corporation)
• Inventors: Yang, Ta-Yung
• Granted Patent: US 7,218,080 B2
• Time in Patent Office: Days
• US Class Current: 323/222
• CPC Class Codes: G05F 1/468 characterised by reference voltage circuitry, e.g. soft start, remote shutdown
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters

Abstract

A soft-start apparatus for a power supply outputs a compensation signal to a PWM controller as the power supply starts up. Meanwhile, a pulse width of a switching signal outputted from the PWM controller gradually increases from an initial value to a determined value. The pulse width of the switching signal is then timely modulated in response to the load conditions thereat so as to prevent a power switch from over-voltage and over-current damage as the power supply starts up.

Claims

1. A soft-start apparatus for a power supply outputting a compensation signal to a PWM controller at a start-up transient of said power supply, comprising:

a start-up capacitor;

a first power source, connected to said start-up capacitor for charging up said start-up capacitor;

a voltage-regulating unit, having a first input, a second input and an output;

said first input of said voltage-regulating unit being supplied with an internal reference voltage, said second input of said voltage-regulating unit being connected to said output of said voltage-regulating unit and said start-up capacitor;

wherein said voltage-regulating unit is used to regulate a reference voltage across said start-up capacitor;

an error amplifier, having a first input, a second input and an output;

said first input of said error amplifier being connected to said output of said voltage-regulating unit for receiving said reference voltage, said second input of said error amplifier being connected to a feedback terminal of said power supply for receiving a feedback voltage;

wherein said error amplifier generates said compensation signal in response to said reference voltage and said feedback voltage; and

a clamping diode, connected between said second input and said output of said error amplifier, said clamping diode being used for clamping said compensation signal at said start-up transient of said power supply.

2. The soft-start apparatus as claimed in claim 1, wherein said first power source is a current source for providing a charge current for said start-up capacitor.

3. The soft-start apparatus as claimed in claim 1, said soft-start apparatus further comprising a first switch connected with said start-up capacitor in parallel, said first switch being tuned on by a reset signal to discharge said start-up capacitor.

4. The soft-start apparatus as claimed in claim 1, said soft-start apparatus further comprising:

a second power source, providing a discharge current for said start-up capacitor;

a second switch, connected to said start-up capacitor and said second power source, said second switch being turned on by a protection signal to discharge said start-up capacitor with said discharge current.

5. The soft-start apparatus as claimed in claim 4, wherein said second power source is a current source.

6. The soft-start apparatus as claimed in claim 1, wherein an initial value of said compensation signal at said start-up transient is equal to a forward voltage drop of said clamping diode.

7. The soft-start apparatus as claimed in claim 1, said voltage-regulating unit comprising:

a differential-pair input amplifier, connected to a current source, wherein said differential-pair input amplifier receives a differential-pair input signal via said first input and said second input thereof, whereby said differential-pair input amplifier generates an output signal; and

an output transistor, having a gate supplied with said output signal of said differential-pair input amplifier generating said reference voltage, said output transistor further comprising a source connected to a ground reference and a drain connected to said output of said voltage-regulating unit.

8. The soft-start apparatus as claimed in claim 7, wherein said output transistor has an open-drain structure.

9. The soft-start apparatus as claimed in claim 7, wherein said output transistor has an open-collector structure.

10. A soft-start apparatus for a power supply outputting a compensation signal to a PWM controller at a start-up transient of said power supply, comprising:

a start-up capacitor;

a current source, connected to said start-up capacitor for providing a charge current to charge up said start-up capacitor;

a first switch, connected with said start-up capacitor in parallel, said first switch being turned on by a reset signal to discharge said start-up capacitor;

a voltage-regulating unit, having a first input, a second input and an output;

said first input of said voltage-regulating unit being supplied with an internal reference voltage, said second input of said voltage-regulating unit being connected to said output of said voltage-regulating unit and said start-up capacitor;

wherein said voltage-regulating unit is used to regulate a reference voltage across said start-up capacitor;

an error amplifier, having a first input, a second input and an output;

said first input of said error amplifier being connected to said output of said voltage-regulating unit for receiving said reference voltage, said second input of said error amplifier being connected to a feedback terminal of said power supply for receiving a feedback voltage;

wherein said error amplifier generates said compensation signal in response to said reference voltage and said feedback voltage; and

a clamping diode, connected between said second input and said output of said error amplifier, said clamping diode being used for clamping said compensation signal at said start-up transient of said power supply.

11. The soft-start apparatus as claimed in claim 10, said soft-start apparatus further comprising:

a second power source, providing a discharge current for said start-up capacitor;

a second switch, connected to said start-up capacitor and said second power source, said second switch being turned on by a protection signal to discharge said start-up capacitor with said discharge current.

12. The soft-start apparatus as claimed in claim 11, wherein said second power source is a current source.

13. The soft-start apparatus as claimed in claim 10, wherein an initial value of said compensation signal at said start-up transient is equal to a forward voltage drop of said clamping diode.

14. The soft-start apparatus as claimed in claim 10, said voltage-regulating unit comprising:

a differential-pair input amplifier, connected to a current source, wherein said differential-pair input amplifier receives a differential-pair input signal via said first input and said second input thereof, whereby said differential-pair input amplifier generates an output signal; and

an output transistor, having a gate supplied with said output signal of said differential-pair input amplifier generating said reference voltage, said output transistor further comprising a source connected to a ground reference and a drain connected to said output of said voltage-regulating unit.

15. The soft-start apparatus as claimed in claim 14, wherein said output transistor has an open-drain structure.

16. The soft-start apparatus as claimed in claim 14, wherein said output transistor has an open-collector structure.