Soft-start apparatus for power supplies
View Patent ↗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.
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.