Switching power supply with slope compensation circuit and added slope circuit
View Patent ↗A current-mode switching power supply is provided, in which there is no unstable operation arising from the fact that signals to generate PWM signals are minute, even when a load is light and a switching frequency is high. In a switching power supply of this invention, an added slope signal is superposed in an early stage of a rise of a current detection signal, so that a combined signal Vsig is caused to reach a certain magnitude even when the load is light and the switching frequency is high, and consequently an output FB of an error amplifier ERRAMP which is balanced with the combined signal is also increased. By this means, even in a current mode, it is possible to eliminate unstable operation arising from the fact that the feedback signal FB which is the output of the error amplifier ERRAMP and the combined signal Vsig are minute.
1. A switching power supply for controlling on/off of a switching element to generate a prescribed output voltage, comprising:
an error amplifier, which compares the output voltage, or a divided voltage thereof, with a reference voltage and outputs an error signal according to a difference therebetween;
current detection means, which detects a current flowing in the switching element;
a slope compensation circuit which generates a slope compensation signal to prevent subharmonic oscillation; and
an added slope circuit, which generates an added slope signal that is reset to zero prior to turn-on of the switching means, begins to increase when the switching means turns on and is stopped from increasing after a prescribed time shorter than a switching period of the switching element,
wherein a first sum signal is determined by adding an output of the current detection means and the added slope signal, and
wherein a second sum signal, obtained by further adding the slope compensation signal to the first sum signal, is compared with the error signal, and the switching element is turned off when the second sum signal becomes equal to the error signal.
2. The switching power supply according to claim 1 , wherein the added slope circuit has a capacitor which is charged by a constant current, and a voltage across the capacitor is used as the added slope signal.
3. The switching power supply according to claim 2 , wherein the constant current is proportional to a switching frequency of the switching element.
4. The switching power supply according to claim 3 , wherein the prescribed time is proportional to the switching frequency of the switching element.
5. The switching power supply according to claim 1 , wherein a rate of increase of the added slope signal is proportional to a switching frequency of the switching element.
6. The switching power supply according to claim 5 , wherein the prescribed time is proportional to the switching frequency of the switching element.
7. The switching power supply according to claim 1 , wherein the current detection means detects a voltage across a resistance connected to the switching element.
8. The switching power supply according to claim 1 , wherein the added slope circuit has a capacitor which is charged by a constant current, and a voltage across the capacitor is used as the added slope signal.
9. The switching power supply according to claim 1 , wherein a rate of increase of the added slope signal is proportional to a switching frequency of the switching element.
10. A switching power supply for controlling on/off of a switching element to generate a prescribed output voltage, comprising:
an error amplifier, which compares the output voltage, or a divided voltage thereof, with a reference voltage and outputs an error signal according to a difference therebetween;
current detection means, which detects a current flowing in the switching element;
a slope compensation circuit which generates a slope compensation signal to prevent subharmonic oscillation; and
an added slope circuit, which generates an added slope signal that is reset to zero prior to turn-on of the switching means, begins to increase when the switching means turns on and is stopped from increasing after a prescribed time shorter than a switching period of the switching element,
wherein a sum signal is determined by adding an output of the current detection means and the added slope signal, and
wherein a difference signal, obtained by subtracting the slope compensation signal from the error signal, is compared with the sum signal, and the switching element is turned off when the sum signal becomes equal to the difference signal.
11. The switching power supply according to claim 10 , wherein the added slope circuit has a capacitor which is charged by a constant current, and a voltage across the capacitor is used as the added slope signal.
12. The switching power supply according to claim 10 , wherein a rate of increase of the added slope signal is proportional to a switching frequency of the switching element.
13. The switching power supply according to claim 10 , wherein the added slope circuit has a capacitor which is charged by a constant current, and a voltage across the capacitor is used as the added slope signal.
14. The switching power supply according to claim 13 , wherein the constant current is proportional to a switching frequency of the switching element.
15. The switching power supply according to claim 10 , wherein a rate of increase of the added slope signal is proportional to a switching frequency of the switching element.
16. The switching power supply according to claim 15 , wherein the prescribed time is proportional to the switching frequency of the switching element.
17. The switching power supply according to claim 15 , wherein the prescribed time is proportional to the switching frequency of the switching element.
18. The switching power supply according to claim 10 , wherein the current detection means detects a voltage across a resistance connected to the switching element.
19. The switching power supply according to claim 10 , wherein the added slope circuit has a capacitor which is charged by a constant current, and a voltage across the capacitor is used as the added slope signal.
20. The switching power supply according to claim 10 , wherein a rate of increase of the added slope signal is proportional to a switching frequency of the switching element.