Switching power supply system
View Patent ↗A switching power supply system, in which a DC voltage obtained by full-wave rectification of an AC voltage is turned on and off by a switching device, converted to a desired DC voltage and outputted, includes a slope compensating circuit and an overcurrent protecting circuit, and a full-wave rectified voltage inputted to a terminal Tm is subtracted from a reference voltage Vref inputted to a terminal Tr to produce a reference voltage waveform signal (OCP correction value) Vocp to eliminate the influence of slope compensation. The reference signal of the overcurrent protection circuit is varied by a signal having a reverse phased waveform relative to the phase of the full-wave rectified waveform. This enables highly accurate overcurrent protection with a net overcurrent protection level unaffected by the slope compensation varied by the input voltage when the system is operated in a current mode.
1. A switching power supply system in which a full-wave rectified voltage is turned on and off by a switching device to be converted to a desired DC voltage, the system comprising:
a current detector configured to detect a current flowing in the switching device and to output a current detecting signal corresponding thereto;
a slope compensating circuit configured to generate a monotonously increasing slope compensating signal; and
an overcurrent protecting circuit configured to detect an overcurrent by performing a comparison among values of the current detecting signal, the slope compensating signal, and a reference signal, and to perform overcurrent protection according to a result of said comparison,
wherein the reference signal has a waveform of opposite phase to the full-wave rectified voltage.
2. The switching power supply system as claimed in claim 1 wherein the overcurrent protecting circuit detects the overcurrent by performing a comparison of the value of the current detecting signal with a signal obtained by subtracting the value of the slope compensating signal from the value of the reference signal.
3. The switching power supply system as claimed in claim 2 wherein the waveform of the reference signal is produced by varying the level of the reference signal in opposite phase to a signal obtained by dividing the full-wave rectified voltage by employing voltage dividing resistors.
4. The switching power supply system as claimed in claim 2 wherein a correction value of the level of the reference signal is determined depending on a product of a slope of the monotonously increasing slope compensating signal and a time elapsed from turning-on of the switching device.
5. The switching power supply system as claimed in claim 1 wherein the waveform of the reference signal is produced by varying the level of the reference signal in opposite phase to a signal obtained by dividing the full-wave rectified voltage by employing voltage dividing resistors.
6. The switching power supply system as claimed in claim 1 wherein a correction value of a level of the reference signal is determined depending on a product of a slope of the monotonously increasing slope compensating signal and a time elapsed from turning-on of the switching device.
7. A method for overcurrent protection of a switching power supply system in which a full-wave rectified voltage is turned on and off by a switching device, converted to a desired DC voltage and outputted, the method comprising:
detecting a current flowing in the switching device;
generating a monotonously increasing slope compensating signal;
detecting an overcurrent to carry out the overcurrent protection by summing the current flowing in the switching device and the slope compensating signal with a reference signal, and
varying a level of the reference signal in opposite phase to a waveform of the full wave rectified voltage.
8. The method as claimed in claim 7 wherein detecting the overcurrent comprises comparing the current detecting signal with a signal obtained by subtracting the slope compensating signal from the reference signal.
9. The method as claimed in claim 8 wherein varying a level of the reference signal further comprises varying the reference signal in opposite phase to a signal obtained by dividing the full-wave rectified voltage by employing voltage dividing resistors.
10. The method as claimed in claim 8 further comprising determining a correction value of a level of the reference signal depending on a product of a slope of the monotonously increasing slope compensating signal and a time elapsed from turning-on of the switching device.
11. The method as claimed in claim 7 wherein varying a level of the reference signal further comprises varying the reference signal in opposite phase to a signal obtained by dividing the full-wave rectified voltage by employing voltage dividing resistors.
12. The method as claimed in claim 7 further comprising determining a correction value of a level of the reference signal depending on a product of a slope of the monotonously increasing slope compensating signal and a time elapsed from turning-on of the switching device.