Control circuit having programmable waveform for limiting output power of power converter
View Patent ↗A control circuit having a programmable waveform for the output power limit comprises an oscillation circuit to generate an oscillation signal. The oscillation signal is used for producing a switching signal to control a switching device in response to a feedback signal of the power converter. A waveform generator generates a waveform signal in accordance with the oscillation signal. The waveform signal controls the control signal and limits the output power of the power converter. A resistor is coupled to the waveform generator to program the waveform of the waveform signal. By properly selecting the resistance of the resistor, which can be used to achieve an identical output power limit of the power converter for the low line and high line voltage input.
1. A control circuit having an adjustable waveform for limiting an output power of a power converter, comprising:
a switch, coupled to a transformer for switching the transformer;
an oscillation circuit, generating an oscillation signal;
a first circuit, generating a temporally variable control signal to control the switch in response to the oscillation signal and a feedback signal for regulating the output of the power converter; said control signal having a temporal width responsive to a temporal width of a current signal;
a second circuit, generating an adjustable waveform signal in accordance with the oscillation signal to control the control signal and limit the output power of the power converter; wherein the waveform signal is synchronized with the control signal, and the control signal is switched off when the voltage of the current signal exceeds the voltage of the waveform signal;
a third circuit, generating a ramp signal in response to the oscillation signal; and
a variable resistor, coupled to the second circuit to program the adjustable waveform of the waveform signal; said resistor externally interposed between ground and the control circuit; wherein the value of the resistor directly adjusts the voltage of the adjustable waveform signal;
wherein the current signal is responsive to low and high input line voltages, such that the power converter produces equal output power for both the low and high input line voltages.
2. The control circuit as claimed in claim 1 , the first circuit comprising:
a flip-flop, generating the control signal in response to the oscillation signal;
a first comparator, coupled to receive the feedback signal and the current signal, and generate a first reset signal, in which the current signal is generated in accordance with a switching current of the switch; and
a second comparator, coupled to receive the waveform signal and the current signal, and generate a second reset signal;
wherein the oscillation signal is applied to turn on the control signal, in which the first reset signal and the second reset signal are utilized to turn off the control signal.
3. The control circuit as claimed in claim 1 , the second circuit comprising:
a fourth circuit, generating a fourth current signal in accordance with the ramp signal and the resistor;
a current mirror circuit, producing a fifth current signal in accordance with the fourth current signal; and
a first resistor, generating the waveform signal in accordance with a constant current signal and the fifth current signal.