Method and system for pulse frequency modulated switching mode power supplies

A signal processing circuit, comprising:

• a first input terminal for receiving a first input signal, the first input signal exhibiting an oscillating waveform of peaks and valleys;
• a second input terminal for receiving a second input signal, the second input signal being characterized by a transition between a first state and a second state;
• an output terminal for providing an output signal;
• a comparator for comparing the first input signal with a first reference signal, the oscillating waveform of the first input signal being characterized by a first oscillation frequency;
• and
• an oscillator for providing an oscillation signal in response to an output signal of the comparator, the oscillation signal being characterized by a 50% duty cycle and a second oscillation frequency that is approximately twice the first oscillation frequency;
• wherein the signal processing circuit is configured to cause the output signal to make a transition between a third state and a fourth state at substantially a peak or a valley of the first input signal in response to the transition of the second input signal.

A switching mode power supply, comprising:

• a power source;
• a transformer having a primary winding coupled to the power source and a secondary winding for providing an output;
• a switch coupled to the primary winding of the transformer, the switch being configured to receive a control signal for turning on and off a current flow in the primary winding;
• a current-sense circuit in series with the switch and the primary winding for sensing the current flow in the primary winding;
• a feedback circuit for providing a feedback signal related of the output of the power supply, the feedback signal exhibiting a ringing waveform when the switch is turned off, the ringing waveform being characterized by a first oscillation frequency;
• and
• a pulse frequency modulated (PFM) controller configured to provide the control signal to the switch in response to the feedback signal, the control signal being adapted to turn on the switch when the feedback signal is substantially at a valley of the ringing waveform;
• wherein the PFM controller comprises:
• an output terminal for providing a control signal to turn on and off the switch in the power supply to regulate an output of the power supply;
• a first input terminal for receiving a feedback signal related to the output of the power supply, the feedback signal exhibiting a ringing waveform when the current in the power supply is turned off;
• a second input terminal for receiving a voltage signal from the current-sense circuit;
• and
• a control circuit configured to provide the control signal in response to the feedback signal, the control signal being adapted to turn on the current when the feedback signal is substantially at a valley of the ringing waveform of the feedback signal;
• wherein the control circuit further comprises:
• a driver circuit coupled to the output terminal for providing the control signal, the driver circuit having a first state and a second state, wherein in the first state the driver circuit provides the control signal for turning on the current in the power supply, and in the second state the driver circuit provides the control signal for turning off the current in the power supply;
• a first circuit coupled to the first input terminal for receiving the feedback signal, the first circuit being configured for providing a first signal in response to the feedback signal;
• a second circuit coupled to the first circuit and the driver circuit, the second circuit being configured to provide a second signal to set the driver circuit to the first state in response to the first signal and when the feedback signal is substantially at a valley of its ringing waveform;
• and
• a third circuit for providing a third signal for setting the driver circuit to the second state;
• wherein the second circuit comprises:
• a comparator for comparing the feedback signal with a first reference signal, the ringing waveform of the feedback signal being characterized by a first oscillation frequency;
• and
• an oscillator for providing an oscillation signal in response to an output signal of the comparator, the oscillation signal being characterized by a 50% duty cycle and a second oscillation frequency that is approximately twice the first oscillation frequency.