| 主权项 |
1. An isolated switching mode power supply, comprising:
an input terminal configured to receive an input voltage; an output terminal configured to provide an output voltage; a transformer having a primary winding and a secondary winding respectively having a first terminal and a second terminal, the first terminal of the primary winding being coupled to the input terminal to receive the input voltage; a primary power switch having a first terminal coupled to the second terminal of the primary winding, a second terminal coupled to a primary ground node and a control terminal; a secondary power switch coupled between the first terminal of the secondary winding and the output terminal of the power supply; a secondary controller having a power terminal configured to receive the output voltage, a first feedback terminal configured to receive a first feedback signal indicative of the output voltage and a coupling control terminal configured to generate a frequency modulation signal based on the output voltage and the first feedback signal, wherein the frequency modulation signal has pulses modulated based on the first feedback signal; a coupled device having an input side coupled between the output terminal of the power supply and the coupling control terminal to receive the output voltage and the frequency modulation signal, and an output side configured to provide a frequency control signal based on the output voltage and the frequency modulation signal; and a primary controller having a current sense terminal configured to receive a current sense signal indicative of a current flowing through the primary winding, a frequency control terminal coupled to the output side of the coupled device to receive the frequency control signal, a second feedback terminal configured to receive a second feedback signal indicative of the output voltage, and an output terminal configured to provide a switching signal to the control terminal of the primary power switch based on the current sense signal and the frequency control signal; wherein the primary controller comprises:
a current limit comparator having a first input terminal configured to receive the current sense signal, a second input terminal configured to receive a peak current signal, and an output terminal configured to provide a current limit signal based on the current sense signal and the peak current signal;a logic circuit having a first input terminal coupled to the output side of the coupled device to receive the frequency control signal, a second input terminal coupled to the output terminal of the current limit comparator to receive the current limit signal, and an output terminal configured to provide a logic control signal based on the frequency control signal and the current limit signal;a load detecting circuit having a first input terminal configured to receive the second feedback signal, a second input terminal configured to receive the switching signal and an output terminal configured to provide a load detecting signal based on the second feedback signal and the switching signal;a startup control circuit having an input terminal configured to receive the current sense signal and an output terminal configured to provide a startup control signal based on the current sense signal; anda selector having a first input terminal coupled to the output terminal of the startup control circuit to receive the startup control signal, a second input terminal coupled to the output terminal of the logic circuit to receive the logic control signal, a control terminal coupled to the output terminal of the load detecting circuit to receive the load detecting signal and an output terminal configured to provide the startup control signal or logic control signal based on the load detecting signal; wherein: a frequency of the switching signal decreases as a load of the isolated switching mode power supply becomes light; and wherein the frequency control signal has a first logical state and a second logical state, wherein a time period of the first logical state of the frequency control signal is prolonged when the load of the isolated switching mode power supply changes from heavy to light, and the second logical state of the frequency control signal evokes an “ON” operation of the primary power switch. |
