Quasiresonant flyback converter

1. A flyback converter, comprising:

an input terminal configured to receive a DC input voltage;

a transformer comprising a primary coil connected to the input terminal and a secondary coil;

a semiconductor switch configured to switch current flow through the primary coil;

a first measurement unit configured to measure the DC input voltage;

a second measurement unit configured to measure a voltage across the semiconductor switch;

a control unit having a synchronization terminal and configured to output a switching signal to the semiconductor switch with a constant frequency in accordance with a synchronization signal input to the synchronization terminal; and

a frequency adjustment unit configured to transmit a synchronization signal to the synchronization terminal based on the DC input voltage measured by the first measurement unit and the voltage measured by the second measurement unit such that the constant frequency of the switching signal output from the control unit becomes variable, wherein the frequency adjustment unit comprises a delay unit configured to delay the DC input voltage measured by the first measurement unit and the voltage measured by the second measurement unit by a predetermined time delay.

2. The flyback converter according to claim 1 , wherein the predetermined time delay is equal to a quarter of the period duration of the parallel resonance oscillation of the transformer.

3. The flyback converter according to claim 1 , wherein the delay unit is a low-pass filter.

4. The flyback converter according to claim 1 , wherein the frequency adjustment unit comprises a comparator configured to

compare a delayed DC input voltage with a delayed voltage measured by the second measurement unit, and

transmit the synchronization signal to the control unit when the delayed DC input voltage equals the delayed voltage measured by the second measurement unit.

5. The flyback converter according to claim 4 , wherein

the frequency adjustment unit comprises a differential circuit configured to calculate a difference between the DC input voltage measured by the first measurement unit and the voltage measured by the second measurement unit,

the delay unit is configured to delay the difference calculated by the differential circuit, and

the comparator is configured to transmit the synchronization signal to the control unit when a delayed difference crosses zero.

6. The flyback converter according to claim 4 , wherein the comparator configured to reset the synchronization signal when the DC input voltage measured by the first measurement unit equals the delayed voltage signal again.

7. The flyback converter according to claim 1 , wherein the frequency adjustment unit comprises an attenuation unit configured to attenuate the DC input voltage measured by the first measurement unit and the voltage measured by the second measurement unit.

8. The flyback converter according to claim 1 , wherein

the switching signal is a PWM signal, and

the control unit is configured to switch the semiconductor switch in synchronization with the synchronization signal.

9. The flyback converter according to claim 1 , wherein the synchronization signal is a square wave signal.

10. The flyback converter according to claim 1 , wherein the frequency adjustment unit configured to transmit the synchronization signal to the synchronization terminal at each cycle of the flyback converter.

11. The flyback converter according to claim 1 , wherein

the control unit comprises an oscillator configured to output the constant frequency, and

the constant frequency is set lower than a desired minimum value of the variable frequency output from the control unit.