Active clamp flyback converter

1 . An active clamp flyback converter comprising:

a main switch electrically connected to one end of a DC power supply;

a primary winding electrically connected in series with the main switch;

a clamp switch electrically connected in series with the main switch;

a clamp capacitor electrically connected in series with the clamp switch;

a controller that controls the main switch and the clamp switch, wherein

the controller performs operations comprising:

controlling a first on-signal that controls the main switch, a second on-signal that controls the clamp switch during a period when the main switch is off, and a third on-signal that controls the clamp switch again after the second on-signal;

setting an on-timing in which a resonance current flowing in a direction of charging the clamp capacitor is reversed after a lapse of a half cycle time of a resonance cycle of a resonance circuit of the clamp capacitor and leakage inductance, which is generated when the clamp switch is turned on, is limited by an excitation current of an excitation inductance of the primary winding;

setting an on-period of the second on-signal to a half or more of a resonance period so that the resonant current discharging the clamp capacitor and the resonant current inverted and the current limited by the excitation current charge and discharge the clamp capacitor once when the clamp switch is turned on;

outputting the third on-signal after the excitation current of the excitation inductance is zero while controlling the clamp switch by the second on-signal; and

controlling a negative current flowing through the excitation inductance while turning on the clamp switch by the third on-signal.

2 . The active clamp flyback converter according to claim 1 , wherein

the controller further comprises a bottom detector that detects the minimum voltage of the clamp switch after the excitation current of the excitation inductance becomes zero, wherein

the controller performs operations comprising:

outputting the third on-signal at a time when the minimum voltage of the clamp switch detected by the bottom detector becomes the minimum;

turning on the clamp switch based on the third on-signal; and

flowing a negative current through the excitation inductance.

3 . The active clamp flyback converter according to claim 1 , wherein

the controller performs operations further comprising:

setting an on-timing of the second on-signal before a time from a half of the resonance period to the resonance period from a time when the excitation current becomes zero;

setting an off-timing of the second on-signal to a time until the excitation current becomes zero; and

setting an on-period of the second on-signal to half or more of the resonance period between the clamp capacitor and the leakage inductance of the primary winding.

4 . The active clamp flyback converter according to claim 2 , wherein

the controller performs operations further comprising:

setting an on-timing of the second on-signal before a time from a half of the resonance period to the resonance period from a time when the excitation current becomes zero;

setting an off-timing of the second on-signal to a time until the excitation current becomes zero; and

setting an on-period of the second on-signal to half or more of the resonance period between the clamp capacitor and the leakage inductance of the primary winding.

5 . The active clamp flyback converter according to claim 1 , wherein

the controller further comprises:

an excitation current detector that detects the excitation current; and

a threshold generator that generates a threshold, wherein

the controller performs operations comprising:

transmitting the second on-signal as the on-timing of the second on-signal when the excitation current detected by the excitation current detector reaches the threshold value generated by the threshold generator.

6 . The active clamp flyback converter according to claim 2 , wherein

the controller further comprises:

an excitation current detector that detects the excitation current; and

a threshold generator that generates a threshold, wherein

the controller performs operations comprising:

transmitting the second on-signal as the on-timing of the second on-signal when the excitation current detected by the excitation current detector reaches the threshold value generated by the threshold generator.

7 . The active clamp flyback converter according to claim 3 , wherein

the controller further comprises:

an excitation current detector that detects the excitation current; and

a threshold generator that generate a threshold, wherein

the controller performs operations comprising:

transmitting the second on-signal as the on-timing of the second on-signal when the excitation current detected by the excitation current detector reaches the threshold value generated by the threshold generator.

8 . The active clamp flyback converter according to claim 5 , wherein

the threshold generator changes the threshold value according to an output voltage.

9 . The active clamp flyback converter according to claim 6 , wherein

the threshold generator changes the threshold value according to an output voltage.

10 . The active clamp flyback converter according to claim 7 , wherein

the threshold generator changes the threshold value according to an output voltage.

11 . The active clamp flyback converter according to claim 1 , wherein

the controller performs operations comprising:

setting the on-period of the second on-signal to a half or more of the resonance period between the clamp capacitor and the leakage inductance of the primary winding and below the resonance period; and

setting the on-timing of the second on-signal to a time in which an amount of charge of the current flowing in the direction of charging the clamp capacitor is smaller due to the resonant current is limited to the excitation current than a half charge amount of the resonant current flowing in the direction of discharging the clamp capacitor when the clamp switch is turned on by the second on-signal.

12 . The active clamp flyback converter according to claim 2 , further comprising a current detector that detects the current in a discharging direction and the current in a charging direction flowing from the second on-signal to the clamp capacitor, wherein

the controller comprises:

an integrating circuit in which the current in the discharging direction detected by the current detector is integrated to obtain a discharge integral amount, and the current in the charging direction is integrated to obtain the charge integral amount; and

a delay circuit that delays the on-timing of the second on-signal so that the charge integral obtained in the integrating circuit is a half or less of the discharge integral amount.