Control method in use of active-clamp flyback power converter
A control method is introduced to operate an ACF power converter under a non-complimentary mode. A high-side switch is turned ON at least twice within a switching cycle of a low-side switch, to provide at least two high-side ON times. One of the high-side ON times follows the end of demagnetization time of a transformer in the ACF power converter, and the other follows the end of the blanking time that controls the maximum switching frequency of the low-side switch.
1. A control method in use of an active clamp flyback power converter including a primary winding, an active-clamp circuit connected in parallel with the primary winding, and a low-side switch connected in series with the primary winding in between two power lines, the active-clamp circuit includes a high-side switch and a capacitor connected in series, the control method comprising:
turning OFF the low-side switch;
providing a blanking signal capable of defining a blanking time in association with a maximum switching frequency of the low-side switch;
detecting whether a demagnetization time of the primary winding ends; and
turning ON the high-side switch at least two times within a switching cycle to generate at least two high-side ON times, one of which begins about after the demagnetization time ends, and the other of which begins about after the blanking time ends.
2. The control method of claim 1 , further comprising:
adaptively adjusting a length of a final high-side ON time within the switching cycle to perform zero voltage switching on the low-side switch.
3. The control method of claim 1 , wherein the active clamp flyback power converter supplies electric power to a load, and the control method comprises:
determining the blanking time in response to the load.
4. The control method of claim 3 , wherein the active clamp flyback power converter provides an output voltage to supply the electric power to the load, the control method comprising:
generating a compensation voltage in response to the output voltage and a target voltage;
determining the blanking time in response to the compensation voltage; and
determining a low-side ON time of the low-side switch in response to the compensation voltage.
5. The control method of claim 1 , further comprising:
detecting a signal peak after an end of the blanking time; and
starting one of the high-side ON times about when the signal peak appears.
6. The control method of claim 5 , wherein the active clamp flyback power converter includes a transformer with the primary winding, a secondary winding, and an auxiliary winding, the control method comprising:
detecting a voltage drop across the auxiliary winding to detect whether the signal peak appears.
7. The control method of claim 5 , wherein the active clamp flyback power converter includes a transformer with the primary winding, a secondary winding, and an auxiliary winding, the control method comprising:
detecting a voltage drop across the auxiliary winding to detect whether the demagnetization time ends.
8. The control method of claim 1 , wherein the switching cycle contains only the two high-side ON times.
9. The control method of claim 8 , wherein one of the two high-side ON times is within the demagnetization time.
10. The control method of claim 8 , wherein one of the two high-side ON times starts about when a signal peak appears.
11. The control method of claim 1 , wherein all the high-side ON times start after the demagnetization time ends.