| 摘要 |
A discontinuous resonant forward converter comprises a polarity matched transformer 206, a switch such as MOSFET 212, an output rectifier such as diode 220 and a smoothing capacitor 222. Switch controller 210 is configured to control the timing of switch 212 such that current only flows to the smoothing capacitor 222 at node X when switch 212 is on and substantially no current flows to the output during a resonant period when switch 212 is off. A small secondary inductor (fig 5c) may contribute to current limiting and a free-wheel rectifier is not necessary. The length of duty cycle is preferably such that the off period includes half a cycle of sinusoidal resonance to reset transformer 206. The duty cycle or frequency are fixed but also can be controlled in response to inputs of the energy level in transformer 206, the primary voltage, an auxiliary winding (fig 6a) voltage, the current through switch 212 and whether operation in a current limiting mode, a standby power mode or mains ripple rejection is required. During start up the frequency may be increased and the converter operated in a non-resonant mode. Current limiting may be achieved by pulse width modulation. In a standby mode switch pulses may be omitted or skipped. The control circuit can be implemented on a single integrated circuit and provides zero volt switching, improved start up, current limiting and regulation, and alleviates the effects of component tolerances in the resonant circuitry. |
