| 摘要 |
A control for a pulse width modulated power supply controls the duty cycle to provide sufficient charging to the output capacitor to satisfy a rated load, yet avoids saturation of the power transformer and overheating of the switch during and after excess line voltage conditions. The control comprises a sense transformer in series with a power transformer, a DC power source and a modulating switch. A rectifier is connected to a secondary winding of the sense transformer. The control also comprises a first capacitor coupled to the rectifier to receive charging current therefrom. The on time of the switch is based on a comparison of a voltage across the first capacitor to a first reference voltage while the power source exhibits rated conditions. During a large voltage transient the first reference voltage rises to an excess level and maintains the excess level after the transient subsides. Thus, the control also comprises a second capacitor coupled to the power source to receive charging current therefrom, and the on time of the switch is based on a comparison of voltage of the second capacitor to a gradually rising second reference voltage after the transient subsides.
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