A synchronized switching system for use in a power converter. A pulse-width modulation controller is provided to alternately turn on and off a switching device and control the on/off duty cycle. When the switching device is turned off, initial commutating or flyback current from an inductor is conducted by a protective diode connected to ground. The diode is paralleled by a more efficient secondary switching device that is synchronized to conduct during the nonconduction period of a main switching device. A sense winding is added on the inductor for synchronizing the secondary switching device. A digital logic circuit controls the conduction time of the secondary switching device based on signals from a pulse-width modulation controller and from the sense winding of the inductor. After the main switching device has been conducting for the required time, the pulse-width modulation controller turns it off. The digital logic circuit senses the voltage level on the inductor and causes the secondary switching device to conduct, just after the main switching device stops conducting. The digital logic circuit prevents the main and secondary switching devices from conducting simultaneously.

A pulse-width modulated current regulator (FIG. 4) includes a current flare out control (406,407,409,412) to limit current output at low output voltages. Current variation in an output filter inductor (408) is monitored and compared with a reference standard (407). A conduction interval is controlled in response to the comparison in order to limit current flare out.

A switching regulator electronic power supply for 115 volt A.C. operation rectifies and filters to provide approximately 170 volts of unregulated D.C. to the primary winding of a power conversion transformer. In series with the primary winding is a transistor power switch controlled by a pulse width modulator whose ON-time is determined by the rectified D.C. voltage level, and whose OFF-time is controlled by an output voltage sensor and a current limiter that provide duty cycles that assure a constant D.C. voltage level from the rectified and filtered output of the power supply.

Power dissipation losses occur during the turnon transition period of the switching transistor of a switching regulator. These losses are due to the voltage appearing across the switching transistor during current conduction within the turnon transition period. A turnon loss reduction network is added to the switching regulator to reduce the power dissipation inthe switching transistor during turnon. This turnon loss reduction network counteracts the voltage normally appearing across the switching transistor and hence reduces the power dissipation therein due to this voltage. A lossless charging arrangement is included in the regulator to reduce the power dissipation that occurs within the turnon loss reduction network.

An overload protection system for a switching power supply includes a variable rate and variable pulse duration drive source for operating the switching transistor. Upon turn-on of the power supply, the switching transistor is driven with low repetition rate pulses having a predetermined time duration. The rate is gradually increased to a normal operating rate to minimize turn-on transients. Load sensing circuitry is used in conjunction with the variable rate drive source to cause the drive source to operate at the low rate in the event that an overload is applied to the power supply, and to turn off the drive source if the overload persists. Circuitry for providing automatic reset following an overload is employed. A feedback circuit is used to vary the duration of the drive pulses to provide automatic voltage regulation.