In general, transistor switches in inverters and converters operating at high frequencies consume power when switching on and off. The power consumption, referred to as power losses, switching losses or turn-on and turn-off losses, arises from the presence of both current through the switch and voltage across the switch during the "turn-on" and "turn-off" processes.
Most inverter/converter loads are inductive in nature. Because the rise rate of an inductive load current is slow, the voltage across the switching device will drop nearly to zero before the current through the switch can reach a significant value. As a result, the power losses during the turn-on period are insignificant.
The power losses during the turn-off period, however, are more significant. As the switching device begins turning off, the inductive load causes a rapid change in current which produces a rapid increase in voltage across the switch subjecting the switching device to high power levels.
To reduce switching losses snubber circuits have been used. A snubber circuit is a capacitor connected across the switching device to reduce turn-off losses. Such a connection delays the voltage rise across the switching device during the turn-off process for the time the capacitor is being charged by the load current. By choosing a capacitor of sufficiently large capacitance, the voltage rise can be maintained at a low level during the decay time resulting in negligible power losses during the turn-off period.
One of the drawbacks of such a snubber circuit is that while turn-off loss is greatly reduced, the circuit will introduce severe turn-on losses and possibly subject the switch to damage as the energy stored in the capacitor dissipates through the switching device during the turn-on period.
In addition to capacitors, snubber circuits also may include diodes, resistors, or inductors. While there are a variety of snubber circuits in existence they generally can be classified as either dissipative or non-dissipative snubber circuits
A dissipative snubber circuit consists of a snubbing capacitor, a resistor which limits the capacitor discharge current during the turn-on period, and a diode that isolates the capacitor from the switch during turn-on. As previously described this type of circuit increases turn-on losses, merely diverting the turn-off loss from the switching device to the snubber resistor.
A non-dissipative snubber circuit is considerably more complex and reduces the switching losses by (1) returning the energy that is stored in the snubber capacitor during turn-off to the input source or (2) diverting the energy to the load. Such circuits generally include inductors in addition to diodes and capacitors. While providing lower losses than the dissipative snubber, the non-dissipative snubber does not eliminate losses associated with the discharge of the output capacitance of the switching devices.
Thus, while there are snubber circuits that reduce or divert turn-off losses to a negligible amount, there is a need for a snubber circuit that can reduce both the turn-on losses and the turn-off losses to a negligible amount.