Power MOSFET protection circuit - has potentiometer controlled diode preceding auxiliary transistor applying voltage proportional to residual source and drain voltage

摘要

The protecting circuit for a power MOSFET (1) uses an auxiliary transistor (2) preceded by a diode (6) which conducts when MOSFET operates. The auxiliary transistor applies a voltage which is proportional to the residual voltage between the source (S) and drain (D) connections of the MOSFET. The diode is connected to a potentiometer (5) so as to control this proportionality. The diode is connected on one side with the drain connected and on the other side by a resistance (7) to a switch (16) at the positive side of a voltage source (8). Thepolarityof the diode is selected so that it conducts when the MOSFET conducts. Also between the gate and source is a Zener diode (12). The auxiliary transistor is controlled by a parallel capacitor (10). The MOSFET control voltage is alternatively supplied by a converter secondary winding (13) in series with a rectifier (14) and smoothing capacitor (15).