A power control device that protects against MOSFET burnout was designed using a well-known gated-oscillator circuit. The circuit prevents excessive On time for the MOSFETs beyond a permitted limit. The R1-C1 time constant forms a timing circuit that limits the On-time pulse period and automatically generates its own maximum permitted On/Off duty cycle (Fig. 1). Without this timing circuit, if the input remained high for any reason it would then generate a continuous On condition for the powerdriver MOSFETs.
R1 is a variable resistor that sets the desired oscillator frequency by keeping the input always On or in one state. Once the limit frequency or On/Off period is set, the circuit is ready for operation with any input signal. A longer zero level (or Off) at the input keeps the zero level intact at the output. However, a longer one level (or On) is prevented automatically (Fig. 2). C1 can be from 100 pF to 10 F, and R1 can vary from 1 k to 1 M.