In typical push-pull driver applications, the npn-pnp transistor pair is alternately turned on and off using a square-wave signal. A simple driver circuit uses an inverter between the V1 and V2 inputs to toggle the output transistors. However, mismatched turn-on and turn-off characteristics of the output transistors can cause cross conduction during the transition periods (Fig. 1). This results in high current surges being drawn from the power supply.
The simple circuit shown in Figure 2 can be used to introduce any desired amount of deadband time between V1 and V2 control pulses. The circuit employs a Schmitt-trigger inverter IC, the 74HC14. With the diodes, different time delays can be obtained for the rising and falling edges of the drive waveform. The deadband is determined by the RC time constant. The waveforms in Figure 3 are self-explanatory.
Using this circuit, any type of transistors can be utilized for the push-pull circuit, eliminating the effort and expense required to obtain matched transistors.
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