bootstrap high side driver in buck converter 01 PUBLIC

A simplified simulation of a bootstrap floating high side driver for a buck (step down) converter.

Note that for this circuit to start up, V(load) must be approximately 0V at startup (T=0).

A simulation of the operation of a simplified bootstrapped high side driver in a buck converter with a diode in place of a low side switch.

As long as the voltage across the load is approximately 0V at startup (T=0) then the bootstrap will still work even if the low side switch transistor is replaced with a diode.

In fact the voltage across the load can be somewhat above 0V but the closer it is to the VCC supply the longer it takes for the bootstrap circuit to esatablish the full (vb-vs) voltage. This could damage the high side transistor by not supplying it with a full Vgs swing for several switching cycles which may cause it to dissipate too much power by not being turned fully on. Generally if the load voltage at startup is less than the normal operating output voltage then the bootstrap circuit should work OK.

In other words there needs to be a DC path to ground to discharge the output smoothing capacitance and there must not be a battery or other supply rail holding the load voltage up at the SMPS output when the SMPS is off.

Normally any external source of supply connected to the SMPS output, such as might be found in a battery charger, would be isolated from the SMPS output itself by a diode to prevent it discharging into the SMPS when it is turned off.

Simulate > Time Domain > Run Time-Domain Simulation

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