I have an application where I need to drive 2 piezoactuators in a push-pull arrangement (voltage 180deg out of phase with each other) where the charge/discharge time is about 50 microseconds and the actuation is not periodic but on-demand and occurs at least 1 time/second. After some very useful help here I started using a proper driver for high side and low side power switching, and everything works when cycled continuously with a square wave, but there are problems when the time domain increases and I try to keep the voltages static. I mirrored the circuit below to drive each stack actuator.

enter image description here

When Vcc and the source voltage are applied, HO defaults to ~3/5-4/5s of Vcc while Hin=0V (Vcc=15V, HO is ~9V in one side, ~12V in the other side), and this seems to allow the voltage at the load/piezos to match HO.

Is there anyway to keep HO=0V on power-up?



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  • \$\begingroup\$ "but there are problems when the time domain increases and I try to keep the voltages static", what exatly you mean with that? \$\endgroup\$ – Marko Buršič Sep 21 '18 at 6:50
  • \$\begingroup\$ Sorry, what I meant was if I'm not constantly switching between HI/LO then the piezo (1.5 microfarad, 150V) raises to Vcc instead of remaining at 0V. \$\endgroup\$ – Marin Sep 22 '18 at 23:27

Note that a bootstrap power supply needs a constant PWM, no DC signals. Further, your schematics is wrong, becasue the MOSFETS have to be connected together D and S.

When lower MOSFET is conducting, then the bootstrap capacitor is charging trough D, then trough 22 ohm, then trough other 22ohm and finally trough lower MOSFET. And this is wrong.

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  • \$\begingroup\$ I was pretty tired when I posted that, I didn't give enough detail. That's not the bootstrap capacitor, that's the piezo. The bootstrap capacitor is the 450 microfarad 150V capacitor. The 22 ohm resistors are to limit the current to the piezo, such that the RC time constant is around 50 microseconds. \$\endgroup\$ – Marin Sep 22 '18 at 23:26

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