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High side driver. No high voltage.

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Member level 3
Jan 5, 2013
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I am testing a build High Side driver made from N mosfets and IR25601. The side I am trying to turn on looks like this:


On the schematic I have showed which state the driver is in (HIN and LIN inputs) and the voltage drops from this state. I am trying to turn on the top mosfet. In order to do that I have put high voltage on HIN and low on LIN. On the HO output I should obtain voltage higher than 12 V but instead I have little above 10 V as shown. Also when load is connected to the mosfets common point and GND, the voltage on HO drops to 0 V. I Dont understand what am I doing wrong.
I would really aprecciate all help in this matter.

If you only want to test the high-side driver, you must tie the VS pin to ground and remove the high-side MOSFET (or put the load between its drain and +12V supply).

But why? In that case the design would be useless. I have a full H bridge built from those. When I open opposite transistors (bottom one on one side and top one on the other) with load between the sides, the top mosfet gate voltage dissapears as well (and also is not higher than 12V).

Learn about bootstrap driver operation, e.g. from the IRF application notes.

The voltage for the high side driver is supplied by the bootstrap capacitor, it's only available when the driver is permanently switching with > 0 low-side on time so that the capacitor can be recharged. Bootstrap capacitor size must be dimensioned according to maximum high-side on time.

Remember the power supply for high side is derived from low side supply by instantly charging the capacitor connected between Vs and Vb pin via diode when low side MOSFET is on.
So the high and low side should switch alternatively to maintain sufficient charge in capacitor.
The capacitor 100nf connected between Vs and Vb seems to be too low. It should be 100MFD.

The bootstrap capacitor (which is feeding the high-side driver output circuit) is only charging when the VS pin is tied to ground.

You could manually do that (for testing purpose only) or you should apply signal pulses to low-side driver input (avoiding the shoot-through, when both low-side and high-side MOSFETs in the same branch are conducting) to periodically recharge the bootstrap capacitor.

Thank you for answers. I Know about the bootstrap capacitor and thats why I tried to charge it first by turning on the bottom capacitor for a while. I Then turned it off and turned on the top capacitor but the voltage was to low already. What should I do then?

What is the time gap between turn off and turn on and its durations?

Like 2 seconds and 2 seconds, I do it manually with my hand. Also, does that mean I cannot use top mosfet constantly?

That is the problem. It should happen at faster rate in the order of khz. Else capacitor will get discharged before you see measurement.

So maybe I place bigger resistor in series with the top gate and place bigger capacitor as well?

IC also draw some current that also you should take into account.

But does that mean I cannot constantly drive a motor with that top transistor? Does that mean that I have to keep like 90% PWM at top, in order to give that 10% to the lower transistor to charge the cap?

You can use a 99% PWM signal for the high-side driver input but you have to increase your bootstrap capacitor accordingly (to keep enough voltage between chargings). Add a 10uF-100uF electrolytic capacitor beside the existing 0.1uF ceramic one.

ok ill try to make it work, thank you for help.

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Could you only tell me is it even possible to drive a load with the half bridge like presented here?


According to what I learned here and read in the meantime, the VS pin neads 0V occasionally and if load is placed like this, turning off the top fet will mean that the cap will have to charge through load.

It works but.. why don't you use the low-side MOSFET only for driving that load? Do you need to isolate it from ground?

I need to use 2 high sides (1 H bridge) to use in multiple configurations. For example 1 dc motor with driving back and forth (full H bridge needed) and driving 2 dc motors 1 direction only (2x half bridge). Also, I think this setup is not possible, because as soon as the RI25601 sees high state both on LIN and HIN it disables outputs.

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I have changed the design like this:


But I still cannot get higher voltage than 12V on HO. I am looking at it at the scope, its not higher than 12V at any point. Why?

The bootstrap capacitor is charged at Vcc level (12V) thus you couldn't go higher than that. VB is the supply voltage for the high-side output circuit (as is Vcc for the low-side one).

Bootstrap voltage works the same as a charge pump supply and requires PWM switch output to charge and dump the voltage with the diode clamp. If you dont use PWM, dont use this chip.

In any case I suggest this is a better driver with logic level inputs and internal clock no external bootstrap required.

Sunny, the motors I intend to drive eventually will have up to 100 V.

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