How to drive two upper MOSFETs with an isolated supply using discrete components?

I have two NMOS MOSFETs in a half-bridge configuration
The problem is that they need to drive the load from -10V to +15V. See the schematic below



Therefore they are both high side MOSFETs and I need to drive them with an isolated supply from say -5V to +5V (also the control signal should probably be isolated, e.g. using an optocoupler)

Can anyone explain how I can do this with discrete components, for example using NPN/PNP transistors?
I have tried the circuit the circuit below which I found on a blog but it never works properly in LTspice. The MOSFETs take way too long to turn off (like 100us) and they only have 0.69n of gate capacitance. And in general it doesn't work.

Best to copy the bootstrap method used in commercial high side drivers. Shifting the drive level to the high side potential is fairly easy but you need to sit the high supply on top of the low supply and a bootstap is the best way to do that.

Brian.

But I thought that bootstrap required the lower MOSFET to be connected to ground?

Here the lower MOSFETs source is connected to -10V rail

For example I am sure I could just buy two HCPL-0302 gate drive optocouplers and use one for each MOSFET, and have separate isolated supplies for each.
But I would like to know how to do this with discrete components.

This MOSFET bridge is actually going to be driving another very large IGBT.

Hi,

I think one improvement is to use a separate gate resistor for each BJT, one for turn-on and the other for turn-off of the MOSFET, and variations on that theme, the detailed explanations are in one of the gate drive pdfs in the rar folder, and there are loads of ideas in all those documents for discrete solutions. Maybe find something in the Semikron manual too if you're driving an IGBT.

You need to show the whole circuit and make it clear what the operating principle is. What frequency are things switching at, what's the goal etc. Proper gate drive circuitry depends on all those things.

Individual isolated supplies and drivers for each fet is the 'always works' solution, bot also the expensive one (of course). The things you're asking about, like non-isolated drive or bootstrapping are simpler and cheaper solutions but only work 'sometimes' and whether this is one of those times depends on a number of things you didn't clarify.

For example I am sure I could just buy two HCPL-0302 gate drive optocouplers and use one for each MOSFET, and have separate isolated supplies for each.
But I would like to know how to do this with discrete components.

This MOSFET bridge is actually going to be driving another very large IGBT.

Click to expand...

The usual way would be to buy an off-the-shelf IGBT driver module.

If you want to design your own IGBT driver, you should start to study existing designs.

The slowness of the post #1 design is mainly caused by the standard optocoupler. You'll either use a fast logic coupler or a standard coupler with a signal regeneration stage, e.g. a comparator or schmitt-trigger.

There's little sense in using two isolated drivers with separate power supply for the H and L side of an IGBT driver, instead the complete driver circuit can be powered by the bipolar supply (e.g. +/-15 V) of the IGBT driver. Using NMOS for the H driver isn't a good idea, better use PMOS or PNP, as said study existing designs.

Bootstrap power supply isn't suited for high power IGBT drivers. You'll use some kind of isolated DC/DC converter. Industry standard IGBT driver modules usually integrate the DC/DC, also IGBT desaturation detection and status feedback.

I've seen design in which they employ an additional gate drive transformer as a small auxiliary and isolated power supply.