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MOSFET and H-bridge question?

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Patrick

Sat Apr 13 2013, 03:31AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im trying to remember, I know when they bring all the wires out, it gives you the advantage of changing the configuration. with the two coils and 4 wires your kinda stuck witht he bi-polar option.

Ash Small

Sat Apr 13 2013, 10:54AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

As a slight aside, and back to the question of high side isolation, would using capacitors for isolation work, in general, for H-bridges, etc. as in the diagram below?

This would push the gate to 20V higher than the source, would it not, regardless of the voltage from drain to source?

1365850470 3414 FT152963 High Side Isolation

Steve Conner

Sat Apr 13 2013, 02:25PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Steve Ward does something similar in his gate drivers. I never liked it.

The problem is, let's say you're switching 320V DC using two N-channel MOSFETs. The high side one needs 12V of gate drive, referred to its source. But its source moves 320V relative to "0V" during the switching event, so you actually need 332V of gate drive relative to 0V. The capacitors allow a change in the DC level, but they won't amplify the signal from 12V to 332. The usual high-side driver circuits address this by bootstrapping themselves along with the MOSFET source.

Steve Ward uses a P-channel device for the high side. Then the source is bolted to the positive rail, and the drain is what moves during the switching event. If we assume the positive rail doesn't change during the switching event, then the required gate drive is just -12V.

The reason I don't like it is because any noise and ripple on the positive rail will simply add to the gate drive voltage. The capacitive level shifter has no way of rejecting them. Steve gets away with it because he has a well filtered and regulated supply. You might get away with it too!

Ash Small

Sat Apr 13 2013, 04:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Now you mention it, I have read about using P channel devices for the high side.

One more question, though. When using GDT's, the source is also at 0V before the switching event, and the GDT pushes the gate to +20v, or whatever, and the source voltage, in your example, rises again to, say, 320V, yet the gate remains at a voltage 20V above this. Why does it work with a GDT, but not when using capacitors for isolation?

(In this application, the +ve rail will only be @~12V anyway, so if I choose MOSFETs that can take, say, 30V on the gate, but only need the gate to be a few volts above the source to fully turn on, I could push the gate to, say, 24V without any isolation, and they'd still be sufficiently above the source voltage not to cause any problems?....Or not?)

I've not actually used MOSFETs or IGBTs before, nor, for that matter, built an H-bridge yet. Maybe I should have a look at some Steve Ward designs, regarding this point? I do like to research things thoroughly before starting to 'let the magic smoke out'.

Ash Small

Sat Apr 13 2013, 11:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Here's Steve Ward's GDT driver, if anyone is interested.

1365894381 3414 FT152963 Steve Ward Driver

For a stepper motor driver, I think you just connect the stepper coil where the GDT would normally connect.

Steve Conner

Sun Apr 14 2013, 08:05AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It works with a GDT because the induced voltage in the transformer secondary appears between the two ends of it. One end is connected to the source and the other to the gate, so when the source flies up during the switching event, the induced voltage follows it up. Capacitors can't do this.

Steve Ward's circuit has no DC path to the high side gate, so the range of duty cycles and frequencies it can handle will be limited.

If you just want 12V at low current, you might like to try some gate driver ICs, like the TC4421, UCC37321 etc. They include a level shift from logic level to whatever the output stage is running off, and probably have enough oomph to drive a small motor.

Years ago I built a discrete high side driver that drove a bank of MOSFETs to switch 24V at 50A and about 50kHz. It is similar to what you'd find inside the IR21xx series of high side driver ICs. See the appendix on inverters at

(pages 18 and 19)

Sulaiman

Sun Apr 14 2013, 09:47AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Since you are working at low voltage have a look here

(I just Googled the above URL in a few seconds !)

I made two small bridges for a roving robot along similar lines,

since you don't need holding/braking the simple circuits should work ok.

I like the idea of TC4421/4422 as I have found them to be quite robust.

Ash Small

Sun Apr 14 2013, 11:18AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Steve Conner wrote ...

It works with a GDT because the induced voltage in the transformer secondary appears between the two ends of it. One end is connected to the source and the other to the gate, so when the source flies up during the switching event, the induced voltage follows it up. Capacitors can't do this.

I'm failing to see something here, Steve. Suppose you place a couple of capacitors on the outputs of the GDT (one between the GDT and source, and one between the GDT and gate), the GDT circuit would still work, would it not, and push the gate to a higher voltage than the source?

Replacing the GDT secondary with, say, a battery, should still push the gate to a higher voltage than the source is at, surely?

What am I missing here?

(I am correct in assuming that the gate-source connection can be modelled as another capacitor, aren't I, and that no current flows from gate to source?)

Steve Conner

Sun Apr 14 2013, 08:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

What you just posted above is correct. The chain of reasoning breaks down when you substitute an ordinary (low-side) driver for the battery. The supply rails of the driver will be in some fixed relation to the supply rails of the main bridge, so the capacitor you put from the MOSFET source to the driver's 0V won't be able to push the driver up. The magic smoke will come out.

By a high-side driver I mean one whose supply rails float relative to the rest of the circuit. It's like your notional battery, a voltage source just sitting there. It would work through two capacitors like you suggested, but it also works when connected directly.

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