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high side mosfet problems

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Ben Solon

Thu Apr 26 2012, 07:41PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

hey... i am embarrassed even to ask a question like this, but is there any way to drive a mosfet half bridge's high side mosfet if they are both n channel without a gtd or isolated drivers? you have to have a reference that is positive relative to the point in between the mosfets. which would be easy in the way that i am used to: high power inverters using gdt's. it would also be easy with a p channel on the high side. but am using small mosfets at low voltages, low to the point where a gdt driving circuit would be as powerful as the inverter itself.

Carbon_Rod

Fri Apr 27 2012, 02:37AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Posts: 1155

Not sure what gdt stand for, but typically I used a cheap voltage doubler to drive low-RDSon balanced H-bridges under 13.5v (i.e. the cheap Fet's I use have a maximum Vgs of around 15.3v.)

For example, on a high current 13.2v battery the lower gates would be around 12.5v before the current limiting resister and passive pull down bias resister (still wise to do this with some FETs). After the doubler, output is around 24.3v relative to ground, but more importantly the high side FET sees its saturation voltage of 10.8v.

The gains these days is usually high enough that even a 555 may provide enough current for the high side gate. I typically add a regulator that acts a bit like a gain follower, but mostly it depends on whats being driven.

It helps to post a schematic of your output section.... =)
Rod

Electroholic

Fri Apr 27 2012, 09:06AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Then can't you just get two highppower gate drivers and use them as half bridge sub assembles?

Ben Solon

Fri Apr 27 2012, 11:59AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

Carbon rod: I am concerned with delays in the doubler. Is this something to be worried about? It will operate up to 100khz and I can't cross conduct. I would use smaller caps to reduce the time constant, but then the signal may have noise.
And gdt is gate drive transformer.

Electroholic: gate drive chips would have to be on a floating supply in order to reference the signal to the source. This is what I am trying to avoid, as it would be way too large for this. Who wants a floating supply to power the inverter for a floating supply?

dude_500

Fri Apr 27 2012, 01:59PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

Look at Steve Ward's output stage of his universal controller:

It works for low voltages/powers where a P-Fet is an option.

Keep in mind it has certain output restrictions, for instance you can't have a high output maintained forever since high side fet is triggered capacitively

Ben Solon

Fri Apr 27 2012, 02:18PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

I am aware of Steve's design. I have looked it over already and run simulations to try to adapt it to an n channel fet, but it will only work with p fet on the high side.

Mattski

Fri Apr 27 2012, 03:45PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

What about going with the bootstrapped gate driver (this may be what you called an isolated driver):

You can buy a chip like that or build one yourself. The basic concept is that when the lower half-bridge FET is turned on the bottom of Cboot is at 0V, and the top terminal charges to Vin. When the bottom FET turns off and the top FET starts to turn on, the bottom of Cs floats up to a higher voltage (eventually Vin when the top FET is fully on), and as the capacitor is charged to Vin, the top terminal of the cap is now at 2*Vin. You need a level shifter circuit to get the gate drive signal up to the floating voltage of the high side.

Other than floating drivers and gate drive transformers the only options I can think of are secondary power supplies like Carbon Rod's. If you have an input voltage Vin and you want to drive your FET to Vgs=10V, then you need a power supply of Vin+10V. You can even use a battery to provide this, it's a bit of a hack but as long as you don't mind occasionally replacing the battery: Put a 9V battery on the positive input of Vin to get a rail at Vin+9V and use this to drive your high-side FET.

Ben Solon

Fri Apr 27 2012, 07:32PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

Ok, thread solved. I like Mattski's drive ic a lot, but there was an even simpler solution to what I was trying to accomplish than using a half bridge. I will probably still try the half bridge as I like the idea more, but I could just as easily use push pull topology.

Patrick

Fri Apr 27 2012, 08:43PM

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

ben123324 wrote ...

... but I could just as easily use push pull topology.

if you can afford the extra copper volume, then yes. Push pull is easy.

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