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Gate drivers - transformer vs bootstrap

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akkudakku

Mon Aug 29 2016, 11:06AM

Registered Member #59455 Joined: Tue Apr 19 2016, 08:29PM
Location:
Posts: 8

Hey guys.

After looking over a bunch of schematics that deal with H-bridges I have found out that you exclusively use GDTs. Why? Isn't the simple bootstrap capacitor approach simpler AND cheaper?

Given that most interrupters are run over fiber (and such are isolated) why even bother with a GTD?

Attaching a simple and cheap high-side/low-side driver schematic. NOT TESTED, but should work. Any tips on it?

Sulaiman

Mon Aug 29 2016, 11:27AM

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

you need twice the number of drivers for a H-bridge as your circuit is for a 1/2-bridge driver

for experimental/diagnostic work it is good to have complete electrical isolation between control and power circuits

a GDT naturally prevents a 'continuous on' condition for the power transistors due to potential driver 'mis-calculations'

for a solid-state solution there are high&low-side driver ICs commonly available that incorporate 'boot-strapping'

akkudakku

Mon Aug 29 2016, 11:43AM

Registered Member #59455 Joined: Tue Apr 19 2016, 08:29PM
Location:
Posts: 8

Sulaiman wrote ...

you need twice the number of drivers for a H-bridge as your circuit is for a 1/2-bridge driver

for experimental/diagnostic work it is good to have complete electrical isolation between control and power circuits

a GDT naturally prevents a 'continuous on' condition for the power transistors due to potential driver 'mis-calculations'

for a solid-state solution there are high&low-side driver ICs commonly available that incorporate 'boot-strapping'

Yes I know there are IC drivers

it's just that they are unavailable in hobby shops where I live. And ordering a bunch of IC's is not very economical.

Also yes what I shown is half-bridge, but it's simple to just make 2 of them.

I agree about the points you made about diagnostics. Less chances to nuke your project and measuring equipment.
Also fair point on the GDT defaulting in the off state.

But with direct drive you dispense with the hassle of making the transformer.

Mads Barnkob

Mon Aug 29 2016, 06:50PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

akkudakku wrote ...

But with direct drive you dispense with the hassle of making the transformer.

If you think winding 15 turns of Cat5 network cable on a ring core and twisting the wires together is a hassle... that is one of the easiest parts to make :)

There are almost no cons against GDT in hobby use, the only reason industry moved away from it, its large, its heavy, its expensive to make, requires more manual/human handling and IC companies does not sell transformers (a actual punch line in a high side driver commercial ).

High side drivers require more complex or tighter control, they are not noise immune as some 4hv members have experienced and moved back to GDT.

Mads Barnkob

Mon Sept 12 2016, 12:18PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I came by the sales presentation that I mentioned and thought I would add it here for your entertainment :)

Sulaiman

Mon Sept 12 2016, 01:44PM

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

Even though biased, I like the honesty, and use of an emoticon.

akkudakku
"I agree about the points you made about diagnostics. Less chances to nuke your project and measuring equipment."
actually I am more concerned about 'nuking' myself !
galvanic isolation is good.

Hazmatt_(The Underdog)

Tue Sept 13 2016, 01:37AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

How do you expect the design to work when the high side source is at Vcc/2?

I could see this working for a low voltage application, but not mains.

If you don't want to use GDT's, and I have thought about this in the past, each driver requires its own isolated power supply, plus and minus, and optical isolation. You want a negative supply to slam the gate off, not just to zero volts. Unfortunately opto's can be slow, which brings us right back to a small iso transformer... so either way....

Patrick

Wed Sept 14 2016, 04:40PM

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

I like the emoticon too, its such a brazen argument. We dont sell it, so dont use it, or screw off.

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