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Half-bridge help

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uzzors2k

Sun Mar 12 2006, 08:49PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I finally got around to building my first half-bridge, but it didn't work quite right. I plan on using it for flyback driving. Basically its a 555 push-pull, running a bipolar transistor, running a GDT, running the half-bridge. The problem is no output from the half-bridge. The GDT is functioning surprisingly, but I don't know if the FETs are running out of phase or not, since my oscilloscope is a speaker torn from a telephone.

I have tried switching leads...

One suspicious thing I have found, is that I get +5v +2.5 and 0v, on the capacitor divider. I thought you where supposed to get +5v 0v and -5v? The GDT is hardly optimized, and might be a problem, otherwise I don't know what is wrong... I'll draw a schematic soon, so any obvious problems can be spotted.

This must seem real newbie.

vasil

Sun Mar 12 2006, 09:21PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

For a half bridge you have to turn a FET high and the other low. How do you do that without inv/non-inv chips? Does the 555 output both polarities?

Dr. Shark

Mon Mar 13 2006, 08:54AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

He is using a GDT with two secondarys I suppose, so no need for inverting driver chips.
uzzors, are you using a totem pole configuration of bipolars to drive the GDT? thats what you should do.
The voltages on your divider could well be correct, depends on your supply voltage, and where your ground reference is. Please give more information.

The two FETs need to be driven out of phase, so one of the GTD secondarys needs to be reversed (doesn't matter which one).

uzzors2k

Mon Mar 13 2006, 06:01PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Ok, here is the schem. The reason I chose a GDT was to save using non-inverting chips, which I don't have at the moment. Powering the circuit and half-bridge from 5volts so I don't blow anything...

[Edit: Fixed oversized picture]

dan

Mon Mar 13 2006, 06:50PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

Umm it seems to me acording to your schematic your mosfets are not connected to the GDT in proper phase.. But you said that you have already tried reversing the polarity of one of the secondaries. But what are the 130 ohm resitors for? I mean if you are using a phone speaker as the load on the bridge (for testing) which usually have a high resistance compared to the 130 resistors. So basically the resistors are shorting out the bridges capacitors. I'm not sure though..

I have never used a GDT before so I don't know.. I usually use a 494 PWM chip to directly drive my half-bridge. Problem is if something goes wrong with the bridge I lose the control circuit too.. :(.

Dr. Dark Current

Mon Mar 13 2006, 06:55PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

uzzors wrote ...

Ok, here is the schem.

There are actually many problems in your schematic. You are driving the GDT primary with a flyback-type converter. This will produce a weird wave on the output of the GDT, far from what would a half-bridge need. You need to drive the GDT primary with another halfbridge/totem pole/gate driver chip and connect the "other" end of the GDT primary to a capacitor divider.

Also, 5V is too low to turn a mosfet on, so I'm not surprised at all it doesn't work. Try 12V, totem pole drive for GDT primary and double the number of GDT secondary turns. back-to-back zeners on fets gates are also great idea.

EDIT: Ahh yes, and swap one of the secondary outputs - they must be out of phase.

uzzors2k

Tue Mar 14 2006, 08:18PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Turns out the GDT wasn't functioning at all... I've never had much luck winding my own xfrmrs. So I've decided to just cut the GDT, since I only intended to use it for inverting purposes.
I've revised the schematic, (and resized

), and just need someone who really knows what they're doing to check that my inverting will work. Should I add coupling caps?

Dr. Dark Current

Tue Mar 14 2006, 08:36PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Sorry but your circuit will not work by any means; the top fet will never turn on, and the bottom one will turn on only half. What are the 130ohm resistors for?
Here's a simple half-bridge using a 555, without GDT:

EDIT: ehm.., it's not right that the upper fet will never turn on, it will, but only half ...

The gate drive section in your circuit is obviously all-wrong.

Marko

Tue Mar 14 2006, 08:51PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Gate driving section is completly wrong now.
A never turns completely OFF and B neverfully ON.

Use some kind of stronger (mosfet or bipolar) halfbridge to drive GDT.

And use zeners on GDT outputs to prevent gate breech.

uzzors2k

Tue Mar 14 2006, 09:40PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Ok, thx for all the help. I'll work away at this and see if I get something running.

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