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HDF49

Thu Jul 25 2013, 12:26PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

I've just built this

but with gate resistors and diodes across drain an source. However the oscillator is only providing + and - 6 volts to the gates, even with a 1:2:2 GDT, and i can't for the life of me work out why. And when I put 6v dc into the bridge I only get out a + and - 1 volt square wave, i assume because the gates aren't being switched properly. What could the cause be?

HV Enthusiast

Thu Jul 25 2013, 06:19PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

1. Look at pin2 of the gate driver. Make sure you are getting the proper input signal there.

2. Look at the voltage rails of the gate driver. Make sure you are getting proper 12V.

3. Look at output of gate driver with gate transformer disconnected. Make sure you are getting nice +12V waveform.

4. Connect gate transformer to driver board and also to MOSFETs on secondary side. Do not apply power to bridge. Run again and check gate waveforms. If you are not getting +12V on the input side and +24V waveforms on secondary side (assuming 1:2:2 turns ratio), you either have an incorrect gate transformer core material, improperly wound gate transformer, or even a bad gate driver IC. Also, make sure the diodes you put from drain-to-source are not shorted and are the correct voltage (>24V).

However, since you are only getting +/-1V, it makes me think you have incorrect diodes on drain-to-source (such as rectifier or schottky diodes) and you are just being clamped by those diodes. You should have 26-30V zeners from drain-to-source - two of them opposed.

HDF49

Thu Jul 25 2013, 10:27PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

I've got these

from drain to source, I assume they're not the right type then.... Would the internal diode in irfp260s be enough? With no power on the bridge I'm still getting only 6 volts on the gates according to my oscilloscope, I've had this with 2 different amplifier chips. Without the gate drive transformer I'm still getting only 3 volts out. I'm using a core found in an atx PSU, how can I tell whether it's the correct material?

HV Enthusiast

Fri Jul 26 2013, 12:34AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

You don't need those diodes.

Okay, if you are measuring the output of the gate driver at no load (no gate transformer attached), and still only getting 3V output, then you have a problem with your gate driver IC and/or the power supply feeding it.

Follow these instructions:

1. Get a new gate driver IC.
2. Before installing, verify you have the +12VDC.
3. Install gate driver IC.
4. Re-verify you have +12VDC at the Vcc pins on the gate driver.
5. Re-test the output with oscilloscope but no gate transformer installed.
6. Make sure you are getting the proper output waveforms.

I'm guessing you have an iron-powdered core from your ATX PSU and that is why its not working and is probably also destroying your gate driver IC.

Take a photo of your core and post it here.

Carl Pugh

Fri Jul 26 2013, 03:56AM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

There must be a connection from D4-4 to U1-1.

Dr. Dark Current

Fri Jul 26 2013, 11:09AM

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

If you can, post scope shots of your gate waveforms. Also , the external Drain to Source diodes are not needed.

HV Enthusiast

Fri Jul 26 2013, 01:06PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Carl Pugh wrote ...

There must be a connection from D4-4 to U1-1.

Yes, that is a good catch. There is a missing GND symbol on the GND of the 78xx regulator. This ground should connect to the ground on your gate driver / 555 circuit.

HDF49

Fri Jul 26 2013, 07:46PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

I rebuilt the oscillator with new chips and removed the diodes from the half bridge. I think it's working now, here are some scope pictures, all with 12v in to the oscillator....
The input to the tc4422

the output of the tc4422

Primary of the GDT, haven't a bloody clue what to make of this.

Secondary of the GDT

All of these are at 5v per division, I changed the gate drive transformer core and that sorted a lot out. I forgot to get a picture off the output of the half bridge, but I hooked it up to a small laptop power supply and connected the output to a flyback, the primary side had a wave looking more sinusoidal than anything, certainly not a square wave. I got some pretty poor sparks out, but sparks nonetheless, there's quite a voltage drop across the half bridge. There's also only a small range for the duty cycle and frequency potentiometers before the wave collapses or loses the positive part, is this usual?

Sigurthr

Fri Jul 26 2013, 11:02PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I've found that my GDTs don't like duty cycles that stray far from 50%, perhaps yours is similar.

Your GDT secondary waveform looks great, is that with or without it being hooked up to the FETs of the bridge? No signs of ringing what-so-ever!

HDF49

Fri Jul 26 2013, 11:18PM

Registered Member #977 Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74

Ah ok, thanks, i think I'll just set the PWM and leave it! Yep that's connected up to the mosfets, I'm getting a nice square wave out of the bridge now too, I'm very pleased I dug out the oscilloscope, it's a great bit of kit. I just vacuum impregnated with oil a 1300 turn flyback secondary I wound, can' 1t wait to rig that up to the half bridge.

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