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1 MHz GDT problems

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Steve Ward

Sat Oct 20 2007, 10:21PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Thanks for checking me, Marko

. I guess i wasnt entirely sure where the compensation capacitor is placed within a scope probe relative to the resistive divider. In any case, if his probe really was 70-80nF, it would probably explode his gate driver thats trying to charge it and discharge it at 1MHz... since thats like driving a big IGBT brick.

Check your cap meter vs some known caps (small ones in the pF range)... maybe the meter is just screwed up.

Sulaiman

Sun Oct 21 2007, 09:07AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

For a 'scope with 1 Mohm input resistance;
at 1 MHz 1 Mohm = 0.159 pF in terms of reactance, i.e. negligible.
SO, to get a clear 'picture' of the waveform shape, (but unknown gain)
just connect a co-ax cable from the 'scope, connect the shield/braid to circuit ground
and connect the inner core to the test point via a small capacitor, e.g. 1pF

The 'scope will have input capacitance of 15 to 25 pF typically, and 1m of co-ax will add 60 to 110 pF typically
So the total capacitance will be 75 to 135 pF
IF your monitoring capacitor is 1 pF then there will be a 75 to 135 : 1 division ratio
BUT the loading on the circuit under test is very low (1 pF in this case)
Just turn up the gain on your 'scope to see the correct SHAPE of the waveform.

IF you do not have a very small capacitor available than you can twist two pieces of insulated wire together
(about an inch of twist) for a low value capacitor.
OR use an inch or two of co-ax.

Dalus

Sun Oct 21 2007, 01:18PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

@ Uzzors
GDT is now changed to 12:12:12. But my zeners are clamping my G-D voltage to 8 volt? Maybe they can't cope with the high frequency. The power consumption of the pre-drive bridge dropped to 6 W after I removed the 15V zeners.

@steve Ward
After I disconnected my probe I measured 0.003 nF. The meter reads ok when measuring a 1nF cap (1.112 nF). But when I put a random resistor in series with this capacitor it always reads 0.005 uF really strange, maybe some error code.
But why would this pre-driver explode I think that the half-bridge of BUZ73A mosfet's is beefy enough to drive this high load.
The probe I'm using is a philips PM 8923 connected to this scope

schematics of the scope are on the last few pages.

@sulaiman
I don't have a co-ax cable here apart from those connected to my probes. But I'll try it when I get the chance to buy some.

Marko

Sun Oct 21 2007, 01:27PM

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

@ Uzzors
GDT is now changed to 12:12:12. But my zeners are clamping my G-D voltage to 8 volt? Maybe they can't cope with the high frequency. The power consumption of the pre-drive bridge dropped to 6 W after I removed the 15V zeners.

Your zeners were shorting it, no wonder you had trouble. I wouldn't surprise if they are dead.
I don't know if you guys don't see that you are creating a short circuit this way.

*always* use zeners of higher voltage rating than your drive voltage, not by a chance lower.

I mean, if you use 12V drive, use 15V zeners, and etc.

Use schottky's on primary side and dampening resistors and with good transformer you won't need zeners at all.

They are just a safety precaution which in properly working circuit don't conduct at all.

Get rid of everything and use UCC's. They were dying because you shorted their outputs.

Dalus

Sun Oct 21 2007, 09:08PM

Registered Member #639 Joined: Wed Apr 11 2007, 09:09PM
Location: The Netherlands, Herkenbosch
Posts: 512

@ Marko I'm already using schottky's on primary side and dampening resistors.

I just measured the G-S voltage with the zeners removed it's a near perfect sine, just 8 V p-p which is just out of the linear range. This low voltage can also be caused by my PSU which is quite sensitive to EM fields and RF noise. I'm going to try to remove the hot glue with a scalpel and put a suitable GDT in there tomorrow. But the strange thing is that the circuit ran for 30 minutes non stop without any significant heating of the H-bridge or any of the other components (50% duty cycle).
One other thing I noticed was that the gate drive power consumption grew when the amperage through the primary of the TC grew.
I'm also trying to make the circuit more stable to do so I'm thinking about using primary current feedback. How do a design a CT for such high frequencies?

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