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Limitation of TC4422 in Steve Ward's flyback driver schematic

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Gregary Boyles

Mon Apr 20 2015, 06:18AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

I have discovered an apparent limitation with this.

My plasma globe was working great while the FET part of the circuit was powered with 80V DC.

But as soon as I powered it with 100V DC I burnt out two TC4422s - 8 pin pdip.

So I assume that there must be some sort of kick back going on in the gate drive transformer that is exceeding the voltage ratings of the transistors in the TC4422s.

And I can see 3 options:

1) I implement a diode based snubber arrangement at the TC4422 output pin.

2) I implement my own push-pull arrangement with higher rated transistors.

3) Or use another higher rated gate driver IC.

If I was to use the T0-22 (or what ever it is) version of TC4422 (as specified by Steve Ward) then does that package use higher voltage rated output transistors? I.E. I need to know whether I am likely to be better off or whether the To-22 packages will burn out with the 100V DC powering my FETs.

And also how do you tell from the data sheet the maximum voltage you can apply to the FET that is being controlled by the TC44422? Assuming there is no GDT between them.

dexter

Mon Apr 20 2015, 05:27PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
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the Tc 4422 is isolated from the power section by the gate drive transformer so as long as the isolation holds should work fine regardless if 80v or 100V on the bridge
i thing there is something wrong somewhere else in the circuit...

Ash Small

Mon Apr 20 2015, 08:56PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

dexter wrote ...

the Tc 4422 is isolated from the power section by the gate drive transformer so as long as the isolation holds should work fine regardless if 80v or 100V on the bridge
i thing there is something wrong somewhere else in the circuit...

I'm not an expert on this stuff Dexter, but the flyback primary is directly connected to the GDT secondary. Won't any spikes on the flyback primary be transferred through the GDT to the 4422?

I'm certainly not sure I'm correct, but that looks possible to me.

dexter

Mon Apr 20 2015, 09:46PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
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well the primary coil is driven by a half bridge and i see many hobbyist half bridges out there that work just fine

only one leg of the flyback primary is connected to the GDT so the spike would have to be quite big to travel back and such spike wouldn't also destroy the transistors?

i still think there is something wrong somewhere else in the circuit

Ash Small

Tue Apr 21 2015, 12:13AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

dexter wrote ...

well the primary coil is driven by a half bridge and i see many hobbyist half bridges out there that work just fine

only one leg of the flyback primary is connected to the GDT so the spike would have to be quite big to travel back and such spike wouldn't also destroy the transistors?

i still think there is something wrong somewhere else in the circuit

By 'transistors are destroyed ' do you mean the IRFP260's, or the output transistors in the 4422? The other end of the primary is connected to two 0.68uF capacitors, so any spikes can't escape that way, as I see it they can only escape into the secondary of the GDT. I'm just guessing, though.

Gregary Boyles

Tue Apr 21 2015, 12:43AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Ash Small wrote ...

I mean the output transistors on the TC4422 - my big FETs on the other side of the GDT are fine. They are rated to 250V.

Although I am lucky my caps did not fail. While I had the FETs powered with 80V I was using 100V 680nF caps. But when I tried the 100V DC I forgot all about those 100V max caps.

I have been fiddling with this spread sheet calculator I have made:

The calculated power does not take into consideration the fact that the duty cycle on my 555 is less than 100% but it still give me an idea of what is going on.

From this I think I actually still have an impedance problem with the cap on the TC4422 output. I changed it down from 680nF to 180nF, but even with this reduction, I still seem to be dissipating a lot more power in the TC4422 (8 pin pdip not TO-220) than it can handle.

So I may have just been lucky that it did not burn earlier with 80V. Perhaps some voltage spikes from the GDT at 100V DC is just tipping my pdip TC4422 over the edge.

It all comes down to how exactly Steve implemented his GDT and what the inductive reactance of the primary was. I used the core of tv flyback transformer and the inductive of the primary is 160uH. The combined inductive and capacitive reactance of the primary and C5 (180nF) still seems pretty low given the pdip TC4422's power rating of 750mW.

So it looks as though I will have to change my pdip to a TO-220 and heat sink it for a power rating of at least 15W. Although that is for the older TC4422 (TO-220) not the newer TC4422A (TO-220), which probably has a slightly higher power rating again.

If I aim for a power output of around 15W according to my spread sheet, then the facts that my duty cycle is less than 100% and that the newer TC4422A can handle more power should add a good safety margin.

dexter

Tue Apr 21 2015, 02:35PM

Registered Member #42796 Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195

Ash Small wrote ...

the IRFP260's
a pulse high enough to travel back through the GDT to destroy the 4422 would also destroy the gate of the IRFP260's

Ash Small

Tue Apr 21 2015, 03:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

dexter wrote ...

.the IRFP260's
a pulse high enough to travel back through the GDT to destroy the 4422 would also destroy the gate of the IRFP260's

Yep, you do have a point there, Dexter.

I've looked back through, and the GDT in question is, I believe, the one discussed here:

where I pointed out that , wound like it is, it will have a fair bit of leakage inductance.

There is a photo of it here:

Now, it's possible that the leakage inductance of the GDT 'could' be playing a part here, could it not?

I can't remember the formula off the top of my head, but there is a formula relating to spikes and ringing, is there not?

It gets reflected back onto the primary of the GDT, at least it does in normal flyback operation. I believe JohnF mentioned this in a thread of mine a while back, but I've not yet got around to investigating it fully.

I'll see if I can find it.

EDIT: Found it.

"Ash
Snubbing is a black art but its need is directly related to your leakage inducance (coupling factor between primary and secondary.
If you have an inductance meter you can get an idea by measuring primary inductance with secondary open circuit and short circuit the difference ratio is your coupling factor. do this again the other way around ie measure secondary inductance with primary open circuit and then short circuit.
The shorted inductances now help with calculating the snubber needed after you power up the circuit and measure the ringing period this then gives the distributed capacitance of the windings. Power Integrations gives good info on their Web site with most flybacks a relatively simple dual diode CR snubber works ok unless you want super efficiency where lossless snubbing will increase your transformer count"

Do you think this could explain the problem here?

Gregary Boyles

Tue Apr 21 2015, 05:43PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

I was playing around with my oscilloscope on the running circuit and I did notice some fairly tall spikes on the leading edge of the square wave.
I was probing the square wave on the 555 side of the GDT - I was trying to make a simple complementary pair
arrangement while I wait for the my TO-220 TC4422 to come. I ended up getting the 13A version of them because it wasn't all that more expensive than the 9A version.

I will take a photo of the scope screen, showing the spikes, next time.

Are those spikes likely to be 'ringing' coming from the secondary windings?

Does anyone have an image of a scope output showing what ringing in the primary winding of a GDT looks like?

Ash Small

Wed Apr 22 2015, 01:51PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I don't have any 'scope shots of a GDT waveform, but I have a couple of flyback 'scope shots with spikes and ringing.

The first one shows a spike and some ringing,

1429710699 3414 FT170706 1312907275 3414 Ft120505 Dscf0494

The second shows the worst ringing I've ever had, the square wave is the output from a signal generator and is inverted, because I used a darlington transister in between the signal generator and 2N3055, with some other protection for the sig. gen.

It's 'probably' the 'fairly tall spikes' that are causing your problems.

I'm hoping to get back to working on my new flyback driver soon, I've had a lot on lately, but I'm catching up with most of the other stuff I need to get on top of first.

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