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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
It will get hot when driving a bridge of transistor at hundreds of kHz...
You can try decreasing the dissipation by connecting a resistor in series with its output. This will slow down the transitions somewhat, but you might find a compromise.
Indeed especially seeing how you're driving a full bridge with just one UCC. Might be a good idea to simply use a UCC27322/21 pair combination. Those are 9A drivers, instead of the 4A one you're using now. Not much changes to the circuit either.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Dr. Dark Current wrote ...
It will get hot when driving a bridge of transistor at hundreds of kHz...
You can try decreasing the dissipation by connecting a resistor in series with its output. This will slow down the transitions somewhat, but you might find a compromise.
I was thinking that would be the case. I know the 555 even gets warm when running at high frequencies.
Just checking, did you see my edit to my previous post? The interrupter doesn't seem to be doing anything to the output. If I am not mistaken, the enable inputs of the 27425 are pulled-up to Vcc, I'm wondering if that may be causing it to run continuously. In which case, how is the enable even useful? The internal schematic can be found here:
loneoceans wrote ...
Indeed especially seeing how you're driving a full bridge with just one UCC. Might be a good idea to simply use a UCC27322/21 pair combination. Those are 9A drivers, instead of the 4A one you're using now. Not much changes to the circuit either.
That's a good point. I have a TC4420 and a TC4429 for this reason exactly--they are rated at 6A each--but I would need some logic gates (AND and NAND) in order to get the enable option.
Now, here's another (probably stupid) question: Would there be any possibility to parallel multiple 27425s? I have never seen it done and I'm not sure if it's even plausible, but thought I'd ask anyway.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Dr. Dark Current wrote ...
What are you using for the interrupter? It should have a push-pull (double ended) output.
Ahh, well that would explain it. I've been using a simple square wave (0-5v) output from an arduino. So you're saying this wouldn't work? It would need to be +/- 5v?
You can use a simple single ended square wave just fine, you need a pull down resistor or a preceding stage which can sink current at low impedance. The UCC37322/21 pair have 100k internal pull ups so it isn't much of an issue to sink using internal output impedance of most logic chips, but a pull down resistor in the 1-10k range should work universally well. I don't remember the threshold voltage it needs for a HIGH on the enable line, but 4.5V is plenty I'm sure.
As above, that single chip will get hot driving a full bridge at low RF, especially running CW. On higher frequency coils I've made I've even paralleled up 2 each of the 9A drivers for just a half bridge.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Dr. Dark Current wrote ...
Well I don't like using pull up/down resistors in a noisy TC environment. A few kOhm is OK, but 100k is not.
I believe he was saying that the built-in pull-up (as shown in the internal schematic that I posted a link to earlier) was 100k, and that a pull-down resistor of 1k would work just fine. I may end up doing that.
I am pretty well convinced that I'll be paralleling two 27425s, since that is what I have.
Thanks guys, you just simplified things. Just to clarify though, the interrupter didn't do anything to the output, but adding a pull-down resistor will likely solve the problem? I was wondering if that would help when I was testing earlier.
Right, I am saying the chips have a built in internal 100k pull up resistor, so if you use a 1k pull down you are forming a voltage divider which will drop the enable pin voltage during the interrupter LOW state voltage to 1% of Vcc. This is only needed when the output impedance of the stage preceding the UCC chip doesn't have a low enough output impedance or can't sink enough current to bring the steady state voltage of the enable pin down below switching transition threshold. When the preceding stage can't bring the enable pin LOW after a HIGH you get a floating input, which due to the internal pull up the chip sees as a HIGH.
Registered Member #3704
Joined: Sun Feb 20 2011, 01:13PM
Location: Vermont, U.S.A.
Posts: 92
Hi guys,
This is a bit off-topic (but only slightly), as it is about a half-bridge instead of the full-bridge that I'm using to drive my coil, but I just picked up the following IGBT brick at the MIT electronics flea market:
Mine is actually the 100 D, but I can't seem to find the specific datasheet for it. This is the closest I could find.
My question is, would there be any major concerns about using this in a Tesla coil application? I'm a bit concerned about the recovery speed of the reverse diode, but I wanted to check with the experts to see what you guys think.
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First SSTC design - Need some critique
