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Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
I think the problem is just some dead diodes. I will replace them this weekend (NZ time)
I can't wait to get this working again with some HGTG30N60B3D IGBTs, because I was really cranking it up with huge 15cm arcs.
Also, I want to show my class this as part of our electronics unit by next term (2-3 weeks time), so if the HGTG30N60B3Ds don't arrive, i can just use IRFP250s again and warn the class the may blow up violently during full power.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Josef, I know that you're having some annoying problems there. Sympthies. Just one thing to think about, before you get obsessed about bigger and bigger and more expensive devices - the exact build of your circuit.
If you look at professional circuits in which MOSFETS or IGBTs are driven on hard, you'll see that lead lengths are very short. The ultra-fast diode should have tiny leads and directly be soldered onto the device you are protecting (obviously provide a heat shunt while you're soldering). The gate lead should be short and as close as possible to the driver / signal. It is very easy to create all sorts of virtual inductors that you don't want, as these will create potentially damaging voltage spikes/ ringing.
33us is not slow at this switching frequency and still gives and ENORMOUS change in voltage over change in time. I'd say you could even multiply your problems by using faster-switching devices. Get things right with a 33 us device and then, maybe, down the road, think about using a device that's going to demand even more care to avoid damaging it.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Hi there. Sorry about the missing accent. I don't lknow how to do this except in a word document, so I'll just have to avoid using your name until I do. Sorry if I hurt your feelings. Anyway, thanks for the construction pix. As I suspected, you have some MASSIVE bits of wiring there in your lash-up. There will be loads of stray inductance that will affect your circuit in ways you didn't intend. The wires to the gates are gigantic and could act as inductors, depending on the switching frequency.
Another thing here is that your circuit has no ground. A ground is a physically large area relative to the rest of the circuit.
Some questions: what is the small diode with an orange body and a black stripe? It's not like any ultra-fast diode I've ever seen, and in fact it looks suspiciously like a small signal diode ( for detecting tiny millivolt signals in radio circuits) Did you buy it specially? You can tell by the tiny body size that this isn't going to be able to dissipate nearly enough power. Where is the other ultra-fast diode shown in the circuit but not in your lash-up?
What is the largish choke that we see in the pictures?
I'm asking these q's with a helpful tone to see if I can shed any light on your woes there. I hope that comes across...
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Sorry, I should clarify my q about the choke. I see you've got quite a hefty choke there. Did you double check the inductance as it says 47uH- 100 uH on the cct diagram and this one looks like it's in the millihenries - just a hunch.
Registered Member #3904
Joined: Sat May 21 2011, 10:32AM
Location:
Posts: 28
astralhighway wrote ...
Another thing here is that your circuit has no ground. A ground is a physically large area relative to the rest of the circuit.
I've never completely understood the role of ground in a circuit. Is it a safety thing? Is it there to act as a reservoir of electrons? Is it a consistent reference voltage in a circuit?
What does its large area actually do? Does it act as one half of a capacitor with the rest of the circuit? Is it for shielding? Or is it simply so that it has a very low impedance?
Registered Member #3395
Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193
astralhighway wrote ...
Hi there. Sorry about the missing accent. I don't lknow how to do this except in a word document, so I'll just have to avoid using your name until I do. Sorry if I hurt your feelings. Anyway, thanks for the construction pix. As I suspected, you have some MASSIVE bits of wiring there in your lash-up. There will be loads of stray inductance that will affect your circuit in ways you didn't intend. The wires to the gates are gigantic and could act as inductors, depending on the switching frequency.
Another thing here is that your circuit has no ground. A ground is a physically large area relative to the rest of the circuit.
Some questions: what is the small diode with an orange body and a black stripe? It's not like any ultra-fast diode I've ever seen, and in fact it looks suspiciously like a small signal diode ( for detecting tiny millivolt signals in radio circuits) Did you buy it specially? You can tell by the tiny body size that this isn't going to be able to dissipate nearly enough power. Where is the other ultra-fast diode shown in the circuit but not in your lash-up?
What is the largish choke that we see in the pictures?
I'm asking these q's with a helpful tone to see if I can shed any light on your woes there. I hope that comes across...
Don't worry about he accent on the 'o', it's just the 'z' (Polish ancestors woot).
In what (bad?) way will the stray inductance affect the circuit? The circuit worked fine with minimal heating on 48v with satisfying arcs. When you mention ground, do you mean that I should ground the flyback cold to the '-' end of the SLAs? I did this once but it poses risk as it tries to arc to the circuit, going to the ground.
The ultrafast diode you are talking about is a UF4007 (1000v 1a) an didn't heat up much at all. They never blew before the MOSFETs went pop. It may look orange but that's just the wire. The other ultrafast diode IS in there, just not shown. That choke is the 100uH inductor.
You are recommending that I resolder this with the MOSFETs pin as the base of the circuit, correct?
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ZVS Driver not working when fixed with 50N321 IGBTs
