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Navrit Bal

Tue Nov 23 2010, 10:47PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Sound good?

Navrit Bal

Tue Nov 23 2010, 10:51PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

Sorry about the double post, but I forgot to upload the coil.

"How about using the diodes to rectify the back pulse into the capacitors?" Could you explain how that would work? That is what I was talking about.

Artikbot

Wed Nov 24 2010, 09:39AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Lol! It looks a lot like my 1st stage acceleration coil's curve.

Yeah it seems to be pretty well dampened. 1.3mm wire diameter seems more reasonable than those massive 2mm... I wouldn't go for such massive wires (2mm) unless the coil is intended to be used on a can crusher or even into an antiair coilgun

!

I think it'll be alright. Will your IGBTs survive the pulse? According to the Toshibas on the first post, they can't withstand more than 200A pulsed in 10ms, so you'll probably need a beefier one.
Consider SCRs, they have crazy surge currents unlike IGBTs, and you don't seem to need proper switching on and off since you're already tweaking your pulse via the coil.
Also they're way cheaper... To put into comparison two models I was browsing lately:

IXYS IGBT, 600V, 100A continuous 120A surge -> 6.5â‚¬
STMicro TYN640RG SCR Thyristor, 600V 40A continuous, 480 (!) surge -> 2.36â‚¬

I ended up getting 4 SCRs for the sake of paralleling them, and hey, for their small packages (TO220) they seem to have a hell lot of power capability.

Saz43

Wed Nov 24 2010, 02:57PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Navrit Bal wrote ...

"How about using the diodes to rectify the back pulse into the capacitors?" Could you explain how that would work? That is what I was talking about.

This is accomplished via a topology known as a "diagonal half-bridge". Take a look at the green section in this diagram:

It requires two switches and two diodes. The coil is fired by opening both switches, connecting the + side of the coil to the power source via the high-side switch (IGBT 1) and the - side of the coil to ground via the low-side switch (IGBT 2). During this time, the diodes are reverse-biased and don't conduct. When the coil is switched off, the coil's magnetic field collapses and the energy turns into a current spike of reverse polarity with respect to the initial pulse. With the switches off, the spike doesn't go back into the capacitors at reverse polarity but rather is piped via the two diodes back into the capacitors at the proper polarity.

This topology requires a special switching IC called a high-side/low-side driver since the high-side IGBT's emitter isn't connected to ground. This adds some extra complexity, and of course needing two IGBTs instead of one adds cost. The benefit is that the reverse polarity spike isn't wasted heating the coil as it would be with the standard anti-parallel diode, but instead goes back into the capacitors, effectively increasing your efficiency. Another benefit is that the reverse current decays much more quickly this way, significantly reducing the effects of projectile suckback.

Navrit Bal

Wed Nov 24 2010, 06:27PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

I have 3 of these SCRs

datasheet:http://www.semikron.com/products/data/ cur/assets/SKT_55_01235921.pdf I asked how much current these could take over 4ms (my target pulse time) and a helpful Semikron engineer told me that they could take 1430A over 4ms @130C, so it looks like I could actually just use one, also I have big heatsinks on them as well, so temperature shouldn't be that high anyway.

What kind of ratings and price would be required for the components for the 'diagonal half-bridge', assuming that my voltage, current, resistance etc are roughly the same. By the way, what are the typical ESL and ESR for computer bus grade capacitors (450V 1.8mF)?

Saz43

Thu Nov 25 2010, 01:29AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Navrit Bal wrote ...

What kind of ratings and price would be required for the components for the 'diagonal half-bridge', assuming that my voltage, current, resistance etc are roughly the same.

Same ratings as the SCR in terms of voltage and current. Given the same coil and capacitor, the half bridge topology won't much alter your discharge waveform characteristics before the turn-off time. You can expect the price to be a little higher, but if you hold out, you can probably find the 1200V/400A bricks on eBay for $20 to $30.

The properties of the reverse polarity spike are more difficult to predict (depends on coil properties and switch turn-off properties). The diodes must handle at least the maximum firing capacitor voltage and to be safe (someone correct me if I'm wrong), let's say half the peak forward current.

Navrit Bal

Thu Nov 25 2010, 07:42PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

In that case, I think that I will have to leave it.

Today I safely charged and discharged my 3 450V 1.8mF capacitors at 20V through a 7kOhm resistor, I would have gone to higher voltages but I wasn't allowed yet as most connections weren't insulated well (using crocodile clips for example).

Would it to be safe/advisable to use sellotape or electrical tape for insulating bare contact points? Since it probably isn't, does anyone have any alternatives for insulating or covering bare contact points? I'm asking because my supervisor won't let me use 450V until I can satisfy her safety demands

Artikbot

Thu Nov 25 2010, 07:48PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Bare contact points you mean bus bars and that stuff?

Yes, you can use electrical tape. It's what it was meant for.

Of course it would be better to use shrink tube, but electrical tape will do the trick (dad and I used to insulate spanners with electrical tape and use them to blow the fuses on our house while we were testing the safety circuits, and we're talking about mains triphasic 400VAC current).

Navrit Bal

Thu Nov 25 2010, 08:04PM

Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116

That made my question sound really stupid now, thanks

that should satisfy my supervisor xD

How did that testing go?

Steve Conner

Thu Nov 25 2010, 09:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Typical ESR for 450V, 1800uF, bigass screw terminal caps would be about 40 milliohms.

Electrical tape (insulating tape in the UK) is acceptable, Sellotape is a bit ghetto. :)

If your supervisor has safety demands, she should also be able to advise you on how to meet them.

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