|
|
|
|
|
|
|
Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
|
By the way Jake, I just had a closer look at the photo of your smoked IBC that was still open in my browser..
Thats one of the very first batch of IBC's ever made - Vintage !.. It has the seperate CNC machined heatsinks for the H-Bridges (with the mounting holes in them) that I had made by CNCBotParts in the US.. wow. memories..
In that case, it probably also still has the standard insulator bushes on the Fets under the bolt heads, which later changed to special-order version that doesnt melt until a much higher temperature, so while you are changing burned Fets, you might like to see about fitting some of them, and Jason now puts spring washers under the nuts.
The idea is that if things do get a little warm as the Fets work hard, the bushes dont soften and deform, and even if they do, the spring washer should help keep tension on the Fets and pressed up hard against the heatsink so they dont get hotter as a result of getting hotter if you know what I mean..
I'd call it Thermal runaway, but its not really, since thats a silicon characteristic that Fets dont exhibit.. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
|
Thu Jun 09, 2005 11:40 pm |
|
|
|
|
Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
|
> Thermal splodification
Something like that.. Mesoplasma inspired smoke production is imminent if it happens..
> are the diodes anything special, they didnt seem to be in the diagram,
You mean the gate-charge-dump diodes ? If so, then yes, they're a high-speed switching diode to speed up the FET turn-off by bypassing the series gate resistor when sucking current out of the Fet Gates.
> any suggestions on local sources for the insulators and non conductive washers?
The standard ones can be had from anywhere, Dicky's, Jaycar etc.. If you want the high-temperature version, you'll have to ask Jason, I think he got them on a special order.. They arent essential, but might help save the Fets from your Thermal 'Splodification if things get a little toasty at the heatsink..
> i presume they go inside the hole in the fet so if something slips a few mm it dosent short.
Yep, keeps the bolt well insulated from the Fet Tab.
> i may look at some kind of "funkyass" cooling, a water boiler or something.
That would probably be very effective at keeping the fets cool, but sounds like it would be heavy and prone to leakage..
quote:
Do you see any problems putting double fets on (well off board) with matching resistors and diodes for ringing etc. - the HIP drive output looks quite good enough for it?
That should work fine.. The HIP chips have been proven to drive up to 8-Fets in parallel sucessfully.
From what I've read of good motor controller design practices, keeping the track length between the driver and fet to a minimum is desireable to stop self-inductance of the track from affecting the gate drive signal transitions..
I'm not certain how big an effect a few cm of wire will have though.. You would probably need to either scope the drive on a nice fast o'scope, or crunch some numbers to see if the theory tells you anything bad might happen..
There are some good Fet-Drive application notes on I.R.'s website and some relevant stuff here . http://powerelectronics.com/mag/power_rid_miller_effect/ _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
|
Sun Jun 19, 2005 1:43 pm |
|
|
|