Power layout for GaN FET

Den onsdag den 20. august 2014 17.48.36 UTC+2 skrev snipped-for-privacy@gmail.com:

seems like a whole lot of trouble for little gain, what switching frequency are we talking about?

-Lasse

Several 100Khz. Target is 200V Vds. If you look at high voltage MOSFETs, the gate capacitance is way up in 3000pf to 5000pf and gate charges of over 100nC. Gate driving circuit would have trouble driving such high cap load.

GaN is less than 500pF and 10nC.

The 600u pitch on the EPC parts (24 mils) isn't especially tight. We do 500-pin BGAs at about that ball pitch, no problem with assembly or occasional rework. I'd think that a short 12 or 15 mil trace is a lot better electrical and thermal conductor than a wire bond. A lot cheaper, too.

EPC recommends heat sinking these parts from the top side if you really need it.

I remember when 0805 resistors used to look scary.

I'd test some of those parts and see at what voltage, and what manner, they fail. I'd be leery of running a 200 volt GaN part at 200 volts. A little inductive spike could kill it.

If it's not a power consumption issue, a better gate driver into a mosfet would be much lower risk. Mosfet Qg's have gotten a lot better lately, too.

SiC is interesting at sub-MHz speeds.

do 500-pin BGAs at about that ball pitch, no problem with assembly or occasional rework. I'd think that a short 12 or 15 mil trace is a lot better electrical and thermal conductor than a wire bond. A lot cheaper, too.

But you can't drive 10A to 20A into 24 mils. It would need more than 10 oz copper traces.

Target part is 100V to 200V Vds Target load voltage is 72V to 144V Target current is 15A to 20A

Those fets have multiple source and drain leads. 20 amps shouldn't be hard.

o 500-pin BGAs at about that ball pitch, no problem with assembly or occasi onal rework. I'd think that a short 12 or 15 mil trace is a lot better elec trical and thermal conductor than a wire bond. A lot Cheaper, too.

oz copper traces.

d.

We figure a 2oz (2.8 mils thick) 16 mils width copper trace, with a 5 mils diameter gold wire bonded along the trace. The gold wire would cost around 25 cents per inch. Not cheap, but not too expensive either.

Says who? IPC trace calculator?

You're using it wrong -- that's for traces very much longer than the width. At a low aspect ratio, heat gets sucked away and the ampacity is vastly higher. That's why, say, 10 mil spokes on a pad/via thermal is perfectly fine for gangbusters amps.

Tim

Could be. But i would rather be safe then be sorry later. The solder bars are 24 mils apart. To allow for 8 mils isolation, the traces are meeting the bars at 16 mils. I just don't feel comfortable passing 20A through 16 mils.

What's the wire bond for? These are just surface-mount parts.