Quick vvr question

[Lindz]

I think I fried a mosfet on my vvr - likely due to inadequate heat transfer. It was bolted to the chassis w/sil pad but not particularly tight (nylon bolt) so I'm guessing the chassis was not pulling heat very well

I'm rebuilding it and have a question about heatsinking it for additional security.

1. assuming a Wreck Express build and Dana vvr3 how big would the heatsink be approximately? The math is a bit over my pay grade

2. Is there a problem if I heatsink the top side of the mosfet vs the bottom?

I was thinking I could more carefully bolt to chassis and also add heatsink on top side of mosfet inside the chassis - any problem doing this or should heat be drawn from the bottom only?

My current install there is no room on the top side of the chassis for a heatsink due to the proximity of the transformers. I could move it but I already drilled holes, made a faceplate etc.

I suppose I could also run leads to the mosfet and then put it elsewhere.

In any event, are there off the shelf heat sinks any of you have used and how would you more experienced builders contend with the install (top, heatsink, bottom, move the mosfet etc)?

thanks!

[Structo]

Does your Express exceed 50 watts?

That is the limit of Hall's fixed bias VVR.

So unless you have a very good heat dissipation scheme, you likely fried it from too much power.

I would message Dana and ask for a recommendation about how to go forward with this.

[Lindz]

I had thought that wreck PT's were kind of purposely undervalued and ran the amps at about 35 - 40 watts?

[dass101]

For a 30-50W VVR you need some serious heat dissipation. You should look into a dedicated aluminium heat sink of around 10/10 cm (or something with a similar surface) and use thermal paste. Connect the heat sink to the side of the Mosfet that provides the biggest contact surface.

I built a VVR for a 50W Marshall, and used a 5/5 cm heat sink, and it was not enough, the heat sink got extremely hot at around 50% attenuation (this is where you get the biggest heat)