VVR3 (fried the MOSFET ... not thinking)

[Espjr215]

Another idea but would a peltier cooler work?

[Tillydog]

The chassis is 1/8" thick aluminum I think. Seemed pretty thick to me as aluminum chassis go.

Yes, it was just me trying to guess from photos.

I did see that Dana worries that some grease is conductive ... causing the mosfet to short out ... or maybe get on something else by accident. So I guess if you were sure it was non-conductive that would help. My understanding is that it's to remove air bubbles ... so laying it on thick is not as important as getting it even. But it sounds like it might help.

Don't use any old grease - use heatsink compound. It is electrically non conductive, but excellent thermally. e.g.

[img:300:300]http://www.reginproducts.co.uk/system/f ... es/Z20.jpg[/img]

One tube will be a lifetimes' supply!

You *need* this with mica washers - as you say it's to get rid of any microscopic gaps between the FET and the washer, and between the washer and the chassis / heatsink. If the mounting faces are as they should be, the smallest smear of grease will be enough, and you will see it oozing out slightly when you screw the MOSFET down.

It will make all the difference in the world, honestly!

Andy

[UR12]

The chassis is 1/8" thick aluminum I think. Seemed pretty thick to me as aluminum chassis go.

Yes, it was just me trying to guess from photos.

I did see that Dana worries that some grease is conductive ... causing the mosfet to short out ... or maybe get on something else by accident. So I guess if you were sure it was non-conductive that would help. My understanding is that it's to remove air bubbles ... so laying it on thick is not as important as getting it even. But it sounds like it might help.

Don't use any old grease - use heatsink compound. It is electrically non conductive, but excellent thermally. e.g.

[img:300:300]http://www.reginproducts.co.uk/system/f ... es/Z20.jpg[/img]

One tube will be a lifetimes' supply!

You *need* this with mica washers - as you say it's to get rid of any microscopic gaps between the FET and the washer, and between the washer and the chassis / heatsink. If the mounting faces are as they should be, the smallest smear of grease will be enough, and you will see it oozing out slightly when you screw the MOSFET down.

It will make all the difference in the world, honestly!

Andy

I get more emails regarding blown mosfets on power up from people who use the grease.. Not ALL Heatsink compound is Electrically nonconductive. There are both conductive and non conductive greases available. IF you get the wrong kind you WILL blow your mosfet on the first fire up even with the mica insulator installed. The same grease that works on devices running 100v or less, like computer CPUs, may be a problem on the mosfet running at 400+ volts. The Dialectric strength of some of the non conductive greases that containe aluminum or silver microparticles may only have a dielectric stregnth of 225 volts and would not be suitable for a VVR. It is much easier for me to recommend no grease that to try and answer all the emails I get about what kind of grease to buy. On any amps of 50 watts and below that the VVR is designed to handle, NO GREASE IS REQUIRED. If you wanted to try and push your luck with a 60 watt or above amp with the VVR (which I don't suggest) then you may need Grease, a larger heatsink and also possibly a fan. Multiple Mosfets with the VVR would require a different setup on the current limiting and if you try it on the VVR without making changes you will probably have problems. I don't know of any product out there using multiple mosfets, or trying to handle 100 watt amps that won't require very large heatsinks and maybe even a fan.

Most of the heat generated is generated when the power control is set to between 3/4 and 1/4. Anything below 1/4 the amp isn't pulling enough current to heat up the device. Anything above 3/4 will be like running the amp without the VVR and will not produce much heat. If you run your amp all the time at 1/2 power using the VVR, that is when the VVR will be dumping the most heat into the chassis. All builds are different and each has it's own challenges to overcome in regards to installing a VVR.

One other thing, the nylon bushing that comes with the mosfet really isn't required to electrically isolate the bolt going thru the mosfet from the chassis. It is more of a cushion to keep from damaging the mosfet from over tightening. You can leave it off and if you are careful can usually get a much tighter connection between the mosfet and the chassis/mica insulator.

[markr14850]

How about a sil-pad? They require no grease and are thermally almost as good as mica+grease. No worries about accidental conduction.

Edit: Also, good discussion here: http://music-electronics-forum.com/t288 ... /#poststop

[Tillydog]

It is much easier for me to recommend no grease that to try and answer all the emails I get about what kind of grease to buy. On any amps of 50 watts and below that the VVR is designed to handle, NO GREASE IS REQUIRED

IMHO, it's not good practice to use *mica* washers like this - you are trusting to luck at getting adequate thermal contact, but experience beats theory every time, I guess.

Andy

[doveman]

Well - the high voltage problem with non-conductive grease makes sense to me. Still thinking about heat sink options. I think I'll be able to work it out.

[UR12]

It is much easier for me to recommend no grease that to try and answer all the emails I get about what kind of grease to buy. On any amps of 50 watts and below that the VVR is designed to handle, NO GREASE IS REQUIRED

IMHO, it's not good practice to use *mica* washers like this - you are trusting to luck at getting adequate thermal contact, but experience beats theory every time, I guess.

Andy

Then I have to wonder why the Mosfet manufacturers don't supply you with the heat sink grease but see fit to include the mica insulator with the device? They evidently think it is enough. It has been my experience that people have used the grease and blew their mosfet. They have then removed the grease, replaced the mosfet and had no other problems. But if you don't think that it's good practice to not use just the insulator and apply the grease, just be warned that you may be replacing the mosfet.

[Tillydog]

Then I have to wonder why the Mosfet manufacturers don't supply you with the heat sink grease but see fit to include the mica insulator with the device?

I wasn't aware that they did, but it's a nice touch, and a convenience for the purchaser, considering that TO3-P(?) insulators aren't as easy to find as some other sizes. But I don't think it matters - I think of heatsink compound as a workshop consumable, but YMMV.

It has been my experience that people have used the grease and blew their mosfet. They have then removed the grease, replaced the mosfet and had no other problems.

If people have used, for example metal loaded grease (sometimes called 'thermal transfer compound') used on computer CPU heatsinks, etc., then it *will* be electrically conductive, and sparks will fly (or MOSFETs will pop) - it simply isn't the right stuff. I can see how it could be a minefield trying to advise people remotely about whether brand 'X' grease is OK or not. You clearly have a system that works very successfully, and I'm not trying to argue with that. The overwhelming evidence is that it works

..if you don't think that it's good practice to not use just the insulator and apply the grease, just be warned that you may be replacing the mosfet.

I remain confident that using the correct heatsink compound with the mica washer gives the most predictable and lowest thermal resistance between the device and whatever it's bolted to, and as such that's what I'd do (using compound from a tube I bought about 30 years ago, which I know has good dielectric strength, and which has worked fine for dozens of switching power supply MOSFETS (and one amplifier HV regulator) over the intervening years). As an alternative, I'd use a silicone pad if I could find one the correct size. It might be overkill, but it's what I'd do as routine. The sort of compound I use is similar to this.

I seem to have touched a nerve, here, which whas honestly not my intention, and I apologise if any offence has been caused.

Getting back to the OP, who seems to be pushing your recommended boundaries for a VVR installation: If indeed the MOSFET in question overheated, then I do think the situation would be greatly improved by either the use of appropriate compound with the supplied mica washer, or substituting a silicone washer for the mica (which doesn't require any additional snake oil ). From what the OP has said, it doesn't sound like the heat from the MOSFET was getting into the metal of the chassis. (The OP has aparently communicated with you about this privately, so you may be aware of things that I am not.)

Humbly yours,

Andy

[UR12]

I seem to have touched a nerve, here, which whas honestly not my intention, and I apologise if any offence has been caused.

No, You havent touched a nerve and no apology is necessary. I am just trying to explain why I don't think it is a good to advise people about going out and buying a generic thermal grease and applying it to a VVR's mosfet. They read stuff like this thread where people come in and advise that they MUST use this heat Sink Compound if they want PROPER thermal transfer. Unlike you, I think most people don't have a grease laying around for years that they have used on numerous high voltage mosfets. They either go to a local electronic/computer store, Radio Shack or some other place and buy what ever "Thermal compound" that they are selling (Thinking they are following the best practice as mentioned by the forum experts) without checking to see if it is suitable for the application they are going to be using it in. I then get an email that they have blown their mosfet and I have to sort out what went wrong. I hope that I have tried to educate people on the proper grease to buy if they just absolutly have to have it and and save them a little money and time in blown mosfets.

That's my only agenda.....

In Your first post:

The mica washer on its own is a disaster, thermally (with all respect to Dana ). One **needs** to use heatsink compound with mica to get good thermal contact. A silicone pad is easier and less messy.

If you didn't use any compound, then I suspect your MOSFET died because it wasn't in thermal contact with the chassis. (Your chassis looked quite thin, to me, in the first picture, sorry).

FWIW, I would just replace the parts where they are now. Take care that the MOSFET sits completely flat against the chassis and that the mounting faces are flat and clean. Just a little heatsink compound (both sides of the mica washer) - it should ooze out all round when you bolt the MOSFET down (or use a silicone pad). Monitor the temperature of the chassis when it's in use. If you can bear your hand on it at all, then it's not too hot for the MOSFET. If it hisses when you spit on it, then you're pushing the envelope

Andy

You didn't mention what kind of compound to use, the dielectric properties needed or any other information that would have lead a person to believe they couldn't just walk into any store, buy any thermal grease and they would be good to go. That can and would have caused a disaster for most DIY's installing a VVR. There are 1000s of VVRs out there working today and over the last 5 years, without the need for a heatsink compound.

[doveman]

Just to bring it back to ground ... so-to-speak. I believe Dana's logic on the grease with high voltage, so I won't try this. My Dr Z amp is rated as 32 watts ... but my guess is it's slightly underrated ... maybe 38-40 watts (especially with the 30w golden lions rather than the original 25w Groove Tubes).

I think I may not have the best choices for mosfet installation although the chassis is fairly thick aluminum. My own guess is that maybe I just blew the original by diming it while installing/biasing the new tubes. I bought two replacements from Dana (just in case).

I think the best solutions for me:

• Double check the mosfet and make sure it's seated as flat as possible then test it at home rather than on a gig. Test it outside the cab and feel how hot it's getting on the chassis.
• If I blow it again then try the silicone wafer and possibly install a heat sink on the inside mounted next to the mosfet. It won't be ideal but might dissipate the heat enough.

Again ... all the ideas and feedback are appreciated. I'd like it to be bullet proof but understanding more about when the real load occurs is helpful.

[doveman]

Ok - Success! I'll put the final solution in all related threads in case someone else is researching this later. The problem was that I had the VVR3 directly following the rectifier. I made an adjustment to slip it in after the first cap but before the rest of power circuit. Dana's VVR3 instructions clearly show the VVR3 following a filter cap but my Dr Z only has 3 caps ... I did not notice that there were 4 caps in his instructions.

Not only does this make sense ... fix the problem ... it's also a neater and more tidy solution. I tested it at the 3:00-3:00-3:00 setttings with the VVR on 5. You can still put your finger on the bolt head holding the MOSFET (hot but not uncomfortable) and the surrounding aluminum is warming nicely (seems to dissipate just fine). Excellent!

Here's the final fix.
[IMG:639:371]http://i104.photobucket.com/albums/m168 ... C_0432.jpg[/img]

Notice it is much cleaner and I will probably simplify the ground next time it's out of the box but I need a connector:
[IMG:639:423]http://i104.photobucket.com/albums/m168 ... C_0431.jpg[/img]

I really appreciate the help guys. Thanks!

[Reeltarded]

Yes! Excellent!

[Tillydog]

Very good!

Glad it worked out

Andy

[katopan]

Go forth and crank that gain with confidence! Glad you got it sorted.