what should i use for insulators between igbt's and heatsink?

[yourboi]

I believe the FGH75T65SHDTLN4 I am using do not have a built in insulator between the collector and metal backing.

[davekni]

Best thermal performance is with separate heatsinks for each IGBT.  Heatsinks electrically live (at IGBT collector voltage), so need to be insulated from each other and ground.  This is the way most of my projects are built.

Next best (and occasionally better) is to add a thick copper heat spreader under each IGBT, then a larger thermal pad between spreader and heat sink.  Copper spreader adds thermal mass to handle short heat bursts such as in DRSSTC enable pulses.  Larger thermal pad increases heat transfer to heatsink.

Of course, either of the above requires good high-thermal-conductivity thermal compound under IGBT.

[yourboi]

Best thermal performance is with separate heatsinks for each IGBT.  Heatsinks electrically live (at IGBT collector voltage), so need to be insulated from each other and ground.  This is the way most of my projects are built.

Next best (and occasionally better) is to add a thick copper heat spreader under each IGBT, then a larger thermal pad between spreader and heat sink.  Copper spreader adds thermal mass to handle short heat bursts such as in DRSSTC enable pulses.  Larger thermal pad increases heat transfer to heatsink.

I am planning to make use a bunch of pc heatsinks I have leftover from old computers that have the fans already connected or common fans I can connect. most drsstcs and sstcs I see built use large wide heatsinks rather then the more narrow heatsinks used for pcs. I imagine I could only attach 2 igbts to each heatsink using this method for a half bridge. while one of the heatsinks I own could only use for a single igbt cooling despite its ability to cool a entire smaller sstc. this is probably the reason I don't see others using them for sstc building.

maybe a full bridge could be squeezed on the bigger one while sacrificing performance. I attached photos to make it clear what heatsinks I am working with.

Of course, either of the above requires good high-thermal-conductivity thermal compound under IGBT.

This will be a non issue as I only have high end artic silver left from my last pc build as a thermal compound. The idea of using liquid metal between the igbt and copper pad has crossed my mind.

my concern is picking a suitable thermal pad. I am also unsure how I would apply the pressure to the thermal pad and copper plate.

Thanks for the reply.

[NyaaX_X]

I think there're some thermal pads made from silicon or mica for single TO-247 or TO-3P transistor.

And in my experience the metal backing of TO-247-3, TO-3P n-channel MOSFET or IGBT are directly connect with Drain or Controller internally.

And if you don't want to drill screw hole to the heat sink to press the transistor . Maybe clamp them.

[davekni]

This will be a non issue as I only have high end artic silver left from my last pc build as a thermal compound.

Yes, arctic silver is good.  That's what I use for most of my projects.  Just be sure to keep it away from IGBT leads since it is electrically conductive.

I think there're some thermal pads made from silicon or mica for single TO-247 or TO-3P transistor.

Mica is terrible for thermal conductivity, though it was used decades ago for lack of better options.

Maybe clamp them.

Yes, some form of clamping that applies pressure over IGBT die is much better than just a screw through the IGBT mounting hole.  This is one good example (not including a heat spreader):
    https://highvoltageforum.net/index.php?topic=2191.msg16147#msg16147
I'm terrible at mechanical design, so don't have any good recommendations for how to clamp to the heatsinks you have.

Here is an example of a heat spreader and 3D-printed clamp.  Not sure about long-term creep stability of 3D printed parts, but it appears to work for Lucasww's project:
    https://highvoltageforum.net/index.php?topic=2621.msg19424#msg19424

And another example of heat spreaders, but using screws.  Screw holes are not best place to apply force.  Final image in this post:
    https://highvoltageforum.net/index.php?topic=2394.msg18907#msg18907

my concern is picking a suitable thermal pad.

Hope others will answer here.  Since I use electrically-live heat sinks, I have limited experience with thermal pads.

[NyaaX_X]

Yes I know mica have good electrical isolate ability, but not good at transfering heat. But I still see there're selling the mica type transistor thermal pads. And the more common one, I think is the silicon type, I guess they are similar to those thermal pads apply on GPU RAM thermal pads.

[yourboi]

I'm terrible at mechanical design, so don't have any good recommendations for how to clamp to the heatsinks you have.

I am also comedically bad at mechanical design

my concern is picking a suitable thermal pad.

yeah, I hope someone passing by gives a suggestion. I'll lurk around and see what others are using while I'm free today. I plan to go the copper heat spreader method like suggested as my goal is to get the igbt to preform more like the 25°C characteristics.

[Anders Mikkelsen]

Sil-pads are easy to use, but the performance is not amazing. For a TO-247 package, they add 2 - 3 k/W thermal resistance with the most common variants, down to around 1.5 k/W for high performance versions. That's 70+ degrees Celcius of additional junction temperature rise at a dissipation of 50W per device. Micas can be around twice as good, at the cost of more drain-heatsink capacitance on account of their lower thickness. Kapton film can perform better, but add even more capacitance for a given thermal resistance, and I've had failures from less than perfect heatsink surface condition and high dV/dt stress. That's 8 kV rated kapton thermal foil that failed after around half an hour at 800 V, 50 V/ns, 75 kHz.

Ceramics are by far the best. Alumina (Al2O3) at 1 mm thickness gives around 0.25 k/w, which can be reduced by going down to 0.63 or 0.50 mm at the cost of some capacitance (which will still be a lot lower than with mica or kapton). Electrical robustness is also extremely good. For even better performance, AlN can be used, which is almost as thermally conductive as aluminium or BeO, at some cost. Note that ceramics are a bit more finicky when it comes to flatness, both of the insulator sheet and the heatsink surface. Some extruded heatsinks are far from flat, others are pretty good, but ones with machined surfaces will always be best.

Spring clip clamping is also superior to using a screw through the package hole, by providing more pressure, more even distribution, and better creepage distances.

Some Aliexpress ceramic insulators, checked or sorted for flatness within say 50 µm, paired with a flat heatsink, a good paste in a not too thick layer, and a good spring clip like the MAX08NG, gives very good performance without being too expensive or difiicult to work with. Beware that some pastes are somewhat conductive, but there are good ones without metallic fillers. Even metallic fillers pastes can be used, as long as it's not smeared near the package pins or across the edge of the insulator.

Edit: as mentioned, direct contact reduces the interface thermal resistance even more, with the same concerns about using a good paste, not applying too much, having a flat heatsink and applying even pressure. Small heatsinks with a lot of air flow can perform better than large heatsinks under any air flow conditions. A bottleneck is how far the heat can spread from the transistor, due to the bulk thermal resistance of the heatsink. Having the fins close to the device and good forced airflow is hard to beat.

[Lucasww]

my concern is picking a suitable thermal pad.

yeah, I hope someone passing by gives a suggestion. I'll lurk around and see what others are using while I'm free today. I plan to go the copper heat spreader method like suggested as my goal is to get the igbt to preform more like the 25°C characteristics.

I used these pads in my QCWDRSSTC: https://www.digikey.com/en/products/detail/t-global-technology/TG-A9000-30-30-0-5/16602234
They are thermally and electrically very good.
do be careful if you decide to use them however, they are very soft and easy to tear.

[VEKTOR]

Funny, I use the same heatsink in my current project. Old Dell machine, socket 775.
Step and FreeCAD files attached, if of any use...


Made good experience with Tgard K52-1 Kapton material.
It has pre applied phase change thermal compound on both sides and is considerably robuster compared to soft pads.
TO247: https://mou.sr/48sW5Cl

If sufficiently heated, the thermal compound becomes viscous. After cooling down it solidifies again and will not become viscous again.
The package is now stuck/glued to the heatsink. Not extremely, but very noticeably.

[Unrealeous]

Digging up an semi-old thread, but I have just come across this issue.  The solutions presented by Dave in the first reply will solve the problem, however another question

Is there a way to tell on the datasheet or the description that an IGBT that they will have a connection between the collector and metal backing? 

The one's I'm currently playing with are here - collector and metal backing are connected.

https://www.onsemi.com/pdf/datasheet/fga60n65smd-d.pdf

[alan sailer]

In my limited experience a device that has the collector electrically isolated from the metal back has horrible thermal
specs. So possibly the thermal junction to case specifications could tell you if this is true.

Cheers.

[klugesmith]

Here's what puzzles me.   The cited datasheet says package is
TO−3P−3LD / EIAJ SC−65, ISOLATED

What could isolated mean, in that context, except electrical insulation between collector and metal thermal pad?
I would guess that isolated devices would have a specified voltage limit for the insulation. Maybe even a value for capacitance.   I've never seen those in a datasheet.
The thermal resistance, junction to case, is given as 0.25 K/W. If there's electrical insulatuion in the path then it must be pretty fancy.

Your ohmeter does not lie.   Might you be playing with knock-off or counterfeit parts that are not isolated?

[Unrealeous]

Your ohmeter does not lie.   Might you be playing with knock-off or counterfeit parts that are not isolated?

I guess I can't rule that out.

The thermal resistance, junction to case, is given as 0.25 K/W. If there's electrical insulatuion in the path then it must be pretty fancy.

I will start paying attention to these.  I'm sure i can find another IGBT without this issue and compare specs.

In my limited experience a device that has the collector electrically isolated from the metal back has horrible thermal
specs. So possibly the thermal junction to case specifications could tell you if this is true.

Good information, thanks

[davekni]

The one's I'm currently playing with are here - collector and metal backing are connected.

https://www.onsemi.com/pdf/datasheet/fga60n65smd-d.pdf

In my limited experience a device that has the collector electrically isolated from the metal back has horrible thermal
specs. So possibly the thermal junction to case specifications could tell you if this is true.

Here's what puzzles me.   The cited datasheet says package is
TO−3P−3LD / EIAJ SC−65, ISOLATED

Agree with all above.  Isolated as applied to a semiconductor device always means electrically.  I suspect an error in this data sheet.  Data sheet author likely copied a package drawing from another part and didn't edit completely (or more likely copied from a different part than intended).  Truly isolated parts also tout isolation as a feature other places within specification.

I would guess that isolated devices would have a specified voltage limit for the insulation. Maybe even a value for capacitance.

Every isolated device I've seen includes isolation voltage specifications.  I have not seen capacitance specified.