Can someone verify my thermal calculations?

I think you have an error in the case to heatsink line, you have 0.255 sq in of sil pad, at 0.3 C/W per square inch so it's 1.176 C/W for the pad. Case to heatsink is 37.5 * 1.176 = 44.1C.

If there is just one heatsink there is a second error - the heatsink temp rise = 150 * 1.4 = 210 so I'm assuming you have a heatsink per device.

SO on that basis the temperature rise junction to ambient = 18.75 + 44.1 + 52.5 = 115 so you have zero margin at 35C ambient.

You might get a small improvement by losing the pad and doing thermal grease directly to the heatsink (feasible with a heatsink per device)  but you won't get it much better - your best bet is to blow the heatsink.

That graph is showing you that the max thermal impedance tends towards 0.5 K/W as the pulse length increases. So 0.5 K/W is the rating you should use. If you were using very short pulses with low mark/space ratio you could assume a lower thermal impedance.

MK

michaelkellett said:

f there is just one heatsink there is a second error - the heatsink temp rise = 150 * 1.4 = 210 so I'm assuming you have a heatsink per device.

Ah, of course!  There is just one heatsink actually and I'm also using it for a Schottky Diode, a Sense Resistor and a Temperature Monitor (all TO-220 packages) so it's likely to be significantly worse.  i do have a fan though.

Thanks for the feedback on this and clarity on the MOSFET specification.  That figure of 1.176 C/W is much closer to the figure I was expecting for silicon pad.  I do have some thermal grease and mica pads so I will use those instead - TBH, I was considering the Sil-Pads as they were cheap and it avoided messing around with grease. 

Rather than try and build 'up' I'm going to revise down to a more manageable Power level.  I'm glad I posted this and appreciate the feedback.

The heatsink temperature looks okay assuming you have one of these heatsinks on each transistor, otherwise the temperature would be much higher. The junction temp looks like it is over 153 which would shorten transistor life. Hopefully the heatsinks aren't where they could be touched since they get very hot. I would try to design for a lower temperatures if possible, (bigger heatsinks) - everything would see less stress and last longer. Note that if the heatsinks are close to each other the effective ambient temperature is increased and everything gets hotter.

Typically active loads in the 150W - 300W class use tube style heatsinks with blowers - but these are a bit pricey as standard parts.

You can expect the heatsink performance to improve by a factor of between 2 and 4 with a decent airflow, so you need to go for about 0.6 K/W convection rated heatsink with a fan. This will result in a heatsink temperature rise of < 45K and a junction max temperatuer of140C.

Farnell have 3813322 at £28 which is rated at 0.113 K/W with fan and better than you need. They have the slightly cheaper 3813292 for about £20 which is rated at 0.4 K/W convection. It should give you no more than 30K above ambient for 150W.

If it's for your own use you can protect yourself by adding a thermal cut out.

If you are going to put 4 devices on one heatsink I suggest you stay with the SIL pads.

Using a MOSFET in a TO247 case will improve the case to heatsink coupling. I'm pretty sure that the 3813322 heatsink has a better surface finish than the 3813292 and that may be worth the extra cost !

MK

AJ my feedback is anecdotal. Thermal dynamics was the one course in college I struggled with. Passing it was critical to graduating. I did get my diploma but avoided electronic design. My heatsink choices were always big is better:)

A system that depends on mechanical means (i.e. fans) to ensure it remains within limits should have a protection circuit to prevent thermal runaway. The transmitters I worked on would back-off power to the finals in order to keep the building from going up in smoke if airflow was compromised. I learned early if you put your toolkit in front of the air intake, you generated alarms. We are talking KV size transmitters.

It would be unfortunate to build such a system to have it go up in smoke because a fan went unservicable.

michaelkellett said:

Typically active loads in the 150W - 300W class use tube style heatsinks with blowers - but these are a bit pricey as standard parts.

I was originally looking at the Fischer Electronic ones which were funky and expensive!  I saw a recent teardown of a DC Load that had these, or similar, inside.

Any particular reason to stay with Sil Pads, I've not actually bought them yet?  Mica seems better - I'm struggling to find data that isn't represented as W/mk which I can't work out how to convert - but they definitely seem easier to use than Mica and I think if the difference between pass and fail is the thermal resistance of the insulating pad then it's a definite fail in any case!

Michael pointed out the flaw in my maths quite succinctly!!  It's my own project, for myself, so I can play around with the specs as I want.  I think the easiest approach is to drop the power down to something more manageable rather than to up spend for a power level I'll likely never bother with anyway.

To be honest, I mostly 'get' it.  The biggest difficulty I have is translating between the different units that manufacturers use, particularly W/mK.  I do have temperature monitoring in place in the design and can automatically turn it on/off as needed.  For initial testing, I also have a means of adding two additional thermistors so I can check temperatures of components.

I haven't used mica insulators for ages, because they are hard you need a very thin film of grease on both sides and if the finish on the heatsink isn't good (and on a classic cheap extruded heatsink it won't be very good) then you get poor results. Good quality sil pads are much more forgiving and far less messy.

MK

Sound like you have given some thought to the possible eventuality.

Environmental monitoring was the norm in high-power equipment. I recall vane-switches, not much different than this beast as a  sensor in a complex system to keep 50KV transmitters cool.

We spent a considerable bit of time doing fan motor and filter maintenance. Few new techs from school had ducting experience. Airflow was critical to keeping the equipment on the air. Banging tin was just part of the job.

Today's system, just look at the BIOS of a desktop computer, have all kinds of monitoring. Flame detection isn't one of them:) Good luck with your project.