Electronic Load Project - TLV171 & IRFP250 with KiCad Files

[Kleinstein]

The IRFP250M is specified with about 0.7 K/W for "junction" to case  and a suggested 0.3 K/W for case to heat sink. With a reasonable size heat sink with 1K/W  the total thermal resistance is 2 K/W. So at 60 W the temperature rise for the chip inside would be at some 120 K and thus maybe 150 C chip temperature.  If one wants more margin it is possible to use a better heat sink (e.g. 0.5 k7W) with a fan.

I know the 214 W maximum power rating is not realistic, but usually something like 1/3 or even 1/2 of the P_tot rating can be realistic.

[spec]

The IRFP250M is specified with about 0.7 K/W for "junction" to case  and a suggested 0.3 K/W for case to heat sink. With a reasonable size heat sink with 1K/W  the total thermal resistance is 2 K/W. So at 60 W the temperature rise for the chip inside would be at some 120 K and thus maybe 150 C chip temperature.  If one wants more margin it is possible to use a better heat sink (e.g. 0.5 k7W) with a fan.

I know the 214 W maximum power rating is not realistic, but usually something like 1/3 or even 1/2 of the P_tot rating can be realistic.

Thanks for the reply- much appreciated.

While I agree with the figure of 0.7 K/W for J/C, you have neglected the thermal resistance of the insulating washer, which is typically 1 K/W overall. 1K/W would be a very good heatsink indeed: large and expensive and nothing like what is being used as far as I can tell.

But even with your figures the IRFP530 junction temperature of 175 C would be exceeded.  Taking into account equipment case air temperature of 70 degC, you get 120 +70= 190 degC. And you would not normally run a transistor at its maximum junction temperature- you would leave a margin for safety and shoot for around 158 deg C.

Yeah, Ptot figures on spec sheets are for the sales people and, to a great extent, so are maximum current figures.

 

[t1d]

The TO247 case is not that bad for thermal dissipation. For heat dissipation it is similar well suited as TO3. So something like 60 W would be a reasonable conservative limit.  20 W is more like the practical limit for the smaller TO220.

Please show your thermal budget getting 60W with an IRFP250M in a TO247 case. Did you miss reply #66 which shows scientifically that 60W is impossible.

EDIT: Oops, I see that this was discussed, in two posts, after this one... So, the below may not have been necessary...

There is a lot of thinking outloud, here, so please bear with me... Remember, I am noobish...

No, I just took this to mean that the FET would burn up, from heat:
Assuming that the ambient temperature inside the equipment case is 70 degC, the junction temperature would be, 282+70 = 352degC, and there, afraid to say, is the problem!

My reply to that was that I thought that the present heat sink/fan combination was cooling well enough to handle what is going on. That came from the thermal tests and your comment that the temperature increases linearly, with wattage, and my resulting math calculation (Case temp ending around 75*C.)

Maybe it is that the temp on the inside of the case is the junction temp and that temp is what can not be violated? It has nothing to do with the efficiency of the cooling system? No, I had thought not... Because, later, you said...In the above, I have assumed that your heatsinks have a thermal resistance of 3 degC/W but, going by their size, their thermal resistance may be higher. Aggressive fan cooling will bring the thermal resistance down, but not much lower than 2 degC/W, I would guess.

I will keep thinking on where I am missing it. If you see my folly, please set me straight.

What do you think the limit might be? If this number was in your prior post, I apologize.

Thanks for your continued help.

[spec]

The TO247 case is not that bad for thermal dissipation. For heat dissipation it is similar well suited as TO3. So something like 60 W would be a reasonable conservative limit.  20 W is more like the practical limit for the smaller TO220.

Please show your thermal budget getting 60W with an IRFP250M in a TO247 case. Did you miss reply #66 which shows scientifically that 60W is impossible.

EDIT: Oops, I see that this was discussed, in two posts, after this one... So, the below may not have been necessary...

There is a lot of thinking out loud, here, so please bear with me... Remember, I am noobish...

No, I just took this to mean that the FET would burn up, from heat:
Assuming that the ambient temperature inside the equipment case is 70 degC, the junction temperature would be, 282+70 = 352degC, and there, afraid to say, is the problem!

My reply to that was that I thought that the present heat sink/fan combination was cooling well enough to handle what is going on. That came from the thermal tests and your comment that the temperature increases linearly, with wattage, and my resulting math calculation (Case temp ending around 75*C.)

Maybe it is that the temp on the inside of the case is the junction temp and that temp is what can not be violated? It has nothing to do with the efficiency of the cooling system? No, I had thought not... Because, later, you said...In the above, I have assumed that your heatsinks have a thermal resistance of 3 degC/W but, going by their size, their thermal resistance may be higher. Aggressive fan cooling will bring the thermal resistance down, but not much lower than 2 degC/W, I would guess.

I will keep thinking on where I am missing it. If you see my folly, please set me straight.

What do you think the limit might be? If this number was in your prior post, I apologize.

Thanks for your continued help.

No problems. 

Yes you are right, what counts after all is said and done, is the maximum temperature of the silicon inside the NMOSFET and that is 175 deg C. If you exceed that temperature the NMOSFET may fail immediately, normally going short circuit drain/source, or its parameters may rapidly or gradually  deteriorate, so its gate threshold voltage might go up, as could its internal resistance. Sooner or later though it would probably fail completely.

Also, it is not wise to run a transistor at its maximum temperature- it would be better to have a margin, and I would say that running at 90% would be the maximum temperature giving a margin for safety of 10%. So that gives a maximum temperature of 157.5 deg C.

As to the maximum amount of power, I think you can safely dissipate 20W.

Two big factors that control the amount of power that can be dissipated are obviously the thermal resistance of the heatsink, but the thermal resistance of the insulating washer is the other and it has a surprisingly large impact.

What insulating washer are you using?

[spec]

I made this a separate post to get away from all the theoretical stuff and focus on some practical measurements.

There is one simple test you can make, the acid test, to establish what power can be safely dissipated.

ACID TEST

[1] Set the voltage across the actual drain and source to a particular voltage.
[2] Adjust the DC current flowing from the drain to the source to a particular current.
[3] Multiply the voltage by the current to get the power being dissipated by the NMOSFET.
[4] Measure the air temperature in the room, assuming that the MOSFET and heatsink are in the open and not in a case.
[5] Measure the case temperature of the NMOSFET after 15 minutes or more.

10W dissipation would be a good choice, say 10V at 1A.

The big problem though, is measuring the temperature of the TO247 case, the part that counts that is, as most of the case is plastic which is a heat insulator. The part of the case that needs to be measured is the metal part underneath which mates with the thermal washer. It is a shame that the NMOSFET was not in a TO3 case, because they are all metal (see reply #80).

Anyway, putting the problem of measuring the TO247 mating surface temperature aside for the moment, once you get, power dissipation, ambient temperature, and the TO247 mating surface temperature, it is a simple matter to work out the the thermal performance of your set up. If you post the figures I will do the calculation for you if you want.

[spec]

Yet another separate post , this time to describe how to measure the temperature of a TO247 mating surface.

One approach you can use is to make a washer of copper sheet, say 3mm thick, to fit between the TO247 case and the insulating washer.

If you make the copper washer slightly larger than the TO247 case, you can then measure the temperature of the copper washer to get a temperature figure to use in the acid test.

[spec]

Yet another post 

As the insulating washer has such a large negative impact, you could consider eliminating it and mounting the NMOSFET directly to the heatsink. This would mean that the heatsink would be at the NMOSFET drain potential though, so the heatsink would need to be electrically insulated from the equipment case.

The other problem is that the heatsink would then act as a capacitor and possibly inductor in the NMOSFET drain circuit and could cause parasitic frequency instability, but in the case of your application this should not be too much of a problem.

[spec]

Guess what!

The single hole fixing of a TO247 case is generally unsatisfactory, and gets worse with time.

In a demanding application like this, a clamp is often placed on the top of the TO247 case to generate a more even pressure between the TO247 mating surface and the insulating washer and the insulating washer and the heatsink.

It would be a good idea to consider a clamp for your application.

[t1d]

What insulating washer are you using?

DS = The TO-247 is similar but superior to the earlier TO-218 package because of its isolated mounting hole.

No insulation pad either. Seats in grease.

For the test rig, I am not using a mounting bolt. The heat sink came with a mounting clip that clamps the FET, to the heat sink.

[t1d]

Anyway, putting the problem of measuring the TO247 mating surface temperature aside for the moment, once you get, power dissipation, ambient temperature, and the TO247 mating surface temperature, it is a simple matter to work out the the thermal performance of your set up. If you post the figures I will do the calculation for you if you want.

Ambient was 26.7*C
At 10 watts, the face temperature was 34.7*C.
My guess is that the ending temp, at 60 watts, will be around 75*C. Please check my math, on the spreadsheet.

Thanks, for your help, with the technical stuff.

[t1d]

Yet another separate post , this time to describe how to measure the temperature of a TO247 mating surface.

One approach you can use is to make a washer of copper sheet, say 3mm thick, to fit between the TO247 case and the insulating washer.

If you make the copper washer slightly larger than the TO247 case, you can then measure the temperature of the copper washer to get a temperature figure to use in the acid test.

Neat trick!

[t1d]

Yet another post 

As the insulating washer has such a large negative impact, you could consider eliminating it and mounting the NMOSFET directly to the heatsink. This would mean that the heatsink would be at the NMOSFET drain potential though, so the heatsink would need to be electrically insulated from the equipment case.

The other problem is that the heatsink would then act as a capacitor and possibly inductor in the NMOSFET drain circuit and could cause parasitic frequency instability, but in the case of your application this should not be too much of a problem.

The FET is mounted directly to the HS, in grease, per the DS. The DS does not make any mention of needing to isolate the HS, but, presently, the HS is isolated. I will check, with my DMM, between the HS and ground, to see if there is anything there.

EDIT: Hmm... Maybe, it would be better to check between the HS and the Shunt Resistor. Seems safer...

[t1d]

Guess what!

The single hole fixing of a TO247 case is generally unsatisfactory, and gets worse with time.

In a demanding application like this, a clamp is often placed on the top of the TO247 case to generate a more even pressure between the TO247 mating surface and the insulating washer and the insulating washer and the heatsink.

It would be a good idea to consider a clamp for your application.

The test rig uses a clamp and so does/will the HS, in the permanent case.

[spec]

What insulating washer are you using?

DS = The TO-247 is similar but superior to the earlier TO-218 package because of its isolated mounting hole.

No insulation pad either. Seats in grease.

For the test rig, I am not using a mounting bolt. The heat sink came with a mounting clip that clamps the FET, to the heat sink.

Ahh, I now see why my thermal calculations were so different from yours and Kleinsteins.

The TO247 case does have an insulated mounting hole, which means only that you do not need a small insulating washer for the mounting bolt. But the IRFP250M drain is still connected to the metal plate that interfaces with the heatsink. This means that unless you go for a live heatsink, you need a large electrically insulating washer between the TO247 case and heat sink (see attached image)

[t1d]

Ahh, I now see why my thermal calculations were so different from yours and Kleinsteins.

The TO247 case does have an insulated mounting hole, which means only that you do not need a small insulating washer for the mounting bolt. But the IRFP250M drain is still connected to the metal plate that interfaces with the heatsink. This means that unless you go for a live heatsink, you need a large electrically insulating washer between the TO247 case and heat sink (see attached image)

Good catch!

It is not noob-friendly (me) to have done their specifications, based on a live heat sink, and to have made no other comment, other than the drawing notes.

I can add a plastic insulator shim, to the test rig, to make it more realistic, when I begin testing, again. I am, presently, waiting on Schottky diodes to arrive.

I think we have a small communication gap, due to terminologies. What do you mean by "a large electrically insulating washer?" I am only familiar with plastic-type insulators and mica (if I have remembered the correct mineral.) I thought an insulating washer was the device to keep the mounting bolt from grounding out. Our FET has an integrated bolt isolator.

Thanks, so much...

[spec]

Ahh, I now see why my thermal calculations were so different from yours and Kleinsteins.

The TO247 case does have an insulated mounting hole, which means only that you do not need a small insulating washer for the mounting bolt. But the IRFP250M drain is still connected to the metal plate that interfaces with the heatsink. This means that unless you go for a live heatsink, you need a large electrically insulating washer between the TO247 case and heat sink (see attached image)

Good catch!

It is not noob-friendly (me) to have done their specifications, based on a live heat sink, and to have made no other comment, other than the drawing notes.

I can add a plastic insulator shim, to the test rig, to make it more realistic, when I begin testing, again. I am, presently, waiting on Schottky diodes to arrive.

I think we have a small communication gap, due to terminologies. What do you mean by "a large electrically insulating washer?" I am only familiar with plastic-type insulators and mica (if I have remembered the correct mineral.) I thought an insulating washer was the device to keep the mounting bolt from grounding out. Our FET has an integrated bolt isolator.

Thanks, so much...

No probs.

Yes, it's my terminology that hasn't been too clear.

I hate to keep on, but can I advise to avoid plastic or foam insulators- they look pretty and are simple to use but they have a high thermal resistance and, long-term, are not that good.

Instead, go for as lower thermal resistance as you can. Aluminum oxide have a very low thermal resistance, but are expensive and easily damaged, ceramic are good but brittle, and thin mica are perhaps the best choice, all things considered.

It is a great shame that the heat conducting area of the TO247 case in not insulated, because that would have made one hell of a good NMOSFET   

[t1d]

Health challenges have prevented the motivation to test the Single-MOSFET board, with a 12v car battery. But, I am hopeful, to soon do so. I didn't want folks to think the thread is dead.

It would be great, if someone wanted a board, on which to do their own testing. The design really needs a more professional approach, than I am able to do, in discovering/defining its capabilities and limits. I would even build one out for you, if that would make it easier for you to participate and you are in the 48. You might help with my parts costs and shipping?

[t1d]

I remembered that I have an IOTA DLS-55 Battery Charger/PSU, for 12v systems. This is a serious battery charger/PSU; 55 amps @ 12vdc. This will be perfect, for testing the e-load. No need to be outside in the cold, no chance of a battery gas explosion, no dirty battery cables to disconnect from the car and I won't have to reset the car's clock. Woot. I think that was just the inspiration I needed!

[t1d]

I did additional testing, with the Single-MOSFET Test Rig. Here are the results:

Beginning Ambient Temperature = 24*C
Highest Volts/Amps = 13.82V @ 2A
Soak Time = 10 Minutes, Temp was settled in only two minutes.
Final MOSFET face temperature = 52.3*C

Heat Sink Considerations
The MOSFET was mounted in grease directly to an Aluminum Plate, being 125mm x 50mm. No insulator was used. This is in keeping with the OEM test method. Direct aeration was provided by a 60mm x 60mm, 12vdc fan. (I originally reported thatIthe fan is an 80mm unit; I was wrong.)

Mathematical Extrapolations
Voltage Extrapolation Factor (The Design Goal is 30V/2A)
30.0V/13.82V = 2.17

MOSFET Rise In Temperature
52.3*C – 24.0*C = 28.3*C Rise

Estimated MOSFET Rise at 30V Design Goal
28.3*C x 2.17VEF = 61.4*C

Estimated MOSFET Temperature at 30V/2A Design Goal
61.4*C + 24.0*C = 85.4*C

Conclusion
I estimate the Single-MOSFET is capable, thermally, of the design goal, and possibly more, with a proper heat sink and cooling. It would be good to do operational testing, at full power, but I will have to think on how to configure that much wattage.

[t1d]

I did additional testing, with the Single-MOSFET Test Rig. Here are the results:

Beginning Ambient Temperature = 24*C
Highest Volts/Amps = 13.82V @ 2A
Soak Time = 10 Minutes, Temp was settled in only two minutes.
Final MOSFET face temperature = 52.3*C

Heat Sink Considerations
The MOSFET was mounted in grease directly to an Aluminum Plate, being 125mm x 50mm. No insulator was used. This is in keeping with the OEM test method. Direct aeration was provided by a 60mm x 60mm, 12vdc fan. (I originally reported thatIthe fan is an 80mm unit; I was wrong.)

Mathematical Extrapolations
Voltage Extrapolation Factor (The Design Goal is 30V/2A)
30.0V/13.82V = 2.17

MOSFET Rise In Temperature
52.3*C – 24.0*C = 28.3*C Rise

Estimated MOSFET Rise at 30V Design Goal
28.3*C x 2.17VEF = 61.4*C

Estimated MOSFET Temperature at 30V/2A Design Goal
61.4*C + 24.0*C = 85.4*C

Conclusion
I estimate the Single-MOSFET is capable, thermally, of the design goal, and possibly more, with a proper heat sink and cooling. It would be good to do operational testing, at full power, but I will have to think on how to configure that much wattage.

Merry Christmas!

I wanted to push the test rig to the MOSFET heat limit, to see what the wattage might be. That is to say that the design goal (30v @ 2a,) expressed as wattage is 60w. But, I only have a ~14vdc/55a DUT supply. So, I could load the test rig at 14v @ 4a.

I upgraded the on-board fuse to 4a and ran the test. The rig performed as before, to the 14v @ 2a point, with some variations... Mainly, that for this similar supplied wattage (~14v @ 2a,) the temperature reading rose to 62.0*C. I attribute this to several things:
- Much larger DUT supply cables were used, which significantly reduced the resistance/heat loss in the cables.
- The heat sink had to be re-seated, in order to change the fuse.
- The thermocoupler contacted the MOSFET's face in a different location.

The most curious result followed, when I attempted to advance the test above the prior range. There was only a small amount of throw left in the pot turn... maybe 1/8-1/4 turn... This put the amperage just over 2a. The temperature only rose to 67.1*C.

I am at a bit of a loss. The DUT supply can supply 55a. So, shouldn't the throw of the pot span this whole amount? I will put the MOSFET onto the oscilloscope, to see what is happening, but, presently, I do not understand. Your thoughts?

[t1d]

Additional 4 amp testing…
I put the scope on the MOSFET input pin.

DC Coupled - x10 - 5V/D - 5uS/D
The first-on pin voltage is small and does vary. But, it jumps to 3.5v, while still sinking 0.00a. Then, the amperage varies readily. The pin voltage was 5.0v @ >2+a (232.7mV [uncalibrated.])

AC Coupled - x10 - 0.2V/D - 5uS/D
At 1/3a, a very pronounced asymmetrical sigh wave, having at least one reflection, develops. Its duration/repetition is 20uS. The wave is erratic, in its form, below 1/3a. It sifts upward, as the pot is advanced.

An explanation of what might be happening would be great. Other thoughts and suggestions are welcome.

[mk_]

show pictures...

[t1d]

show pictures...

Sorry, not available, as explained in the thread... I rather imagine the AC wave is normal mains hum... Your thoughts?

[mk_]

show pictures...

Sorry, not available, as explained in the thread... I rather imagine the AC wave is normal mains hum... Your thoughts?

Sorry, I didn`t read the whole thread to look why there are no pictures.

My thoughts? I`m bad in imagination of "writen" pictures - specialy when it comes to debug a circuit were the last schematic is 3 months old (date-field)  and shown 2 months ago... .

Mains hum can`t be seen on a 5us/D so something is oscillating.

[t1d]

Mk, I forgot to welcome you and thank you for joining in. I can use all of the help I can get!

I`m bad in imagination of "writen" pictures -

I will be joking here... I do not have a wife... But, I do have a budget... So, just for you, I will (really) be buying (today, r-e-a-l-l-y) that new oscilloscope that I have been wanting - I mean needing - And, blaming - I mean justifying - it on you. Actually, I was given some Christmas love/$ for just such a purpose.

specialy when it comes to debug a circuit were the last schematic is 3 months old (date-field)  and shown 2 months ago... .

The greater a person's age, the longer it will take to complete

Mains hum can`t be seen on a 5us/D so something is oscillating.

Ahh, this is a great clue. Can you give me some ideas of what to look for and how? Given that I find what you suspect, what cure do you want me to try? Please and thank you. I suspect that the (poor) test board layout hasn't helped. The Two-MOSFET board layout should be better...

I have offered to share the PCB board. As far as getting true test results, someone with real technological knowledge needs to document the circuit's performance. If you would like a board, please send me a pm.