Electronic Load Project - TLV171 & IRFP250 with KiCad Files

[t1d]

Just sharing, here..

I took a current reading, of the circuit's draw at 5v/1a = 0.0199a. This was an ac reading and my first (ac reading.) I have a Brymen 869s DMM and it does all the ac computations (very accurately.)

I still want to put the PCB in the case, for the advanage of the powerful, 120v fan, and run the wattage up. I will check the draw, at the upper limit, too.

[Kleinstein]

There usually is no need for a high power fan. The worst case power per MOSFET is around 60-100W - so a usual computer fan (e.g. 120x120 mm² and usually around 1-5 W) should be large enough.

The Electronic load circuit is usually a rather simple circuit, so that for a small version with 1 or 2 MOSFETs a prototype board can be sufficient. It is not that fast that short distances are really important. The OPA172 is relatively fast, but only chosen because of relatively low noise (and single supply), not for speed. The other end would be the classic OP 07 here the speed may be at the low end, but it can still work well.

For the shunts some extra distance to the MOSFETs can help to keep the temperature low.  If done right, the extra trace resistance kind of adds to the R_On of the MOSFET, but should not directly cause an error. The point is to keep the trace from the shunt to ground short and low resistance - here some errors might come in, though there are ways too to reduce this error.

The shunt can be a costly part. This is the reason why one might consider a version with only 1 good shunt for the main control loop and 2 auxiliary resistors responsible for current sharing only.  However with just 2 MOSFETs I tend stick to the simple case with 2 shunts.
The shunt used in the first version is reasonable choice for a low cost version - not really instrument grade, but definitely OK for a DIY version with limited accuracy (the OPA172 is also not very precise). The higher power rating can compensate the not so great TC to some extend.

[t1d]

Thanks, Kleinstein. I will spread out the components and run the new layout by you.

[t1d]

I have made some good progress, on both sides of the project... Single-Fet testing and Double-Fet layout development.

I decided to not install the test board, in the permanent case. I took Kleinstein's advice and just attached a 12v, 80mm x 80mm fan, to the test jig.

The board rectifies the fan supply, to dc. But, it does not regulate it. So, I needed to add the regulator. I bodged one up, out of used parts. The total draw, of the fan circuit that I added, after the rectifier and before the regulator, at a 5v/1a load, is 0.1105a. When added to the prior complete circuit test, at the 5v/1a load, the total draw should be 0.1304a.

I am considering adding space, for a fan regulator, on the board. Now that I have increased the size of the Double-FET board, I should be able to work it in.

According, again, to Kleinstein's suggestion (This guy knows so much!) I spread out the Double-FET board, to the full size, of a 100mm x 100mm, flat-rate board. I also changed the shunt resistors to the single-per-MOSFET HSA10R10J-0.1R-10w, as used on the Single-FET test board.

I am pleased with the layout. If Kleinstein gives the thumbs-up, the board will nearly be ready to order, after fulling testing the test rig.

[t1d]

I am about to push the load, to find the test rigs upper capacity. I propose to find that limit, by watching the heat level, of the MOSFET. IIRC, it is good, to 125*C. So, maybe 100*C would be an appropriate safety margin.

Is using the MOSFET heat level a reasonable means to finding the rigs abilities? Of course, looking at the signal, at that level, too. I would think that observing how that much heat effects the rest of the board would be important, also.

[t1d]

I arranged a nice setup, to do the load/temperature testing. However, getting the thermocoupler wire to keep contact with the face of the MOSFET was a bit fiddly...

My top sunken wattage was just 5v x 2.0a = 10 watts @ 34.7*C/94.46*F. The fan cooled very well.

I was not able to go further, because my PSU started to go into constant current mode, at 2.14a. I am sure that I can put my two power supplies in series, to come up with more wattage. But, I doubt, even with the two of them, that I will be able to get up to the top limit the e-load can handle. I would think that the second unit could only provide 10 watts, similarly to the first... That would only make 20 watts. Any suggestions on how to safely get more wattage would be appreciated.

[Kleinstein]

Testing the limits is rather difficult, as this test tends to be destructive. At least it gets rather difficult to find the edge of the SOA curve. In addition there is quite some scattering between the parts. So some may be good for 150 W and other may fail at 80 W.

The temperature measurement is more like a test for the fan and heat sink. So it still makes sense, but this would be more for the safe  design limit. Here I would consider something like 70-80 C the upper limit for the MOSFETs.

With forces air cooling, the temperature rise is about proportional to power. So one can do the test with less power. The upper limit is not such a well defined limit anyway, as there is quite some safety margin included. It is only the check of the SOA at higher voltage that should be done with the full power - as this is for checking for a possible faulty, weak MOSFET.

[spec]

+ t1d

Just been reading this thread with interest.

Maybe I missed it, but can I ask what your design objectives are for the load.
What is the maximum load voltage?
What is the maximum drain current through each IRFP250N?
What is the maximum power dissipation of each IRFP250N?
When the IRFP250N is dissipating maximum power what is the maximum VCE of the IRFP250N? 













;

[t1d]

Welcome, Spec! I am glad you have joined in; I can use all the help I can get. lol.

I am hopeful that
- the Single-MOSFET test board will handle 30v/2a = 60w and
- the Two-MOSFET final model 30v/4a = 120w.

The original circuit was developed by Jay-Diddy-B. His thread link is in a prior post. It is easily adaptable to your choice of Op Amps and MOSFETs. It is also easily scaled up, to whatever capacity you desire.

As for your other questions, here are the Data sheets, in two posts, due to the file sizes. I am using the International Rectifier brand MOSFETs. But, the Vishay DS has additional information.

The MOSFET maximum voltage and amperage will be in AC. Those numbers must be significantly derated, for DC operations. IIRC, the rule of thumb is 1/3 of the AC specifications. I am in the process of doing power-on testing, to find the SOA that I am comfortable stating as a specification.

If you would like a test board, to build one out, just send me a pm.

Thanks, for your interest.

[t1d]

Vishay Data Sheet... Even zipped, it is too big to post. So, you can find it, at Mouser.com. Sorry...

[spec]

That's very kind of you t1d 

Thanks for the information and the offer of a board.

I would love to take up your offer, but I do not have a lab at the moment: temporary house while refurbishing target house.

[t1d]

Oooo... No lab... That's gotta hurt... Maybe look for a local club, like MakerSpace...

Just let me know, when you are ready for a board...

[spec]

  That's an idea, but I need all my gear: scope, solder station, microscope etc to be permanently set up on a bench.

Also, as I am doing the work on the target house, don't have much time, or energy for other activities- yaking on EEV doesn't take much physical effort and keeps the brain ticking over.

You asked for comments about the design. Does that include areas that may be a problem?

Have you got a part number for the heatsinks?

[t1d]

That's an idea, but I need all my gear: scope, solder station, microscope etc to be permanently set up on a bench.

Well, that was sort my point... They have, often, lots of great equipment. The groups are formed to get good equipment and share it. So, maybe all your goodies can stay packed.

You asked for comments about the design. Does that include areas that may be a problem?

Absolutely. That's why I posted the project... More heads are better than one. But, at this point, hopefully, I have weeded through the majority of the design problems and any remaining are due to required compromises. And, the test rig board is not laid out as well as the final Two-MOSFET board. So, layout comments might best be aimed at that board.

Have you got a part number for the heatsinks?

No, because my final heat sink is from a donor big, old school, rack-mounted amplifier. And, I have a 120v fan, mounted on the case. So, for me, I think I am covered. Other users can determine what they need.

The test rig heat sink is just from a scavenged TV. I added a donor 12v fan, to it. I did not use any particular specifications, because the rig was really just to determine if the circuit would be operational, that is, mostly, that it would not oscillate.

The test rig is working so well that I decided to push it, to see what it might do. So far, I am pleased.

I needed more wattage, to run the tests up, farther. So, I just did some research on paralleling PSUs. Here's the thread link, for that.
https://www.eevblog.com/forum/beginners/how-to-connect-two-psus-to-get-more-current/

Thanks!

[Kleinstein]

The simple electronic load circuit has a few problems. Some of these points also applies to some of the commercial versions:

One it that it is very difficult to protect from too high a voltage applied. So overload is a possible problem. Turning down the current, once the voltage is too high is a partial solution. Turning down the current fast on an inductive source could cause even higher voltage.

When de voltage gets too low, the regulator can go into saturation. If than the external voltage rises, it takes some time to limit the current.
Under certain conditions the saturation might also lead to a kind of oscillation, when just at the boundary to saturation, especially with 2 MOSFETs.

[t1d]

The simple electronic load circuit has a few problems. Some of these points also applies to some of the commercial versions:

One it that it is very difficult to protect from too high a voltage applied. So overload is a possible problem. Turning down the current, once the voltage is too high is a partial solution. Turning down the current fast on an inductive source could cause even higher voltage.

When de voltage gets too low, the regulator can go into saturation. If than the external voltage rises, it takes some time to limit the current.
Under certain conditions the saturation might also lead to a kind of oscillation, when just at the boundary to saturation, especially with 2 MOSFETs.

Good stuff, good sir.

You will recall that I made a noob mistake, when first testing the unit, by starving it. This caused the MOSFET to go into saturation and sent me on a goose chase, to find a problem that didn't exist...

[spec]

+ t1d

You asked for comments about the design. Does that include areas that may be a problem?

Absolutely. That's why I posted the project... More heads are better than one.

That is a good positive view and the way I see constructive and valid comments about my designs too. But I always feel a tad guilty about finding problems with other people's work, especially when they have put a lot of effort into them, as you obviously have.

Thanks for defining your target output voltage, current, and power specification. That has been a big help, especially as I thought you were aiming at much higher figures. Just to reiterate:

I am hopeful that
- the Single-MOSFET test board will handle 30v/2a = 60w and
- the Two-MOSFET final model 30v/4a = 120w.

As everyone, no doubt, knows, the trouble with active loads, just like lab power supplies, is that you never know what is going to be connected to them. It's also well known that, ultimately there are two limiting areas, output power transistor maximum junction temperature (TJmax) and safe operating area (SOA).
So out of interest, I had a look at these two areas on your active load.

IRFP250M NMOSFET RELEVANT CHARACTERISTICS
The IRFP250M is a beefy NMOSFET: Case type= TO247, VDSmax= 200V,   IDmax=30A, ThR J/C= 0.7DegC/W, Tjmax= 175DegC. 

TEMPERATURE BUDGET
The total thermal resistance junction to ambient (TTRJA) = TRJC(0.7degC/W) + TRCH(1DegC/W)(mica washer) + TRHSA (3DegC/W) (assumed) =  4.7DdegC/W

The required dissipation = 60W, so the temperature difference, J/ambient = 60 * 4.7 = 282degC

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!

The actual allowable IRFP250M dissipation with your heat sinking arrangement is, (TJmax- Tamb)/TRtot = 105/4.7= 22W, which agrees with the general rule of thumb that you can't safely dissipate more than around 20W in a TO247 (or TO3) case.

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.

SAFE OPERATING AREA (SOA)

As there are no SOA lines for 100ms or DC, it would seem that the IRFP250M is intended mainly for pulse amplification, whereas for an active load the signal will include DC: battery load testing being an example. So, by extrapolation, I have added orange lines for 100ms and DC, as shown on the attached image. But these lines are not necessarily accurate: they are just a guide.

The nice thing about the SOA graph is that it applies at the maximum junction temperature of 175 degC, which is excellent.
The good news is that the SOA for the IRFP250M looks OK for your application- but you already knew that

Apologies for being the bringer of bad news about the junction temperature: I will get my coat.

[t1d]

It's also well known that, ultimately there are two limiting areas, output power transistor maximum junction temperature (TJmax) and safe operating area (SOA).

Good information.

So out of interest, I had a look at these two areas on your active load.
 
TEMPERATURE BUDGET
The total thermal resistance junction to ambient (TTRJA) = TRJC(0.7degC/W) + TRCH(1DegC/W)(mica washer) + TRHSA (3DegC/W) (assumed) =  4.7DdegC/W

Well, you have to remember that I am still significantly not knowledgeable. So, here, I don't know if this is summing different devices, or summing the characteristics, of just the MOSFET.

If the latter, I searched the data sheet for the proper mounting procedures. I did not find any instructions, other than, on Page 2, in the thermal resistance ratings, it reads "Case-to-Sink, Flat, Greased Surface." And, that is how I mounted the MOSFET, that is, in grease, without an isolator. Have I done something wrong?

The required dissipation = 60W, so the temperature difference, J/ambient = 60 * 4.7 = 282degC

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!

The actual allowable IRFP250M dissipation with your heat sinking arrangement is, (TJmax- Tamb)/TRtot = 105/4.7= 22W, which agrees with the general rule of thumb that you can't safely dissipate more than around 20W in a TO247 (or TO3) case.

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 have done some thermal testing. Here is the log.
Soak time, at each wattage, was at least 10 minutes.
The test was not precise. Particularly, keeping the thermocouple positioned on the face of the MOSFET was fiddly.
The temperature gain does not evidence an exponential nature, but I can’t believe that it will be linear, in fact.
I expect the case heat sink and fan to be more efficient, even with both the single and double MOSFET models installed. The heat sink is just plain huge and the fan is 120v.

SAFE OPERATING AREA (SOA)
The good news is that the SOA for the IRFP250M looks OK for your application- but you already knew that

Apologies for being the bringer of bad news about the junction temperature: I will get my coat.

Leave your coat, on the rack. I am off to clarify the definition of “junction temperature” and “Safe Operating Area.”

[t1d]

I am off to clarify the definition of “junction temperature” and “Safe Operating Area.”

Okay, I had those, in mind, mostly correctly.:-)

[spec]

You say that the heatsink is huge and has a powerful fan- that is very good news

The junction temperature rise with power dissipation is linear not exponential. So, for a given total thermal resistance from the junction to ambient, if you double the power dissipation you double the junction temperature.

Working out junction temperature is dead easy.

(don't let the term 'junction' cloud the issue. MOSFETS do not have a junction like BJTs. The term is just historical)

[t1d]

The junction temperature rise with power dissipation is linear not exponential. So, for a given total thermal resistance from the junction to ambient, if you double the power dissipation you double the junction temperature.

Well then, maybe it is okay to put our hope in the actual power-on test sample data; 74.7*C. (See prior post temperature spreadsheet.) I hope so. However, this is at the face of the MOSFET, not its internal temp)...

[t1d]

I fooled around with attaching my two PSUs in parallel, to get more current, to push the e-load. I did not have the exact/proper parts and the load only partially balanced, between the two PSUs. Even in such a state, it appeared to me that the test rig can handle more current. So, woot, for that encouragement...

[Kleinstein]

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.

The SOA curve is a little tricky, as many DS don't show the DC curve.  An old Fairchild DS does show an DC curve. However there are IRFP250 and IRFP250N and the N Version is the newer one with a likely smaller chip and lower SOA. The IRFP250 is also rather similar (better selection and thus higher specified voltage) to the IRFP150 that is used in some older HP linear power supplies.
Still it is a MOSFET that is usually known to work reasonably well in linear mode, but for most manufacturers there is no DC SOA spec. So it's in a area that will likely work, but not tested / specified unless one gets some old non N version from Fairchild.

With not individually tested MOSFETs one would ideally do a possibly destructive SOA check anyway. So make sure the fuse is good (ideally the power source has a reliable limit) and than apply some worst case (depending on how brave you are) relatively high voltage (e.g. 40 V), a little higher than the later limit and do the test at the limiting current. If is does not blow, chances are good it can withstand a somewhat lower voltage / power in the future.

[t1d]

With not individually tested MOSFETs one would ideally do a possibly destructive SOA check anyway. So make sure the fuse is good (ideally the power source has a reliable limit) and than apply some worst case (depending on how brave you are) relatively high voltage (e.g. 40 V), a little higher than the later limit and do the test at the limiting current. If is does not blow, chances are good it can withstand a somewhat lower voltage / power in the future.

Yes, a practical, power-on test is what I was working toward. Thank you, for the confirmation; I feel more confident, now.

I purchased extra parts, for the possibility of failures. I have lots of PCB boards, too, so there are no worries... I hope...

You have not made any comments, in regard to the new Two-MOSFET Board layout. May I assume that you would have spoken up, if you had seen a problem? Post #53

What about adding an isolated copper pour/heat plane, on the top layer, just under the shunt resistors. I wonder if it might absorb as much, or more, heat, from the main heat sink, as it would dissipate? I guess it would not add capacitance, because it is not in the power loop?

Isn't the shunts' aluminum housing adequate. Ahh, the answer is in the Data Sheet, of course, so I will look, there.

[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.