Author Topic: IGBT vs MOSFET for dummy load  (Read 4336 times)

0 Members and 1 Guest are viewing this topic.

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #50 on: September 24, 2018, 10:29:10 am »
It's going to be used to test small DC power supplies and battery packs so it doesn't need crazy high frequency response ... even a few kilohertz would be sufficient. as I mentioned before my maximum ratings are 50 votls/20 amps/ 200 watts (maybe 250 to 300 watts if I can find a suitable cooling solution).

There isn't yet an exact "topology". I'll be using an ESP 32 so I can have wifi functionality and write testing software on my PC. there would be a mosfet with a 16 bit DAC connected to it's gate to get a really good resolution over the entire range (~1 mA) but the accuracy would be questionable so I have to test the circuit in action.
I'll also be using a "common shunt" with a 24 bit ADC(I don't know why ADCs are so much cheaper than DACs but I won't complain lol), but any other FET/IGBT/BJT would have it's own shunt connected to a much lower resolution (10 bit) ADC. This will allow for a course current adjustment on the parts that take up the majority of the load, but allows a fine adjust with the one FET. This entire project should cost less than 70 bucks and would be a pretty killer device for the average hobbyist.

so far I'm really liking the ADS1219 but haven't decided on the DAC. I also don't know which device is best for linear application ... BJTs should be the cheapest and most robust but I still haven't figured out a circuit diagram that works with them :D

There would be temperature monitoring for each individual part so thermal runaways are not a possibility.
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #51 on: September 24, 2018, 01:10:20 pm »
I watched a few teardowns of the commercially available DC loads ... the BK Precision 8500 uses IRFP250N as the power mosfet. checking the datasheet, there's no SOA mentioned which makes everyone jump to the conclusion that they're not rated for linear applications  ???

Obviously BK Precision doesn't just use random parts and have done their own testing to make sure that this mosfet meets their requirement. How can somebody know if the part is suitably rated without destructive testing?
The more I dig into this subject the more confused I am as to which part performs the best for a given price budget ...
 

Offline MiDi

  • Regular Contributor
  • *
  • Posts: 173
  • Country: de
Re: IGBT vs MOSFET for dummy load
« Reply #52 on: September 24, 2018, 04:07:15 pm »
what topology do you use for extremely high frequency dynamic loads? whats the road map look like?

What is extremely high frequency 1MHz, 100MHz, 10GHz?
If you go into the MHz your wirering begins to behave as transmission lines with impedances that are frequency dependant and a lot higher than dc resistance.
There it starts to become a whole different design.

I watched a few teardowns of the commercially available DC loads ... the BK Precision 8500 uses IRFP250N as the power mosfet. checking the datasheet, there's no SOA mentioned which makes everyone jump to the conclusion that they're not rated for linear applications  ???

Obviously BK Precision doesn't just use random parts and have done their own testing to make sure that this mosfet meets their requirement. How can somebody know if the part is suitably rated without destructive testing?
The more I dig into this subject the more confused I am as to which part performs the best for a given price budget ...

I stumpled over this too.
You can bet a dollar they tested this and perhaps they have a test method to sort them.
If the part is not rated for linear mode this does not exclude it per se, the manufacturer did not test nor garanties a SOA at DC.
And as you can see, there are quite an amount of them in parallel that would suggest a higher power rating...

There are few modern mosfets with linear mode SOA in datasheet and they all suffer from not giving SOAs for other die temperatures than 25°C.
There is one exception I know: IXYS has a series of linear rated MOSFETS that give another SOA for more convenient 75°C (I hopefully remind correctly).
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11859
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: IGBT vs MOSFET for dummy load
« Reply #53 on: September 24, 2018, 04:40:10 pm »
I watched a few teardowns of the commercially available DC loads ... the BK Precision 8500 uses IRFP250N as the power mosfet. checking the datasheet, there's no SOA mentioned which makes everyone jump to the conclusion that they're not rated for linear applications  ???

It's going to be hard to beat in cost.

There's nothing else you need to shop for.  Only power dissipation, with a full SOA, and cost.

FWIW, FQA9N90C is one of the best deals I've seen among high voltage FETs.  Offhand, it's probably going to be hard finding anything with as big of a die (power rating correlates with die area) for as cheap.

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline TurboTom

  • Frequent Contributor
  • **
  • Posts: 437
  • Country: de
Re: IGBT vs MOSFET for dummy load
« Reply #54 on: September 24, 2018, 05:10:18 pm »
I watched a few teardowns of the commercially available DC loads ... the BK Precision 8500 uses IRFP250N as the power mosfet. checking the datasheet, there's no SOA mentioned which makes everyone jump to the conclusion that they're not rated for linear applications  ???

Obviously BK Precision doesn't just use random parts and have done their own testing to make sure that this mosfet meets their requirement. How can somebody know if the part is suitably rated without destructive testing?
The more I dig into this subject the more confused I am as to which part performs the best for a given price budget ...

Fairchild publishes DC SOA specs for their IRFP250.
 

Offline Le_Bassiste

  • Regular Contributor
  • *
  • Posts: 114
  • Country: de
Re: IGBT vs MOSFET for dummy load
« Reply #55 on: September 24, 2018, 05:28:08 pm »
Using a vacuum tube might still be attractive at high voltage, as they tend to fail open - while semiconductors tend to fail short. However the minimum working voltage is rather limited.



...
What about oldy BU208 and BU508 transistors? Linear 1500V TO3 package
The BU208 / BU508 are not that good SOA wise.  The right MOSFET types (usually higher voltage types even for just 30 V use) are still the better choice at higher voltages. The BJTs (especially audio types) might be a good choice up to about 50 V, maybe 100 V)

never said that. please quote me correctly next time.  ;D
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #56 on: September 24, 2018, 08:11:27 pm »
Fairchild publishes DC SOA specs for their IRFP250.

I had a look ... they have a tiny SOA ... no wonder they used 10 of them for a 250 watt unit!

I know that the die area and power dissipation are correlated, which is exactly why I'll be using 2 devices under 1 heat sink. this will spread out the heat and allow them to run cooler for the same amount of load.

I looked at the IGBTs again and man ... they have ridiculously low thermal resistance!
FGH50T65SQD-F155 is rated at 0.56 degrees C/W ... that means a delta of only 28 degrees for my application under full load!(50 watts per device).
Any generic 95 watt CPU cooler should be able to handle them while keeping them under 70c! that's just crazy!

for some reason the mosfets that I'm finding are in the neighborhood of 1 to 1.5 degrees C/W which means 2 to 3 times higher delta values!

I'll order a few of these bad boys for testing. I'll publish the results later  ;)
 

Offline mzzj

  • Frequent Contributor
  • **
  • Posts: 707
  • Country: fi
Re: IGBT vs MOSFET for dummy load
« Reply #57 on: September 24, 2018, 08:34:58 pm »
Fairchild publishes DC SOA specs for their IRFP250.

I had a look ... they have a tiny SOA ... no wonder they used 10 of them for a 250 watt unit!

I know that the die area and power dissipation are correlated, which is exactly why I'll be using 2 devices under 1 heat sink. this will spread out the heat and allow them to run cooler for the same amount of load.

I looked at the IGBTs again and man ... they have ridiculously low thermal resistance!
FGH50T65SQD-F155 is rated at 0.56 degrees C/W ... that means a delta of only 28 degrees for my application under full load!(50 watts per device).
Any generic 95 watt CPU cooler should be able to handle them while keeping them under 70c! that's just crazy!

for some reason the mosfets that I'm finding are in the neighborhood of 1 to 1.5 degrees C/W which means 2 to 3 times higher delta values!

I'll order a few of these bad boys for testing. I'll publish the results later  ;)
FQA9N90C mentioned by Te51acOi1 has 0.45 C/W

Old generation high voltage parts with large dies are winner here.
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #58 on: September 24, 2018, 08:40:03 pm »
you are correct, but look at the current rating! only 9 amps maximum!
If I want to make a large number of these products later (I intend to sell them if they meet my expectations), I can still have the same voltage and current rating, but just drop the power rating to match the cooling.In any case I'll buy a few of those as well. they seem like quite a nice option as well.
 

Offline David Hess

  • Super Contributor
  • ***
  • Posts: 7922
  • Country: us
  • DavidH
Re: IGBT vs MOSFET for dummy load
« Reply #59 on: September 24, 2018, 11:05:14 pm »
what topology do you use for extremely high frequency dynamic loads? whats the road map look like?

The next step up in dynamic performance uses a cascode output with a low voltage transistor driving the emitter/source of a high voltage transistor.  This isolates the reverse transfer capacitance of the output transistor yielding better high frequency performance.
 

Online Hydron

  • Frequent Contributor
  • **
  • Posts: 334
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #60 on: September 25, 2018, 12:45:01 am »
Fairchild publishes DC SOA specs for their IRFP250.

I had a look ... they have a tiny SOA ... no wonder they used 10 of them for a 250 watt unit!

I know that the die area and power dissipation are correlated, which is exactly why I'll be using 2 devices under 1 heat sink. this will spread out the heat and allow them to run cooler for the same amount of load.

I looked at the IGBTs again and man ... they have ridiculously low thermal resistance!
FGH50T65SQD-F155 is rated at 0.56 degrees C/W ... that means a delta of only 28 degrees for my application under full load!(50 watts per device).
Any generic 95 watt CPU cooler should be able to handle them while keeping them under 70c! that's just crazy!

for some reason the mosfets that I'm finding are in the neighborhood of 1 to 1.5 degrees C/W which means 2 to 3 times higher delta values!

I'll order a few of these bad boys for testing. I'll publish the results later  ;)

If you're playing the low thermal-resistance game then check out some of the other Fairchild parts, e.g.:
http://www.onsemi.com/PowerSolutions/product.do?id=FGA60N65SMD (0.25 K/W J-C)
http://www.onsemi.com/PowerSolutions/product.do?id=FGY75N60SMD (0.2 K/W)
(also see FG60N60SMD for another case variant of the first one, and there's some others in the series as well, probably including cheaper diode-less versions)

These have thin (75 micron IIRC) dies with fairly large area to give those numbers. Note that while these _do_ give a DC SOA, they also specify that it's a "single pulse" (i.e. not really DC after all). Cheap though, so feel free to test one out to see if it lets out the magic DC linear region smoke.
I can confirm that they can take some abuse in switching though - these parts are well loved in the solid state tesla coiling scene (I've had the 75N60 hard switching >200A @ 350kHz, albeit at a low duty cycle)!

Edit:
Remember that a sil-pad or similar insulation could easily double the thermal resistance when dealing with stuff this low. In my use I've got isolated heatsinks to avoid having anything other than thermal grease in the way.
Also massive "brick" style IGBTs and power BJTs are available as surplus for reasonably prices. These will have rated dissipation in the kW region and are easy to mount with inbuilt isolation, but again, linear DC operation is _not_ what they are designed for, and the use of multiple dies internally will probably make any hotspot issues even worse.
« Last Edit: September 25, 2018, 12:50:40 am by Hydron »
 

Offline coppercone2

  • Super Contributor
  • ***
  • Posts: 1393
  • Country: zw
  • 🚫_🚫
Re: IGBT vs MOSFET for dummy load
« Reply #61 on: September 25, 2018, 12:47:41 am »
do they ever use more advanced current mirrors and stuff like that? as inefficient was it would be
« Last Edit: September 25, 2018, 12:50:04 am by coppercone2 »
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #62 on: September 25, 2018, 02:14:02 am »
The only devices that are connected to each heat sink, would be the IBGT or MOSFETs which will have a common drain anyways. No need to use a pad :) just some high quality thermal paste. I have a lot leftover after I changed my laptop and xbox thermal paste. I think it's 4 watt per meter kelvin  :P absolutely no issues there.
 

Offline drussell

  • Frequent Contributor
  • **
  • Posts: 979
  • Country: ca
  • Hardcore Geek
Re: IGBT vs MOSFET for dummy load
« Reply #63 on: September 25, 2018, 02:39:49 am »
The only devices that are connected to each heat sink, would be the IBGT or MOSFETs which will have a common drain anyways. No need to use a pad :)

I thought we were talking high enough voltage that it needed devices in series, or is that two different conversations going on in here that I missed something?  :)
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #64 on: September 25, 2018, 02:49:55 am »
The only devices that are connected to each heat sink, would be the IBGT or MOSFETs which will have a common drain anyways. No need to use a pad :)

I thought we were talking high enough voltage that it needed devices in series, or is that two different conversations going on in here that I missed something?  :)

Yeah  ;D there are two different conversations going on in here. I just mentioned that IGBTs usually have higher voltage ratings for similar price at the same current capability. however, I will be using multiple devices in parallel to do load balancing and increase the overall die area which helps with cooling.
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 4752
  • Country: de
Re: IGBT vs MOSFET for dummy load
« Reply #65 on: September 25, 2018, 02:54:28 am »
Having the heat sink connected to the output terminal can be an issue, as it requires the heat sink to be well insulated from the case and thus well inside. It depends on the overall case if this is an option or not.  The other point is the option of using individual fuses for the MOSFETs - these would normally be on the drain side.

Due to the SOA limitations one usually can only use a small fraction of the maximum P_tot. Thus the mounting on the heat sink is normally not that critical. I would consider something like 20 W for a TO220 case and 47 W for a TO247 case as a practical limit. In an electronic load there is no real practical protection to a higher than recommended voltage - thus for robustness its best to have some reserve, so that even a load intended for 50 V could withstand something like 100 V or 200 V at least for a short time until protection (turn off) can kick in. Another troublesome case is a large series inductor and thus possibly some energy to dump in avalanche mode as a result of an attempted fast turn off.

At least those old type 400-600 V MOSFETs are not that expensive. One might need a few more to get a low minimum drop and high current at low voltage anyway. The limited current rating is usually not an issue, unless one needs an electronic load for very low voltage use (e.g. single cell battery dicharge / testing or CPU simulator), it is more the SOA at higher voltage and R_on al low voltage that is limiting.

IGBTs usually make no sense for low voltage and also not for high power dissipation. So the IGBT is not a good idea  :horse::horse:,....
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #66 on: September 25, 2018, 05:31:29 am »
If you read my earlier post, I intend to use high precision parts as well as a mosfet to gain that accuracy at lower voltages and a very precise current over the full 20A range as well as IGBTs to handle the majority of the load
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 4752
  • Country: de
Re: IGBT vs MOSFET for dummy load
« Reply #67 on: September 25, 2018, 05:48:06 am »
If you read my earlier post, I intend to use high precision parts as well as a mosfet to gain that accuracy at lower voltages and a very precise current over the full 20A range as well as IGBTs to handle the majority of the load
Please read the other posts too, and forget about the IGBTs idea - essentially all of them are not suitable in the normal linear operating electronic load.

One can likely get away with just MOSFETs in the power stage. If needed it may help to have different size circuits for low and high currents, so one does not need to use the 20 A capably circuit for less than some 100 mA.  A slight limitation of the normal MOSFET circuit would be there gate current, but this is usually a really tiny current.

For high precision the more difficult part would likely be the choice of the right shunt resistor - expect to spend more on the resistors than on semiconductors.
 

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11859
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: IGBT vs MOSFET for dummy load
« Reply #68 on: September 25, 2018, 07:19:36 am »
you are correct, but look at the current rating! only 9 amps maximum!

Well yeah, that was an example.  Mine is higher voltage than yours --



Read it as encouragement, to go shopping and find something effective and affordable, and at your power level I'd consider TO-220 as well as TO-3P and TO-247. :)

Tim
« Last Edit: September 25, 2018, 07:24:07 am by T3sl4co1l »
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline coppercone2

  • Super Contributor
  • ***
  • Posts: 1393
  • Country: zw
  • 🚫_🚫
Re: IGBT vs MOSFET for dummy load
« Reply #69 on: September 25, 2018, 08:20:32 am »
you should put 10 turn pots on that if your not selling it

also grounded chassis or plastic knobs
« Last Edit: September 25, 2018, 08:22:41 am by coppercone2 »
 

Offline OM222O

  • Regular Contributor
  • *
  • Posts: 117
  • Country: gb
Re: IGBT vs MOSFET for dummy load
« Reply #70 on: September 25, 2018, 09:44:08 am »
I'm mostly looking at TO 247 packages because they have more metallic surface area which has the least thermal resistance and they are physically larger too which means they're easier to cool. The power circuitry would be just a switch mode PSU (like the laptop chargers) to provide 12 volts input which is then split to other rails using LDOs. At these sort of voltages grounding the body doesn't really matter. even if the load line somehow shorts out to the body, it would be 50 volts max (voltage sense circuit will cut the current if over voltage is detected) so it's pretty safe. You can't even feel 50 volts DC for the most part. Honestly the exact value of current shunts doesn't matter at all. I have access to high precision tools and can measure each one and calibrate for them in the software. I just need high temperature stability and I'll make sure to put them in an area that they receive air from the cooler blowing down on them.

I ordered the IGBTs so it's too late to not try them out lol. I honestly don't see why people are so skeptical about using them, given that they have guaranteed DC SOA in the data sheet. The manufacturer doesn't warn that they're not intended for use in their linear region anywhere on the datasheet as far as I can tell  ???

Anyways, I'll update you when I get the parts and do my testing on them. I ordered a few extras for the destructive testing purposes just to see how far I can push them with the cooling solution that I have and what external case temperatures might be dangerous as there is no way for me to directly read the junction temperature.

Just to make sure I also emailed ON semi technical support and described my applications and specifications and asked if there would be any problems running them in their linear region. If I get any answers, I will post them here.
« Last Edit: September 25, 2018, 10:03:38 am by OM222O »
 

Offline coppercone2

  • Super Contributor
  • ***
  • Posts: 1393
  • Country: zw
  • 🚫_🚫
Re: IGBT vs MOSFET for dummy load
« Reply #71 on: September 25, 2018, 10:20:48 am »
well some IGBT require a negative voltage or at least recommend a negative voltage for switching off.

If it has a DC soa then it seems fine, unless there is something weird about IGBT sinking transient/AC while biased or bias change while under load.
 

Offline boB

  • Regular Contributor
  • *
  • Posts: 71
  • Country: us
    • my work www
Re: IGBT vs MOSFET for dummy load
« Reply #72 on: September 25, 2018, 02:56:37 pm »
you are correct, but look at the current rating! only 9 amps maximum!

Well yeah, that was an example.  Mine is higher voltage than yours --



Read it as encouragement, to go shopping and find something effective and affordable, and at your power level I'd consider TO-220 as well as TO-3P and TO-247. :)

Tim


Where's the heat sinks, Tim ?   One per transistor ?

boB
K7IQ
 

Offline BravoV

  • Super Contributor
  • ***
  • Posts: 5410
  • Country: 00
Re: IGBT vs MOSFET for dummy load
« Reply #73 on: September 25, 2018, 03:19:08 pm »
I'm mostly looking at TO 247 packages because they have more metallic surface area which has the least thermal resistance and they are physically larger too which means they're easier to cool.

Ever consider SOT-227 body ?

My IXYS IXTN46N50L 500V VDSS 46A FET still sitting idle as I don't have time to build mine yet, planned to use a desktop oc-ing cpu heatsink.



 
The following users thanked this post: thm_w

Offline T3sl4co1l

  • Super Contributor
  • ***
  • Posts: 11859
  • Country: us
  • Expert, Analog Electronics, PCB Layout, EMC
    • Seven Transistor Labs
Re: IGBT vs MOSFET for dummy load
« Reply #74 on: September 25, 2018, 07:33:59 pm »

Where's the heat sinks, Tim ?   One per transistor ?

boB

Those are the switching channels (using same transistor for convenience, though a much smaller type would ideally be used), the linear ones are on the back row.  I put together a bigass heatsink for them, two heatsinks strapped together, supported on fiberglass insulators and monitored with a thermistor. :)

Ed: this will make more sense: https://www.seventransistorlabs.com/Images/ActiveLoad2.jpg

Tim
« Last Edit: September 25, 2018, 07:36:48 pm by T3sl4co1l »
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 
The following users thanked this post: boB


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf