Author Topic: IGBT vs MOSFET for dummy load  (Read 12376 times)

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Offline OM222O

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IGBT vs MOSFET for dummy load
« on: September 21, 2018, 01:55:11 am »
Hello, I was trying to build a dummy load and it seems everyone is recommending to use a mosfet in it's linear region to control the current and therefore dissipate all the heat.
As far as I know, IGBT is basically a mosfet with a pass transistor on it's output. They have way higher voltage and current ratings as well as wider safe operation area, they supposedly react faster than mosfets and in most cases are cheaper than a suitably rated mosfet for a given power target.
Is there a reason that nobody is using them and instead they use mosfets? in which case would it make sense to create the same effect using a discrete mosfet and BJT?
are there any drawbacks to using IGBTs? they seem to be the superior choice. The only think I can think of, is not being able to go below ~0.7 volts which is not an issue for me as even then 1.5v alkaline batteries have a cut off voltage of 0.8 volts. I think they're also easier to cool compared to mosfet as they're designed and manufactured with that voltage drop in mind witch makes them even more suitable in a situation where they're only purpose is to transfer electrical energy to heat?
 

Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #1 on: September 21, 2018, 02:05:05 am »
No linear SOA.

IGBTs are constructed expressly to have the most current density possible, damn anything else -- which means you put any voltage across them and they melt real damn quick!

You could, however, make a switching dummy load, where the input goes through an inductor, the IGBT chops the current through the inductor, and the flyback is dumped into a fixed dummy load, like a resistor or MOV or something.  Or recycled into a source battery, say.

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Offline OM222O

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Re: IGBT vs MOSFET for dummy load
« Reply #2 on: September 21, 2018, 02:22:45 am »
I can't quite understand what you mean ...
The SOA for mosfets isn't perfectly linear either?
and I'm not sure what you mean by:"you put any voltage across them and they melt real damn quick"
As I mentioned, they're rated for much much higher voltages than a mosfet. for example the "IKW15N120BH6XKSA1" is rated for 1200 volts! that seems insane compared to a few hundred volts maximum for a mosfet. I know they're meant for the most current density, which is exactly why I think they're a good candidate for a high power dummy load. I'm trying to make a dynamic electronic load, so the fixed method is not really good at all. I'm planning on using 4 of the chosen device (either IGBT or MOSFET), each one controlled via an op amp and a MCU) so I can spread the heat and check each one's temperature using NTCs to balance current and heat output between the 4. I'll most likely aim for 50 volts, 20 amps, 200 watts and spread the 4 into 2 groups of 2 (again for increased surface area and better cooling), each cooled by a CPU cooler, so each device needs to handle roughly 50 watts which is not that large.

I'm not planning as using this for a current source or anything, specifically testing PSUs and batteries, hence the MCU to allow for data logging. It would be great if you could explain a little more about what you wrote. thanks
« Last Edit: September 21, 2018, 02:24:36 am by OM222O »
 

Offline David Hess

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Re: IGBT vs MOSFET for dummy load
« Reply #3 on: September 21, 2018, 05:10:33 am »
Toshiba used to make IGBTs for linear service in audio power amplifiers but that was years ago and only up to a couple hundred volts.  They even made complementary pairs!

IGBT safe operating area is not any better than that of MOSFETs.  If they are not showing it in the datasheet, then it is because they are not intended for linear operation; it is not square or even flat.  Even worse they use minority carriers like bipolar transistors which allows for a much smaller die (lower cost) for a given current and voltage, especially voltage, but this also results in a lower power rating because of the small die size.

The big advantage of IGBTs over MOSFETs is that at high voltages, MOSFET die size is proportional to the square of the voltage.  IGBTs like bipolar transistors rely on minority carriers so they so not suffer from this limitation and can have a much smaller die size.  Bipolar transistor are more economical though.

If you are building big 1000 volt electronic loads, then you do not need my help.  You will require lots of devices in series and parallel to distribute the power and handle the worst case safe operating area no matter what power pass element you use.  Those big linear MOSFETs from IXYS start to look like a deal if they cut the number of pass devices down.
 

Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #4 on: September 21, 2018, 09:56:11 am »
"No linear SOA" as in, no DC (linear operating mode) SOA curve at all, or if there is one (I forget if I've ever seen a DC curve on an IGBT), it is tiny, a small fraction of the total power rating.

MOSFETs are available up to 4500V -- with pretty reasonable ratings at that, too* -- I'm not sure where you're finding "a few hundred volts maximum".  :-//

*I used to observe that, heater power aside, vacuum tubes still held their own in this one domain: at high voltages and modest to very high powers.  For example, a 6LQ6 horizontal output tube might have a saturated plate "resistance" of 70 ohms, and a peak voltage rating of about 7kV (strictly under cutoff conditions, mind).  The heater and screen power requirements make overall efficiency rather worse of course (about 20W between them, plus a plate dissipation rating of 30W), nevermind the requirement for two extra supplies (heater and screen), plus a fairly large grid voltage (about -200V is recommended in peak cutoff).

Over the last 5 years or so, MOSFETs have been released with ratings around 2500V 10A and 4500V 2A (30 ohm?), completely removing any claim tubes might've still held in this domain. :)

(Vacuum tubes for high frequency (klystrons, TWTs, etc.), high power (some industrial and radio finals; magnetrons, etc.), and various physics and research applications, still dominate those fields.  Of those, TWTs are probably the next to fall: they've been flying on satellites for literally as long as satellites have been flown in space, and are extremely well understood, reliable and still perform quite well despite their old fashioned nature.  However, microwave transistors -- Si LDMOS, GaAsFET, PHEMTs and GaN FETs -- have been in commercial use for many years now, and it's my understanding that some are finally getting rad-hard and space-ready approval..?)

Ahh... I digress.  Fun stuff.

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Offline OM222O

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Re: IGBT vs MOSFET for dummy load
« Reply #5 on: September 21, 2018, 10:16:55 am »
This IGBT is fairly cheap and seems fit for the job. It has a SOA chart unlike what previously has been said, and given the numbers I provided, it seems like a fairly decent choice.

https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/FGH50T65SQD-F155?qs=sGAEpiMZZMv4z0HnGdrLjmAnkg%2f6XuvzWCGzpW%2fDvWZ3y7weEXwbsA%3d%3d


Can you please confirm I'm not making a mistake in reading the datasheet? It has way better SOA for similarly priced mosfets.
(Please keep in mind I'm limiting the power of each part to 50 watts, so @ 50 volts 1A would be the current limit and as you decrease the voltage, the current can go up accordingly, so my intended area of operation falls way below the provided SOA)
« Last Edit: September 21, 2018, 10:21:02 am by OM222O »
 
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Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #6 on: September 21, 2018, 12:10:20 pm »
Well, if it's in the SOA, there you go...

I'd test it very carefully to be sure, though.

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Offline rancor

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Re: IGBT vs MOSFET for dummy load
« Reply #7 on: September 21, 2018, 02:21:26 pm »
Don't forget temperature derating. That SOA curve is with a nearly impossible to hit 25C Tcase. What's your heat sink you plan on using?
« Last Edit: September 21, 2018, 02:27:16 pm by rancor »
 

Offline OM222O

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Re: IGBT vs MOSFET for dummy load
« Reply #8 on: September 21, 2018, 05:25:47 pm »
I'll be using cpu coolers rated for 100 watts to cool 2 IGBTs each with a load of 50 watts under the heat sink. I'm expecting tempretures in the high 70s or mid 80s wich I believe still gives me a good enough margin. I'll be dissapating about 5 watts in the shunt resistor under max load which drops the actual IGBT load to 45 watts (so a total of 90). I'll be monitoring temperature of each individual IGBT with an NTC thermistor (10k THT mounted in the hole of the TO-247 packaging which gives accurate enough temperature readings when compensated for the package thermal resistance (which again, in this case is way less than a similarly rated FET). I still didn't get a proper answer tho: given the superior performance in every aspect when compared to a fet, why aren't IGBTs used in every single electronics load? other than SOA no other parameters were brought up?
 
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Offline 001

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Re: IGBT vs MOSFET for dummy load
« Reply #9 on: September 22, 2018, 08:03:25 am »
Why not BJTs?  :-//
 

Offline Le_Bassiste

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Re: IGBT vs MOSFET for dummy load
« Reply #10 on: September 22, 2018, 08:19:44 am »
there are two more aspects icould think of:
1. VCESAT vs. RDSON: if you want to go to really low load voltages (say, 2V and below), an IGBT isn't probably the best choice because of the always present VCESAT. also, datasheet VCESAT is dependent on the actual speed rating of the IGBT: slow IGBTs will reach down to 0.9 V, whereas fast IGBTs may have VCESAT beyond 2V. so, if you don't require too fast transient responses of your load, you would chose a slow IGBT to shave off some of the VCESAT.
2. MOSFET hot-spotting: although RDSON does have a positive tempco, which greatly helps in paralleled _switching_ applications, the VGSth actually has a negative tempco, which can be desastrous in _linear_ applications. a MOSFET itself consists of thousands of paralleled cells on a single MOSFET die. if the MOSFET runs in the VGSth region (as in your application), some of the cells in that sea of cells may get a little hotter than the surrounding ones, which in turn reduces VGSth in that area, which then increases the local drain current, which increases the local temp,... you get the picture. countermeasure is indeed heavy heatsinking, and ample over-dimensioning of the MOSFET. 
idk, IGBTs may have the same effect, perhaps other forum members with more experience on IGBTs can comment on this.

hth





 

Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #11 on: September 22, 2018, 08:25:17 am »
2. MOSFET hot-spotting: although RDSON does have a positive tempco, which greatly helps in paralleled _switching_ applications, the VGSth actually has a negative tempco, which can be desastrous in _linear_ applications. a MOSFET itself consists of thousands of paralleled cells on a single MOSFET die. if the MOSFET runs in the VGSth region (as in your application), some of the cells in that sea of cells may get a little hotter than the surrounding ones, which in turn reduces VGSth in that area, which then increases the local drain current, which increases the local temp,... you get the picture. countermeasure is indeed heavy heatsinking, and ample over-dimensioning of the MOSFET. 
idk, IGBTs may have the same effect, perhaps other forum members with more experience on IGBTs can comment on this.

That's the mechanism, but it doesn't much matter for practical purposes: it's rolled into the SOA.  At least, hopefully, if the manufacturer has done their due diligence...


Why not BJTs?  :-//

Almost as bad as IGBTs.

BJTs for amplifiers are made to avoid hot-spotting and therefore 2nd breakdown.  These aren't available much beyond 250V.  Switching BJTs (rated 1500V or more) are optimized in the opposite way, giving rather small SOAs.

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Offline Le_Bassiste

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Re: IGBT vs MOSFET for dummy load
« Reply #12 on: September 22, 2018, 08:28:32 am »
Why not BJTs?  :-//

just my 0.02:
have a look at the datasheet of one of may favorites:
https://www.digchip.com/datasheets/parts/datasheet/456/BUT30-pdf.php
this is a real unit in current handling capability and power dissipation, so it seems to be a good choice. however, BASE current requirement of several amps and the fact that it is absolutely not up to linear operation (look at the SOA curve: just 2 A IC at 20 V VCE: pathetic!) basically prohibits its use in electronic loads.

 

Offline Le_Bassiste

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Re: IGBT vs MOSFET for dummy load
« Reply #13 on: September 22, 2018, 08:29:41 am »
@: Tim: beat me to it! ;D
 

Offline 001

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Re: IGBT vs MOSFET for dummy load
« Reply #14 on: September 22, 2018, 08:35:05 am »
Why not BJTs?  :-//

just my 0.02:
have a look at the datasheet of one of may favorites:
https://www.digchip.com/datasheets/parts/datasheet/456/BUT30-pdf.php
this is a real unit in current handling capability and power dissipation, so it seems to be a good choice. however, BASE current requirement of several amps and the fact that it is absolutely not up to linear operation (look at the SOA curve: just 2 A IC at 20 V VCE: pathetic!) basically prohibits its use in electronic loads.

What about oldy BU208 and BU508 transistors? Linear 1500V TO3 package
 

Offline Le_Bassiste

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Re: IGBT vs MOSFET for dummy load
« Reply #15 on: September 22, 2018, 08:39:35 am »
dsheeesus, why aren't you just looking up the datasheet numbers?  :-DD
 

Offline 001

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Re: IGBT vs MOSFET for dummy load
« Reply #16 on: September 22, 2018, 08:45:42 am »
dsheeesus, why aren't you just looking up the datasheet numbers?  :-DD

 :-//
 

Offline mzzj

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Re: IGBT vs MOSFET for dummy load
« Reply #17 on: September 22, 2018, 09:00:22 am »
This IGBT is fairly cheap and seems fit for the job. It has a SOA chart unlike what previously has been said, and given the numbers I provided, it seems like a fairly decent choice.

https://www.mouser.co.uk/ProductDetail/ON-Semiconductor-Fairchild/FGH50T65SQD-F155?

That's interesting, most IGBT's don't promise anything for DC SOA.

Fairchild's trench stop technology based IGBT's seem to have DC SOA specification, added benefit of trench stop or just hopefull specification?
 

Offline TurboTom

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Re: IGBT vs MOSFET for dummy load
« Reply #18 on: September 22, 2018, 09:14:45 am »
We once tried to use a real bad-ass IGBT that's been left over form another project in linear mode as a load, far below it's rated maximum dissipation (10kW). Sure enough, no DC SOA data was available. It died at a single-digit percentage of it's rated dissipation in switching operation. So I guess, unless there's proper data available, consider an IGBT as a switch and not as a resistor.
 
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Re: IGBT vs MOSFET for dummy load
« Reply #19 on: September 22, 2018, 03:10:34 pm »
If you are building big 1000 volt electronic loads, then you do not need my help.  You will require lots of devices in series and parallel to distribute the power and handle the worst case safe operating area no matter what power pass element you use.  Those big linear MOSFETs from IXYS start to look like a deal if they cut the number of pass devices down.
At that high voltage, wouldn't tubes start making sense?
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Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #20 on: September 22, 2018, 04:10:29 pm »
Nah, there are 2500V+ FETs on the market, cheaper than sweep tubes (the audiophiles and TV restorers keep their prices surprisingly high) and far easier to use.  If nothing else, lower voltage parts can be cascoded.

Might be tempting for stupid high voltages (10kV+, single digit to fractional amperes?), but those transmitter tubes aren't any cheaper, and if you're doing quite that much power you're probably a power company that has a better way of dealing with that anyway (e.g., braking resistors?). :)

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Offline Kleinstein

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Re: IGBT vs MOSFET for dummy load
« Reply #21 on: September 22, 2018, 05:28:10 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)
 

Offline TurboTom

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Re: IGBT vs MOSFET for dummy load
« Reply #22 on: September 22, 2018, 05:40:28 pm »
I wouldn't subscribe to that: If a vacuum tube is severely overloaded, usually the vacuum gets compromised (crack, seal breach, outgassing of internal components) which will eventually lead to a discharge - hence the breakdown. That's what's actually utilized in thyratrons to switch high currents in tubes.
 

Offline OM222O

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Re: IGBT vs MOSFET for dummy load
« Reply #23 on: September 22, 2018, 06:56:27 pm »
the Vce(sat) voltage was an interesting point that was brought up ...

How about using 3 IGBTs and one fet?
the fet can be used to short out the output as well as allowing for low voltage applications. It could also be connected to a 16 bit DAC combined with a matching ADC on a common shunt to get <1mA resolution going all the way to 20A.

IGBTs are then used for higher power applications to take the load off of the mosfet.

I've seen "Marco Reps" use a huge IGBT as a load and he didn't seem to have any problem with it either?
« Last Edit: September 22, 2018, 07:01:02 pm by OM222O »
 

Offline T3sl4co1l

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Re: IGBT vs MOSFET for dummy load
« Reply #24 on: September 22, 2018, 08:46:03 pm »
My preferred method, switched resistors in parallel with a (much smaller) current sink.  Cheap, needs much less heatsinking.  Very robust.

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