IGBT vs MOSFET for dummy load

[OM222O]

You have mentioned that before, but it's not a dynamic load if it's used in that way! that's the main point you're missing tim. I want to build a dynamic dummy load with as much precision as I can.

[T3sl4co1l]

How dynamic you want?  I get >10kHz bandwidth with that method.

Tim

[Kleinstein]

You have mentioned that before, but it's not a dynamic load if it's used in that way! that's the main point you're missing tim. I want to build a dynamic dummy load with as much precision as I can.

In addition to poor SOA that make the IGBTs essentially useless to linear operation they are also slow - very slow compared to MOSFETs.
For the precision one might consider combining MOSFETs and BJTs in a kind a IGBT replacement circuit. This could reduce gate leakage, but a don'T think gate leakage would be a big deal anyway.

If high precision / good resolution is wanted it is more about the shunts / resistors.  From an older electron microscope (needs quite a few constant current sources for the magnets, in this case some 6 channels of some 5 A at around 50 V) a remember a set of resistors of significant size: likely 50-200 W power rating wire would each. The resistors could be a more difficult (and expensive) part compared to the power semiconductors. It is not just the power rating needed, but also low TC if it needs to be accurate.

[OM222O]

the absolute value of the shunt is not a big deal as I have access to high precision lab equipment in our uni so I can calibrate that out in software, I'd be more considered about the temperature stability of the shunts

And by dynamic load, I don't mean the frequency response, rather being able to have constant current,constant power. a simple resistor cannot do that.

In addition to poor SOA that make the IGBTs essentially useless to linear operation they are also slow - very slow compared to MOSFETs.
For the precision one might consider combining MOSFETs and BJTs in a kind a IGBT replacement circuit. This could reduce gate leakage, but a don'T think gate leakage would be a big deal anyway.

If high precision / good resolution is wanted it is more about the shunts / resistors.  From an older electron microscope (needs quite a few constant current sources for the magnets, in this case some 6 channels of some 5 A at around 50 V) a remember a set of resistors of significant size: likely 50-200 W power rating wire would each. The resistors could be a more difficult (and expensive) part compared to the power semiconductors. It is not just the power rating needed, but also low TC if it needs to be accurate.

what do you think about my last idea? 3 IGBTs to take up most of the load, plus a linear fet for the added precision?

[David Hess]

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?

I think the DC SOA shown is a lie.  IR shows the same thing in their datasheets but in their application notes gives a warning that they are not characterized for linear operation.

Get some samples and test them to find out.

[David Hess]

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?

At higher voltages, I have seen EIMAC transmitter tubes used as power pass elements but their main advantage is ruggedness when overloaded and not cost.  At those prices, linear rated power MOSFETs become competitive even when many devices have to be used in series and parallel.

[OM222O]

I think the DC SOA shown is a lie.  IR shows the same thing in their datasheets but in their application notes gives a warning that they are not characterized for linear operation.

Get some samples and test them to find out.

I highly doubt that the manufacturer is going to lie about their product details. I will get some for testing for sure, but I have high hopes for this device

[OM222O]

some people mentioned BJTs as another way to dissipate the energy into heat. They are current controlled devices tho ... how could they be implemented in a similar fashion? can anyone give examples of a circuit that uses a BJT?

I would imagine they are probably cheaper and higher rated than the IGBTs 

I might be able to use a FET for the precision and the BJTs for the bulk of the load

[David Hess]

I think the DC SOA shown is a lie.  IR shows the same thing in their datasheets but in their application notes gives a warning that they are not characterized for linear operation.

Get some samples and test them to find out.

I highly doubt that the manufacturer is going to lie about their product details. I will get some for testing for sure, but I have high hopes for this device

If they are not intended for linear applications, then why would they bother to destructively characterize them for it?  And if they do not destructively characterize them for it, what are they going to show on the datasheet?  I will not believe it until someone destructively tests a good sample size.

This application note has some comments about SOA testing.

some people mentioned BJTs as another way to dissipate the energy into heat. They are current controlled devices tho ... how could they be implemented in a similar fashion? can anyone give examples of a circuit that uses a BJT?

I would imagine they are probably cheaper and higher rated than the IGBTs???

The big advantage of bipolar transistors is that excluding secondary breakdown, they will be the least expensive option for a given power level simply because they cost less per square millimeter of silicon area.  Unfortunately secondary breakdown makes a big difference so linear rated power MOSFETs may be more economical especially now due to economy of scale.

[OM222O]

I will not be pushing the devices as hard as I can. I will allow a lot of headroom, so the secondary break would not be a problem. Can you please provide schematics of how a BJT can be used instead of the mosfet? is it the same circuit with the output of the op amp connected to the base of the transistor?

[boB]

some people mentioned BJTs as another way to dissipate the energy into heat. They are current controlled devices tho ... how could they be implemented in a similar fashion? can anyone give examples of a circuit that uses a BJT?

I would imagine they are probably cheaper and higher rated than the IGBTs 

I might be able to use a FET for the precision and the BJTs for the bulk of the load

The company, TDI  (Transistor Devices Inc, now astrodynetdi.com ?) has made dynamic loads (Dynaload) for many years and we have bought and used many of these from E-bay for the past 25+ years.  They have different modes, constant voltage, constant current, pulsed, external control, etc....

They used TO-3 BJTs in parallel and  emitter resistors, just like an audio power amplifier, for transistor  sharing.  Also good heat sink and fans for cooling. We have had to fix a couple of them and we would replace the BJTs with better parts like the On-Semiconductor  MJ15024 parts or similar. The old ones at least used not very good BJTs like, 2N3055s as I remember.   MJ15024  (and that general family) are 250V parts with a good SOA and we used these in audio power amps for many many years as well as other amplifier companies.


I didn't think you needed higher voltage ?

boB

[David Hess]

I will not be pushing the devices as hard as I can. I will allow a lot of headroom, so the secondary break would not be a problem. Can you please provide schematics of how a BJT can be used instead of the mosfet? is it the same circuit with the output of the op amp connected to the base of the transistor?

It is the same circuit however because the base current of a bipolar transistor contributes to the emitter current, either a Darlington configuration or power MOSFET is used to drive the output transistor for better accuracy.

[Kleinstein]

Some manufacturers are more honest with SOA curves than others. There a quite a few questionable to likely wrong FBSOA curves in MOSFET data-sheets.

A problem with the SOA curves in some data-sheets is that for switching applications it relates to the dynamic thermal resistance. So they measure / calculate the pulsed thermal response and than calculate the FBSOA from that.  However this way is ignoring thermal instabilities and is thus valid only for short pulses.  So the curves for 1 µs up to maybe 1 ms may be valid - but the longer pulse times and DC may not be real.

So if there is a pulsed thermal response curve (which is normal for more modern switching devices) one should be careful with the FBSOA, if it does not include the steeper roll off due to thermal instabilities.

Another problem is that not all samples may behave the same - so sample testing may not be good enough to get reliable devices. For this reason there are individually SOA tested BJTs for audio applications available and also linear rated MOSFETs may be SOA test. However this comes at a price.

[David Hess]

How would you non-destructively test SOA?  Motorola's old application notes on the subject only mention destructive SOA testing and specifically warn against trying to grade parts based on SOA for this reason.

[Kleinstein]

Non destructive SOA testing can be tricky. My idea for at least the near DC case would be to run the DUT in forced constant current mode and carefully monitor the gate voltage. When instability starts I would expect to get a significant drop in gate voltage, faster  than from the pure homogeneous warming. Still this is tricky and might cause some destructive events.

The other point is an SOA check, to see if the samples survive a give load pulse. This would be nondestructive for the good ones and destructive for the bad ones. 

[OM222O]

so if I intend to use a BJT, I should add a base resistor which is then driven by a darlington pair that is controlled by an OP amp? wouldn't that be more complicated than a straight up IGBT? I'm still not sure about the overal schematic ... like how much base current would I need? what value of base resistor should I use? where would the op amp feedback come from? It would be awesome if someone can post a picture of the schematic. Thanks.

[Siwastaja]

I highly doubt that the manufacturer is going to lie about their product details.

Happens all the time, day in day out. Completely normal business practice, regardless of how highly you doubt it.

Yet, you have to trust something. Try it out.

[T3sl4co1l]

I think the preferred way is to run increasingly longer pulses and stop when the pulse just starts to curl over.  Then repeat that at different voltages and currents to sample points in the SOA plane.

Vgs drop is a tricky basis, because it's a microscopic part of the transistor that's hogging all the current and it doesn't take much change for it to burn through.  That's probably the first thing you need to establish, how much droop or what rate of droop is acceptable.  If neither varies with current, you may need a lot of transistors...

Tim

[Kleinstein]

I think the preferred way is to run increasingly longer pulses and stop when the pulse just starts to curl over.  Then repeat that at different voltages and currents to sample points in the SOA plane.

Vgs drop is a tricky basis, because it's a microscopic part of the transistor that's hogging all the current and it doesn't take much change for it to burn through.  That's probably the first thing you need to establish, how much droop or what rate of droop is acceptable.  If neither varies with current, you may need a lot of transistors...

Tim

This sounds like a better and practical method, though it might still be tricky to see the fine difference of just starting to become unstable before blowing up.

so if I intend to use a BJT, I should add a base resistor which is then driven by a darlington pair that is controlled by an OP amp? wouldn't that be more complicated than a straight up IGBT? I'm still not sure about the overal schematic ... like how much base current would I need? what value of base resistor should I use? where would the op amp feedback come from? It would be awesome if someone can post a picture of the schematic. Thanks.

The IGBT is normally just not a real option due to the very limited SOA - so a big ass IGBT might be no better than an TO220 MOSFET like IRF510.

The BJT solution could be the IGBT equivalent circuit, with a N-Channel MOSFET driving a BJT in a Darlington like circuit. A simple darlington would be another possible way, though slightly less accurate.

[OM222O]

Can you please provide some sort of schematic?

I'm definitely going with the mixed method. A single mosfet for the high accuracy with 16 or 24 bit ADC and DAC (they're both fairly cheap) and some other linear device to take up the majority of the load. if a BJT or darlington could be used, I would appreciate if you could provide some sort of schematic as how to implement them.

[MiDi]

I will not be pushing the devices as hard as I can. I will allow a lot of headroom, so the secondary break would not be a problem. Can you please provide schematics of how a BJT can be used instead of the mosfet? is it the same circuit with the output of the op amp connected to the base of the transistor?

Maybe Bob Pease could help you 

[boB]

OM222O said  "Can you please provide some sort of schematic?"

From an earlier EEvlog posting,  https://www.eevblog.com/forum/testgear/dynaload-dlf-100-100-1500-(100v-100a-1500w-dc-load)-teardown-repair/
there was one PDF with a schematic of a 100 watt TDI Dynaload...  The large Dynaloads are very similar though from what I can tell.

boB

[OM222O]

they're using mosfets tho ... I was asking about a BJT equivalent

[boB]

they're using mosfets tho ... I was asking about a BJT equivalent

TDI uses (used) BJTs.

[coppercone2]

what topology do you use for extremely high frequency dynamic loads? whats the road map look like?