How to detect invidual load across parralled MOSFETS - Page 1
Forum section: electronics / testgear

author: SpottedDick
author_rank: Regular Contributor
timestamp: June 30, 2022, 07:30:11 pm
content: Say you had the following configuration:Is it possible to detect how much load each MOSFET is getting electrically without modifying the circuit (for example adding a shunt)?I know thermally checking will give some indication, but they're quite well thermally joined.

author: Martin72
author_rank: Super Contributor
timestamp: June 30, 2022, 09:17:23 pm
content: If you had balancing resistors inbetween, you could measure over them to see possibly differences.But in this configuration....

author: SpottedDick
author_rank: Regular Contributor
timestamp: June 30, 2022, 09:37:13 pm
content: It's just not possible sure it's not?They're basically connected together.I shouldn't have even asked the question! Thanks anyway.

author: PeteH
author_rank: Unknown
timestamp: June 30, 2022, 09:44:13 pm
content: Is this controlled in linear mode (current controlled?), or hard switched on/off?

author: SpottedDick
author_rank: Regular Contributor
timestamp: June 30, 2022, 09:45:15 pm
content: Linear region (current controlled).

author: PeteH
author_rank: Unknown
timestamp: June 30, 2022, 09:52:46 pm
content: You should ideally have independent gate resistors, and some kind of ballast resistor setup for these paralleled gets, depending.Paralleled MOSFETs in the linear region have a tendency to "hog current" when their temperature increases, opposite to what you'd like to see... (look at the Id vs Vgs curve, across temperature)

author: PeteH
author_rank: Unknown
timestamp: June 30, 2022, 10:01:21 pm
content: To add a bit of background, ballast resistors on the lowside (usually where current shunt feedback is anyway, for current control feedback) helps pinch off the Vgs slightly for MOSFETs taking a disproportionate share of the current. Assuming the currents are high enough where it matters thermally.Gate resistors help prevent any parasitic oscillation between FETs.Ideal world you'd have an amp for every FET which enforces current sharing regardless of temperature. Can get by with what you're doing if you have lots of margin.

author: PeteH
author_rank: Unknown
timestamp: June 30, 2022, 10:06:49 pm
content: I am usually always waiting for T3sl4co1l to post (from the comfort of a physical keyboard) with the background and long justification on a question like this...Shoot if you have specific questions.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: July 01, 2022, 04:36:24 am
content: Quote from: SpottedDick on June 30, 2022, 07:30:11 pmAs already mentioned quite a few times by different people in your other threads related to this topic, we’d never have this configuration for this particular application.Your own thermal imaging of the 6 mosfets was nice though

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2022, 05:26:38 am
content: Yep as others have said this is NOT the right way to do it.Mosfets have too much manufacturing tolerance in what the shape of each transistors linear region looks like. So some transistors will be pulling a lot of current while another transistor is barely turning on. This is also unstable with temperature so if a particularly happy to conduct transistor starts getting too hot it will hog more and more current, heading to a thermal runaway until it blows up.Just add shunt resistors to ground for each transistor. They will not only let you see the current but also give each transistor its own mini feedback loop that balances the current across all of them. The more current the transistor draws the more voltage drops appears on the resistor, this lifts the transistors pin up so that the gate sees less voltage, closing down the transistor so its current reduces.

author: SpottedDick
author_rank: Regular Contributor
timestamp: July 01, 2022, 10:42:42 am
content: Quote from: Berni on July 01, 2022, 05:26:38 amThat's an interesting idea for the modded DL24 I have, but looking at the shunt (0.05 Ohm), I'd only be dropping 0.05V at 10A across the whole array, so I doubt that would make a significant difference.

author: switchabl
author_rank: Frequent Contributor
timestamp: July 01, 2022, 10:55:32 am
content: Well, yes, this circuit clearly is a bad idea and I believe the OP already knows that. So assuming this is mostly about the educational value ("I really want to see just how bad it is") and at the risk of stating the obvious: a current probe should work. I assume the DUT is the one from the other thread (https://www.eevblog.com/forum/testgear/example-of-why-people-say-you-sholdnt-use-mosfets-in-parrallel-as-dummy-load/msg4247347/#msg4247347), with point-to-point wiring between the MOSFETs. In this case, just about any clamp-on-style probe or even clamp meter (that goes to DC) would work. In a more conventional circuit, you might have to resort to something like an I-Prober.

author: Berni
author_rank: Super Contributor
timestamp: July 01, 2022, 10:57:41 am
content: Quote from: SpottedDick on July 01, 2022, 10:42:42 amThat still means the too exited transistor hogging most of the current gets 50mV less drive signal on the gate, turning it on slightly less so it lets the other transistors do more of the work.If 50mV is enough is a question of what kind of FET it is. For transistors with a very sensitive gate that can conduct a fair bit of current already at 1.5V gate voltage, then taking away 50mV can mean quite a bit. For particularly insensitive transistors that need something like 7V to pass any reasonable current those 50mV is not much.The point is that you choose a shunt resistor value that is reasonably small, yet at the same time still large enough to stabilize the transistors behavior. You can use these resistors to actually measure the imbalance in current sharing and see if it is working well. To give it a worst case scenario you can intentionally loosen one of the transistors on the heatsink, letting it get hot, so that its characteristic drifts particularly far from the others, if it can still current share reasonably well when its 50°C above others then it will work fine for pretty much all cases.

author: t1d
author_rank: Super Contributor
timestamp: July 03, 2022, 07:43:57 am
content: I also researched a similar balancing topic and found the possible methods to be much more broad than I had anticipated. In addition to balancing with resistors, there are other methods/components... For example, diodes. Each method has its own pros and cons. In addition to passive methods, there are all sorts of active methods that work on feedback... arrangements of transistors, op amps driving transistors/MOSFET/Etc.Just for your considerations in learning, attached is a circuit I adapted from an EDN design. As I used it as a learning tool, it may, or may not be exactly correct. But, it will surely give you some ideas to broaden your thinking... The link to the EDN design is on the schematic sheet, in the information block. I have my original KiCad files, if someone wants them.I also had proto PCB boards manufactured. They are incorrect, but they can be bodged. I would be glad to share them, if anyone wants to send me a private message. I can't remember if I corrected the schematic, after I discovered the boards were faulty. I would have to check, but it is rather likely.I do not want to high-jack the OP's thread with comments about the circuit. If someone wants to discuss it, please let me know and we will move to a separate thread.Anyway, my point is to suggest that you research different load leveling methods. They generally will include some method of reading the load on each path.

author: RCinFLA
author_rank: Contributor
timestamp: December 27, 2022, 08:00:15 pm
content: Some info that might be of interest on DL24P with addition for extra load devices.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2022, 08:01:51 pm
content: Quote from: PeteH on June 30, 2022, 10:06:49 pmOh hey.Well, a bit late it seems...Tim