Balance resistor circuit for paralled linear mosfets? - Page 1
Forum section: electronics / beginners

author: trilerian
author_rank: Regular Contributor
timestamp: May 31, 2024, 06:04:42 pm
content: The subject of the post says it all... I understand the need for the balance resistors for paralleled mosfets operating in their linear region.My choice of mosfets is going to be the IXYS linear L2 line. I currently have two IXTK90N25L2https://www.mouser.com/datasheet/2/240/media-3321496.pdf. Not sure if these will be the final component I use or not, but most likely I will use a mosfet from that family. I have read lots of posts about needing the balance resistors on the source. I'm not quite sure where the other leg of the resistor goes, and how to choose the resistor value. Also, since I using this family of mosfets, I need an op amp to boost the out voltage of the mcu I will use to power the gate of the fet, since I will need approximately 8V on the gate for my application. And since this family of fets has a high gate capacitance, the op amp need to be able to handle the load. Any help with which attributes in the data sheet I should be looking at for op amp (gate driver) choice would be welcomed as well.Since someone will eventually ask. I am building a device that can sink 200A for 2-5 seconds at a time. I setup a POC using a variable voltage power supply to drive the gate and a modified HP server power supply as the DUT. I put a 200A current shunt on the low side to measure the voltage drop across it to verify current. I was able to sink 80A with this test, so a good start. But I think I may need to parallel two of these fets to get to 200A and 1600W. Unless it is ok to go over the max power rating of 1040W for the 200A version of the mosfet for 2-5 seconds. The data sheets indicates that pulses can go over the max Id2S so long as temperature is managed, not sure on the wattage though.EDIT: There is the real possibility that because these are short bursts, the balance resistors are not needed at all, however this device should also be able to provide a continuous 40A sink. Although one of the chosen mosfets should be able to do this as well, so let's just call this an opportunity to learn...Thanks

author: shapirus
author_rank: Super Contributor
timestamp: May 31, 2024, 06:30:30 pm
content: Quote from: trilerian on May 31, 2024, 06:04:42 pmThe other leg goes to ground. The idea is that as the voltage drop across the resistor increases, the Vgs voltage decreases, working towards turning the transistor off, which decreases current via the respective branch. As for the values... I would assume that you'd want something that at 1/2 the max current would reduce Vgs to the value where Rds begins to increase non-linearly (fast).Someone else will give a better advice on the balance resistors and whether they're going to work well with mosfets, but I will suggest looking into controlling each transistor individually with its own op amp (and current sensing resistor) to target 1/2 the total current (or, generally, 1/N for N transistors in parallel). This way each op amp will have less load capacitance and you will also be able to achieve almost equal current sharing.

author: trilerian
author_rank: Regular Contributor
timestamp: June 01, 2024, 03:05:35 am
content: I'm still trying to let this sink in a bit...Quote from: shapirus on May 31, 2024, 06:30:30 pmIf I am following this, that would be for a P channel mosfet? If I am using N channel, would the resistor go between the drain and gnd? And if so, that raises more questions.

author: Whales
author_rank: Super Contributor
timestamp: June 01, 2024, 06:15:16 am
content: Question:Are you building a 200A mosfet switch or a 200A 1.6kW mosfet load?I suspect that source degen resistors are not useful when using mosfets as loads in the linear region (please someone correct me if I'm wrong). The mosfet itself will act as a resistor that can change wildly in value. shaprius' suggestion of individually current-regulated mosfets might be wiser.Extra notes if you're making a mosfet load:* This is a lot of power. This thing will be easily capable of explosions, severe burns and fire.* Use a lot of forced air over very big heatsinks to dissipate the full 1.6kW* Investigate the Safe Operating Area (SOA) graphs on the mosfet datasheets.* Calculate mosfet junction temperature (using thermal resistances provided in the datasheet and estimated thermal resistances of your heatsinks) and check it's not too high.* Use a lot more fets than you think you will need.* Fuse it with a fuse rated for DC, not AC! DC is harder to fuse than AC.

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 07:32:40 am
content: Quote from: trilerian on June 01, 2024, 03:05:35 amNope. N-channel mosfets (when used as switches or, like your case, current controlling elements) are normally connected between load and ground: source to ground, drain to load, gate is the controlling pin. Controlling voltage is applied between gate and source, which is "by default" the same as ground.If you add a resistor between source and ground and still apply your controlling voltage between gate and ground, then theactualcontrolling voltage, Vgs aka gate-source voltage, that the transistor sees, will be a difference of the gate-ground voltage and the voltage drop across the resistor (the higher the current, the higher the voltage drop).To sense and control current, a current sense resistor (typically very small, like mΩ-small) is added typically between source and ground. It is selected to be small enough for the planned load current so as not to have a significant effect on Vgs. A balance resistor, on the other hand, should be large enough to affect Vgs when necessary.It's the reverse with P-channel mosfets. Their source connects (typically) to the positive power rail and drain goes to load (the other terminal of the load goes to ground).More details on this (basic) topic can easily be found on the internet, e.g.https://www.electronics-tutorials.ws/transistor/tran_7.htmlAt the same time, you said that you built a 80A current sink successfully, so now it's my turn to be confused, meaning... it's time to show your circuit!

author: trilerian
author_rank: Regular Contributor
timestamp: June 01, 2024, 05:23:54 pm
content: Quote from: Whales on June 01, 2024, 06:15:16 amI am trying to learn how to parallel multiple mosfets together when using them as the load itself (operating in their linear region acting as variable resistors). 1.6kW will only need to be dissipated for short bursts, 2 seconds. Then the mosfet and heat sink will be able to cool. I have tested approximately half that load with a single mosfet attached to a 120mm x 67mm x27mm aluminum heatsink via a screw and thermal compound, no forced air. The mosfet gets to approximately 60°C on the case. I'm pretty much at the limits of what the data sheet says are the max. However the data sheet says the max Id can be higher if you control the temp, but doesn't state anything about max power being higher. This is the reason I want to parallel two or more, but these mosfets are $35 a piece, so using less is ideal...Quote from: shapirus on June 01, 2024, 07:32:40 amYep, I'm with you here, that is how it is setup.Quote from: shapirus on June 01, 2024, 07:32:40 amDo you mean to break the path between source and gnd with a resistor? Still not following how this works for the VGS, this is essentially what I am confused on.Quote from: shapirus on June 01, 2024, 07:32:40 amYep, in my POC I am using a Riedon current shunt, 200A/100mV, so the resistance is 500µΩ. The actual unit would use a 200A - 400A hall sensor so I don't have to deal with tempco issues.Quote from: shapirus on June 01, 2024, 07:32:40 amAs it pertains to this thread, yes. It was just a POC . I'll attach a pic of the setup.Not pertaining to this thread, I did build a device that can do a sustained user selectable 1A-45A discharge using mosfets (different mosfets that what I am testing with here) as switches (much easier to parallel in that config). I have used this to do 2 second bursts of 180A @8V. But the size when using it for 180A is a bit much...

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 06:03:25 pm
content: Quote from: trilerian on June 01, 2024, 05:23:54 pmIf that's so, then you need to elaborate on the "gate driver" in your attached schematic. It can't be just a constant DC voltage source, it has to be something (typically an op amp) that adjusts the gate voltage gradually so as to keep the load current at a set level using some kind of negative feedback: for example, the voltage drop across a shunt resistor.Quote from: trilerian on June 01, 2024, 05:23:54 pmConsider the following:Assume, for example, GATE_CTL = V(G,GND) = 15V, R1 = 1Ω. If current I(R1) = 0, then V(S,GND) = 0 and V(G,S) = V(G,GND) - V(S,GND) = 15V - 0 = 15V. The transistor is fully on.Now, as I(R1) increases, the voltage drop V(S,GND) across R1 increases proportionally. When I(R1) = 10A, V(S,GND) = 1Ω * 10A = 10V.Since V(G,GND) still equals 15V, we have V(G,S) = V(G,GND) - V(S,GND) = 15V - 10V = 5V. See how it works now? As I(R1) continues to increase, V(G,S) will continue to drop, eventually reaching the level where the transistor will start to turn off, increasing its own drain-source resistance and preventing any further increase of the current flowing through it.That's how these balancing resistors work with multiple transistors in parallel. If one transistor's characteristics are slightly different and it becomes more conductive at the same gate voltage, it will start passing more current, but the voltage drop across the balancing resistor will increase, thereby reducing V(G,S) of this transistor, making it less conductive.Playing with this in simulation will make it much easier to understand (and find specific values of resistors and other parts, too).

author: trilerian
author_rank: Regular Contributor
timestamp: June 01, 2024, 06:14:38 pm
content: Quote from: shapirus on June 01, 2024, 06:03:25 pmThe power supply in the left of the picture is what I used to drive the gate in the POC. It is variable voltage from 0-10V. The mosfet starts conducting about 4.5V on the gate, starts ramping up fast around 7V and close to 8V is all the DUT can put out. I have also switched out the power supply I used for a DUT and used a 2s LiPo that got up to 100A before I said "enough is enough", lol.The unit I will build will use a micro controller DAC to power the gate via an op amp (gate driver), I'll use a current sensor for a feedback loop to control the gate voltage.I'm looking at the rest of your reply to digest the balance resistor still. I may take your original advice and just make two separate control circuits for the mosfets, but I want to understand the balance resistor because with more than 2 mosfets the separate control circuits isn't really scalable.

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 06:21:40 pm
content: Quote from: trilerian on June 01, 2024, 06:14:38 pmThat makes sense so far. I'd only suggest concept-proofing the idea with much lower currents first (say sub-1A), keeping the control circuitry and logic the same, before scaling up. Less risk of causing fire, shorting out LiPos (god forbid), or blowing expensive transistors. And, of course, I'll never get tired saying that simulating the circuit before wiring it all in hardware can easily save a lot of time and effort.

author: trilerian
author_rank: Regular Contributor
timestamp: June 01, 2024, 07:12:05 pm
content: Quote from: shapirus on June 01, 2024, 06:21:40 pmI have not found a simulation software that was easy to use yet. I tired LTSpice?, but it was very unintuitive. I use KiCad to build schematics and PCBs and pretty much just build something and have a pcb maker send me pcbs and I solder on the components on and test. For this I started out with an Uno with an RC filter on a pwm out to control the gate of the mosfet. My intent was just to do small amounts like you said to verify things would work. I have already done this with smaller mosfets... But the Vgs threshold of this fet is high and I couldn't turn it on with the Uno despite throwing a 4.7V dc signal to it. The data sheet says the threshold is min 2v, max 4.5V, but as said, the Uno couldn't turn it on. So I removed the Uno and used the vari power supply. Anyway, I'm sure someday I'll learn a simulation software, but right now I am still trying to learn C++ in my spare time, and maybe a round of golf this afternoon...Quote from: shapirus on June 01, 2024, 06:03:25 pmThat is starting to make sense in my mind. It will probably take a few hundred more times reading it to fully sink in. And yes, I'm sure a sim software would help, recommendations on easy to use sim software?

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 07:58:10 pm
content: Quote from: trilerian on June 01, 2024, 07:12:05 pmWell, I can mention Proteus as one that's easy to start using. It's very intuitive and straightforward, all the parts have sim models, the simulation is interactive, so basically plug and play. One problem: it's ridiculously expensive if you want to use it the "white" way. There are other ways, like with any other non-free software, but that's left to each one's own decision.I used to use Proteus, but then made the effort to switch to Kicad and use ngspice that it's integrated with for simulation. Besides the fact that I wasn't happy using a pirated copy of Proteus and that I had to use wine to run it under linux and also that Proteus 8 basically sucked performance-wise (because they switched to the bullshit .NET or whatever it was UI) compared to Proteus 7, Kicad is way better at PCB design.Simulation in Kicad+ngspice is much less straightforward, but it's not super difficult, either. Since you already use Kicad, you may find it reasonable to go this way.There are some things that are now obvious to me, but which took a good chunk of time to realize and get along with, so let me list them here to help others to make the entry threshold easier:- the only elements that have built-in models are generic BJTs and mosfets, diodes, resistors, inductors etc.; for anything else you'll have to download models from the internet.- the good thing is that a lot of models are compatible with ngspice out of the box -- for example, nearly any model from TI that I tried worked fine, and those cover most of what I've had to simulate so far. Diodes, inc. have spice models for a lot of, well, diodes, including Schottky diodes, which can be useful. Some manufacturer's models (e.g. Infineon) are sometimes not working right away, but can be fixed more or less easily.- spice simulation models are plain text files. Yes their format is very 1970's-ish, but it is comprehensible. Eventually you'll get to modifying them and creating your own models. I for example created some to simulate bouncy switches. It gets easy enough as you grasp the concept.- watch out for pin assignment for non-builtin models: they are more often not matching the Kicad symbols and the actual part pin ordering than they are. Kicad allows to assign the symbol pins to the model pins in the symbol properties. If you get weird errors when you run simulation, then pin assignment is the first place to check.- enable ngspice compatibility modes: ps lt a (it's in the settings in the Kicad simulation window)- it's neither realtime nor interactive. It is both good and bad. I learned to live with it. Interactive elements can be simulated using e.g. voltage-controlled switches.- it's not exactly plug and play: you need to define a simulation command in a text box placed anywhere on the schematic.- begin with something dead simple: VDC source, resistor, ground. Yes, don't forget to always add a ground node. Spice needs it. If you get a "singular matrix error", then it's likely a missing path to ground in some of the elements.- use Kicad 8. Don't bother with earlier versions.- always save your schematic before running simulation. There are crash bugs in ngspice. A crash in ngspice will crash entire Kicad.- yes, timestep is too small. Life's hard. But there are workarounds. When you get to that, post in the EDA section of this forum :).- one problem is that most tutorials are made with older versions of Kicad, sometimes much older (e.g. 5), and it can be a bit of a problem, since a lot changed since then. But usually it's possible to translate that to the current version.For example, the following vids are good enough to help get started:https://www.youtube.com/@holger8105p.s. a beauty of Kicad is in the fact that it is an all in one solution for everything: draw schematic, simulate it, design a PCB. Yes, there are some shortcomings and some workarounds may be necessary, but it can be quite viable.

author: DavidAlfa
author_rank: Super Contributor
timestamp: June 01, 2024, 08:52:40 pm
content: Quote from: shapirus on June 01, 2024, 06:03:25 pmThis is somewhat acceptable for low-medium current circuits using BJTs, as you onlu need to drop 0.7V to disable transistor.But it's an awful circuit for mosfets, current can't be regulated easily and the resistor will dissipate a gigantic amount of power.10A*15V = 150W!That's why shunt resistors exist, we can amplify a small voltage drop and make it way more efficientWith 1mR shunt, 99% of the power goes to the transistor.10A * 1mR = 10mV, just 100mW.And for higher currents, it's still doable:50A * 1mR = 50mV, 2.5W.A very basic current control loop would look like this.To parallel different mosfets in very high current/power loads, multiply the shunt/comparator circuits, feed the same reference signal to each one.Though the circuit would needs plenty of enhancements, like different protections, sleeve rate control, no input detection...

author: shapirus
author_rank: Super Contributor
timestamp: June 01, 2024, 09:09:13 pm
content: Quote from: DavidAlfa on June 01, 2024, 08:52:40 pmWhat exactly? Is my math wrong?Quote from: DavidAlfa on June 01, 2024, 08:52:40 pmIt was a general case explanation of how current balancing resistors work (to answer exactly that question). Of course it's not (normally) the way to control or set current.Good point about power dissipation. When it comes to specific applications, it of course has to be considered. At 100 A, a resistor dropping, say 5 volts, will have to dissipate 500 W of heat. And 1 kW for 10 V. But the same can be said for the mosfets, and it was mentioned in earlier posts. The entire device will need to have a huge heat sink and/or powerful fans. Once those are present, they can be used for both the mosfets and the resistors.Quote from: DavidAlfa on June 01, 2024, 08:52:40 pmWhy not amplify the R1 voltage using U1:B directly? Is there any particular reason to use a separate (even if it's another unit in the same package) op amp for that?BTW U1:B is not really a comparator, it's an amplifier.Quote from: DavidAlfa on June 01, 2024, 08:52:40 pmYeah that's what I suggested in one of the first posts. It just feels like it'll be easier to design and troubleshoot that way. I don't have a solid proof, but it feels like using resistors for current balancing (even for lower currents) won't work well with mosfets, unlike BJTs.