1200W (50A 25V) Adjustable Current Load Bank Design Questions - Page 1
Forum section: electronics / projects

author: jeffmhopkins
author_rank: Newbie
timestamp: May 02, 2018, 08:41:56 am
content: Hello!I'm in the process of creating a 1200W resistor load bank for discharging large LiPo battery packs and for testing power supplies. It will operate as a 0.75Ohm or 2.75Ohm load on 3S-6S batteries.I'd like the capability to do an adjustable constant current driver for it, but I'm unsure the best way to go about it.I had originally thought a switching MOSFET design would work, but I have no experience with 50A DC circuits and after some research they seem more complicated than I first though at this power level, and thought that perhaps a large collection of 2N3055 transistors might be easier.Any suggestions for constant current 50A 25V designs?

author: ogden
author_rank: Super Contributor
timestamp: May 02, 2018, 09:07:47 am
content: Quote from: jeffmhopkins on May 02, 2018, 08:41:56 am1200W DC CC load is no jokeYou can consider configurable, switched resistor bank, like R/2R/4R/8R/16R which will give you 31 resistance steps. Obviously MOSFETS are used as on/off switches, not heaters.If you want to try bumpy DIY road of hi-power DC loads, then I suggest to start with Kerry Wong videos:

author: jeffmhopkins
author_rank: Newbie
timestamp: May 02, 2018, 09:18:39 am
content: One thing that should be mentioned is that I only really care about sourced current, not current into the load.I'm comfortable with multiple legs on the resistors, but was looking for an adjustable current that I could control, hopefully by microcontroller.I think that the switching MOSFET idea doesn't work well, which makes me think that using 2N3055s might be the best bet.Kerry Wong's channel looks interesting, and I'll take a look.

author: Amper
author_rank: Frequent Contributor
timestamp: May 03, 2018, 10:03:49 am
content: I have recently built a shunt regulator as a load for a small hydro electric emergency generator. Power is about 600W peak. I didnt want any switching going on , since it has to be as reliable as possible, so no capacitors and not to many active parts. I settled with a 1Ohm 1kW resistor and a TL341 driving an IXYS linear mosfet to burn the top 10V. If you need to go down to 0V it will be a bit more challenging, since the usual ixys fets only go up to about 800W. You can actually parallel them but dont underestimate what 1200W means in cooling for semiconductors. Even a large sink is no guarantee, since it is even hard to spread the heat over its entirety. Water cooling and a computer radiator might be a solution.This circuit can be made adjustable with a simple DAC i think, maybe you need to throw in an OP and a cap to mix it in. The result will have much less noise than a pure switcher. If you are just interested in your batteries capacity you could also just skip the regulation, since your voltage will not vary much, you can just calculate power and integrate that.I hope that helps, if you are interested, i can write more on this.EDIT: I forgot to mention, i built a constant voltage load, that will lead to much fire in your case, but i tried wongs circuit as well, it works nicely and should be perfect for your application sinde you can provide the reference voltage for the OP with a PWM pin of your arduino through a divider and a smoothing cap.

author: jeffmhopkins
author_rank: Newbie
timestamp: May 03, 2018, 12:56:12 pm
content: I can already do battery capacity testing through my traditional chargers/dischargers, but they are low amperage (500mA), I'm more interested in rapid discharging (20A+ primary goal) and high current testing of the batteries (lower priority nice to have secondary goal)Rapid discharging I can handle with a fixed resistance (I have the supplies for this already), but having a microcontroller do pulse and arbitrary current over time programs is something that would be quite interesting.I currently plan on operating my fixed resistance load at around 4S 16.8V 22A 375W, with the option of 6S operating at 850W. In order to increase the wattage to 1200W I think it will be necessary to do a mineral oil cooling setup, but this would only be if I plan as a dynamic load for high current battery testing.So my 1200W is really derated to around 375W or 850W for normal use, with four 24V 0.5A fans blowing down on some rather large heat sinks.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 03, 2018, 01:05:15 pm
content: I have seen hybrid designs:where the bulk of the power is handled by a switchable bank of power resistors, let's say in 100 watt increments, and the gaps filled in with a much lower power, Mosfet-based circuit.I'm curious, why would a 2N3055 work better than a Mosfet?

author: jeffmhopkins
author_rank: Newbie
timestamp: May 03, 2018, 01:23:23 pm
content: I was thinking that a larger collection of 2N3055s would be easier due to it being basically a bunch of transistors to ground. Current mirroring, and that's all you need, just a large amount of heat sink area. The switching MOSFET I was worried about handling the voltage spikes at the large currents and what I need to do in order to not kill the MOSFET. I think it's doable, I just couldn't find a nice schematic at the power levels I was looking for.I'm still all for a MOSFET switching design, as a PWM driven from the microcontroller would be easy to control.I found the IPP020N08N5AKSA1 MOSFET, and currently have two of them for some testing.They are 80V, 120A, 2mOhm resistance, 40ns rise, 102ns fall MOSFETS. I think maybe 4-8 of them would work well, but I just don't have the experience in designing high power switching systems and fear just frying them.

author: Amper
author_rank: Frequent Contributor
timestamp: May 03, 2018, 03:23:17 pm
content: The disadvantage of normal transistors is, that you need to drive them with a lot of power. I know this setup from linear laser powersupplies (that might actually be a nice source for a bank of linear burning transistors if you want to go down that road). With a Fet you can go directly into the gate coming from your OP. Just have a look at the IXTX200N10L2 for example. It alone can burn 1kW of power in a single device, driven as described and controlled by a voltage coming from your controller.Keep in mind, with a switcher you need to do control as well, if you just give it a fixed PWM you basically have a variable resistor with linear UI curve. Though the choice of fets is endless there, you just need a proper driver and a low impedance setup to not fry it.

author: nctnico
author_rank: Super Contributor
timestamp: May 03, 2018, 04:17:22 pm
content: Quote from: Amper on May 03, 2018, 03:23:17 pmNo it won't. Just do the math on the thermal resistances alone. The rule of thumb is that you can burn less than 100W per semiconductor in a DC load.

author: Amper
author_rank: Frequent Contributor
timestamp: May 03, 2018, 04:30:25 pm
content: Why ? I did the math and it comes out nicely. Maybe you are talking about TO3?IXTK-TX200N10L2 datasheet (http://ixapps.ixys.com/DataSheet/DS100239(IXTK-TX200N10L2).pdf)says in TO264 Package the safe DC operating area at 75°C Case temperature (not junction!) goes up to 600W, if you are able to cool it to 25°C (not realistic) it will be 1000W continuous on a single device. Thats why i told that burning those 1200W is no problem with a dump resistor and one of there for regulation.

author: nctnico
author_rank: Super Contributor
timestamp: May 03, 2018, 04:42:55 pm
content: Quote from: Amper on May 03, 2018, 04:30:25 pmBut you can't cool it to 25 degrees. Let's do the math with real parts you can buy: The Rth junction-case is 0.12 K/W and the Rth case-sink is 0.15 (typical). So with a live heatsink and the best thermal compound you can get the Rth junction-sink is 0.27 K/W. If you manage to get the heatsink to have an Rth sink-ambient of 0.2 for a single device then Rth junction-ambient adds up to 0.47K/W. If you want to keep the maximum junction temperature at 125 deg and the environment is 30 degrees C then you have a temperature difference of 95 degrees which means a maximum dissipation of slightly over 200W. Increase the ambient temperature to 40 degrees and lower the junction temperature to a much healthier 90 degrees then you are looking at a realistic number of little over 100W.There is a good reason professional DC loads use many transistors. I bought an old 1500W load from TDI a while ago. That one has no less than 32 TO3 MOSFETs to dissipate the power (46W per MOSFET).

author: nctnico
author_rank: Super Contributor
timestamp: May 03, 2018, 04:51:28 pm
content: By the way I like to build constant current loads from bipolar transistors. Sure they need more drive current but the collector acts as a natural current sink. You only need a slow control loop to undo the set point drift introduced by the temperature change in the transistors. Because the transistors act as a true current sink the input impedance is high so if your PSU is prone to oscillating, it will do that.

author: Amper
author_rank: Frequent Contributor
timestamp: May 03, 2018, 05:09:01 pm
content: Well, cooling them down to 90°C is not really any possibility. I assumed running at the 125°C mark and with 0.1K/W sink in a SOT227 package, that works out to about 475W. You will not have less cooling troubles with your bipolars. You can spread them out nicely, but sinking that amount of power is not easy in any air cooled setup. In laser supplies there is usually a bank of about 20-50 TO3 package bipolars with balancing resistors on a large copper plate that has soldered on piping. Thats pretty much the only way if you want to burn all of the 100W without a resistor.

author: BravoV
author_rank: Super Contributor
timestamp: May 03, 2018, 05:12:26 pm
content: Quote from: nctnico on May 03, 2018, 04:42:55 pmThat Amper mentions is the niche special linear Mosfet made by IXYS, it has FBSOA specifically stated capable of running high current in linear mode, even quite hot.Watch these two FBSOA charts at different temperatures, in this example is IXTN46N50L.I deliberately highlighted in yellow.Regarding cooling, some form like SOT-227 form can be cooled quite effectively if mounted on good heatsink. Here example of my IXTN46N50L, yep, its quite expensive compared to ordinary non linear (switching) mosfet.Still don't have time to build my load, example when I was trying to find of how to mount it at good CPU overclocking heatsink.

author: nctnico
author_rank: Super Contributor
timestamp: May 03, 2018, 06:02:06 pm
content: Quote from: Amper on May 03, 2018, 05:09:01 pmOfcourse the heatsink will need forced air but you also need to take into account that the heat has to flow away from a very small spot if you use a single transistor. All in all the thermal resistance a single transistor sees will be much higher than 0.1K/W. Also running transistors near their maximum junction temperature makes them have a short life. I have more DC loads and they all top out at less than 100W per transistor. The numbers in the datasheet are theoretical limits only.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2018, 06:14:42 pm
content: Because semiconductors are not that expensive anymore, but good heatsinking still is, it's oftencheaperto use more transistors.A beginner mistake is to assume that the same amount of heatsinking is needed to get rid of the same amount of watts, regardless the number of transistors. This is not the case: if you increase the number of transistors, you parallel the junction-to-case and case-to-heatsink thermal resistance values, as well as spread the power more evenly to the heatsink, meaning you can run the heatsink hotter (while keeping the junction temperature, which matters, cool), meaning smaller and cheaper, possibly simpler heatsink for the same power dissipation. A good resource is:http://ludens.cl/Electron/Thermal.htmlIn some cases, throwing in extra $20 by doubling the number of transistors means you can save $200 by ditching water cooling, for example!This is case-by-case; there is always the sweet spot depending on the requirements. Do the actual math.The sweet spot is almost never close to the maximum ratings of the transistors, although that might be case with some specialized, expensive parts.

author: jeffmhopkins
author_rank: Newbie
timestamp: May 03, 2018, 10:57:57 pm
content: So let's say I try to go the MOSFET route.The IPP020N08N5AKSA1 MOSFET has a 2 mOhm resistance when switched on. So at 50A would be dissipating 100mV or 5W.If I reconfigured my resistor bank to be 0.2 Ohms, would it be appropriate to use two of these off of PWM? 100% duty cycle would be about 80A discharge, and 60% duty cycle would be 50A.What frequency would I drive the PWM, and any extra considerations for capacitor / inductors for this kind of control? I don't actually care about how clean the load side is, but smoothing out the battery side would be nice.

author: Amper
author_rank: Frequent Contributor
timestamp: May 04, 2018, 05:52:08 am
content: I think something in the order of 20KHz should be good. That will not give you much ripple and not make switching the fet difficult at all. On the input side you will need at least a capacitor, though it will not have to be to large, just have low enough esr. If you really want smooth current you also need inductance in some place. Maybe just build a boostconverter? It will have smooth input current, but need a large inductor and should also have more frequency in the order of 100kHz.Its also very important at these kind of currents to have a bit of snubber circuit or at least a cap around the mosfet. You will have longis lines going to the resistor and most likely it will be a wound one, so a pretty nice inductor. This will lead to a lot of peaks when switching off. They will almost certainly destroy your fet. I worked with a car inverter once testing one phase dumping it into a resistor. The input Voltage was around 24V and the current 50A, it ended up being a nice boost converter charging the large capacitor to over 200V.

author: jeffmhopkins
author_rank: Newbie
timestamp: May 04, 2018, 09:35:02 am
content: I had planned on adding a flyback / snubber diode, because yeah, the wirewound resistors are going to be quite the induction source.20KHz seems quite reasonable.This is what I'm looking at right now:MOSFET: IPP020N08N5AKSA1https://www.mouser.com/datasheet/2/196/Infineon-IPP020N08N5-DS-v02_01-en-531352.pdfSnubber: BYC20-600,127https://www.mouser.com/datasheet/2/848/BYC20-600-1105142.pdfI also planned on adding a cap on both source and load side. A Panasonic 2000uF ElectrolyticCapacitors: 2 x ECA-1HM222E 2000uF 50Vhttps://www.mouser.com/datasheet/2/315/ABA0000C1218-1131230.pdfFirst testing is planned with a function generator to control the PWM, but later testing will be with a microcontroller.Pretty simple schematic, with the MOSFET between the battery and load, flyback diode and some caps.... I don't have much experience with this large of a power circuit, does anyone else want to yell at me to stop and modify something? I'll have a week or more before I actually start construction of the dynamic controller while I wait on some parts.

author: Amper
author_rank: Frequent Contributor
timestamp: May 04, 2018, 11:19:00 am
content: Sounds pretty reasonable.How will you drive the fets? if you dont have a large heat sink for other stuff you should try to switch it fast, so better get a driver too. in this case id recommend to use some optocoupler driver, they remove a lot of headache. Just throw in 12V drive supply and have a completely potential free input with no risk of ground loops. Also it will be easy to implement a high side driver if you want to play around a bit.Also maybe you will need a bit of a beefier cap, at 50% duty your cap will be loaded and unloaded with 25A if i didnt get something wrong here.

author: jeffmhopkins
author_rank: Newbie
timestamp: May 04, 2018, 12:23:57 pm
content: Good call on the caps.What about something like four of the Kemet PEG226KL4270QE1 in parrallel?https://www.mouser.com/ProductDetail/KEMET/PEG226KL4270QE1?qs=sGAEpiMZZMvwFf0viD3Y3bzn7LR4wbx4yZm%252bpLmSnBM%3d2700uF40VDCRipple Current, 12.8A13mOhm ESRAnd probably just on the load side of the MOSFET.After proof of concept and setting up the microcontroller I'll be looking at an optocoupler for sure.

author: Amper
author_rank: Frequent Contributor
timestamp: May 04, 2018, 02:05:25 pm
content: Here, have a look:https://www.mouser.de/datasheet/2/427/vo3120-244747.pdfId rather buy one of these than to fiddle around with discrete stuff even for a prototype. They make your life so much easier, just a single component, maybe a gate resistor and a cap, but no more.About the Caps probably you should do a quick run in LTspice? to me its usually very helpful with these things to just see it and tweak it. Maybe you could even go down to film caps at this frequency, cheaper, longer life and smaller.

author: nctnico
author_rank: Super Contributor
timestamp: May 04, 2018, 02:43:54 pm
content: Don't put a big capacitor across the load! It will cause huge rush-in currents (easely 500A) and ruin the connectors.