DC Load MOSFET Question: PWM or Linear Control? - Page 1
Forum section: electronics / projects

author: timb
author_rank: Super Contributor
timestamp: August 26, 2014, 05:58:47 am
content: Hey guys, I’m working on a high power Programmable DC Load. I’m trying to spec out the MOSFETs but have a few questions…I know most MOSFETs aren’t designed to run in the Linear region due to uneven heating of the internal holes. From what I understand, at lower Id they have negative tempcos as well. I suppose most MOSFETs are used in switching applications, where these problems don’t show up.After doing some comprehensive research, I’ve found there are several MOSFETs designed specifically for a SOA in the Linear Region. This would be ideal for my DC Load; one issue is that they tend to be rather expensive.So I’ve been thinking, is there any particular reason you can’t just use a normal MOSFET with PWM control? I’ve been specifically looking at IRF’s DirectFET line, which basically puts the die directly under a metal shell, allowing pretty much the most direct two sided cooling you can get.To summarize, is there any reason running MOSFETs with PWM instead of DC Servo control wouldn’t work for a DC Load?

author: bktemp
author_rank: Super Contributor
timestamp: August 26, 2014, 06:21:47 am
content: Think again: When the mosfet only acts as a switch, where does the power go? The only way this could work is building a stepup converter pumping the voltage into a fixed resistor, so simulating a variable load.

author: timb
author_rank: Super Contributor
timestamp: August 26, 2014, 06:49:50 am
content: When the switch is on, the power goes through the switch, where some of it is turned into heat due to the resistance, the rest goes to ground. The same thing that happens when you run the MOSFET in linear mode. The difference is that in linear mode you’re controlling the power flow by how much the switch is open. In switching mode you’d be opening the switch fully each time, but controlling how long it’s on.If you switch at a suitably fast speed and use a few large capacitors to smooth the ripple out…Or am I thinking about this from the wrong perspective?

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 26, 2014, 07:05:49 am
content: I'm working on a similar project (1600W) and am using very expensive IXYS MOSFETS (SOT-227 package) that are intended for linear operation. In my circuit each MOSFET has it's own current sense resistor and feedback loop. This ensures equal current sharing between devices to within 1%. The linear MOSFETs ensure that they are reliable in linear operation.Note that the SOA of switching MOSFETs indicates that they are suitable for linear operation and are dissipation limited only. This is incorrect and the semiconductor manufacturers have admitted this. They are not reliable when operated within the SOA at high Vds and moderate current hence the emergence of special linear MOSFETs.Before I opted for the linear MOSFETs I looked into using switching MOSFETs and PWM to discharge a capacitor bank via a high power load resistor. It can be done using PWM and probably has some advantages over the linear option. I opted to go the linear route as there was no noise produced (serious considerstion for this application) and cost wasn't too critical.Dick

author: bktemp
author_rank: Super Contributor
timestamp: August 26, 2014, 07:11:09 am
content: Quote from: timb on August 26, 2014, 06:49:50 amYou mean using the Rdson as the load resistor?The current flowing will be extremely high, also the ripple current. At high voltages the current will be so high that the mosfet die melts almost immediatly. Just select a mosfet, use your input voltage and calculate the peak power dissipation, it will be in the kW region.For low voltages it could work, but there is no benefit over linear operation: Some mosfet datasheets show SOA curves for DC. At low voltages, you can use the full power dissipation capability.At higher voltage, the SOA curve gets smaller and your method even increases the current. It will be counterproductive.

author: timb
author_rank: Super Contributor
timestamp: August 26, 2014, 08:26:32 am
content: Quote from: Richard Head on August 26, 2014, 07:05:49 amInteresting idea on discharging the capacitor bank into a large load resistor. And yeah, the IXYS parts is what I was looking at as well. I’ve read some of their app notes, along with AN’s from MicroSemi.My original plan was to monitor low value current shunts on each FET with INA213s. The idea was to use a TI C2000 processor to control the feedback to the controlling op-amps.What have you found in terms of heatsinks for your project? I’m still looking, butOhmite’s C40 Series looks nice.(You can double them up and attach a fan directly to it.Another option would be something like this from eBay.The idea being you could tap sides of the case and mount the FETs directly.

author: dannyf
author_rank: Super Contributor
timestamp: August 26, 2014, 10:34:42 am
content: QuoteNo. There are gazillions of them doing precisely that.There must be something unique about your application / understanding / terminology that leads you to that particular conclusion.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 26, 2014, 12:23:26 pm
content: Quote from: bktemp on August 26, 2014, 06:21:47 amEven better: step-up the voltage and feed it back to the source's input to reduce your power supply testing power bill so you only pay for conversion losses... you save 70-80% of your energy costs compared to dumping all that power into resistors!

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2014, 06:15:42 pm
content: What kind of derating would be necessary on MOSFETs intended for switching applications to use them in linear mode with appropriate ballasting?Do any of the manufactures provide this information?

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 26, 2014, 06:22:30 pm
content: Quote from: timb on August 26, 2014, 05:58:47 amIRF’s DirectFET is used specifically for high speed applications, used to minimise package parasitics what is so high speed about your DC load?

author: tom66
author_rank: Super Contributor
timestamp: August 26, 2014, 06:29:52 pm
content: Quote from: DanielS on August 26, 2014, 12:23:26 pmEven better, feed the output of your boost converter into the input so you can generate infinite power**!

author: nctnico
author_rank: Super Contributor
timestamp: August 26, 2014, 06:56:58 pm
content: I'm also working on a dummy load and choose to use resistors and switch these into various combinations to create a wide range of resistances. The advantage is no control loop and resistors may run much hotter than transistors so you can get away with a smaller heatsink. I've build a similar dummy load for a customer and it works pretty well.If you really want to build a linear dummy load I'd always choose transistors. The collector of a transistor is a natural current source (or sink in this case) so you only need a slow control loop to take care of thermal runaway. In most cases you want to set a dummy load to a certain current.

author: Pedram
author_rank: Regular Contributor
timestamp: August 26, 2014, 07:00:38 pm
content: IRFPx50 (like IRFP150 ) works well on semi-high current loads... refer tohttp://www.eevblog.com/2012/05/23/eevblog-281-bk-precision-8500-electronic-load-teardown/

author: dannyf
author_rank: Super Contributor
timestamp: August 26, 2014, 07:06:05 pm
content: QuoteMost SOA or temperature-based derating.The role of thumb is that they tend to dissipate no more than 1/3 of what their package is rated for: 10w for to220 and 30w for to247/3p.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 26, 2014, 07:18:01 pm
content: Quote from: Pedram on August 26, 2014, 07:00:38 pmSpot on.I've noticed they parallel them, I still dont get why those TL074 quad are arranged that way.

author: Pedram
author_rank: Regular Contributor
timestamp: August 26, 2014, 07:31:01 pm
content: Quote from: diyaudio on August 26, 2014, 07:18:01 pmi'm not sure about BK's one but this agilent one use separate resistor and 2 opamps for each mosfethttp://cp.literature.agilent.com/litweb/pdf/5951-2828.pdf( page 90 )

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 26, 2014, 09:55:08 pm
content: Quote from: Pedram on August 26, 2014, 07:31:01 pmI have this manual its a good reference document.

author: timb
author_rank: Super Contributor
timestamp: August 26, 2014, 09:57:58 pm
content: So the BK Precision unit is using those resistors for balancing *and* current sense? Interesting, that's what I was suspecting after watching a tear down video of it. (Gerry's I think; Dave didn't go into much detail about them in his.)Is there a minimum size that should be used for balancing? I'm planning on using a 0R002 resistor on the low side of each FET.Sent from my iPhone

author: tom66
author_rank: Super Contributor
timestamp: August 26, 2014, 10:21:07 pm
content: 0.002 ohm is 18 times smaller than IRFP150N on resistance, how much difference will it make? The lead resistance is probably at least that much.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 26, 2014, 10:26:36 pm
content: Quote from: tom66 on August 26, 2014, 10:21:07 pmNote the RDS on is only relevant in switching applications. however taking current samples from shunts at that ohmic region is another story, current shunt theory should unveil the answer.

author: timb
author_rank: Super Contributor
timestamp: August 26, 2014, 10:38:16 pm
content: Sorry guys, I meant 0.020, not 0.002! Stupid phone. That said, I've gotten very accurate current readings with an INA215 and 0.005 shunt before, so 0.002 would be possible assuming the right layout.Sent from my iPhone

author: dannyf
author_rank: Super Contributor
timestamp: August 26, 2014, 11:08:06 pm
content: QuoteThe size of the current sensing resistor has no relation to the Rds. As long as the current sense amplifier has sufficient gain, the current sensing resistor can be as low as you want.

author: Mr Carlsons Lab
author_rank: Newbie
timestamp: August 26, 2014, 11:26:41 pm
content: Quote from: dannyf on August 26, 2014, 11:08:06 pmYou may need this amp for that low resistance though

author: timb
author_rank: Super Contributor
timestamp: August 27, 2014, 02:05:27 am
content: Right, I understand that. I'm asking if I'll need additional resistance to balance the FETs. (I'm talking about running in Linear mode here, not PWM.)Sent from my iPhone

author: DanielS
author_rank: Frequent Contributor
timestamp: August 27, 2014, 03:08:24 am
content: Quote from: timb on August 27, 2014, 02:05:27 amIf you want to parallel multiple MOSFETs, a source resistance between the source and sense resistor would make the circuit less sensitive to slightly mismatched devices.Or you could give each MOSFET its own error amplifier/integrator and make each channel track 1/Nth of the desired current.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 27, 2014, 03:52:44 am
content: Hi,Check this project:https://www.eevblog.com/forum/projects/dynamic-electronic-load-project/I show an example of how to use one op-amp per MOSFET. I used two MOSFETs but this could be extended to more.I also show how to stabilize the control loop.Regards,Jay_Diddy_B

author: timb
author_rank: Super Contributor
timestamp: August 27, 2014, 04:20:25 am
content: Quote from: DanielS on August 27, 2014, 03:08:24 amOh right, duh. I’m not sure what I was thinking! I will be monitoring each FET’s current individually and controlling them individually as well, so I won’t have to worry about the balancing resistors. That’s what I get for posting without some caffeine in me!

author: timb
author_rank: Super Contributor
timestamp: August 27, 2014, 05:17:02 am
content: Quote from: diyaudio on August 26, 2014, 06:22:30 pmWell, if you read my first post, my original plan was to control the MOSFETs via PWM instead of using the usual Linear method used in DC Loads. That would be a high speed application.Beyond that, the DirectFET package isn’tonlyused to reduce package parasitics. It’s also used in applications where space and optimal cooling are the primary concerns. I’ve seen a dozen or more of them crammed on a small board used for 3-phase motor control. It was for an electric scooter driver; the guy was able to mount a nice aluminum heatsink on top of them. The whole thing was about 1/4 the size of a conventional driver board with the same output power.Because the DirectFET package is essentially a MOSFET die with the drain bonded directly to the metal case and other pads coming off the back. That’s what gives it the low parasitics. It’s also what gives it up to 80% greater cooling efficiency compared to other packages. (Specifically SMD packages like D2PAK, which generally have to be cooled through the backside of the PCB.)So I was thinking, it would be nice to make a super compact, high power DC Load with a bank of them. But since I’ve scrapped the idea of PWM control I guess that’s out the window now. Though, I might still evaluate a few of them and see how they work in Linear mode.Quote from: Jay_Diddy_B on August 27, 2014, 03:52:44 amThanks, I’ve seen a similar method in other DIY Load projects as well. Your thread will be an interesting read!

author: mzzj
author_rank: Super Contributor
timestamp: August 27, 2014, 10:14:37 am
content: Quote from: timb on August 27, 2014, 02:05:27 amIf you use unmatched mosfets you need unpractical source resistor size, probably something that drops 2-3 volts with the intented full maximum current. With Vgs-th matched mosfet you can probably manage with 500mv sorce resistor drop. You will probably now see why independent current control loop for every mosfet looks viable option..

author: timb
author_rank: Super Contributor
timestamp: August 27, 2014, 01:09:58 pm
content: Quote from: nctnico on August 26, 2014, 06:56:58 pmThat's a rather interesting idea, actually. You can pickup rather high current BJTs for next to nothing. You can even get 20 and 30A 200V Darlingtons in TO-247 for under a dollar. (Or one of my favorite packages, the classic TO-3!)There's got to be some downside to using BJTs over FETs, or everyone would be doing it, right?Sent from my iPhone

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2014, 01:28:50 pm
content: Quote from: timb on August 27, 2014, 01:09:58 pmContinuous base current requirements are high although a darlington configuration helps with this.BJTs suffer from secondary breakdown at high voltages which will limit current capability. Vertical MOSFETs suffer from a similar problem however which it is not well documented or specified.

author: nctnico
author_rank: Super Contributor
timestamp: August 27, 2014, 01:59:14 pm
content: If you look at SOA graphs from both MOSFETs and BJTs you'll see they are very similar. A BJT needs a lot of current but that can be helped by using a darlington emitter follower. MOSFETs have a large input capacitance which can have a negative effect on the control loop response which is most problematic in constant current applications.But still it may be wise to consider using resistors. For my 1kW dummy load I have a heatsink with a thermal resistance of a about 0.4K/W. At 1kW it's temperatur will rise 40 degrees making it 70 degrees. When using transistors at a maximum die temperature of 120 degrees I would have 120-70=50 degrees to spend between the die and the heatsink. A transistor mounted (isolated) on a heatsink easely has a thermal resistance of 1.5K/W. That means that the maximum dissipation for the transistor is 50/1.5=33W. In order to dissipate 1kW I would need at least 30 transistors. Preferably more. Semelab has some nice audio BJT transistors BTW.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2014, 02:59:10 pm
content: Quote from: nctnico on August 27, 2014, 01:59:14 pmThey have some nice MOSFETs designed and characterized for linear operation as well.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 27, 2014, 05:46:09 pm
content: Quote from: timb on August 27, 2014, 01:09:58 pmYou can use BJT's and get a fairly good response ,I have built them ,one of the reasons that most use mosfets is because the loads are speced to work down to a few mV input's .With a Mosfet you have the required low Rds to enable highish current's even at low DUT input voltages of say =<100mV .With a Darlington you can't work below input's of ~0.7 V because of min Vce drop. Single BJT solve that but then you have the problem of base current required to drive them .Sometimes you see a small mosfet driving single BJT as a Darlington pair which solves the base current drive prob ,but stilll leaves you with 0.6V dropout .If your working with higher Dut voltages then this minimum V drop might not mean anything to you ,but for a standard bench load then 'minimum resistance' or sometimes stated as 'min burden Voltage' is usually given as a spec .And you get that lower easier with mosfets .Regards

author: timb
author_rank: Super Contributor
timestamp: August 28, 2014, 12:59:41 am
content: @nctnico How are you switching the resistors in and out to vary the load current/voltage?Sent from my iPhone

author: dannyf
author_rank: Super Contributor
timestamp: August 28, 2014, 01:10:34 am
content: QuoteUse mind control?or dark force,