MOSFETs heat up when driving in switching circuit - Page 1
Forum section: electronics / projects

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 27, 2024, 02:42:23 pm
content: I was testing some gate drive circuits for a switchmode power supply and I came across a strange issue. I couldn't get some MOSFETs to switch properly, causing a big D-S voltage drop and them heating up significantly. I then tried out different MOSFETs and some didn't experience this issue, were driven properly and there was minimal drop/heating. I'm trying to understand why. The totem pole transistors never heat up, so I don't think there's insufficient drive current.Attached is the circuit with some waveforms of the gates and load. The transistors I've tested:- IRF530: 0.04V drop- 6R650CE (genuine) - 0.75V drop- IRF640 (both genuine and chinese) - 6.7V drop- K2645 - 9.2V drop- IRF510 (genuine) - 6.7V drop- IRF840 - 5.6V dropAm I missing something?

author: madires
author_rank: Super Contributor
timestamp: December 27, 2024, 03:00:29 pm
content: My guess is exactly that, a current drive issue caused by C9 and C12.

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 03:21:14 pm
content: Can you show us one switching cycle to see the rise and fall times of the gate voltage?Is it measured dirctly across R3/R6?What is the reason for C9 and C12?

author: wraper
author_rank: Unknown
timestamp: December 27, 2024, 03:32:24 pm
content: What's that EE10 IC? Not to say where its power? There is none on the schematic.Ah it's a transformer that's not drawn as transformer. Why in hell you cannot use a proper symbol, not to say describe anything properly?QuoteNot 100% clear what's what, but I guess upper one is for high side gate and lower on for output. There need to be 2 waveforms for gates, one for output. High side side mosfet needs at least a few volts above +15V (relative to GND) on the gate even for MOSFETs with the lowest possible threshold voltage.

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 03:52:10 pm
content: I assume it’s a transformer with two secondary windings - as it’s connected between gate and source, it will deliver a gate drive voltage independent of the potential.

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 27, 2024, 04:02:06 pm
content: QuoteSorry about that. It's an EE10 transformer, didn't have a symbol so I drew a quick one.QuoteThe gate signal is measured directly across R3 (it's identical for R6, I just don't have an isolated probe to measure them simultaneously).QuoteThose are DC restore caps so that the signal is restored to 0-15V instead of -7.5-0-7.5.

author: jnk0le
author_rank: Regular Contributor
timestamp: December 27, 2024, 04:32:08 pm
content: If it's supposed to run as a H bridge (or half-H), then you are likely getting shoot-through due to not having enough dead time.

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 27, 2024, 04:35:31 pm
content: QuoteBoth turn on at the same time (it's for a 2-switch forward topology). And both get the same waveform, same drops, heat up the same.Also increasing the drive voltage to more than 15V does nothing.Attached is a switching cycle.

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 06:06:18 pm
content: Can you even zoom in more to the slopes?Stretch it over at least three divs.Switching appears to be below 100ns though.I have no clue so far, I have to admit

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 27, 2024, 06:33:24 pm
content: I'll get some more waveforms tomorrow when I get back to the lab.Meanwhile I replaced the coupling caps with 2.2uF polyester, no change.I then shorted each transistor from drain to source, leaving the other switching, and it still gets hot.

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 06:43:45 pm
content: Btw, is the measurement from a good or a bad MOSFET?

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 27, 2024, 06:48:00 pm
content: It's using the 6R650CE, new and genuine (bought from Farnell).

author: inse
author_rank: Super Contributor
timestamp: December 27, 2024, 07:06:45 pm
content: K2645 would be most interesting

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 28, 2024, 01:58:10 pm
content: Some rise and fall times attached.I went and tested each transistor by simply applying a 15VDC voltage to the gate, and trying to turn on the bulb, then measured VDS drop:IRF530: 0.15V6N60C: about 7VIRF740: 2.5VIRF510: 0.5VK2645: 4.5V6R650: 1VWhat is going on?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 29, 2024, 06:04:12 am
content: Some are damaged.Why use the transformer, you have a current sense that is not isolated at all ?That will have slow turn off issues if the drive suddenly ceases.A resistor may be a better test load than a light bulb.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: December 29, 2024, 11:28:45 am
content: Quote from: bogdan2014 on December 28, 2024, 01:58:10 pmHave you thought about using MOSFETs with much lower "on" resistance? I use DMT6002 types in high current protection circuits, they have a typical "on" resistance of 2~3 milli Ohms, the IRF530 has 160 milli Ohms when fully switched on.The datasheet is:https://www.diodes.com/assets/Datasheets/DMT6002LPS.pdfMouser have them in stock, around 2,200, and available in 1 off quantities for $1.73 each.SJ

author: kosine
author_rank: Regular Contributor
timestamp: December 29, 2024, 11:45:57 am
content: Multiple FETs in parallel can also reduce power dissipation significantly. Since power is RI^2, sharing the current between devices makes a considerable difference.

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 29, 2024, 02:17:02 pm
content: Quote from: PCB.Wiz on December 29, 2024, 06:04:12 amThis topology has a high side MOSFET that needs a gate transformer (half bridge drivers don't work because both transistors turn on at the same time). This transformer does not offer isolation, it's just for driving the gates. The isolation is done by the power transformer, and everything in this circuit is on the primary side.QuoteI could, sure. But for the ones that I tested, Rdson is much higher than the datasheet figures. So I thought it's a matter of improper drive.

author: langwadt
author_rank: Super Contributor
timestamp: December 29, 2024, 02:30:39 pm
content: Quote from: bogdan2014 on December 29, 2024, 02:17:02 pmthere are "half bridge" drivers that can turn on both transistors at the same time, like the IR2101

author: SCSKITS
author_rank: Regular Contributor
timestamp: December 29, 2024, 04:22:47 pm
content: Never mind, I missed the switcher part..I do not see the point of using two MOSFETs to turn on the load.Or is the load just a test load, and not the real one?Should be able to remove the top FET.Or, if for some reason the top FET is required, maybe use a P-Channel MOSFET on the top then only have to pull the gate down to ground.

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 29, 2024, 04:40:26 pm
content: Quote from: SCSKITS on December 29, 2024, 04:22:47 pmI've attached the full schematic of the power supply.

author: SCSKITS
author_rank: Regular Contributor
timestamp: December 29, 2024, 04:52:11 pm
content: Do not over think the MOSFETs.Pick a MOSFET that can handle the current with a low enough gate capacitance to be able to turn ON/OFF quickly.If there is too much gate capacitance, you may be in linear operation too long.

author: inse
author_rank: Super Contributor
timestamp: December 29, 2024, 06:02:19 pm
content: Quote from: bogdan2014 on December 28, 2024, 01:58:10 pmThat is the irritating fact as rise time and gate charge are irrelevant.Picking the 6N60C: if it’s tested with a 12V/20W bulb, 5V will remain for the bulb making it consume ~1A.That means the Rdson of the MOS would have to be ~7Ohms -> impossible as it‘s specified 2Ohms (oops!) at 10V G-S.I just noticed your MOS selection is rather diverse

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 29, 2024, 06:33:06 pm
content: I tested the Rdson (at 12V) of all of my power MOSFETs for fun:IRF740: 550mohm specified, 1.9ohm measuredK2645: 1ohm specified, 2.5ohm measured3N80: 4.8ohm specified, 6ohm measuredIRF640: 180mohm specified, 700mohm measuredIRF710: 3.6ohm specified, same measured6N60: 1.7ohm specified, 4.2ohm measuredIRF840: 850mohm specified, 1.5ohm measured12N60: 800mohm specified, 4.2ohm measuredIRF530: 90mohm specified, 500mohm measured4N80: 1.95ohm specified, 1.6ohm measured6R650: 650mohm specified, 800mohm measuredI tested at least 5-6 from each batch, so it can't be that they're all damaged.

author: inse
author_rank: Super Contributor
timestamp: December 29, 2024, 06:39:30 pm
content: How did you measure the Rdson?With a constant current (1A, e.g.) or with a multimeter?

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 29, 2024, 07:55:41 pm
content: This time with a multimeter (resistance directly over D-S) and with a transistor tester.

author: inse
author_rank: Super Contributor
timestamp: December 29, 2024, 08:07:52 pm
content: It appears every measurement is flawed, your lab must be jinxed, thats the only explanation.Get an exorcist, quick!

author: bogdan2014
author_rank: Regular Contributor
timestamp: December 29, 2024, 08:35:15 pm
content: Quote from: inse on December 29, 2024, 08:07:52 pmAt this point it's the only explanation. I'd better move already.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 30, 2024, 02:27:11 am
content: Quote from: bogdan2014 on December 29, 2024, 07:55:41 pmIt takes a very good multimeter to measures mOhms.Maybe inject 100mA ~1A thru a power resistor, and measure the mV drop instead ?