Why MOSFET fails? - Page 1
Forum section: electronics / general

author: Vytautas
author_rank: Regular Contributor
timestamp: February 13, 2022, 03:22:38 pm
content: Hello, everyone.I had ordered two soldering controllers from China for my double station project. The Hakko T12 works with it excellent. Heats up fast, excellent power control. Overall, happy with the controller. The problem is using it with the JBC C245 iron. Even though the C245 draws much more current than Hakko, the controller is supposed to work with this handle, according to the manufacturer. But the output MOSFET quickly failed. It is the D403, shown in the picture and it is controlled through the 470Ohm gate resistor (circled in the picture). The fail of this MOSFET is no surprise, as the datasheet for this component shows it is hardly capable of handling the current needed for the C245. So, I ordered much more powerful MOSFET to replace the original one. The one I chose is SQM120P06, which are a bit larger TO-263 package devices, but still possible to fit on the board. These are quite an overkill, and are able to handle a MUCH larger current, way beyond that required for the C245. Still, the MOSFET died after a time again! Meanwhile, the Hakko T12 works with no problems whatsoever.Could anyone guess what could be the reason for this?Thank you.

author: MadScientist
author_rank: Frequent Contributor
timestamp: February 13, 2022, 08:38:22 pm
content: How’s the heat meant to get conducted away. SMD devices tend to need large areas of copper to dissipate the heat. Doesn’t look like that’s available here.

author: thinkfat
author_rank: Unknown
timestamp: February 13, 2022, 09:02:10 pm
content: The mosfet is a P-Channel type, so it acts as a high-side switch? How is the gate driven, through separate driver circuit or directly from a GPIO of the controller? What is the source-drain voltage? Can you provide an oscilloscope trace of the gate-source voltage? What is the micro controller referenced to, can it pull the gate low enough, fast enough so that the mosfet is truly switching and not dwelling in its linear region for prolonged time? What is pulling the gate high again, how high, how fast?That mosfet can switch a lot of current, yes, but it only achieves a low RDSon if the gate voltage is at least 4.5V lower than the source, and it should return to source voltage quickly when it switches off (input capacitance is around 10-14 nF)

author: wraper
author_rank: Unknown
timestamp: February 13, 2022, 09:07:59 pm
content: 20V max G-S voltage of the replacement may be the issue. As it supposedly sits on the high side, gate likely is pulled straight to GND. Therefore you may need a MOSFET with 30V G-S voltage, considering 24V supply.

author: wraper
author_rank: Unknown
timestamp: February 13, 2022, 09:22:08 pm
content: Quote from: Vytautas on February 13, 2022, 03:22:38 pmYour statement has no connection to reality whatsoever. 70A max current is not enough, or what? Replacement you have chosen has no advantage over the original AOD403 at all. RDS(on)is the same, which means the same heat dissipation. Maximum current/power ratings do not matter here whatsoever since PCB heatsinking capability is the limit here. And FYI it has a higher current/power ratings only due to a larger package which can transfer more heat away. Again this does not matter since there is no huge heatsink to take any advantage of that. However the replacement also has10V5V lower maximum G-S voltage, which most likely is the cause of failure.EDIT: on a second thought, you are probably one of those beginners who think that the power rating of a transistor means how powerful load it can drive. This has no connection to reality at all. This figure means the maximum power transistor (not the load) can dissipate in ideal conditions with abnormally powerful cooling. And it has nothing to do with the power of the load it drives.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 06:29:58 am
content: Thanks everyone for helping me to learn. Concerning the 70A rating of D403, that is not for practical applications and no heatsinking, I suppose. The Safe Operating Area graph shows it is not even able to handle more than 4A (of DC) at 24V. It works well with Hakko, as that draws only about 3A maximum. But it dies when C245 is connected. I will try to replace it with one with higher Vgs. And, maybe, even solder a small copper plate to have a little heatsinking.

author: wraper
author_rank: Unknown
timestamp: February 14, 2022, 07:27:12 am
content: Quote from: Vytautas on February 14, 2022, 06:29:58 amWhich shows a total lack of understanding. RDS(ON)is 0.0062 OHm. Voltage drop over the MOSFET (V=I*R) at 4A is 4A*0.0062OHM=0.025V , not 24V. Which results in measly 0.025V*4A=0.1W power dissipation.

author: wraper
author_rank: Unknown
timestamp: February 14, 2022, 08:07:22 am
content: Yeah, and the gate resistor is 4.7k, not 470 Ohm. But original AOD403 looks suspicious. Probably it was a counterfeit part.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 10:28:40 am
content: Yes, I am trying to learn and do lack knowledge. (Hope, you are not amused by that). Just read this explanation and understood (hopefully, correctly) that not the supply voltage is that counts but the voltage dropped on the Rds: "The MOSFETs dissipate power and generate heat in the drain-source resistance: RDS(ON). Thermal constraints, including the ambient temperature and any heatsinking available for the MOSFETs, set limits on how much power can be dissipated. This maximum allowable power dissipation drives the selection of the MOSFETs based on the RDS(ON) value."Thank you. I hope, I will learn.

author: wraper
author_rank: Unknown
timestamp: February 14, 2022, 12:14:28 pm
content: Quote from: Vytautas on February 14, 2022, 10:28:40 amYes, that's right.

author: thinkfat
author_rank: Unknown
timestamp: February 14, 2022, 03:42:42 pm
content: I'd still like to see an oscilloscope trace of the gate voltage during switching. I find the 4k7 gate resistor somewhat suspicious. Even the original mosfet (if genuine) should be able to handle that current, but the resistor slows down the switching.The overall circuit is definitely daring. The AOD403 is quite close to its abs max rating for Vgs and adding a Zener diode to protect the gate would be advisable.

author: thinkfat
author_rank: Unknown
timestamp: February 14, 2022, 03:51:38 pm
content: Quote from: thinkfat on February 14, 2022, 03:42:42 pmThe SOT-23 marked with 'Y1', above the gate resistor, is likely a BZX84C11-V. If you trace out the circuit you might find it connects to the mosfet gate. I guess they already thought of adding the protection I mentioned.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 04:59:50 pm
content: Here is a quick copy of the oscilloscope showing 12V on the gate. (Sorry, my USB stick does not work with this scope, so a photo, not a screenshot.)

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 05:15:29 pm
content: And here is a shot of both gate to minus (yellow) and gate to source (blue) traces. I connected the minus of the scope to gate so as to have the common ground and not to create a short. So, in my limited understanding, I have no idea why the mosfet dies, if gate so source is only something like 12V.

author: thinkfat
author_rank: Unknown
timestamp: February 14, 2022, 05:51:04 pm
content: Can you speed up the timebase? I want to see one switching cycle only, once triggered on the rising edge and once on the falling edge. Make it so that the full transition is visible. Also, which type of mosfet is this, the original or your replacement?The voltage only going to 12V is what I expect, the BZX84C11 is an 11V zener diode. It is there to limit Vgs and protect the mosfet gate.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 06:08:25 pm
content: Here are the shots on rising and falling edges. The mosfet is the AOD403. Also, I suspected that the mosfets die on switch-on. So, took a single shot of the waveform when switching on in the last picture. Vgs is large and opposite polarity on switch on, do not know how to interpret that, my understanding and knowledge limited. By the way, these tests done with T12 tip, as C245 would blow the mosfet.

author: thinkfat
author_rank: Unknown
timestamp: February 14, 2022, 06:34:35 pm
content: Well, to me that looks like the gate is driven waaay too slowly. Nothing you can do besides maybe adding a heatsink. But even then, the junction-to-case thermal resistance is still there. I suspect that the mosfet gets killed thermally when you try to solder something and the regulation increases the duty cycle.The switching time should be in the 10-of-nanoseconds range, not hundreds of microseconds.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 14, 2022, 06:39:58 pm
content: Thank you very much. I must have been too optimistic about this Chinese controller.

author: wraper
author_rank: Unknown
timestamp: February 14, 2022, 10:02:17 pm
content: Quote from: thinkfat on February 14, 2022, 06:34:35 pmI doubt it fails from overheating. Most likely it uses a very slow control. Switching a few times in a second at most. Therefore gate drive speed is not that critical. As of waveforms, they are a total meaningless junk, as measurements were made in AC mode. Op should redo them properly in DC mode for them to have any meaning.

author: thinkfat
author_rank: Unknown
timestamp: February 14, 2022, 10:22:15 pm
content: Quote from: wraper on February 14, 2022, 10:02:17 pmhuh. now that you mention it, I was actually wondering why the rise and fall time was _that_ long. AC mode, doh. I should have payed more attention to what else was on the screen...

author: Vytautas
author_rank: Regular Contributor
timestamp: February 18, 2022, 07:07:37 pm
content: Instead of the AOD403 soldered in a FQB22P10. Used the C245 iron a lot today. So far, everything works fine. Maybe, the original Mosfet is truly a fake.Thank you for help.

author: wraper
author_rank: Unknown
timestamp: February 18, 2022, 07:29:05 pm
content: Quote from: Vytautas on February 18, 2022, 07:07:37 pmIt's a very bad replacement though. RDS(ON)is 20 times higher. It will have 0.5V voltage drop and 2W power dissipation in ON state.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 18, 2022, 08:14:47 pm
content: I counted similarly (learned that). And why is that a problem? The mosfet will not be able to dissipate it without cooling? Seems to be able (Ta - 3.75). Again, excuse my ignorance. I am concerned that if I order the original AOD403 mosfet (just to be sure its not fake), at 0,03 or so Voltage drop it will not be able to handle the current, if I understand the safe operating area graph correctly.

author: wraper
author_rank: Unknown
timestamp: February 18, 2022, 09:51:33 pm
content: FQB22P10 - Thermal resistance, Junction to ambient (minimum pad of 2oz copper) 62.5oC/W. As the pad on PCB seems to be minimal, thermal resistance should be about that. If it's switched on permanently (say when soldering something very heavy), MOSFET will be 125oC above ambient.QuoteYou used a much worse MOSFET as replacement and are concerned that AOD403 will not be able to handle a small fraction of what it's rated for. That graph clearly says RDS(ON)limited. Bold uninterrupted line on the left side does not show that it cannot handle 120A at low voltage drop, it shows there will be a higher voltage drop at larger current. Dotted line shows the actual SOA limit without taking into account if it's actually physically possible to achieve such current/voltage drop relation.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 18, 2022, 10:24:48 pm
content: Thank you again. Simple logic should have implied to me that if a component is able to handle a current at larger V drop, it should be able to do that also at lower voltage. Silly me.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 23, 2022, 03:35:43 pm
content: Hi, its me again. Bought ten AOD403 mosfets, as these were only 58 euro cents each. They are from a respectable supplier, so they must be original. Soldered in one. The C245 worked for a time and then again, at switch on, the mosfet shorted (I have to be on guard to switch the iron off instantly as it would get red very quickly). Hakko T12 handle works perfectly at least.

author: wraper
author_rank: Unknown
timestamp: February 23, 2022, 03:48:43 pm
content: That's strange. Another thing I can think about is PSU. AOD403 is rated for 30V, which is not a large margin with 24V power. Therefore increased voltage pulses caused by unstable SMPS at the moment when heater is turned off could kill the MOSFET. I suggest to look with oscilloscope what happens with power when JBC heater is used. Maybe set a trigger a bit above PSU voltage to capture any increased voltage pulses if they are present. What PSU do you use? I understand you are using external PSU, as controller itself came without it.

author: Vytautas
author_rank: Regular Contributor
timestamp: February 23, 2022, 04:05:36 pm
content: I do not use SMPS. I use a 240W torroidal transformer rectified through a 35A diode bridge and 8 paralleled 470uF capacitors. Could inrush current have an effect on switch on? But then, the same I suppose would happen to the controller used for T12? It is fed by the same supply. The voltage under load, by the way, is about 23.5V (transformer is about 19V).

author: wraper
author_rank: Unknown
timestamp: February 23, 2022, 04:13:33 pm
content: Quote from: Vytautas on February 23, 2022, 04:05:36 pmWhat matters is the rectified voltage when not loaded.