Help checking correct substitute for KMB2D0N60SA mosfet

[re-atari]

Hi all,

I'm looking for advice in the selection of a replacement for a defective KMB2D0N60SA mosfet. This one is used in a standby PSU, where it controls external current limit of a TOP258MG off-line switcher. I don't know why and how the mosfet died. I measured the other components on the PCB with my DMM, and they all appear to be OK.

Finding a source for this mosfet proves to be quite a challenge. I either end up with brokers in Asia that only do deals in large quantities or a webshop that charges $ 10 shipping for a part costing 20 cents

So, I started looking for a readily available substitute. Luckily I could find its datasheet, which showed it to be a relatively standard run-of-the-mill N-channel type. Using this datasheet and alltransistors.com/mosfet/crsearch.php's mosfet cross-reference search engine, I found a number of more-or-less equivalent types. The fact that the replacement needs to come in a SOT23 package did complicate finding a suitable one a bit. I ended up with a Vishay part (Si3208 to be more precise) that (according to its datasheet) appears to fit the original mosfet's specs largely.

Before ordering, I would like the BadCaps experts' opinion, though, to make sure this really is a proper replacement.

The specs of the original part are:
Vds = 60V
Vgs = +/- 20V
Vth = 1.5V
Id = 2A
Is = 1A
Idss = 0.5 uA @ Vgs = 0V, Vds = 60V
Igss = 100 nA
Pd = 1.25W
Rds(on) = max. 160 mOhm, typ. 125 mOhm @ Vgs = 10V, and max. 220 mOhm, typ. 155 mOhm @ Vgs = 4.5V
Id(on) = 6A @ Vgs = 10V, Vds => 4.5V, and 4A @ Vgs = 4.5V, Vds => 4.5V

The Vishay Si3208 looks like a good substitute. Its specs are:
Vds = 60V
Vgs = +/- 20V
Vth = 1V
Id = 2.3A
Is = 1.39A
Idss = 1 uA @ Vgs = 0V, Vds = 60V
Igss = 100 nA
Pd = 1.66W
Rds(on) = max. 156 mOhm, typ. 130 mOhm @ Vgs = 10V, and max. 192 mOhm, typ. 160 mOhm @ Vgs = 4.5V
Id(on) = 8A @ Vgs = 10V, Vds => 5V

The Vishay part has better specs in general, but Rds(on), Vth and Idss differ with the original part. I don't know if these have to be completely identical or closer to the original part's values.

What do your expert eyes think, can I safely use a Si3208 as a replacement?

re-atari

[Agent24]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Do you have a schematic of the circuit it's used in?

Are you sure Si3208 is the correct number? I can't find that part easily.

[R_J]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Did you mean Vishay Si2308cds by chance?
The specs look very close, that very small difference in Rds on will not effect anything, It's only a difference of 4 miliOhms

[re-atari]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Thank you both for your replies. Much appreciated!

No problem of course, here's the schematic of the standby PSU. The mosfet in question is located immediately to the left of the 9-pin TOP258 IC.

The specs I mentioned in post 1 are of a Vishay Si3208BDS, a webshop here has them in stock. It costs just 66 cents, with shipping at € 4.95. I might as well order a few extra spares, just in case.

Looking at the schematic, I cannot see a reason why the mosfet blew. I measured all other components on the faulty PCB with my DMM (on diodetest) and compared it with my measurements on an identical working PCB. The mosfet was the only component that gave different values, and after desoldering turned out to be defective. I checked the caps with my ESR meter, and they were all OK. I don't know yet if the TOP258 IC is defective, measuring the IC pins on the 2 PCB's gives identical results. They are only € 3 ea, so it's not the end of the world if it turns out blown as well and needs to be replaced. For now I think it's OK.

I've attached the KMB2D0N60SA and Si3208BDS datasheet as well.

re-atari

[re-atari]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Checking the standby PSU schematic and the datasheets I noticed every manufacturer has a different way of numbering the mosfet's pins. Denon calls the gate pin 3, drain pin 1 and source pin 2. KEC (KMB2D0N60 manufacturer) calls gate pin 2, drain pin 3 and source pin 1, where Vishay says gate is pin 1, drain pin 3 and source pin 2

Thankfully the numbering of the various pins is the only aspect in which they differ, the parts all have the same signal on the same pin. Gate is the pin bottom left pin, source bottom right and drain is the top pin
Isn't the pin numbering of these parts standardised by JEDEC?

Looking at the Denon schematic, could a leaking zener ZD4160 (leads to the mosfet gate) be responsible for the mosfet having failed? I measured it in circuit with my DMM on diodetest and compared the reading with a known good identical standby PSU, and the readings were just about the same. I haven't measured it connected live to 220V AC. I could just replace the zener with a new one, of course, to be on the safe side. The 1M resistor between gate and source checked out OK, as did the other components between the AC inlet and the mosfet's gate/source.
What's your opinion on this?

re-atari

[Agent24]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

I'm not sure about the pin numbering differences. Maybe when they drew the schematic they weren't sure on the component footprint they would use, and never back-annotated it after the PCB was designed. Lots of service manuals have 'mistakes' like that.

Most of the time the schematic is never drawn for servicing, only for design/production. Then whoever writes the service manual just puts in whatever schematic was available. Often they never go to the trouble to write proper service manuals with listed waveforms\voltages etc and describe circuit operation.

Anyway...

What was the failure of the transistor? ZD4160 could be the problem if it was shorted\leaky, but the FET could have just failed by itself.

I see they write:

The EXTERNAL CURRENT LIMIT (X) pin can be used to reduce
the current limit externally to a value close to the operating peak
current, by connecting the pin to SOURCE through a resistor.
This pin can also be used as a remote ON/OFF input.

I don't think they are using it to adjust the current limit, it looks more like an ON/OFF control by way of detecting the presence of the AC line power.

[re-atari]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Most of the time the schematic is never drawn for servicing, only for design/production. Then whoever writes the service manual just puts in whatever schematic was available. Often they never go to the trouble to write proper service manuals with listed waveforms\voltages etc and describe circuit operation.

Yeah, it looks like manufacturer's focus these days lies with hastily announcing and introducing new equipment that has 'all the latest technology and standards' (that will of course be obsolete or abandoned in 3 years' time). No time for after sales activities like repair jobs. If it fails, too bad, just replace it and again have 'all the latest technology and standards' (for the time being, that is).
We probably have to be lucky that service manuals are not just a pile of handwritten sketches or drawings. Luckily the difference between the parts only lies in the way their pins are numbered in the datasheets, the parts themselves are pin-compatible and interchangeable. It would be quite a challenge to connect reroute wires on a SOT23 device.

What was the failure of the transistor? ZD4160 could be the problem if it was shorted\leaky, but the FET could have just failed by itself.

I checked all components on the failing standby PSU for obvious faults like open R's or shorts with my DMM on diodetest, but found nothing. I then got an identical PCB on loan, by doing measurements on both PCB's and comparing them I found the failed mosfet. After desoldering and measuring it, the values show it is defective. That is, they are not how a mosfet should typically measure. Measuring the zener in circuit on both PCB's gives more or less identical values. Not shorted, so probably OK. I didn't do any measurements to the standby PSU live connected to AC, though.

I don't think they are using it to adjust the current limit, it looks more like an ON/OFF control by way of detecting the presence of the AC line power.

Your suggestion that it's probably used as an On/Off switch makes sense, as it is a standby PSU. In a Denon X2000 receiver to be more precise.

Some background info:
I got this receiver in non-working condition, and have already fixed a number of damages done to it by the previous owner. In his efforts to fix it himself, he just made matters worse. Lifted and missing traces, loose and broken loose connectors, to name a few. I have already fixed all of these damages, and think I now have the receiver back in the state it was when it began showing the initial failure.
The error the receiver now shows is a boot-loop, where after pressing the on/off switch it would permanently repeatedly startup and switch off in a 1 second cycle. It appears this is not an uncommon failure with Denon receivers.
After installing the working standby PSU, my receiver started up and worked OK, so my standby PSU must be the cause of my receiver failing.
I then started the above mentioned measuring of the 2 standby PSU's and compared their vaules, and as mentioned there then found the failed mosfet. I didn't do measurements on the standby PSU live on AC. I only measured the 5,2V it has to output to the HDMI and CPU board, and found this voltage rises to 5,2V and falls to 0V with the rhythm of the bootloop.

re-atari

[re-atari]

Re: Help checking correct substitute for KMB2D0N60SA mosfet

Hi All,

I finally got around to order a replacement mosfet. As was kindly suggested, the Vishay item indeed turns out to be a valid drop in equivalent for the defective Korean one. As they were only 60cents each, I decided I might just as well order a few extra spares. You never know, they might come in handy some day

After soldering in the replacement I did some final measurements on both the mosfet and the surrounding components. Funny, all of a sudden resistor R4145, value 1M and connected between the mosfet's Gate and Source (probably as a protection for the mosfet) read open circuit. I did measure its nominal value a few weeks earlier. But after desoldering, it still read open circuit, so had to be replaced as well. I think this resistor was already flaky, and this was the reason the mosfet had died in the first place.

I didn''t want to go through the trouble of online ordering a single SMD resistor, so just used a standard type I had in my spare parts box. It took a little effort bending both wires and soldering them in place.

After switching it on, the receiver immediately came to live without a problem, where it had been locked in repeatedly switching on and off previously. The receiver has been test running just fine all evening. No overheating, no strange clicking of the STDBY relay, no 'electrical' smell. Looks like a result to me

re-atari