Can I replace 70 ampere mosfet with 60 ampere?

[Gaber Mohamed Boraey]

Hello everyone
I repair ups with burned mosfets , the mosfets are 200v 70 ampere “ 20N70” , and I only have irfp59 “ which is about 59 ampere and 200v ,

do you think ampere can reach the 70 ampere at full load? , or it’s not safe if I replace with 10 ampere lower mosfet?

Please let me know how you think, I think designer only chose 70 ampere for safety, but for reality current can not reach this value

 

[D.A.(Tony)Stewart]

Why did the FET burn up if rated for more than you think is necessary? You may try it, but will you test the worst case heat rise safely??

 

[KlausST]

Hi,

it´smore than the current rating that makes a MOSFET suitable or not. Like:
* switching time
* r_th
* R_DS_ON
* SOA
* body diode
...

I´m with Tony: There is a reason why the original one burnt. Installing any other type of MOSFET may be risky.

Klaus

 

[FvM]

I'm unable to identify transistor "20N70" with 200V 70 A rating. Do you mean 70N20?

Neither "IRFP59" appears to be commonly known. Please clarify.

 

[danadakk]

The forum often gets asked of a device with lower ratings can be used
or substituted.

To answer professionally one has to know what the designer decided his
margins needed are. Especially on designs that concern human safety.

MOSFETs typically more involved because of switching characteristics
as well as DC ratings.

So smart choice is use same part. When that is no longer available then
you pick parts as close as possible, and evaluate them in design. Hence
must or should know a lot about architecture of design.

Or just throw mud against the wall and hope it sticks as the saying goes.


Regards, Dana.

 

[Gaber Mohamed Boraey]

I'm unable to identify transistor "20N70" with 200V 70 A rating. Do you mean 70N20?

Neither "IRFP59" appears to be commonly known. Please clarify.

Yesterday 70N20 sorry

--- Updated Dec 31, 2023 ---

Yes 70N20 sorry

--- Updated Dec 31, 2023 ---

Why did the FET burn up if rated for more than you think is necessary? You may try it, but will you test the worst case heat rise safely??

Don’t know reason, maybe overload , my job is only fix , I opened the ups and found mosfets are shorted and can not get same part locally

That’s why I’m asking here
I still little worry of trying , but college said designers often consider 35% higher for safety

 

[danadakk]

I still little worry of trying , but college said designers often consider 35% higher for safety

That MAY be a current or a voltage rating, but not the capacitance ratings hence
switching performance and power dissipation of the driver driving the MOSFET
or a whole lot of other things. College is vastly over simplifying something they
have not verified, rather pulled out of thin air, the 35%. I know designs that used 5%,
10%., and ones that used 100%. The design of driving a high power MOSFET,
keeping it operating in its safe area (SOA) is not trivial. The motor guys,
welding guys, plasma cutter guys, know a lot in this area and do not make a
substitute based on "well my college said".......

Do not guess, it will catch up with you in very ugly ways, including lawsuits and
death in some product areas.

Post some links to the two datasheets so folks can do a better examination of the
problem.


Regards, Dana.