MOSFET replacement advice - Page 1
Forum section: electronics / beginners

author: jmar
author_rank: Newbie
timestamp: August 12, 2015, 10:44:18 am
content: Hello all!First post here.I'm repairing a somewhat old Samsung LCD TV, mostly for training purposes. After a power surge the TV went out and i have debugged the power supply, deducting that a MOSFET has shorted out causing the fuse to blow immediately upon replacement.The original MOSFET is this one:https://www.fairchildsemi.com/datasheets/FQ/FQPF7N65C.pdfThe replacement one i found:https://www1.elfa.se/data1/wwwroot/assets/datasheets/rrSTM_Power_MOSFET_STx_10NK60Z_EN.pdfWith some help from a StackExchange post i have determined that the parameters appear to be equal or better to the original:http://electronics.stackexchange.com/questions/56629/how-to-find-a-replacement-mosfetParameterOriginalReplacementNameFQPF7N65CSTP10NK60ZFPId7 A10 ARds(on)1.2 O0.75 OIdm28 A36 AVgs(th)2-4 V3-4.5 V (is this ok?)Vdss650 V650 VAs far as i understand this should be a good enough replacement, but i would really appreciate some input!Thanks!

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2015, 11:19:16 am
content: Looks good to me

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: August 12, 2015, 11:31:10 am
content: Why not replace with original...they are available here in Australia from RS for $2 $AUD ea....RS UK should be able to obtain them.Blow me down...RS Sweden has them for 11,268Kr ea in pkt of 5http://se.rs-online.com/web/p/mosfet-transistors/6715301/?searchTerm=FQPF7N65C&relevancy-data=636F3D3226696E3D4931384E4B6E6F776E41734D504E266C753D7376266D6D3D6D61746368616C6C7061727469616C26706D3D5E5B5C707B4C7D5C707B4E647D2D2C2F255C2E5D2B2426706F3D313326736E3D592673743D4D414E5F504152545F4E554D4245522677633D424F5448267573743D46515046374E36354326Always best to try to use OEM and the OEM is cheaper than suggested replacement (although in pkt of 5).

author: firewalker
author_rank: Super Contributor
timestamp: August 12, 2015, 11:49:30 am
content: Also the smaller Rds(on) could lead to higher current that could be potentially damaging. It heavily depends of the circuit (I doubt there will be a problem).Alexander.

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2015, 11:52:34 am
content: If that's a mosfet you can easily obtain, slap it in and see.Im 95% sure it will work fine.AcHmed99 raise a good point though, about the package case.You want to check your not going to short something out using a non isolated case fet in place of an isolated one.

author: jmar
author_rank: Newbie
timestamp: August 12, 2015, 11:58:46 am
content: Thanks a lot for the helpful replies!I would definitely prefer to replace it with the same part, but the only sources i managed to find until RS was suggested has had shipping costs of close to 30 euros. RS is only 12, which seems more reasonable. I will most likely place an order for the original part with RS, but for the sake of discussion:The original is TO-220F, and the suggested replacement is TO-220FP. I understand they have slightly different thermal characteristics (source is again StackExchange,http://electronics.stackexchange.com/questions/128449/what-is-the-difference-between-to-220f-and-to-220fp-cases).I have added an image of the power supply board in case it would help anyone to elaborate a bit on the gate charge issue. The MOSFET in question is mounted to the large heatsink on the right edge of the board.

author: Psi
author_rank: Super Contributor
timestamp: August 12, 2015, 12:30:28 pm
content: The gate charge refers to the fact that the gate behaves like a capacitor. Needing to be charged up to switch on the fet.The gate charge tells you how much energy is needed to charge up the gate.This becomes more of an issue the faster you try and switch the mosfet on/off.You want it to move between on/off and off/on states very fast because time spent in the transition burns up a lot of energy as heat in the fet. Since the gate needs to be charged you need more current to charge it up quickly and reduce the losses.The reason this is an issue when driving the fet fast is that number of on/of transitions per second is higher.So more time is spent in this transition state and more heat lost. This is why mosfets get hotter when you switch them fast.If you were turning it on/off once per second it would have an entire second to cool down from this transition change heat. if you were turning it on/off at 1mhz it would melt into a ball of fireUnless you pump 10Amps into the gate to make it turn on ludicrous fast and reduce this heat loss. Note that i'm grossly over simplifying this trying to make it easy to understand. There are limits to have fast you can do things. (The datasheet provides the fastest on/off times possible under Turn on and Turn off times.)

author: jdraughn
author_rank: Regular Contributor
timestamp: August 12, 2015, 12:41:37 pm
content: Please post back and let us know if replacing the mosfet fixed it.

author: jmar
author_rank: Newbie
timestamp: August 12, 2015, 03:03:33 pm
content: That explanation makes a lot of sense, thanks Psi! What a great forum, very happy to be here.Will post an update once i'm done replacing the MOSFET.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: August 13, 2015, 01:46:35 am
content: Quote from: jmar on August 12, 2015, 11:58:46 amJeez... 12Euro's .....that's a bit rich...I get free free local delivery from RS in Australia