UPS - Blown Mosfet Cross Reference Question [ IRLB8748 ] - Page 1
Forum section: electronics / projects

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: October 12, 2012, 09:20:30 am
content: Hi ,Have This UPS With One Side Mosfet Blow Up .In India We Do Not Have The Same In The Market Commonly .Some Of The Common Mosfet Are Here1 : IRFZ44N " 55 Volt ,49 Amps,RDS 17.5 milli ohm "2 : P55N06 " 60 Volts , 55Amps ,RDS.022 Ohms " [ I Am Not Sure Of This ]Ps : The Original was 30 Volts ,78 amps ,4.8milli ohms ,I do not think there is much load on the ups as such .

author: Rerouter
author_rank: Super Contributor
timestamp: October 12, 2012, 09:36:42 am
content: neither of those 2 would be a safe subsitute, you will want the resistance close to the original (say 2-6 milli ohm,) atleast the same or larger current capacity, a higher voltage, and most importantly a very similar gate to source voltage and curve of current vs gate voltage,

author: Kremmen
author_rank: Super Contributor
timestamp: October 12, 2012, 09:53:06 am
content: All true and additionally, the gate charge information should be closely similar to the original to avoid issues with driver current sourcing/sinking capability if the switching frequency is significant.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: October 12, 2012, 10:25:56 am
content: Quote from: Rerouter on October 12, 2012, 09:36:42 amSir ,The voltage is more then enough in both the substitutes .Ampere i think is not important as i know load on ups is not much .RDS could be a issue here ,what you say

author: Rerouter
author_rank: Super Contributor
timestamp: October 12, 2012, 10:41:45 am
content: without more info i can only give a basic outline,in general you will want to match the current capacity because there is generally a reason to it,if you want us to agree with you cutting it down we need to know where it is, what its doing, some infor on the actual model UPS etc,from the current rating i would guess that its rated for about 500W of power draw for whatever its capacity,assuming its driven off a 12V lead acid, lets work out the worse case current for this thing,most low voltage shut offs are at 10.8-10.5V, lets say 10.8V, to draw 500W at mains, you would be drawing (500/10.8)almost 50A off the battery, but that is RMS current, it will be peaking at 1.414 times that, or 70A, close to the rating of the original, now while yours may be rated for less, i just want you to be aware there is generally a reason for picking such a high capacity mosfet,

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: October 12, 2012, 10:59:02 am
content: Quote from: Rerouter on October 12, 2012, 10:41:45 amWell,in india these are genrally 500 va or 650va ups running off 12 volt 7 ah battery .this company is beetal .i bet none of you have ever seen or used one

author: rsjsouza
author_rank: Super Contributor
timestamp: October 12, 2012, 11:29:15 am
content: Hi,What others are saying is something you can see in figures 1 and 2 of each of the transistor's datasheets:IRFZ44andIRLB8748.the IRFZ44 requires more voltage in its gate to provide the same current output when compared to the IRLB8748 - this is what Rerouter mentioned in his first reply.Also, the resistance drain-source (RdsON) is directly related to the device's maximum dissipated power (ohm's law), therefore if the circuit uses the transistor at its limit, then it becomes risky to use one with a higher resistance.Also, both the voltage and current are important if the transistor is swithing inductive loads. For one, compare the pulsed drain current (front page of the datasheet) of each transistor: it is 160A versus 370A. If the transistor is switching an inductive load, this parameter is significant.All in all, it always depends on where the transistor is placed in the circuit.

author: krish2487
author_rank: Frequent Contributor
timestamp: October 12, 2012, 01:41:20 pm
content: A relatively safer substitute is the STP80NF55As an indian, and moreover in the power electronics business myself, I can say with certainity that you will find them in india.However the Vds and the Ids are only a small part of the story as explicitly explained by others.Looking aat the gate charge and capacitance vis-a-vis the original IRL series the gate cap for the STP series is almost double and the charge is almost ten times.This effectively translates to a much "stiffer" gate drive to get the same performance as the original.I d look carefully at the driver stage and make the necessary changes to the drive circuitry, otherwise you are either left with a smoking pile of non functional electronics or a UPS that fails to regulate under load.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: October 12, 2012, 02:06:20 pm
content: Quote from: krish2487 on October 12, 2012, 01:41:20 pmWell,Hello And nice to meet you ,what place in india are you from ,i am from chandigarh .No point doing repairs on this UPS due to lack of parts .I have reassembled the same and would return the same back to the customer tommorow as arranging 2 mosfets for the same is not practical .

author: SeanB
author_rank: Super Contributor
timestamp: October 12, 2012, 03:37:18 pm
content: Amarbir, the most likely reason for them failing in the first case is the battery being connected in reverse, or using an old battery with a dead cell. Other cause is a poor heatsink, as most of the time they are run too hot with too small a heatsink area.

author: Amarbir[Lynx-India]
author_rank: Frequent Contributor
timestamp: October 12, 2012, 04:22:56 pm
content: Quote from: SeanB on October 12, 2012, 03:37:18 pmSean ,The Heatsink in This Case Should Not Be a Issue,I Will Click Some Photos And show you folks .The battery voltage when measure out of ups was 1.1 volts .I could still manage to charge the same and it could sustain the charge .Tested it on a battery tester that given proper load to a 7AH battery .Damn Thing Works .Well i do not know if the mosfets could be a issue but then i had no idea they had blown up i was getting approx 5 volts output in the charge wired to the ups .anyways the matter is closed cos i cannot find this mosfet here .i will send it to the service center and let them handle the case .

author: krish2487
author_rank: Frequent Contributor
timestamp: October 12, 2012, 07:15:47 pm
content: Theres your problem. SeanB hit it right on the head.a 12V Sealed lead acid battery when fully discharged, bottoms out at 10.5VIf you are reading 1.1 then the battery is dead.Replacing the mosfets will do you no good.A dead battery will still show as picking up the charge and show full float voltage in open circuit condition.Load it and the voltage drops fast.With the 500VA/800VA units which are , generally, discrete versions and not microcontroller based, the lower the battery voltage drops, the current draw from the battery goes up non linearly.As for the UPS, it is most likely a cheap chinese unit rebranded beetel.@SeanBYou are rightThe heatsinking is woefully inadequate with the smaller offline UPS systems(500-800VA).The idea being, the cheap one off units are designed to give a backup for 15-25 mins with a 12V 7A SMF battery.Hence, if the temperature rise is with in the SOA of the power devices under full load for the rated duration, they have met their specs.However try with a increased Ah rating battery and the unit conks out in the first 10s of cycles of usage.