APC XS 1000 with bad MOSFETs - Is high RDS(on) bad enough to replace?

[dmulligan]

I am repairing an old APC XS 1000 which went bad on me a few years ago.  First the batteries were dead, no problem I replaced those then I got the dreaded clicking noise.  I let it sit for years and now I am finally repairing it. 

I replaced all of the 22uf capacitors, testing them with my mega328 transistor tester showed they were all quite bad.  The click of death went away but now the overload led lights and the buzzer whines.  I checked the supply voltages and output of the op amps and they look good so I moved on to the MOSFETs on the DC-AC inverter section.  When testing I found Q3 and Q4 are shorted.  Q5 and Q6 read 0.5 ohms for their rdson.  The datasheet states <0.18ohms and 0.15 typical.  Q1 and Q2 read 0.1 and 0.2 ohms.

The mainboard I have is a 640-0253c and the only schematic I could find was for the 640-0253e.  The vast majority of the components match except the MOSFETs in the AC-DC inverter section.  The schematic just shows "0.18 OHM 200V PWR" for all 6 MOSFETs.  On my board Q1-Q4 are PNP20NQ20T and Q5-Q6 are STP19NB20T.  Obviously I should replace Q1 and Q2 and DigiKey has them in stock. 

My question is about Q5 and Q6.  Should I replace them based on the high RDSon reading?  If so what should I replace them with? DigiKey nor Mouser stock the P19NB20.  It looks to be very similar to the PNP20NQ20T other than the latter has a lower RDSon, which should be better right?

Thanks.
David

[tooki]

I'm pretty sure those Q5 and Q6 are just fine.

The measured R_DS(on) is almost certainly because the tester isn't testing with V_GS = 10V. Those little testers use 5V, so you'll get the higher R_DS(on) of the MOSFET being not fully turned on. Yes, 5V is above the V_GS(th) threshold voltage of 4V, but that doesn't mean it'll have reached its lowest R_DS(on). The datasheet clearly says that R_DS(on) is shown for V_GS = 10V, as is typical for non-logic-level MOSFETs.

[dmulligan]

That makes sense.  Q5 and Q6 were also consistent with each other too.  Thank you for your help!

[dmulligan]

@tooki Thanks again.  I now have a working APC XS 1000 UPS.