Acer AL1916 monitor PSU repair (DM0565R blown)

[Radio Fox]

I'm repairing an Acer AL1916 monitor PSU at the moment. The 400v high voltage smoothing capacitor C114 is measuring completely O/C. The case of U101 has blown apart. Fuse F101 has blown. ZD121 reads 6 Ohms in both directions & ZD120 reads 99 Ohms in both directions. While looking at the data sheet of U101 (DM0565R), I noticed that the absolute maximum voltage for Vcc is 20v. However, the zener diode across the Vcc pin to GND is a 1N4748A (22v).

Now, whether that zener is used to constantly regulate the voltage, or whether its's used to protect the Vcc voltage from rising too high, I can't understand why 22v was chosen, given that the absolute maximum voltage for that pin is 20v? Wouldn't a 20v or 18v zener have been more appropriate?

Until the replacement components arrive, I dont know what voltage the auxiliary transformer winding is supplying to the Vcc pin of the DM0565R.

https://dalincom.ru/datasheet/FSDM0565R.pdf

[CapLeaker]

NO! You'll screw up the OVP using the 18V zener. Read the data sheet! Also the 20Vcc is NOT the maximum. It's more like when the IC goes into OVP. 22V zener is correct. The normal running voltage is more than 12V for the IC to start switching.
The 22V zener limits the Vcc for OVP condition only. It’s there to protect the IC instead of blowing it up. Under normal conditions, the zener does nothing.

[Radio Fox]

Thanks for reply.

Yeah, I hadn't read further down the data sheet to read how the OVP works. I can see now that an 18v zener can't be used on that pin.

However, the data sheet still clearly states that the absolute maximum voltage for Vcc pin is 20v. There's nothing in the table to say that it's the absolute maximum voltage that trips the OVP. That's why it's confusing. That Vcc pin must have a maximum voltage that would break it, so what is it? All we can tell is that it must be greater than 22v, as that's the value of the zener.

It's obviously correct anyway, as the PSU has been OK for many, many years without problems.

[CapLeaker]

20v + 10% = 22v
my data sheet sez 20v but not absolute.

[Radio Fox]

Oh right. The only 2 data sheets for that part that I can find are Fairchild Rev 1.0.5 dated 2005 (that I linked to), or an earlier Rev 1.0.4. Both say absolute.

Can you upload or link to the datasheet you have for comparison?

Cheers.