Can a failed input smoothing capacitor destroy the switching IC?

[John843883]

Hi,

I'm looking at a DVD player (Tevion QS808) which has not been used in about 4 years, on the power supply its 1 amp glass PCB fuse is blown and the VIPer22A switching IC is shorted 60 ohm between drain and source with a hole blown out the side on pin 2 (source).

The main 400v 22uF "Acon" input filter capacitor is swollen and measures just 271pF, could this have caused the switcher IC to have blown? I can't find any other obvious faults on the board with a meter.

Thanks.

[stj]

Re: Can a failed input smoothing capacitor destroy the switching IC?

yes, it could blow the viper chip.
and it really should be 47uf anyway!!

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

yes, it could blow the viper chip.
and it really should be 47uf anyway!!

Thanks. So just how does a bad input smoothing capacitor destroy the IC? Also its the big 400v mains input smoothing capacitor and not the standby capacitor for the IC which has gone bad.

Here's the PSU, on the back of the DVD player it reads "power dissipation 20w".

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

Another update: I've found that the 10nF orange ceramic capacitor for the switching transistors RCD snubber reads as 8.8nF out of circuit rather than the full 10nF, is this likely to be a problem? Its value seems to drop by a further 1nF from just my body heat when I've handled it.

Also I've just been told that infact the DVD player has been left in standby for all those 4 years of disuse so plenty of time for the capacitor to fail and the machine to blow its fuse noticed.

[R_J]

Re: Can a failed input smoothing capacitor destroy the switching IC?

The switching ic needs to switch DC, when the main cap went open, the dc from the bridge would have vey high ripple, this would cause the ic to not operate properly and in the end shorted it's internal mosfet.
You should also "replace" that small (startup) electrolytic near the ic, it filters the vcc voltage and usually dries out as well. The 10nf is likely ok and within tolerance but replace it if you have another one.

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

The switching ic needs to switch DC, when the main cap went open, the dc from the bridge would have vey high ripple, this would cause the ic to not operate properly and in the end shorted it's internal mosfet.
You should also replace that small (startup) electrolytic near the ic, it filters the vcc voltage and usually dries out as well. The 10nf is likely ok and within tolerance but replace it if you have another one.

Thanks, the ceramic capacitor is currently an orange disk ceramic type, would blue safety class capacitors be acceptable there?

And yes I'll check that standby cap too.

[momaka]

Re: Can a failed input smoothing capacitor destroy the switching IC?

Thanks, the ceramic capacitor is currently an orange disk ceramic type, would blue safety class capacitors be acceptable there?

No, safety Y1/Y2 caps are rated for only 250/400V. Snubber cap needs to be rated for at least 1 kV or preferably 2 kV.

Is there a letter written after its capacitance code? (i.e. 103J, or 103K or 103M?) The letter would signify the tolerance (J = 5%, K = 10%, M = 20%). Most likely, it's a 10% part. So at 8.8 nF, that appears to be just slightly out of spec... though this could also be due to slight error in your measuring equipment too. So all in all, it's probably OK to re-use it.

And yes I'll check that standby cap too.

Just replace all of the electrolytic caps in this PSU with a good quality brand so you won't have to deal with them anymore in the future.

Here's the PSU, on the back of the DVD player it reads "power dissipation 20w".

and it really should be 47uf anyway!!

+1
For PSUs rated around 10-15 Watts or less, 22 uF might just be enough. But as sjt suggested, 47 uF really would be more appropriate here.

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

Hi, there doesn't appear to be any tolerance markings on the original orange ceramic capacitor.

Also I'm not sure if the ones I found in my parts box are actually safety caps or not but they're blue and have the capacitance and voltage ratings on them, some are 1kv and others are 2kv, they don't have all that tiny text you often see with safety class capacitors however.

They look like this.


Regarding the input capacitor size, I wonder if its related to the inrush current as there's no inrush limiting and the bridge rectifier diodes are 1 amp types, on 240v mains the inrush could be quite large I take it. The PCB fuse is a 1 amp glass type.

[stj]

Re: Can a failed input smoothing capacitor destroy the switching IC?

no, input capacitor size relates to the load because as it goes up and down you need to keep the power stable.

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

Yes I am well aware of that, but when its first plugged in the 400v 22uF capacitor is empty and needs to charge up basically representing a brief short to the mains and with no inrush liming on the PSU wouldn't a larger one risk blowing the little 1 amp fuse or diode bridge?

Anyway its currently back working again using junked parts for now.

[stj]

Re: Can a failed input smoothing capacitor destroy the switching IC?

the inrush is not much until your over 100uf,
and frankly the maximum holding current is minimal.
the best caps from rubycon(BXC series) are as follows.
22uf 400v 760mA
47uf 400v 1180mA

and i can promise you that what manufacturers use is probably only half that good!

[John843883]

Re: Can a failed input smoothing capacitor destroy the switching IC?

the inrush is not much until your over 100uf,
and frankly the maximum holding current is minimal.
the best caps from rubycon(BXC series) are as follows.
22uf 400v 760mA
47uf 400v 1180mA

and i can promise you that what manufacturers use is probably only half that good!

Thanks. Well the DVD player belongs to a relative of mine who only lives a few minutes away so if it fails again then it isn't much trouble to get it back over here. I don't think they use it much anyway what with everything coming down the internet these days.