SB315 and SB320 Shottky Diodes Voltages - TV PSU

[MicroCap]

Hi all. I'm working on a Samsung TV Power supply board (BN044-00501a) and found one of the SB315 diodes on the cold side to be shorted. I removed it and ordered a 3A 20V PRV replacement as the 3A 15V PRV is difficult to order quickly. The TV sprung to life. After a few seconds, it went dead again...same symptoms. Did diode check again and the replacement diode had blown again. This diode is in parallel with 3 other SB315's. I am now seeing in places that "SB315 = SB3150". Is this true?

Is the SB315 on the PSU actually 3A 150V PRV and NOT rated at 15V? If it is actually supposed to be 15V and I've more or less replaced it with the 20V (I know I'm gonna get shot for that), then what can be causing the same diode to blow in the circuit? Had any of the other 3 diodes blown I would have assumed a fault further down the circuit, but since it's the replacement that blew, I'm thinking I may have made a mistake with interpreting the diode PRV.

All input will be helpful. Any pieces of missing info, please let me know. Thank you.

[sam_sam_sam]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

First of all you need to use a 100 watt incandescent light bulb instead of the fuse and if your light bulb is very bright then you still have something shorted in the system and needs to be investigated to find where the short is

It would also help if you posted some pictures of the front and back of the board in question

[MicroCap]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

Thanks. Hot side is fine. Voltages up to transformer are fine. Problem is on the cold side. Will the bulb still brighten if replaced with the fuse? As I'm guessing the osco is disabling some of the cold side rails since voltage is 0V once the diode fails and creates a short. Please tell me if I've got it wrong.

Any light on if "SB315 = SB3150"?...

[MicroCap]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

Power board images (front & rear)

[budm]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

Your pictures are not in focus, I cannot read any printing on the board.
Did you replace all 4 diode with SB320? See spec of SB3XX, too low of the Voltage.
What is actual printed on the diode? I cannot find anything on SB315. Are you sure it is not printed as SB3150?
PS board BN44-00501a.

[MicroCap]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

Greetings Budm. I will re-upload. Also pics of the SB315. Writing on the diode: 1st line "SB315", 2nd line "0C40". That's it.

No, I only replaced the one. The remaining three are still SB315. If the SB315, indeed has a PRV of 150V, I can understand how the TV worked for a few seconds and then died since the culprit was the zapped 20V diode I had just inserted.

As for availability of the SB315, I'm only finding one entry, manufactured by EIC and labelled as SB315SVF. But, there is an abundance galore of SB3150. I just need clarification whether the SB315 labelling infers SB3150???

The other rail also has 4 schottky diodes (SR506) which are 5A 60V.

[budm]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

1) They parallel 4 diodes to get more current handling but the bad part is that if the Vf are not closely matched the diode with the lower Vf will conduct the most and that will cause thermal run away. It is bad design practice to parallel diodes. https://www.daenotes.com/electronics...de-in-parallel
2) I look through some of the Samsung power supply, I.E BN44-00440A, the 13V power supply uses one 150V 10A rating, so I would suggest to replace all 4 diodes at the same time with SB3150.
3) You also need to verify that the RC snubber for the didoes are still good.

[redwire]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

Found a partial schematic for BN44-00501A

The two sets of quad SR506's 5A 60V, one for the low voltage 5V and other for 13V rail; now SB560's. I think they need to be a matched set of four or else current sharing (heat) would not be great if one oddball was in the set. 20V is overvoltage for a 13V rail diode.
The LED backlight 135V 240mA and uses four HV diodes.

[petehall347]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

replace them all and it will be fine . have done this a couple of times now .

[MicroCap]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

@budm: Thanks a lot dude. Just those few paragraphs enlightened me more than what I could muster in a week. Very grateful for the knowledge share, much appreciated. I also started digging for BN44-* schemas hoping to find similarities in design and diode usage but fell asleep on the job. Thanks for the rapid find. The snubber checks out ok, no short/open so far. Ok, so you reckon I should go for the SB3150?! So, do I...keep your toes crossed.

@Redwire: Likewise, thanks for the current impact info on throwing in a rogue valued diode.

@petehall347: Thanks for the input Pete. What was the spec of the diodes and was it the same PS board?

I've ordered a batch of SB3150's and they should be here next week. I will do the replacement and provide an update of the outcome accordingly. But, in the meantime should anyone know about SB315's = SB3150's then do add your two cents. Thus far, budm's PS board find seems promising.

[petehall347]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

cant remember now what exact board. was bn something also had 4 in it . nor can i remember what diodes i used .

[eccerr0r]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

I suspect that the real part number is "SB3150 C40"
In any case yes, 15 V part for 13V is way too low. 20V part is still too low.

I'd think that Schottky diodes are extremely sensitive to overvoltage due to their intrinsic leakage, and anything that exasperates it will cause it to quickly fail.

[petehall347]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

if i remember i will see if i bought extras .. thinking it might have been 1N something .

[MicroCap]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

UPDATE: Success

Indeed, SB315 = SB3150.

Successful replacement of diode SB315 with SB3150. TV came to life and maintained a continuous pulse verified via an empirical study conducted by playing a couple of games on xbox.

Thank you for everyone's input and help.

[eccerr0r]

Re: SB315 and SB320 Shottky Diodes Voltages - TV PSU

I wouldn't say SB315=SB3150, but rather SB315 0C40 = SB3150 ...

The board analysis probably would have told me to get at least 60V units but well, if it was all potted it'd be hard to tell...