Unknown Component Function (for replacement) - Page 1
Forum section: electronics / repair

author: Vincenzo
author_rank: Regular Contributor
timestamp: October 25, 2022, 08:20:54 pm
content: I have a fancy (around $400) 72V 1500W ebike charger that was connected to a battery in reverse polarity. There were two high specs MOSFETs on large heatsinks that blew up to pieces.I ordered the MOSFETs from digikey and inspected the few other large components near the outputs and there was still a short.My suspects were two MOVs that were not short, 680uf/100v 3 parallel caps that measured good (capacitance, leakage, and ESR), and finally a small SMD diode that had a crack and when inspected off the board was shorted.This diode is connected across the caps (and the output to the battery) in reverse bias. My guess is that it is put there for reverse connection protection but I am not sure. I am trying to make an educated guess to what to replace it with because consulting websites that list SMD components' symbols was not fruitful so far.I am hoping that someone with more experience and knowledge have come across a similar situation. Any advice is much appreciated

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 25, 2022, 09:44:06 pm
content: Just replace it with a standard diode that uses the same package. Just sort by cheapest price when you go to buy on Digikey. It's what they did when they looked for a diode to use in the PSU. It's probably 100 Volt or better 1 or 2 Amps.Often when the high side MOSFETs blow the driver circuity is damaged as well. Are there resistors on the gates of the MOSFETs? Sometimes the resistors are enough to save the driver IC.

author: fzabkar
author_rank: Super Contributor
timestamp: October 26, 2022, 03:27:27 am
content: I expect that "PK" is a unidirectional TVS diode, possibly SMBG130A or SMBJ130A (130V working voltage).http://www.smdmark.com/en-US/search/code?id=pkhttps://www.vishay.com/docs/88392/smbj.pdfhttps://www.farnell.com/datasheets/2358777.pdfhttps://www.diodes.com/assets/Datasheets/ds19002.pdfThe polarity of the capacitors appears to be reversed in your annotations.Perhaps 130V is too high.Here is a 30V ESD protection diode (SMF30A):https://www.vishay.com/docs/85881/smf5v0atosmf58a.pdfThe logo belongs to General Semiconductor which is now owned by Vishay.

author: wasedadoc
author_rank: Super Contributor
timestamp: October 26, 2022, 11:03:45 am
content: Those TVS diodes are available in unidirectional and bidirectional types. The band indicates this one is unidirectional. Band end is the cathode which will be the more positive end when used as a TVS.

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 26, 2022, 08:55:14 pm
content: If it is a TVS diode, is it intended to be a sacrificial over Voltage protection device?

author: wasedadoc
author_rank: Super Contributor
timestamp: October 26, 2022, 09:16:52 pm
content: Quote from: 2N2222A on October 26, 2022, 08:55:14 pmThey are intended to clamp transients. That is not the same as sacrificing themselves if exposed to sustained over voltage.

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 27, 2022, 12:03:29 am
content: Why would there be a need to clamp transients when it is in parallel with a rather large capacitor?

author: fzabkar
author_rank: Super Contributor
timestamp: October 27, 2022, 06:05:44 pm
content: Quote from: 2N2222A on October 27, 2022, 12:03:29 amMy question would be, why is the diode's working voltage greater than the rating of the capacitor? What would be the point of clamping transients at 140V - 150V if the supply rail is less than 100V?