The value of the Zener diode - Page 1
Forum section: electronics / repair

author: shamooooot
author_rank: Regular Contributor
timestamp: October 24, 2020, 05:27:03 am
content: Hello guysI have been always using this wonderful forum but it is the first time that I post something and I need your help.I have a problem in an ultrasonic cleaner board, which is completely dead:I tested the two power MOSFETs and they were shorted inside. No short in the circuit found.I replaced the MOSFETs but the board is still dead.I suspected the Zener diode in the circuit and I believe one of them is damaged, as it broke in half when I de-soldered it.Two questions:- Can this faulty Zener diode make the whole board dead?.- What is the logical value of this Zener diode depending on the other circuit components?.

author: jonpaul
author_rank: Super Contributor
timestamp: October 24, 2020, 05:36:01 am
content: Dead FET>>serious circuit fault, damaging other parts in FET driver ckt.FET short is the symptom not the cause of the problem.Zener is FET gate protection and requiredSuspect driver is blown as well.Suggest:Get schematic or trace it outUse LV PS on driver ckt WITH MAINS POWER DISCONNECTED, probe drive to find faults.Good luckJon

author: wraper
author_rank: Unknown
timestamp: October 24, 2020, 06:19:47 am
content: This should be elma ultrasonic cleaner. IIRC when I repaired them I needed to change only MOSFETs and fuse. This zener seem to be connected as GS protection. Did you check if there was actually voltage coming to those mosfets to begin with?QuoteIf you tried to do this with one soldering iron. it's not surprising it broke.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 24, 2020, 06:35:01 am
content: I appreciate the feedback guys@jonpaul but it is as well common for the FETs to go faulty in SMPS, isn't it?Everything seems to work good in the circuit including the driver, and only this diode was suspected.@wraper yes it is an Elma ultrasonic cleaner, and yes the voltage was there on one of the FETs.I just need to know what is the value of this Zener diode, can I determine it from the given circuit?

author: wraper
author_rank: Unknown
timestamp: October 24, 2020, 06:44:03 am
content: Quote from: shamooooot on October 24, 2020, 06:35:01 amDesolder another zener and test it's clamping voltage. Also check if low voltage PSU is working. I vaguely remember that probably I once had an issue with it too.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 24, 2020, 06:57:41 am
content: Yes I thought of doing that, but I only have a power supply with 12V. Will that be enough?By the "low voltage PSU" do you mean the half bridge FET driver ?

author: wraper
author_rank: Unknown
timestamp: October 24, 2020, 07:06:02 am
content: I mean LNK501 and small transformer. MCU and the rest of the logic needs to be powered from something.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 24, 2020, 07:17:27 am
content: Quote from: wraper on October 24, 2020, 07:06:02 amWill do, thank you.Can you please confirm if the 12V would be sufficient to find the clamping voltage of the Zener diode?

author: drvtech
author_rank: Regular Contributor
timestamp: October 24, 2020, 08:57:30 am
content: If the zener is there for g-s protection then all you need to do is look up the maximum Vgs of the FET from the datasheet and use something like 60-75% of that for the zener voltage. The actual voltage is not critical, it just needs to clamp low enough to prevent the voltage on the gate exceeding the maximum the FET can handle, whilst being high enough to allow the FET to switch on fully. Typically a zener of 10V satisfies those criteria but you should check the datasheet.

author: wraper
author_rank: Unknown
timestamp: October 24, 2020, 01:30:08 pm
content: Quote from: drvtech on October 24, 2020, 08:57:30 amVCC of gate driver is up to 14.6V. So if it's powered from 12V, then you may clamp it during normal operation if you use 10V zener. I don't remember actual VCC since I repaired them quite a time ago. There are no zeners in the datasheet.QuoteDunno, may be enough or may be not. If 12V is not enough for clamping you would at least know that it's likely 15V or 18V.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 24, 2020, 02:41:06 pm
content: Excuse my ignorance guys, I thought this (glass) package of diodes is only for Zeners. But today I learned that it might be for normal diode as well !I searched for some circuits for the half bridge driver and I found the attached circuit, "MOSFET Power Driver for Lighting Reference Design using part L6384E by STMicroelectronics".I need to determine what actually the diode is before proceeding.Any advise would be appreciated.

author: Pulsepowerguy
author_rank: Regular Contributor
timestamp: October 24, 2020, 02:50:06 pm
content: The gate protection zener must not clamp the gate driver's output, so it must have clamp voltage higher than the driver's supply, but lower than the FET's maximum allowed voltage. Also of note, a FET failure will often it take out the zener and the gate driver.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 25, 2020, 06:57:08 am
content: ok guys so I de-soldered the other (working) diode and it tests 0.26V in forward bias, which makes me strongly believe it is a Schottky diode now and not a Zener one.I am too scared to test it in reverse bias as Schottky has a low reverse breakdown voltage. And I don't want to risk losing the only diode.How would you guys proceed?

author: wraper
author_rank: Unknown
timestamp: October 25, 2020, 12:55:11 pm
content: Quote from: shamooooot on October 25, 2020, 06:57:08 amNothing will happen. Reverse breakdown with limited current will not kill it. Zener is a diode with controlled reverse breakdown voltage after all.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 25, 2020, 02:06:59 pm
content: To test a zener you need a variable power supply with a setting that enables very low current. I have blown zeners using too much current. You can put the zener between your fingers to see if it heats up during the test.If you use a 12 V power supply, you need to restrict current through zener with a series large value resistor.If the zener is mounted between the gate (cathode of zener) and source (Ground, Anode of zener) it is a protection diode and the comments as to the value are appropriate.However, if it is a protection diode, the FETs should work without the diode, unless they are blown again.Wally

author: wraper
author_rank: Unknown
timestamp: October 25, 2020, 02:09:32 pm
content: Quote from: Wallace Gasiewicz on October 25, 2020, 02:06:59 pmNope, you need a current limiting series resistor. PSUs usually have output cap and current limiting speed is not prefect. If you try connecting zener diode directly, there is a big chance you will damage it regardless of which lab PSU you use.

author: shamooooot
author_rank: Regular Contributor
timestamp: October 25, 2020, 04:07:52 pm
content: Thank you guys for the very valuable feedbacks.I will go with the Schottky diodes as it is the more logical option. Any ideas about the values i need to pick ?

author: shamooooot
author_rank: Regular Contributor
timestamp: November 11, 2020, 11:22:55 am
content: So I replaced the Schottky diodes but it seems that I have only 2.5V on the input of the 5v voltage regulator. I also replaced these three caps shown below but without success:Any advise ?

author: shamooooot
author_rank: Regular Contributor
timestamp: November 20, 2020, 09:23:56 am
content: Just a couple of questionsCould the shortened FET's have toasted the l6384ed IC?Should the 5v regulator be alive and powering the IC's even if the output circuit has problems?