Failing to clamp transient with schottky. What is going on? - Page 1
Forum section: electronics / beginners

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 15, 2013, 01:12:24 am
content: Hello,I wound a flyback transformer on a pretty big core. ETD59, I think.I am switching it low side with an NMOS. As expected I was getting large voltages rising when I switch the FET off.To my surprise I cannot clamp these transient voltages with a diode forward biased to VDD. The diode current doesn't go higher than 7mA before I reach MOSFET drain source breakdown voltage of 80V.LTSpice predict that it should be able to clamp the transient. Why doesn't this doesn't happen? What can I try and do to nail down source of the problem?Thank you for your time.Kind regards,Marius

author: c4757p
author_rank: Super Contributor
timestamp: March 15, 2013, 01:14:42 am
content: How solidly is the cathode connected to VDD? If there's a good bit of impedance between them the spike might not want to go anywhere - fast edges don't like inductance.

author: Rerouter
author_rank: Super Contributor
timestamp: March 15, 2013, 01:17:47 am
content: put 2 35V zeners cathode to cathode across the inductor instead, this will limit your voltage positive or negative to 40V,

author: micro_freak
author_rank: Contributor
timestamp: March 15, 2013, 01:24:33 am
content: Have you tried a TVS diode, they act fast and can dissipate a lot of energy.It has to be selected so that it's breakdown voltage exceed your supply voltage, for the power factor you will have to look up data sheets to make sure you select one that can repeatedly dissipate enough energy without overheating.regards

author: qno
author_rank: Frequent Contributor
timestamp: March 15, 2013, 07:41:38 pm
content: Where does the charge go?If you feed the circuit from a battery you are ok.If you use a powersupply you need some clamping or large capacitors

author: king.oslo
author_rank: Frequent Contributor
timestamp: March 15, 2013, 09:14:32 pm
content: Yes, charge goes through the MOSFET which dies after about a minute.I will try the advice which has been given to me.Thanks.M

author: Marco
author_rank: Super Contributor
timestamp: March 15, 2013, 09:28:39 pm
content: Quote from: king.oslo on March 15, 2013, 01:12:24 amSomething is obviously just plain wrong in your circuit. (Getting 10s of Volt forward voltage across a functioning Schottky? Nope.) On top of that ... if the Schottky was connected and functioning properly your flyback transformer wouldn't work. A flyback transformer is still a transformer, when the secondary voltage becomes positive the primary voltage also has to be allowed to become positive (relative to Vdd in your circuit). If you clamp the primary with a Schottky you are also clamping the secondary.

author: Neilm
author_rank: Super Contributor
timestamp: March 16, 2013, 11:56:05 am
content: Quote from: Marco on March 15, 2013, 09:28:39 pmNo it isn't. A "flyback transformer" in a SMPSU is a pair of closely coupled inductors. Current is conducted in the primary but the secondary does not conduct until the supply to the primary is removed. The energy that has been stored in the magnetic circuit is then transferred to the secondary. A subtle but important difference.The rectifier diodes in a flyback topology should be as fast as you can get. The problem the OP has could be a symptom of the secondary rectifiers not being fast enough. It could also be due to poor layout. If there is a significant amount of stray inductance it will radically alter the behavior of the snubber network.When this was simulated in LTSpice, were "real" values and components used or did you just go for ideal components? Also, did you measure the leakage inductance of the transformer. (measure the primary inductance with the secondary shorted). The coupling in the simulator can be set to ideal (value of 1) and a separate inductor can be put in series with the primary inductance to simulate it. This might show that the original simulation has greatly underestimated the energy in the turnoff spike.For more information on snubber networks look atthisapp note from Fairchild.Neil

author: AndyC_772
author_rank: Super Contributor
timestamp: March 16, 2013, 12:29:00 pm
content: One beer says this is layout related. Can you post a photo of your setup? I bet there's significant inductance in series with your diode.Do also consider trying a transient voltage suppressor rather than a Schottky diode - they're faster acting and designed to absorb significant energy. But the layout has to be right first.

author: A Hellene
author_rank: Frequent Contributor
timestamp: March 16, 2013, 12:56:49 pm
content: Quote from: Neilm on March 16, 2013, 11:56:05 amCorrect.That is because:Quote from: A Hellene on July 22, 2012, 05:52:46 pmQuote from: A Hellene on July 21, 2012, 10:14:07 pm-George

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2013, 01:28:44 pm
content: Quote from: Neilm on March 16, 2013, 11:56:05 amAs I said, a transformer.QuoteGood thing transformer action is a voltage transformation then isn't it, so it doesn't actually preclude my statement.QuoteIn a normal transformer you generally want it to store as little energy as possible ... but that's not really my point. You generally still want the coupling in a flyback to be as high as possible (in the smaller ones it's something like 0.99) and with close coupling transformer action (ie. voltage transformation based on turns ratio) is in full effect. If you clamp the primary to some low voltage, you're also clamping the secondary.A normal RCD snubber in a flyback topology like he has will allow the primary voltage on the switch to rise above Vdd+nVo.

author: croberts
author_rank: Regular Contributor
timestamp: March 16, 2013, 02:39:52 pm
content: My apologies if this has already been mentioned but when trying to measure transients like this very narrow spike, it is important to have a very short ground on the scope probe (1" or less) and make the measurement right across the diode. I think they make ground probes that snap right on the scope probe ground collar just behind the probe tip.http://www.google.com/imgres?imgurl=http://www.cliftonlaboratories.com/userimages/Update202.jpg&imgrefurl=http://www.cliftonlaboratories.com/july_2007.htm&h=284&w=400&sz=11&tbnid=2KVfX9IRNlJiuM:&tbnh=110&tbnw=155&zoom=1&usg=__v_ByYVmcNbNh2BVahJrqDIaVgX4=&docid=GxR4oJqSXMWk4M&sa=X&ei=CohEUfmxHomi4APq5YCYAg&ved=0CHEQ9QEwCQ&dur=364

author: Icarus
author_rank: Regular Contributor
timestamp: March 16, 2013, 04:06:02 pm
content: Diodes have also forward recovery time. May be your diode is just too slow ?

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2013, 09:53:51 pm
content: Quote from: Icarus on March 16, 2013, 04:06:02 pmIt's almost never relevant (usually swamped by inductive effects). Also it doesn't apply to Schottkys.

author: Icarus
author_rank: Regular Contributor
timestamp: March 17, 2013, 09:03:45 am
content: I see, but Jim Williams says something different about that. If I got that correcthttp://cds.linear.com/docs/en/application-note/an122f.pdf

author: Marco
author_rank: Super Contributor
timestamp: March 17, 2013, 01:13:48 pm
content: He didn't exactly say something different ...He didn't say Schottky's suffer the effect (as far as I can see the negligible width of the Schottky barrier will prevent resistance modulation during forward conduction, aka forward recovery, just as much as it prevents reverse recovery).He didn't say the overvoltage was large enough to overcome the inductive effects of a through hole component either.Look at what he measured and compare it to the component the OP is using .... do you think something like a 750 mV overvoltage excursion would be relevant in a circuit with a 80 V MOSFET switch?As I said, rarely relevant ... not never.

author: Icarus
author_rank: Regular Contributor
timestamp: March 17, 2013, 01:17:35 pm
content: He also didn't say roses are red and more over he didn't say violets are blue.It's only matter what he said

author: Marco
author_rank: Super Contributor
timestamp: March 17, 2013, 01:33:35 pm
content: I was trying to show by example some things he should have said to contradict me, to make it obvious why he did not, but just for you here is a shortened version of my previous reply ...No, he didn't contradict me. You didn't get that correct.Better?