Scoping hi side drive of Two Transistor Forwrd SMPS - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 29, 2024, 11:24:31 pm
content: Hi,We will use the attached (LTspice and .PNG) method for scoping the hi side gate drive of an 100-265vac offline two transistor forward.The opamps will be OPA657, not the ones shown.OPA657..https://www.ti.com/lit/ds/symlink/opa657.pdf?ts=1719671367891&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FOPA657%253Fbm-verify%253DAAQAAAAJ_____3biydGeXMMf_px9rIv0cV__DR8gc64yfTnj_dWiJVdPERQX9iI_9bz2MU7-TU83KtF_OTB4i3WIKBz6vbcx00pDrYmt9W8VXP9iA9NnIJBIpvFrfJ_nNjgZR2SGKJQ80d0BG0drd8lfm5a5aaP9qKHP3hWFekiMnXKJi3uYsmWNNHDa4xr1ysKVNngAq5MJnlzrvPbmyf5xglIczpY9RsdTUQ3inY09yyQ7CNLQjevP5iPZfniScGTb1fV8FJWVq9_w7jX_twBwsjLv9pECGNx5GFMrt5-zR9W5cfF8soz4GEJwRqH6ENIn8p31VIYs_sP-WVmSYZx4V4sjQnZAoB2cVYRlojNvqlUyHbhn_AIt will be much cheaper than buying a £800 diff probe. It will show it reduced in amplitude, but that doesn't matter too much.....its the pulse shape we want to see.We can always calibrate it to see if its roughly the right voltage level.We are just interested in seeing how "ringy" the upper gate drive is, and the attached will do this, and doesn't cost the £800 rip-off.Would you agree?

author: Faringdon
author_rank: Super Contributor
timestamp: June 29, 2024, 11:49:58 pm
content: In fact, the attached technique is another option.We will simply view it on two channels...and we will mentally superimpose, and it will be easy to see if there is any untoward ringing in the hi side gate drive. would you concur?

author: Faringdon
author_rank: Super Contributor
timestamp: June 30, 2024, 12:02:33 am
content: In fact, we will add 9MEGs (+compensating capacitance) to the DIY coaxials, as attached, to turn them in to 10:1 probes and so the scope doesnt load down the waveforms and distort them.Can you think of a better way to scope the hi side gate drive?We just need to see if it has deleterious ringing.

author: Faringdon
author_rank: Super Contributor
timestamp: June 30, 2024, 12:33:46 am
content: Here is the full thing with the compensating caps (for the DS1054Z scope)

author: Faringdon
author_rank: Super Contributor
timestamp: June 30, 2024, 07:51:54 pm
content: Hi,Actually, due to concerns over capacitively loading the gate drive go and return traces, it has beendecided to opt for a 20:1 DIY probe as attached (LTspice and .PNG).This uses cheap , readily available parts (10MEG resistors and 10p COG capacitors).The signal chain is tight at all places.This appears to be the cheapest way to satisfactorily scope the two tran forward hi sidegate drive.All that's needed is to get an idea of how ringy it is (ie, is it ringing so wildly that the ringinggoes below the FET Vgs(th).Would you agree this is the cheapest way to do it?....ie, not buying an £800 diff probe.Two 100:1 probes was the alternative, but they cost £25 each.-And also, the cheap ones have the 99MEG and comp cap in the tip, so you unfortunately can't cut the tip offto solder the probe into circuit.Dave Jones video gives good info related to this..

author: jonpaul
author_rank: Super Contributor
timestamp: July 02, 2024, 05:40:58 am
content: FTTS (Farrington Treeze Time Sink)In another post you said you have suceed to "sell" your "company" off.Logically, you no longer have the power supply firm, and you have finally retired. WHAT A RELIEF!!!We should never hear from you if that is true.Just more fantisy?For the technical question, (cfg is DOUBLE FWD)The measurement is useless and your suggested probing techniques in your FOUR notes are inaccurate or worse dangerousRefs: Tektronix Circuits Concepts BOOKS (1980s) , Oscilloscope Probe Measurements and circuitsBon chance and HAVE A GREAT RETIRMENT!||JonPS: I designed, protoed and mfg (among others ) a 750W, IEC/UL medical compliant double fwd, in 1990s,We NEVER had any such issues. many 1000s built and deployed.

author: Faringdon
author_rank: Super Contributor
timestamp: July 02, 2024, 06:15:41 am
content: Thanks, yes i appreciate tektronix sell good stuff for this, and have great app notes for them...but its too expensive....this is about doing a "satisfactory" solution/version thats cheap.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: July 02, 2024, 11:40:48 pm
content: Transformer (?

author: jonpaul
author_rank: Super Contributor
timestamp: July 03, 2024, 05:31:46 am
content: FTTS: No reason to even make the measurement. Never had to do it in decades of SMPS design and dev, mfg.The title and premise for the question are nonsense as always.The TEK Circuits Concepts books on probes give you all info to maky a DIY probe for ~ zero cost.JPS: in previous posts you asked about sale of a power supply company....So, You sold your power supply "company"Did you retire???If yes GOODBY! AND GOOD LUCK!

author: Faringdon
author_rank: Super Contributor
timestamp: July 03, 2024, 05:39:24 pm
content: Hi, thanks..the attached shows the Vgs waveforms of both FETs in a 130W offline two transistor forward SMPS. These are both taken withdiff probes.As you can see the yellow trace shows the high side FET , which is driven by a gate drive transformer.It is saturated in common mode noise.The lower FET is just driven directly from the controller (no GDT)We could add resistors to the probe points, but that would filter the signal away aswell.As such, the better method depicted in this thread is needed.

author: jonpaul
author_rank: Super Contributor
timestamp: July 04, 2024, 05:05:10 am
content: FTTS, Sorry, nonesense. Better probes will not solve a lack of SMPS good engineering and poor measurement trchnique.0/ Poor choice of topology. A 175W fwd does not need a double FWD. Even above 500W its optional.1/ "saturated with noise" Again nonsense.both traces of high and low side reveal severe HF ringing due to very poor probing technique and poor layout, wiring prectice and/or component choice.2/ gate drive and transformer design is also suspect.3/ suggest again to read the TEK Circuits Concepts book already referenced.Enjoy your retirement!ByejRead app notes and ppaers on gate drive design.

author: Faringdon
author_rank: Super Contributor
timestamp: July 04, 2024, 05:23:14 am
content: Thanks...I assure the layout is very tight, the GDT is PH9572.111.Single fwd needs reset coil and then volts on transistor when offis 2 x vin. (if n(reset) = n(pri) )That's why this is double fwd.I am sure we would all agree that 100W to 500W say, offline, is good double forward territory.That noise seen would be there no matter how tight the layout.Its common mode noise picked up by the diff probe leads.QuoteQuoteThanks i looked but did not find.I doubt they will have details on the lines described, as it would hit their own sales of their own super-expensive diff probes.(all diff probes offshelf are so expensive)

author: Andy Chee
author_rank: Super Contributor
timestamp: July 04, 2024, 11:38:41 am
content: Quote from: Faringdon on July 04, 2024, 05:23:14 amPictures...or it didn't happen

author: temperance
author_rank: Super Contributor
timestamp: July 05, 2024, 12:12:10 pm
content: What are you hoping to see by changing the way you are probing this supply? You must first address the common noise problem and probing the gate's isn't going to help much.I think you are using those "modern" Infineon or similar MOSFET's which are prone to gate oscillations around Vgs-th? Those oscillation might be the cause of the gigantic CM noise.To shed some light on the (sometimes frustrating) behavior of those MOSFET's:https://www.infineon.com/dgdl/Infineon-ApplicationNote_650V_CoolMOS_C7_Mastering_the_Art_of_Quickness-AN-v01_00-EN.pdf?fileId=db3a30433e5a5024013e6a966779640bInfineon had an older other app note dealing with the same thing for earlier coolmos devices. But it seems I can't find it anymore.You will need some ferrite beads able to handle the peak drive current without saturating to solve this. But those are available from several manufacturers. Older MOSFET's are much easier to work with and you should ask yourself if you need those new MOSFET's.Does the supply employ synchronous rectification? If so, reduce the problem by reverting to Schottky diodes while solving the gate drive problems. The high Q network formed between the probably large Coss and the leakage inductance is an other source of trouble.

author: jonpaul
author_rank: Super Contributor
timestamp: July 06, 2024, 05:39:10 am
content: Temperance: "Infineon ... prone to gate oscillations "I am not familiar with Infiniton or its oddities, but ALL FETs have very high gain in the linear region.An SMPS FET gate drive, (floating or not) design includes a series gate R close to the FET pkg, ~ 10..100 Ohm to damp the parasitic L and C of the FET and drive circuit.We used ferrite beads or a combination of bead and damping R.Applies regardless of the FET type, Infiniton or other.As FTSS (Farrington/Treeze/Time Sink) will never reveal a schematic or layout, (top secret!) just another FTSS time sink pipe dream.Anyway he said a few weeks back in another FTSS post, he sold his firm so I assume he is retired (:-:)Bon weekendJ

author: mzzj
author_rank: Super Contributor
timestamp: July 06, 2024, 10:16:16 am
content: Quote from: jonpaul on July 06, 2024, 05:39:10 amSuperjunction mosfets used today are indeed quite different animal compared to "traditional" mosfets.