Help with 2-switch Forward 140A Welder - Page 1
Forum section: electronics / projects

author: staffy
author_rank: Newbie
timestamp: May 24, 2014, 06:00:37 pm
content: Hi, new to this forum, so please be patientI have problem with welder that I am designing - Forward inverter designed for 140A.Problem is that when I connect main power supply ( I have driver - UC3845 - running from separated source) I have this ripple on upper pair of the mosfets(Drains still connected to +300V - transistors T2 and T3).Size of this ripple is variable on main voltage - not on main current(tried to measure it under different power).I tried different GDT, I tried to switch gate drivers PCB(determin so I's not caused by bad gate driver), tried powering it from batteries(to ensure there is no problem with HF ground loop) I still got the same ripple. This is the voltage between drain and source of the badly driven mosfets:Frequency is 100kHz. Mosfets used are 47N60CFD by Infineon."Healty" pair of mosfets have this between drain and source.And its voltage between gate and source:I am using isolation transformer and variac to power the inverter - scope is connected straight to mains.I am attaching schematic diagram and PCB layout, driver's boards are separated and now are soldered into the main board. All pictures are here:https://plus.google.com/u/0/photos/110879269093265321205/albums/6015529961650829425

author: planet12
author_rank: Regular Contributor
timestamp: May 25, 2014, 08:36:17 am
content: It's not clear which, if any, of the scope traces are the gate-source voltage for the top pair if MOSFETs.The top image, with the excessive ringing - where was that measured? Is that the top pairs Vgs?

author: staffy
author_rank: Newbie
timestamp: May 25, 2014, 09:06:27 am
content: Yes, that is top pair of MOSFETs.

author: planet12
author_rank: Regular Contributor
timestamp: May 25, 2014, 11:23:28 am
content: And just to confirm, the top one is the gate-to-source voltage?What does the top gate waveform look like when there's no 300V supply?Have you tried removing the transformer primary and replacing it with a suitable purely resistive load?Also interesting would be a trace of the voltage over the transformer primary.

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2014, 11:24:48 am
content: My first concern is the ripple on the rising edge, which would suggest the gates are not being driven hard enough.Not voltage, current!Either there is not enough current supply or your driver is struggling to meet your design.Try more capacitance for the driver supply and you could scope it to see if it is managing the current load.All power devices respond to having over-capacity current drive on their gates.My moto is "hammer the gates".

author: staffy
author_rank: Newbie
timestamp: May 25, 2014, 12:04:10 pm
content: Yes, the top one ist gate-source voltage.This is gate-source voltage with no main voltage connected:With purely resistive load the was no ringing - tried it with 66Ohm/450W power resistor.For example, this is secondary of GDT with no +300V:This is with power voltage connected:Other secondary of GDT is fine, no matter what supply voltage is!This is voltage over main primary transformer:"Hammer the gates" is good motto, but what I see is that other pair of mosfets are hammered enough(no ringing). But I will try to upper the capacitance of driver supply.

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2014, 12:11:49 pm
content: Scope the V+ driver power supply and see what going on.

author: staffy
author_rank: Newbie
timestamp: May 25, 2014, 06:36:41 pm
content: V+ of driver, near GDT with no main power supply connected(1V/div) - 18V driver supply, AC coupling:V+ of driver, near GDT with main power supply connected:I tried applying 1mF capacitor across supply, as near as possible to the GDT, almost no change.Another strange pic, I tried different scope probe - all pictures above are taken with Tektronix 465 and it's P6105 probe. Picture belowis taken with same scope but with HP10434A probe, it was properly compensated, I checked it. Trace below is gate-source voltage of badly driven MOSFETs:Everything measured with that probe is worse looking. Tomorrow I am going to see my bachelor's thesis supervisor and I hope he will unleash some magic

author: tautech
author_rank: Super Contributor
timestamp: May 25, 2014, 08:50:15 pm
content: QuoteWhat is a mF?If it is a Micro Farad always refer to it as uF.There is 0.7V ripple on the supply at switching, I would think the current source is not stiff enough possibly all the way to the gates. Is there a low value series resistor in the gate drive.I normally use 2.7 ohms, but driving 2, you may want less.Traces to gates IMO want to be 1mmYou have 470uF on the 18V supply.Go to 1000uF

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 26, 2014, 12:40:29 am
content: Looks like C13 ringing with the stray inductance of that big open loop where it says "C10".I suggest eliminating C13, and tuning the stray inductance so it works with your snubbers to give a good damped response. Do this by calculating the inverter switch impedance Vsupply / Ipk and choosing a suitable commutation time, say 100ns, then solving for L ~= Z*t_sw and C ~= t_sw/Z. If C is less than Cdss (if your transistors specify "total drain charge, time related", use that divided by the test's Vds to get the equivalent average Cdss), go the other way and solve for t_sw (which will be the minimum you can do) and L. You can always go higher (L, C (adding more C to the ttransistors, D-S) and t_sw), but this burns more power in the snubber resistors. You'd then want to research resonant or quasi-resonant ("lossless") snubbers, or dump the snubber power into a common rail that drives another converter that recycles the power into the main supply.Finally, use a voltage clamping snubber from high side drain to low side source (i.e., across the supply, but at the transistors) to handle the ringing of the inductance.For transistors this big, you really can't justify a GDT. At this voltage (assuming ground bounce can be satisfied), HVIC type drivers can be used, or you can roll your own from opto or magnetically coupled drivers and isolated DC-DC supplies.Why not H-bridge? The peak current ripple in the supply caps must be astonishing.Otherwise, circuit looks good: you'll probably have to figure out the power line filter if this goes to EMC testing, but it's better than nothing obviously, and good prudence to begin with. Precharge handles turn-on transients, so there's that; just beware of line dropout (does the circuit reset if the line drops?) and fault conditions (if the inverter fails shorted, is it really a good idea for the precharge to switch in and mangle the relay contacts?). I'm not sure what all the control part is doing; I can't find a supply for the TL074, but it seems to be used near ground; just be sure that's got extra supplies, or use an R2R type like TLV2374 (if the total supply is within ratings).Layout is good, straightforward, give or take the stray inductance of the loops discussed above (think of that more as component value than layout style). Clearance to HV traces could be higher, but if I'm thinking in the right scale, it should still meet UL.Tim

author: megajocke
author_rank: Regular Contributor
timestamp: May 26, 2014, 12:55:49 am
content: How are you measuring the high-side Vgs and Vds waveforms?If you just put the scope ground on the high-side transistor source you're likely to see lots of ringing which isn't really there. At high frequency, your circuit isn't well isolated from ground, so you can't really attach scope ground to nodes with high HF voltage and expect to get useful results. Especially if EMI filter ground in your circuit is connected to actual ground (and the scope ground).

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 26, 2014, 09:59:31 am
content: Megajocke is correct. That ringing you see is almost certainly due to a poor ground connection.You need an isolated probe like a Tektronix P5200 or similar for looking at the high side FET.How are you presently measuring the high side gate drive while the circuit is operating? Do you connect the scope ground to the Source of the high side FET and float the scope? If this is the case then you are slewing the scope ground up and down at 100khz and the scope is unable to reject the common mode signal sufficently. If you were to connect the scope probe tip to the source while it's operating you would expect to see a flat line but will almost certainly see the common mode signal, which is the ringing component.Solution: Don't float the scope and use an isolated HV probe.Why didn't you go full bridge? It's a much better option at these power levels. Better flux utilisation in the power transformer and a smaller output inductor.

author: Rerouter
author_rank: Super Contributor
timestamp: May 26, 2014, 10:29:05 am
content: by 1mF i would imagine your using a 1000uF electrolytic, instead try a big tantalum or ceramic as there esr is much lower expecially towards the 100Khz your seeing your jumping at, and if the big electro is just as resistive as the traces you wont see much change0

author: staffy
author_rank: Newbie
timestamp: May 26, 2014, 04:48:56 pm
content: Problem solved! It was caused by bad scope/probe. I measured it with digital scope(properly isolated) and all traces are good! Now I will focus on getting current regulation to work. So far, thank you all for your advices, hope I will soon show you fully working welder.

author: mzzj
author_rank: Super Contributor
timestamp: May 27, 2014, 07:08:30 am
content: Quote from: staffy on May 26, 2014, 04:48:56 pmI noticed that you have connected the parallel mosfet gates directly to each other. This is usually bad idea, prone to parasitic oscillations and hell breaks loose in EMI-testing.Use separate gate resistor for each mosfet, ie something like 10 ohms. Or ferrite bead(model has to be selected by experiments) instead of resistor.