Fullbridge LOAD TRANSIENT RESPONSE - Page 1
Forum section: electronics / projects

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 30, 2023, 07:38:21 am
content: Hello, i think i need to add another GDT transformer. At the moment i am using one with 4 secondary windings (Schematic is using wrong transformer).Yellow trace in pictures is Vgs, blue trace is Vds. measured on transistor Q2Turning on is okay, howeever turning off is quite bad. There is some ringing probably due miller capacitence coupling to gate of mosfet.Fullbridge control chip is TL494.When GDT is off, both gate drivers is off. I though that transistor will turn off via gate resistor, it does but not nicely =D.I think i need to make another GDT. One for controling Q1, Q4 and for Q2 Q3. Only difference will be, that driving voltage for gate will go directly to negative side. So there wont be a gap where voltage on GDT is zero. That should solve my problem. What do you think?Thanks

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 30, 2023, 08:08:03 pm
content: Ok so i have added another GDT and rework old one.But to be honest it actually worsen the waveform.Again measured on Vgs, VDS of Q2Now there are strange oscilations

author: moffy
author_rank: Super Contributor
timestamp: January 30, 2023, 08:47:10 pm
content: I am replying to your first set of waveforms. You are not measuring current which would tell you more than your voltage waveforms. When the bridge transistors are all OFF together, the bridge is in a high impedance state and the load and parasitics will dictate what is happening with the voltage, Vds, you have of course the body diodes of the MOSFETs to limit extremes. The only true voltage measurement is a differential one across the load.

author: 2N3055
author_rank: Super Contributor
timestamp: January 30, 2023, 08:50:54 pm
content: There is also fascinating topic of measuring with a scope... You need to say more on your measurement setup.. It can easily be that what you see on a scope is not actual waveform...

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 10:25:51 am
content: Adding waveforms, measured directly on transformer.I am trying my best to limit EMI/EMF. I have designed everything to be as close as possible to fight parasitic inductance. i am using curled wires for feedback, mosfet driving... By curled i mean two wires close together, sorry for bad english =D...

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 10:41:38 am
content: You have a lovely LC oscillation across the transformer, have you tried damping it with an RC snubber?

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 10:58:32 am
content: I was thinking about adding one 😅. Howeever i though that magnetizing current from primary will recuperate through mosfet body diodes back to bulk caps. So it wont be a necessary. Seems i was wrong 😑

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 11:35:35 am
content: It looks like your first set of waveforms are better damped than the second set. Can you go back to the first configuration? Or are there other issues involved?

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 11:50:27 am
content: I just noticed on your first post that you have 2.2nF caps in series with 6.8 ohm resistors across each FET. The resonance is around 600kHz so the caps have an impedance of about 120 reactive ohms, which means the 6.8 ohm resistor is doing very little at that frequency. The 2.2nF caps might be contributing to the ringing you might want to raise the value of the 6.8 ohm resistor.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 12:11:42 pm
content: I was doing some changes, there are 50ohms resistors and 1nF caps used in RCD.I have only added another GDT which is turning mosfet on and off way better (actually only off better, turning on is same). There was positive overshoot on gate which turned again mosfet on, so that why i improved GDT to go to negative voltage ass well.And reason why i did this change was that Q2 as only transistor was getting hot. Dont know why. But with added GDT now all transistors gets hot. Temp of Q2 is lowerm howeever there is rise of temp on others. Its not critical temperature, you can hold finger on it.

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 12:54:16 pm
content: Does the output transformer have a load on it? That would tend to dampen oscillations. 1nF/50R is a bit better but you need to kill the 600kHz so I would be tempted to try 4.7nf/50R. If you can keep your finger on it it's not hot, but that depends on whether it is driving a load, no load should be cool. The body diodes tend to be slow and since they are conducting some of the heat might be coming from them.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 01:35:26 pm
content: PSU is designed for 200V 5A, so far i have tested only aprox 700W. 130V on 5+Amps. So yes, i am measuring with load on it

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 01:49:42 pm
content: Quote from: MacIntoshCZ on January 31, 2023, 01:35:26 pmIf you can touch the FETs without burning your finger, you are doing pretty well, dissipation wise.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 05:05:06 pm
content: Mosfets exploded. I looked inside and i think i have bought fake transistor. This should be irf460A. Also measured RDS and its about .44ohms. in datasheet is 0.27 ohms...What a nice buy.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: January 31, 2023, 05:19:00 pm
content: So after explosion, two transistors are gone, zener protection of gate of those two transistors are gone and i have also noticed one of 1nF cap is gone. But this cap was on other side where transistor survived...

author: moffy
author_rank: Super Contributor
timestamp: January 31, 2023, 11:14:17 pm
content: Sounds like excessive dv/dt, overvoltage. Those snubber components look awfully small for a 1kW psu.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 01, 2023, 06:29:31 am
content: Maybe i am using wrong material for caps. Those ones are polypropylen 1kVBut there are two 100R resistors in parallel. I have added one on other side.

author: moffy
author_rank: Super Contributor
timestamp: February 01, 2023, 06:50:31 am
content: Polypropylene is a great material, very low dissipation, but they need to be physically large to handle the power needed. If the caps are blowing and the resistors are not getting hot, then the resistors are not doing their job i.e. the snubber is not working.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 02, 2023, 10:47:23 am
content: With combination of 4n7 caps and 50R resistors, it looks damped well. Measured Vds of Q4ThanksSo its better to have higher resistor value than reactance/impedance of capacitor.

author: Wolfram
author_rank: Frequent Contributor
timestamp: February 02, 2023, 11:01:28 am
content: How are you generating your PWM and enable signals? Keep in mind that GDTs will enforce a zero volt-second balance due to their magnetizing inductance. Any time asymmetry in your upper and lower drive pulses will also lead to a voltage asymmetry, and there is also the potential to excite resonances between your magnetizing inductance and DC block cap.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2023, 01:21:17 pm
content: Show the control circuit and connection.Also show GDT input (before and after the coupling cap).Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 02, 2023, 06:22:14 pm
content: I am using two GDTs, one for OutA and second for OutB.Upper zener diode tempt to blow up and positive voltage on output of GDT is higher in absolutive values than negative one. Its probably as Wolframe say.Yellow trace - Out A on input of gate driversBlue trace - output of UCC37321P (Just noticed that i am using UCC37321p in place of UCC37322p and vice versa). So cap before transformer is near UCC37321p.Purple - Output/secondary of GDT, without load on it (mosfet not connected)

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 02, 2023, 06:38:27 pm
content: here can be better seen voltage assymetryY - input of gate driverP- secondary of GDT(I have replaced noninverting and inverting driver between each other, so now its as in schematic. And also replaced outputs of GDTs. Nothing happen, but now its as in schematic =D)VCC is about 12,5V aprox

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2023, 06:49:53 pm
content: What is PWM_stav?Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 02, 2023, 07:14:48 pm
content: When there is GND at enable pins of gate drivers, it shutdown. But i am not using it at the moment.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 02, 2023, 07:21:17 pm
content: So i should use lower voltage for GDTs or use only one GDT with 4 secondary outputs (two noninverting drivers on same board and two inputs, outA and outB from tl494) as in the begining... What seems to be a better option? I think having two different GDT is betterJust for ilustration. Blue and Yellow are outputs from TL494Purple is output of GDT with 4 secondaries and two noninverting drivers.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 03, 2023, 02:42:38 pm
content: So this is overshoot causing mosfets to blow up. IRF460 is rated for 20V driving voltage and my overshoot is equal to 32V aprox. Seems its in rated specs...QP35 - measured on outputQP36 measured on output, with added two 12V zener diodes in series (with common anode) across primarybtw VCC was 12,7V

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2023, 04:02:02 pm
content: Do you mean IRFP460, by the way? The non-'P' version is TO-3...Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 03, 2023, 04:40:31 pm
content: Yes, i mean irfp460.With to 247 package

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2023, 11:38:24 pm
content: Quote from: MacIntoshCZ on February 03, 2023, 02:42:38 pmIf you are having overshoot problems on your drive transformer, you could try loading the secondaries with resistors which will tend to dampen any overshoot.The waveforms you see might be a result of scope probe capacitance, if you can measure the waveform with say a 100R resistor in series with the probe, that can reduce the capacitive influence of the probe somewhat.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 04, 2023, 09:22:18 am
content: Thanks very much =)1) I tried first to measure overshoots with 100R with probe in series, Overshoot get down to 28V. So i have tried 50R, it gets down to 25V. 33R did nothing at all. So yes, "real" value of overshoot was way lower but still enough to blow up mosfets.2) Now i have added 100R across secondary, overshoot was gone. But resistor was too hot. So i have added another one 100R in series-> 200R. This is the waveform. I think it will work now =)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2023, 09:44:26 am
content: Oh, I think I see what's going on now.It's a constant guessing game since no setups are specified for each waveform here, but I think the last waveforms must've been scope probe on secondary, unloaded (no gate connected), primary drive as shown (TL494 + UCC drivers). So, you get the response of an unterminated transmission line, and apparently the winding is about that long (10-20ns electrical length, so, about 2m of wire?), so that's that.This waveform is nothing of concern: the gate resistance swamps transmission line behavior, and leakage inductance dominates.2m of wire at ballpark 100 ohms Zo is 0.7uH. The gate average capacitance is 20nF per transistor, so the LC series resonant circuit has Zo (that's a different Zo now) 5.9 ohms, so the 6.8 ohm gate resistor will do well enough, and it'll have a time constant around 180ns. We can expect about 1/3 that or 60ns commutation speed at the drain. (Actual values may differ, but will likely be within 2x or so of these.)To run faster, smaller gate resistors are not feasible: that'll just result in ringing. A lower transmission line Zo is required to enable that. That is, use more twisted pairs in parallel, or star quad(s).The slanted waveforms shown earlier are suggestive of excessive magnetizing inductance; rewind with more turns or a higher A_L core.Also consider an R+C in parallel with the GDT coupling capacitor, to dampen startup transients. Probably there's some low frequency ringing in there. Same calculation, R = sqrt(L/C), where L is magnetizing inductance and C is the coupling cap; use a series capacitor >=2.5 times the coupling cap.Tim

author: moffy
author_rank: Super Contributor
timestamp: February 04, 2023, 10:02:24 am
content: Quote from: MacIntoshCZ on February 04, 2023, 09:22:18 amThat is a good result. Any time there is inductance(transformer leakage) and some capacitance(possibly the gates) you will have ringing, that is where a suitably placed resistor will absorb the energy and dampen the oscillation.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 05, 2023, 11:50:29 am
content: There is one thing i dont understand. I rewinded transformer for driving gates of mosfets.I am using this toroidhttps://semic.cz/!old/files/pdf_www/Lj_195BT3615C_CF.pdfIt has 40 turns (BTW before it has 10 turns only), which should be equal to 9,28mH and energy which was stored in coil after 50k cycles should be 39mW.But when i byppas capacitor, which is in series with transformer, or i place paralel diode to transformer, it actually go to short circuit. I though those 39mW would "dissapear by heat" in chip itself, well actually i was nice to gate driver and placed 1R5 resistor in series with transformer, but still AMPS have a high ground =D.

author: moffy
author_rank: Super Contributor
timestamp: February 05, 2023, 01:20:25 pm
content: Quote from: MacIntoshCZ on February 05, 2023, 11:50:29 amWhat is possibly happening when it "goes to short" is that the current is not being reset after each cycle, so a DC current and magnetic field builds until the core saturates and you get your short circuit. It is important to always balance the current through the core so that you don't saturate it. Any DC voltage across a coil will only be limited by the series resistance.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 06, 2023, 07:44:27 pm
content: Just curious what is allowed negative spike which could possibly turn on next pair of mosfets? I cant get rid of it. At the moment its 1V aprox, i dont think 1V will turn mosfets on.QP 38, measured on secondary of GDT, with mosfet connectedQP40, measured on secondary oif GDT, 20R and 25nF cap connected in series directly at output of secondary side, mosfet connectedBlue - Primary winding of transformer,Yellow - secondary winding of transofmer,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2023, 08:55:12 pm
content: Why is the first one asymmetrical (different response around 0V vs. peak)?Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 06, 2023, 09:47:19 pm
content: Probably i dont know what you mean, but if i undestand correctly, first picture is without snubber and second one is with snubber.I though you see a filename of pictures. One is named quickpicture38.png and second one quickpicture40.png.

author: uer166
author_rank: Super Contributor
timestamp: February 06, 2023, 10:18:50 pm
content: Quote from: MacIntoshCZ on February 05, 2023, 11:50:29 amWell, yes, that is expected since you're applying a DC voltage to transformer, which saturates the core and it becomes a short. In that circuit youneedthe series cap to maintain volt-second balance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2023, 10:40:42 pm
content: I mean compare the two rising edges (or fallings), overlaid here for example.Is that direct connection to G-S, nothing on D-S? Or with 6.8R gate resistor? Did you try other resistor values?There's no need for an RC in parallel with the gate, that needlessly slows things down and consumes more drive power.Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 07, 2023, 06:51:37 am
content: This is circuitry i measured on. With this setup capacitor before primary can be omitted.WHy its assymetrical i honestly dont know.I did not try other values of gates resistor. SHall i play with it?I added RC parralel to secondary becouse i am afraid of negative spikes when turning off, could lead to turn on others pair of mosfets which could lead to short circuit..

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 07, 2023, 06:47:46 pm
content: Looks good to me, except horrible ringing at gate waveform... I did not use 100R in series with probe so, who knows if its real...I was scared that this ringing turns mosfet on but then i realized that it could be just body diode of mosfet that starts conduct when no mosfet its turned on.BTW there is no RC parelel on secondary output of GDT transformer, should i think to add some?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2023, 07:34:21 pm
content: Wait you probed across the primary while both sides are driven? Do you have a floating scope/probe??Quote from: MacIntoshCZ on February 07, 2023, 06:47:46 pmSo C33+R21 are there, or aren't?Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:12:54 pm
content: Sorry i forgot to explain what which waveform represents.100V scale is measured Vds of Q2. 10v scale is Vgs of Q2.I was checking for excesice high dv/dt.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 07, 2023, 10:13:41 pm
content: C33 R21 are not connected.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 08, 2023, 08:41:12 pm
content: First sorry for duplicite topic, i started another one, but then realised it would be better to keep just this one alive.I tried to do load transient response test. Turn ON time is probably too low i should make him larger.In the video i was playing with RV3.https://youtu.be/q_M8uPSxmOo?t=25It seems "bumps" are alive =D.Blue trace is just gate of mosfet that turns load on and off. BTW load is 23R, so at 188V its about 1,5kW aprox...yellow trace is voltage at R9. I think i should not use this resistor to sense voltage. Becouse its also used by error op amp of tl494..I already changed RV3 from 10k to 100K. I should play with C27 until there would be no "bumps" am i right?