QCW questions

[davekni]

I know this is probably fine by now but.. When phase lead tuning, whats 'best' to watch out for? Get minimum 'spike' (yellow,Vce) or the ringing(of Vce) when Vge(cyan) turns ON?

Or just disregard that ringing of Vce(Vge turn-on) b/c its insignificant already(?)

Yes, I'd disregard the Vce turn-on ringing, at least within the range you show in the video.  It's low amplitude and does not change that significantly over the range shown.  Important feature of turn-on is that Vce is already close to 0V before Vge rises above threshold voltage (above ~5V).  That appears to remain true over range shown.

Turn-off Vce spike is larger when phase lead is larger.  Turn-off is farther before zero current.  The larger remaining current causes a larger voltage spike when that current transfers to the high-side IGBT's diode.  Within shown range, the minimum Vce turn-off spike (minimum phase lead) is still sufficient.

Even though the Vce turn-off spike isn't large enough to be a likely problem over any of the shown range, you could for learning figure out what is causing it.  Leave scope probe ground as connected for low-side Vce scoping.  Trigger on Vge or current or any other signal in order to have fixed trigger timing.  Then move scope probe tip from low-side collector to high-side emitter.  See how much of the spike is still present.  Then move probe tip to high-side collector.  See how much spike occurs there.  If a spike on high-side collector (Vbus), that is a result of inductance in snubber cap leads or bridge construction.  If the spike is not on high-side collector but is on high-side emitter, then spike is due to high-side IGBT lead inductance or diode forward recovery time.  This testing will be easiest with a large spike to measure, so set phase lead high (large Vce spike setting) just for this experiment.

[Rafft]

David



**LO Vce, Vge as trigger

**HI emitter leg , LO emitter gnd reference
I notice a shorter spike

**collector leg(Vbus) LO emitter gnd reference
Are these spikes on Vbus too much? Im using 1uF 275v~ mkp x2(4pc in parallel) parallel-wired using 2 layer pcb(planes) and wired as short as possible

**fullbridge

[davekni]

Thank you for the scope traces.  Makes it clear that the majority of spike voltage is due to snubber capacitor inductance, with a little contribution from IGBT lead inductance.  Don't see much evidence of diode forward-recovery time issue.

Are these spikes on Vbus too much? Im using 1uF 275v~ mkp x2(4pc in parallel) parallel-wired using 2 layer pcb(planes) and wired as short as possible

No, these spikes should be fine.  Only issue would be if they went far enough above Vbus to exceed IGBT Vce rating.  Seems quite unlikely.
Your construction is better than most.  No need to worry.  If you want to be excessively perfectionistic:  Snubber inductance is likely dominated by the short wires down to IGBT board.  Pairs of Vbus+ and Vbus- wires closely-spaced will reduce inductance.  Multiple pairs and close wire spacing both help.  Or, snubber caps on the same board as IGTBs.  That can be better (lower inductance) even if snubber caps need to be physically farther from IGBTs.  Overlapping parallel planes have lower inductance than paired (twisted or adjacent) wires unless many pairs are paralleled.
BTW, I'm presuming overlapping parallel planes already exist separately on the two boards, IGBT board and snubber capacitor board.

[Rafft]

Makes it clear that the majority of spike voltage is due to snubber capacitor inductance

Im relieved. I have no proper snubber caps at the moment, had to use whats on the bin. as noted from your previous reply --> mkp x2 as excellent spike catchers

Snubber inductance is likely dominated by the short wires down to IGBT board.

Ive mine constructed on 2 layer ECB(overlapping planes) and "wired" to IGBT board(also 2 layer planes) as short as possible. around 10mm or so leads.

these spikes should be fine.

glad to hear this as well. 

Pairs of Vbus+ and Vbus- wires closely-spaced will reduce inductance.

mine is twisted and short , to 5mm bullet connectors(used on RC toys), w/c connects to twisted wire from buck output

BTW, I'm presuming overlapping parallel planes already exist separately on the two boards, IGBT board and snubber capacitor board.

yes correct! overlapping planes for IGBT board and another for the paralleled snubber caps

Thank you 

EDIT: forgot to mention,  scope shots where done with the -worst- spikes possible, so when I adjust back again the phase lead, spikes will all go down 

EDIT 2: changing gate resistor from 1/4watt to 1watt made a small difference. Now I see tripple transition. I will adjust this to have the smallest Vce spike

/>

[davekni]

Im relieved. I have no proper snubber caps at the moment, had to use whats on the bin. as noted from your previous reply --> mkp x2 as excellent spike catchers

Yes, X2 caps aren't ideal for snubbing.  I still suspect at least half of the inductance is in 10mm wires rather than in the cap leads, given there are four parallel caps.

mine is twisted and short , to 5mm bullet connectors(used on RC toys), w/c connects to twisted wire from buck output

I was referring to the 10mm wires from snubber board to IGBT board, not your buck-to-H-bridge connection.  Yes, very short.  But pairing will still reduce inductance even for those short wires.  Best I could tell from image, the two 10mm wires were not adjacent.

EDIT: forgot to mention,  scope shots where done with the -worst- spikes possible, so when I adjust back again the phase lead, spikes will all go down 

Yes, that is exactly what I'd suggested you do for this experiment.  Could see that in scope traces too.

EDIT 2: changing gate resistor from 1/4watt to 1watt made a small difference. Now I see tripple transition. I will adjust this to have the smallest Vce spike

Minimum phase lead in this video is a bit too little.  Shows small triple-transition on rising edge.  More concerning is that falling edge isn't quite complete at the point where lower IGBT starts turning on.  One evidence of this is the longer plateau on Vge at turn-on, as well as observing that Vce isn't to 0V at the start of that plateau.  (Would still be OK, except that IGBTs will slow down as they get hot, so need a bit more phase lead to compensate.)

[Rafft]

I can see a small plateau of Vge at 0v. Vce is also below 0v now.

Edit (2 shots 1 diagram)
Same phase lead setting
**1 lo Vce - where I can see worst rising spike

**2 lo Vce - best Vce

**fb /ocd/ 3rd ct connections

[davekni]

Edit (2 shots 1 diagram)

I see three scope captures.  First two look like plenty of phase lead.  Third looks like barely enough phase lead.  That does minimize current at switching time, so minimizes overshoot spike.  Good as long as IGBTs do not slow down too much when hot.  The ones with more phase lead are safer in that respect - margin for IGBT heating.

**fb /ocd/ 3rd ct connections

As expected.  Thank you for confirming.

[Rafft]

Hi David

First scope shot was with different phase lead L value

Next 2 shots, are from another L value. Are taken from 'side 1 Lo Vge' & another 'side 2 Lo Vge' w/c is reffered in the diagram.

I read in another web post, fb/ocd should be connected on the other side, like bridge output before MMC(?)

[davekni]

First scope shot was with different phase lead L value

Next 2 shots, are from another L value. Are taken from 'side 1 Lo Vge' & another 'side 2 Lo Vge' w/c is reffered in the diagram.

Are you certain?  First two look quite similar.  Third looks like less phase lead (lower L or higher R).  If the second and third shots are with same phase lead, then there is a significant unexpected difference between side 1 and side 2 (different gate R, or perhaps quite different internal IGBT die).

I read in another web post, fb/ocd should be connected on the other side, like bridge output before MMC(?)

Doesn't matter which side.  Just not in the middle between primary and MMC.  Voltage is much higher there, risking insulation breakdown within CTs.

[Rafft]

Yes very certain    1st and 2nd scopeshots are similar because they are from same '1 lo side igbt' aka the worst Vce turn-on spike. I can also see '2 lo side' Vce/Vge to have better phase lead. Yes I know they look different(Vce Vge side1 & side2). different IGBTs as you say. Also, all those Vce Vge shots where taken from 'side 1' w/c has worst Vce spike.

Anyways I will be testing cbb21 470nF 630v caps for snubber.. Spikes do look smaller than with mkp x2. Wonder if this is because of lower value capacitance(1.4uF vs 4uF) or because leads are very short.

EDIT:
Since I have changed -snubber- caps to lower value, why has the bridge output become wavy?

I have also been too preoccupied having a good looking wavefirm. Am I correct in saying that these IGBTs can switch on-off in less than 100nS? If so, then its not that bad of an IGBT? These are sold as 'stick welder' IGBTs(about 2$ each).
Btw what is right term for this(turn-on time? Rise time?)

And could it possibly why im getting higher Vce spike because im scoping on the PRImary coil side and not on MMC side? (Just a thought)

[davekni]

Anyways I will be testing cbb21 470nF 630v caps for snubber.. Spikes do look smaller than with mkp x2. Wonder if this is because of lower value capacitance(1.4uF vs 4uF) or because leads are very short.

Because of short leads, including that caps are directly on IGBT board, so no additional lead/wire length to separate board.  Cap impedance is very small at the spike frequency whether 1.4uF or 4uF.

Since I have changed -snubber- caps to lower value, why has the bridge output become wavy?

That is due to the reduction from 4uF to 1.4uF.  It is the resonance of 1.4uF with wire inductance from H-Bridge to buck converter.  Happens to now hit 2x coil frequency, which is exactly the dominant frequency of H-Bridge supply current.  One easy fix for this is to add back in 4uF too, even with somewhat longer leads.  Local 1.4uF will still reduce spike.

I have also been too preoccupied having a good looking wavefirm. Am I correct in saying that these IGBTs can switch on-off in less than 100nS? If so, then its not that bad of an IGBT?

Yes, under your operating conditions, IGBTs are turning off in 100ns or a bit less.  However, IGBT data sheet specifications are usually at hard-switching conditions.  Switching is faster at ZCS conditions.  Thus it is hard to say how these parts compare with genuine ones.  May be close.

Btw what is right term for this(turn-on time? Rise time?)

IGBTs typically have four time specifications.  Turn-on delay, rise time, turn-off delay, fall time.  Rise and fall here refer to IGBT current.  Generally measured in hard-switching conditions, half-bridge feeding an inductor.

And could it possibly why im getting higher Vce spike because im scoping on the PRImary coil side and not on MMC side? (Just a thought)

No, that does not directly matter.  However, routing of scope probe and ground lead can make quite a difference.  Loop formed by probe and ground lead will have induced voltage from adjacent fields.  Routing scope ground lead along probe tip as much as possible will help.  Some people wrap (spiral) ground lead around probe, then connect to circuit as close to probe point as possible.

[Rafft]

Wow! Those 'snubber' capacitor lead inductances DO matter, how they should be mounted/connected. Ive tried adding the 4uF mkp x2 above the 220nF(4p) but still, resonance can be seen & it looked like the 4uF isnt doing anything. So I just removed all caps, removed the 1uf 4p caps from its 'board', and just connected them to the igbt board. But I only manage to fit 3uF(no more space on the board) . result is almost no Vbus resonance and very little spike.

Ive also taken photos of #1 lo Vce Vge & #2 lo Vce Vge. Same phase lead settings.

Also tried short and long breakouts. Longer breakout point makes longer sworksparks. Almost same primary peak currents.

[davekni]

result is almost no Vbus resonance and very little spike.

First scope capture shows the normal Vbus resonance at end of enable pulse, about 80kHz in this capture.  (Presume this is normal DRSSTC mode.)  80kHz is so far from twice 310kHz operating frequency.  Makes me think my original explanation for 620kHz resonance was incorrect.  This 80kHz resonance is probably due to snubber capacitance and wire inductance to buck converter.  Not sure where the 620kHz resonance came from.  Are there any other caps across VBus on the H-bridge board?

Ive also taken photos of #1 lo Vce Vge & #2 lo Vce Vge. Same phase lead settings.

These two captures look similar enough, as they should be.  They do show that current measurement is about 90ns ahead of actual current.  That can be caused by CT inductance and/or by inductance of burden resistor.  Is burden resistor wire-wound?

Final picture shows one scope probe with no ground clip (I think).  Which signal is on that probe?  High frequency details will not be accurate with an ungrounded probe.  The loop from one probe ground to other probe is quite long with lots of inductance and lots of loop area to pick up signals.

[Rafft]

Vce scope shots is my final hardware. 1uF mkp x2 3parallel. 3uF total. As snubber.Those are the only caps left on the igbt board. Yes DRSSTC mode. No secondary coil.

CT burden resistor is 1watt carbon

Final picture. Vce has the GND clip. The unGNDed probe is Vge. I leave the GND clip for Vge open(when checking the bridge output/s

[davekni]

CT burden resistor is 1watt carbon

If I recall correctly, burden resistor is 1 ohm.  So only 90nH is needed to make 90ns of phase shift.  Could easily get that much with lead wires even though not a wire-wound resistor.  Ideal to have both CT secondary and scope connections close to resistor body (short resistor leads), with scope leads heading opposite direction from resistor as CT secondary leads.  Of course, now that a bit of phase shift is clear, can be just mentally compensated for in viewing scope traces.  I'd guessed way back that there was some phase shift (lead) in CT scoping.  It was clearer in recent captures.  (CT inductance is almost certainly not an issue with 1 ohm burden resistor.  CT inductance can become an issue with high-value burden resistors.)

[Rafft]

Just another -fine tuning- update:
 
As per filename shows:  primary current difference between high and low coupling. My coil is telling me 'something' (on that faster rise on ramp peak) but I dont quiet get it yet 

- does it want more ramp voltage? More ON-time perhaps? Is it indicating coil is resonating better? Is it at its prime of resonance? Any thoughts and insights welcome.

Hardware: same secondary and topload. Same PRI inductance(to maintain k to sec winding). I adjust MMC only.
Sec Fres vs Pri Fres, Fres is always higher(upper pole)

Just for reference, highK=400kHz lowK=355kHz

Thank you

[davekni]

As per filename shows:  primary current difference between high and low coupling. My coil is telling me 'something' (on that faster rise on ramp peak) but I dont quiet get it yet 

My guess:  High coupling case looks good.  Low coupling case shows current rising quickly at the end.  That is likely arc loading pulling secondary frequency too low.  With lower coupling, primary-to-secondary power coupling drops off more rapidly as frequencies move apart.  Without much power transferring to secondary, primary current ramps up.  I think that is why QCW coil designs typically have high coupling.  Normal DRSSTCs would also be better with high coupling, except that high coupling causes racing sparks in normal DRSSTC mode.

[Rafft]

I have a question regarding the UD2.1b

During no oscillation, what is the state of comparator output?

Im instead using MAX913 as my comparator. Pin#2 is on a voltage divider @1.599v(as per diagram). When I used this, pin#7(non inv out) is at active high. During test on DR mode(no sec), I get erratic output. So I adjusted that voltage to about 1.542v(pin#7 is now active low). Is my hardware corrent? Or it doesnt matter if active low or high on pin#7 output?

Reason I asked is,  a single big glitch(Iprim) near ramp top. Was wondering if this has something to do with comparator output/s state.

Ive tuned phase lead to the best output(clean Iprim) I could get.

Blue(single channel) is comparator pin7(non-inv out) with pin2# on 1.599v. On 1.54v it is inverted

Phase lead capture. Yellow(Vce) cyan(Vge) magenta(Iprim)

No idea where/what this glitch is comming from. Maybe cause of slow/fake IGBTs?

A small video. Just random ramp area to check Iprim. Ignore the 'glitch' on ramp-down. Scope memory error perhaps. qcw test done with 300vbus.

/>
And a short phase lead tuning

/>


Edit:
Issue Fixxed !
On higher ramp ON-times, 'ripples' on the ramp edges hinted me of something. Looks like inverter bridge resonance(?) Or buck output 'problems'. Tried increasing buck output LC Filter capacitance. Seems to have -fixxed- it.

[davekni]

Im instead using MAX913 as my comparator. Pin#2 is on a voltage divider @1.599v(as per diagram). When I used this, pin#7(non inv out) is at active high. During test on DR mode(no sec), I get erratic output. So I adjusted that voltage to about 1.542v(pin#7 is now active low). Is my hardware corrent? Or it doesnt matter if active low or high on pin#7 output?

The default state should be with pin 7 high (1.599V on pin 2).  Pin 3 is one diode drop lower when quiescent due to current through 100k resistor R27.  I'm a bit surprised that such a small change of pin 2 voltage switches state.  If D1 and D2 diodes are 1N5819 or similar, then perhaps that is reasonable given the small current through R27.

Do you have any bleed resistor across H-bridge outputs?  If not, initial state is indeterminate.  Perhaps your H-bridge initial state tends towards one direction or the other due to imbalances in IGBT leakage current.  If so, changing comparitor initial state may be providing a better initial half-cycle signal to get oscillation started reliably.  Startup is particularly problematic at low bus voltage as in your QCW use.

And a short phase lead tuning

Looks like good setting at the end.  Video is a great way to show the range.  Behaves roughly as expected.

Issue Fixxed !
On higher ramp ON-times, 'ripples' on the ramp edges hinted me of something. Looks like inverter bridge resonance(?) Or buck output 'problems'. Tried increasing buck output LC Filter capacitance. Seems to have -fixxed- it.

Glad it's fixed.  My first guess is that some noise coupling from buck to UD2.1 is causing an H-bridge transition at the wrong time when current is high, causing that glitch.  Just a guess.

[Rafft]

Yes Im using 5819 there. On the 1k/470 pin#2, ive used multiturn in place of 470. I have bleeder resistor on bridge output(33k 3watts). 

Coil is running OK now (electronically).

Thanks