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Pulse Skipped DRSSTC Waveform

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zzz_julian_zzz

Fri Sept 30 2016, 02:42PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi Guys,

Any idea what would UD3 (with pulse skipping) logic would be like? This control feature is shown on Philip's Skinny coil - I am trying to replicate it with purely analog circuitry like in UD2 (i.e., without uControllers/ Expensive Intelligent Chips) for the reasons that I am not well versed on those chips and not have enough resource to carry on..

however, I have attempted to replicate it (pic below) and will be glad to hear your comments if it is somehow "getting to the ballpark" or my thinking of pulse skipping method is different from Steve W's...
i.e. using OCD to detect OC event then zeroed the next pulse (of both legs of inverter) and let current flow and subside thru diodes.. while switching is IN-Sync with the coil oscillation/no hard switch; then let the pulse come again 'til the next OC event... in that way that the IGBT will be helped by its copack diode in terms of energy handling..

Below is the Primary Current waveform, any ideas on the subject is highly appreciated.
Thanks in advance!

Hydron

Fri Sept 30 2016, 03:59PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I sketched up a UD2.x addition for Kizmo to do exactly this.
I don't have the diagrams handy, hopefully he has saved them somewhere!
From what I recall it is quite easy to do, only one or two extra logic chips.
My solution was to turn off only one leg of the inverter, letting the current free-wheel via one co-pack diode (off-leg) and one IGBT (on-leg). This method rings down slower than when turning off both legs, which forces the current to flow back into the DC bus capacitor.

zzz_julian_zzz

Sat Oct 01 2016, 02:43PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hydron,

thanks for the useful inputs- for 1 leg off of the inverter, waveform will look like in the pic below...

Here are my concerns for this waveform:
1. Unbalance loading for IGBTs (i.e., in 1st IGBT Brick(half of the full-bridge); only 1 IGBT is switching full time and the other is skipping - which is same scenario on the 2nd Brick ... that it could lead also to unbalance heating? or duty?

2. Not Symmetrical canopy (top and below) - in my pic, you can see that the bottom part of the waveform is constantly straight unlike in the TOP peaks - it is fluctuating obviously; will there be any issues on this?

My observations/questions:
1. For Slower Change in current (fluctuation); I find it that coupling plays factor- Low K~ slow dI/dt from Sims

2. Is inductance of Pri also plays role here? how about the total surge impedance? what is ideal Z?

Do you have pics or vids of your pulse skipping attempts?
once again, thanks for the input-can't find anything relatively useful from web browsing..

zzz_julian_zzz

Fri Nov 04 2016, 06:02AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi All,

just to give an update on this thread - I was able to design a circuit that skips a certain "one pulse" every time that Primary Circuit of my DRSSTC hits a threshold which is being monitored cycle by cycle (both negative and positive going) - Skipping is done on LOW side switches only. The resultant waveform is below.

I think I am satisfied on the Current waveform (yellow on the 1st pic) as it is constantly plain on its tops and bottoms (which indicates slow dI/dt) even when skipping is happening.

The problem I am facing right now is that because IGBTs are being driven by 2 GDTs (both of the GDTs have skipping characteristics on one of its side, which is [intended for driving LOW SIDE IGBT] and without skipping on the other side [intended for driving HIGH SIDE IGBT])--- So when skipping occurs, funny thing happens - the FLUX WALKS! {EDIT: just to clarify, 1 GDT drives 1 High side and 1 Low side IGBT}

And because of this Flux walk problem â€“ during a â€œskipâ€ on a â€œWALK phenomenaâ€ , the gate voltages of IGBTs which are supposed to be 0 volts will be a bit higher (see pics, it is approximately ~11volts) I think this is the effect of walk, it moves the center of the waveform so it is not in 0 volts (as seen in Pics below) and causing the â€œsupposed to be OFF IGBTâ€ to be "ON" (for a short time) which is my concern because there will be huge current during skips and IGBT will experience hard switch (due to non-zero switching) and Voltage spikes(on my pic shots for Vge vs- Vce) and probably will cause heating/death.

I believe this flux walk is a typical response of transformer when there are unbalance drive on its primary/ which is introduced when drive skips

I have tried changing the capacitance on the GDT primary from 1 uF to 47 uF, still the FLUX WALKS
I have tried changing from 24v drive to 15v drive, still the FLUX WALKS
I have tried changing from BIG GDT core(1.5 inch to Small GDT Core(.75 inch) , still the FLUX WALKS

Pics are below â€“ you can see there are SKIP that happens during the WALK (and so Vge greater than Zero)

With this, I would like to seek inputs from you guys on how to resolve itâ€¦ My other option is just utilize OPTOCOUPLER â€“ but as much as possible, I would like to find other solution in this and still use GDT.
Thanks in advance.

Julian

hen918

Fri Nov 04 2016, 10:38AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

The skipped pulse should be both positive and negative. i.e. you need to skip two pulsed one positive and one negative. Alternatively, you could alternately skip positive and negative pulses to keep the flux balance. Using an optocoupler won't help, it might just make things worse as the imbalance will be shifted from the GDT.

zzz_julian_zzz

Fri Nov 04 2016, 03:02PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hen918,

Thanks for your comment. It makes sense perfectly to me. I will revise the PCB, maybe some jumper wires will do :) I'll do the simulation on it first, but I think the name "Pulse Skip DRSSTC" should be changed to "Cycle Skip DRSSTC" because of the GDT imbalances.. Just a thought haha

I have no reference on Pulse Skip Drsstc except for the Youtube Videos of Philip and that's it, So when I read "Pulse Skip" I then designed for pulse skipping circuit ...

BTW, I have made some progress on the optocoupler approach and here is the result..

My approach is get the non-skipping sides from the 2 GDTs and put it on 1 GDT - meaning this GDT do not suffer Flux Walk.. then, I put the secondaries from this non Skipping GDT to high side IGBTs and the skipping drive for the low side IGBTs are passing OPTOs (for the two AC legs). So now, I am using only 1 GDT. I also introduced some negative drive to at least mimic the GDT wave characteristics & to avoid accidental ON..

based on the pics below, I am seeing not so very bad waveforms, transition between GDT driven and opto driven are quite nice.. in the aspect of no cross conduct. Clearly there is a bit dead time on the turning on-off and off-on transitions..

BLUE - OPTOCOUPLER (driving the low side IGBT)
YELLOW - GDT (driving the high side IGBT)

another thing, have you ever tried doing Pulse Skip DRSSTC? Do you have video of it? thanks!

What do you think? will this work ok?

Hydron

Thu Nov 17 2016, 07:42AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I sketched up my proposed way to do the pulse skipping in another thread (using 2 GDTs and skipping whole cycles to keep DC balance). Also did some simulations, let me know if you want the .asc file

zzz_julian_zzz

Fri Nov 18 2016, 02:59AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi Hydron,

I received ASC, thanks for that! I already plugged my coils parameter and working ok...
here's what I see the differences in our approach:

1. Yours is using both AC legs of the inverter to be skipping (both high and low side), mine is only one side (which is low side)
2. Yours (I don't know) where to put limits on current on where to trigger skip events, mine is sort of like OCD circuit

i can send you my Asc files if you want to see them too..Thanks!

Hydron

Fri Nov 18 2016, 09:53AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Yeah send the Asc through, it would make it much easier to understand how you are trying to do it.

The limit in my circuit is set by H2 - the last number (500 in the schematic i emailed) is the current limit. It was done this way to simplify the simulation - we already know that the OCD circuit with comparator etc works!

zzz_julian_zzz

Mon Dec 05 2016, 02:31PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Hi All,

with the help of Hydron and Phillip, I was able to apply the PSD Driver to my Coil..
however, I am facing another problem when running on a freewheel drive...

Primary Coil tuned lower to Secondary (forcing to Lower Pole):
the primary current tends to be like a "sperm" shaping.. see pics below, (pic 1 &2)

Primary Coil tuned higher to Secondary (forcing to Upper Pole)
the primary current at first look, is ok since the tops and bottoms are fairly plain..
but the problem is that it experiences "notching" on the firsts few cycles - seen in the photo belows, which I believe causes the spark to be short..

Hope someone can help me on this.. Thanks!

Julian

and here is my coil spec:
-------------------------------------------- --------
Secondary Coil Inputs:
------------------------------------------ ----------
Current Profile = G.PROFILE_LOADED
2.175 = Radius 1
2.175 = Radius 2
2.51 = Height 1
13.75 = Height 2
1170 = Turns
32 = Wire Awg

--------------------------------------------- -------
Primary Coil Inputs:
------------------------------------------ ----------
Round Primary Conductor
3.25 = Radius 1
3.25 = Radius 2
1.625 = Height 1
2.5 = Height 2
5 = Turns
10 = Wire Awg
0 = Ribbon Width
0 = Ribbon Thickness
0.146667 = Primary Cap (uF)
0 = Total Lead Length
0 = Lead Diameter

---------------------------------------- ------------
Top Load Inputs:
------------------------------------------ ----------
Toroid #1: minor=4.35, major=16, height=15.75, topload

----------------------------------------- -----------
Secondary Outputs:
----------------------------------------- -----------
163.55 kHz = Secondary Resonant Frequency
90 degÂ° = Angle of Secondary
11.24 inch = Length of Winding
104.1 inch = Turns Per Unit
0.00166 inch = Space Between Turns (edge to edge)
1332.4 ft = Length of Wire
2.58:1 = H/D Aspect Ratio
216.8408 Ohms = DC Resistance
54523 Ohms = Reactance at Resonance
0.25 lbs = Weight of Wire
53.058 mH = Les-Effective Series Inductance
51.946 mH = Lee-Equivalent Energy Inductance
49.544 mH = Ldc-Low Frequency Inductance
17.848 pF = Ces-Effective Shunt Capacitance
18.23 pF = Cee-Equivalent Energy Capacitance
25.264 pF = Cdc-Low Frequency Capacitance
7.51 mils = Skin Depth
15.846 pF = Topload Effective Capacitance
305.4158 Ohms = Effective AC Resistance
179 = Q

----------------------------------------------- -----
Primary Outputs:
----------------------------------------- -----------
151.93 kHz = Primary Resonant Frequency
7.11 % high = Percent Detuned
90 degÂ° = Angle of Primary
8.51 ft = Length of Wire
8.5 mOhms = DC Resistance
0.073 inch = Average spacing between turns (edge to edge)
1.02 inch = Proximity between coils
0 inch = Recommended minimum proximity between coils
7.482 ÂµH = Ldc-Low Frequency Inductance
0.12657 ÂµF = Cap size needed with Primary L (reference)
0 ÂµH = Lead Length Inductance
98.794 ÂµH = Lm-Mutual Inductance
0.162 k = Coupling Coefficient
0.13 k = Recommended Coupling Coefficient
6.17 = Number of half cycles for energy transfer at K
19.98 Âµs = Time for total energy transfer (ideal quench time)

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