Author Topic: Mains Ramped SSTC - Power Supply Issue  (Read 830 times)

Offline ZakW

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Mains Ramped SSTC - Power Supply Issue
« on: December 19, 2021, 02:33:54 AM »
Hello,

TL;DR – SSTC will not run using a stepdown AC transformer to power the H-bridge but works with a variac and DC power supply.

I was recently inspired by a ramped SSTC I saw on YouTube https://www.youtube.com/watch?v=02WiBYO8dbI&ab_channel=Magneticitist

I decided to build it and have found it works really well, 12in arcs @ 120V from a 5.5” secondary. However, I am having a strange issue with powering it and I feel like it is a really simple problem but I cannot figure it out for my life.

Schematics I used:

- He is using Steve's SSTC 5 mini as the base design https://www.stevehv.4hv.org/SSTC5/miniSSTCsch.JPG
- He is also using LoneOcean's staccato interrupter but he made a few modifications. https://imgur.com/ZVc5PYW

Layout and setup:

•   Logic side
        I am powering the staccato controller with a 12VAC transformer, rectified with a single diode. That feeds a LM7805 to power the 556 and SN74HC. I have a separate 12V 1A power supply feeding the UCC27425 to drive the MOSFETs.

•   H-bridge:
        IRFP460’s
        I use a 30v 10A lab bench power supply for testing, works just fine. At 30V is get steamers and everything appears normal.
        I also use a variac transformer for higher voltage, rectified with a single diode, works just fine. I have tested up to 120v no problems.

Here is the issue: (attached schematic) It is really simple. I can provide more details and pictures but I at least wanted to get the question out there first.


        I tried scoping the gate and source of the MOSFETs only to trip the GFCI with the ground lead of my scope. I was not expecting my variac to be grounded to mains earth (somehow).  Even though I am plugging into the variac with a 2 prong type plug (hot and natural) it must be tied to ground internally.
        I want to be able to scope the bridge while the board is powered up, so I grabbed a random transformer I have that outputs around 60V AC (not sure of the current but it is a decently sized transformer so testing should be fine).
   When I wire everything up (see diagram), the coil does not have its usual output but instead has a sporadic popping sound with really thin arcs that only happen when I bring my hand close to the break out point.
   Reversing the AC input polarity causes there to be zero output or popping … The H-bridge is not grounded to mains earth.

Things I have tried:

•   0-30V DC bench power supply
        The negative lead has something like 2.8M Ohms of resistance to mains earth but it works to power the bridge. It doesn’t appear to have its negative lead tied to mains earth so
        I can also scope the MOSFETs without any issues. [/pre][/pre]
•   Varic transformer
        Connected to mains earth internally (a few ohms according to my DMM) but it powers the bridge just fine.
       Cannot scope the bridge without causing a short.
•   Step down transformer to isolate the bridge from ground via the variac transformer
       Does not cause normal operation.
       I tried connecting one leg of the transformer to mains earth but that did not cause it to operate normally.
       I even tried a second transformer to rule out a bad transformer. Same result.
•   Reversing primary coil polarity
•   Reversing current transformer polarity
•   Reversing the 12VAC XFMR input of the staccato interrupter
•   Scoped the output of the XFMR’s to make sure the output was correct. Both are outputting the correct voltage with the expected half wave rectified output.
•   Connecting the stepdown transformer directly to mains bypassing the variac. Same result :( 



What is so frustrating about all of this is that I have a working coil right next to me that uses a stepdown transformer from mains that is not grounded to mains ground (except for the secondary coil). Everything works just fine. I even used my DMM to compare both setups by probing different sections of each coil to make sure I was not missing a ground connection or anything. I did not find any differences.

I am really at a loss here. Other than complete user error and or a lack of understanding I am not sure what would cause this behavior.

Thank you, any advice would be greatly appreciated!
-Zak


Offline davekni

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #1 on: December 19, 2021, 03:32:31 AM »
Quote
I tried scoping the gate and source of the MOSFETs only to trip the GFCI with the ground lead of my scope. I was not expecting my variac to be grounded to mains earth (somehow).
Variac is connected to line.  Grounding with a scope probe is shorting line to ground.  You are lucky that the breaker tripped before frying your scope's internal ground wiring or ECB traces.

Quote
Step down transformer to isolate the bridge from ground via the variac transformer
       Does not cause normal operation.
Two thoughts come to mind.  First, transformers have difficulty with single-diode (half-wave) rectifier loads.  The core hard saturates due to the DC load current.  Usually causes excessive transformer heating and hum sound more than failed output.  You could measure voltage under load to see.  (Variacs have somewhat less of that issue.)
Second, perhaps the transformer output voltage is not high enough for long enough (one half-cycle of line) for SSTC oscillation to start.  DC supply provides lots of time for SSTC to start.  Variac has similar half-cycle time, but perhaps more voltage.  (What Variac voltage is needed for successful operation?)  If this is the issue, kick-start oscillator U1 frequency can be tweaked to match your coil frequency, making startup faster.
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Offline ZakW

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #2 on: December 19, 2021, 05:23:09 AM »
Hello davekni,

Thanks for taking a look!

To your first point, you are right. I am fortunate that I didnt blow anything up! I made sure my power strip was on the same circuit as my GFCI outlet to catch things like that.

Either all I had to do was post a question here or its magic. Its like calling IT but then it starts working... sort of. Still a few things I am not sure about (see scope shots below).

Minimum voltage is around 18V AC.

I did not know transformers can have issues with single diode rectification. Per your suggestion I just went and did a number of tests. First thing is I kept my setup the same but powered the transformer up with the variac set at 120V, instead of ramping it up like before. By doing that I achieved breakout but it was very choppy and skipping beats.

The smaller transformer that I had on hand did not work even when powered up at 120V. Sounds like you were right, either the voltage was too low under load or that the peak is not long enough to power the coil.

Still I tested the larger transformer (that works now) directly connected to mains and it did not work. I guess I will put a pin in that for now.

Some other concerning things I noticed, now that I can scope the bridge. Please see the final questions below as well.

Transformer without load - connected to rectifying diode



Transformer with a 4.7R load



Two pictures probing the AC input on the bridge during operation. Why such huge voltages? That doesn't seem right.




Gate/Source measurements. One MOSFET voltage is higher than the other 18.8V (yellow) vs 16V (purple)... What would cause that? Is that a concern?
Here is what I am using for a GDT https://www.mouser.com/ProductDetail/871-B64290L618X35. I have used it before without issues. 11 primary turns & 15 secondary turns.





Primary coil output




MOSFET Gate/Source (the one with lower voltage 16V) - What could those spikes towards the end be caused by? I have not scene that before.




MOSFET Gate/Source (the one with higher voltage 18.8V) - what might cause this MOSFET to have those spike in the beginning compared to the other?





CT input before diodes (yellow) Gate/Source of a MOSFET (purple) - is the phase shifted by almost 90 degrees or could I have probed the wrong MOSFET? It seems like the MOSFET switching is lagging behind a lot.





Another strange observation - when connecting my probe ground lead to the negative leg of the AC input, the coil stopped skipping and missing beats and its output increased. Almost like when there is interference or when you use an antenna the coil can have a intermittent output. There was even a little spark to the alligator clip. The performance change happened when I connected the GND clip to the source of the MOSFETs as well. Do you know why that might be?

Lastly, while trying to scope both gates/sources of the MOSFETs at the same time, the output of the coil was drastically decreased when connecting the second GND clip to the source of the MOSFET. Is this normally the case? I have scene scope pictures before showing both MOSFETs signals.

There is a lot here, sorry for all of the questions. Thanks again.

- Zak







Offline davekni

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #3 on: December 19, 2021, 06:19:03 AM »
Quote
Two pictures probing the AC input on the bridge during operation. Why such huge voltages? That doesn't seem right.
My first guess is the scope is set for 1x probe and you are using a 10x probe.
A diagram of your actual circuit would be very helpful, with labels that you can reference for where you are probing.  It is not clear what you are measuring here.  Does not look like power into bridge, unless there is a serious issue with the circuit.  (Or perhaps with scoping technique.  You need to understand scope ground connections - see later answer.)
A schematic of your actual circuit will show how you connect a CT to the antenna input.  Needs to be AC-coupled (series capacitor), as CT output has 0V average, while HC14 input is ~2.5V average.

Quote
11 primary turns & 15 secondary turns.
Core is fine.  Trifilar-wound is better (1:1:1 ratio), as leakage inductance will be lower.  That's what Steve's schematic says to do (the one you referenced initially).  That will clean up your gate waveforms some.

Quote
What could those spikes towards the end be caused by? I have not scene that before.
 - what might cause this MOSFET to have those spike in the beginning compared to the other?
Likely an issue with physical construction - interference (stray electric or magnetic fields) causing your interrupter to misbehave.

Quote
CT input before diodes (yellow) Gate/Source of a MOSFET (purple) - is the phase shifted by almost 90 degrees or could I have probed the wrong MOSFET? It seems like the MOSFET switching is lagging behind a lot.
Delay from CT output to Vgs is the delay through the HC14 inverters and gate-drive chip.

Quote
Another strange observation - when connecting my probe ground lead to the negative leg of the AC input, the coil stopped skipping and missing beats and its output increased. Almost like when there is interference or when you use an antenna the coil can have a intermittent output. There was even a little spark to the alligator clip. The performance change happened when I connected the GND clip to the source of the MOSFETs as well. Do you know why that might be?
Your coil (especially bottom of secondary winding) needs a good ground connection.  A grounded counterpoise is ideal, but not critical for SSTC.  Grounding the primary (half-bridge - supply) also helps, but must be connected to ground through a small (2-10nF) capacitor to avoid shorting line-to-ground.

Quote
Lastly, while trying to scope both gates/sources of the MOSFETs at the same time, the output of the coil was drastically decreased when connecting the second GND clip to the source of the MOSFET. Is this normally the case? I have scene scope pictures before showing both MOSFETs signals.
Again, you are lucky you didn't fry your circuit.  Connecting scope ground clips to both sources is shorting them together.  You must have been running at low power (low voltage and/or low duty cycle) for the upper FET to have survived.

Even scoping only the high-side FET alone can cause artifacts.  The high-side FET source is the half-bridge output, so switching at relatively-high frequency with rapid transitions.  Grounding that node induces this high-frequency signal across the transformer supplying power (or from DC supply to ground).  Parasitic capacitance in the supply or transformer loads the half-bridge output and can couple into your drive electronics.
« Last Edit: December 19, 2021, 06:22:17 AM by davekni »
David Knierim

Offline ZakW

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #4 on: December 20, 2021, 01:15:41 AM »
Quote
Two pictures probing the AC input on the bridge during operation. Why such huge voltages? That doesn't seem right.
My first guess is the scope is set for 1x probe and you are using a 10x probe.
A diagram of your actual circuit would be very helpful, with labels that you can reference for where you are probing.  It is not clear what you are measuring here.  Does not look like power into bridge, unless there is a serious issue with the circuit.  (Or perhaps with scoping technique.  You need to understand scope ground connections - see later answer.)
A schematic of your actual circuit will show how you connect a CT to the antenna input.  Needs to be AC-coupled (series capacitor), as CT output has 0V average, while HC14 input is ~2.5V average.

Here is the link to the probe I was using. It is set to 100x in the oscilloscope so I am not sure why the voltage was so high. I also tested it using the scopes square wave frequency generator and it was accurate. https://www.amazon.com/1pc-P4060-BNC-Oscilloscope-Probe/dp/B07F6348B1/ref=sr_1_12_sspa?crid=1TIY5GOG6F88C&keywords=100x+oscilloscope+probe&qid=1639955175&sprefix=100x+o%2Caps%2C259&sr=8-12-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFGQTdWOEE3WEQwWTUmZW5jcnlwdGVkSWQ9QTA5MTg0ODYzVEk1STRGQk5SUzcxJmVuY3J5cHRlZEFkSWQ9QTA5NDcwMzYzOURKRUJBVkUxSDBaJndpZGdldE5hbWU9c3BfbXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

I attached the driver schematic below. Hopefully it is not too ugly, I was not planning on sharing it  :-[. I drew it using KiCad so I could use the auto route feature to help layout the board that I etched it by hand. There are a couple things I have changed with the design and I think it would be best for me to rebuild the driver at this point. I added a sperate power supply for the gate drive IC as well as a 7812 regulator.

The DC blocking cap is on the H-bridge board to save space.

I agree, I only know how to basic things with my scope and I have a lot of learning to do. It has been many years since building a half bridge coil and so I am learning what is okay as I go. The last coil I built was Steve's class e which was a lot easier to trouble shoot.


Quote
11 primary turns & 15 secondary turns.
Quote
Core is fine.  Trifilar-wound is better (1:1:1 ratio), as leakage inductance will be lower.  That's what Steve's schematic says to do (the one you referenced initially).  That will clean up your gate waveforms some.

I will rewind the GDT as per Steve's suggestion and measure the gate/source signal again.

Quote
What could those spikes towards the end be caused by? I have not scene that before.
 - what might cause this MOSFET to have those spike in the beginning compared to the other?
Quote
Likely an issue with physical construction - interference (stray electric or magnetic fields) causing your interrupter to misbehave.

I attached the H-bridge schematic as well. I also added test points to show you where I was measuring from.

At this point, now that I have it working some what reliably @120v I think I would like to rebuild the inverter as well. With the PCB I etched I did not leave a lot of room for a larger heatsink. After a few short runs a full power the heatsink gets warm to the touch. If I am going to go that far I might as well upgrade to a full bridge. Can you recommend any links or resources for topology/layout best practices? I will spend some time Googling ideas but if you knew of any that would be appreciated.

Quote
CT input before diodes (yellow) Gate/Source of a MOSFET (purple) - is the phase shifted by almost 90 degrees or could I have probed the wrong MOSFET? It seems like the MOSFET switching is lagging behind a lot.
Quote
Delay from CT output to Vgs is the delay through the HC14 inverters and gate-drive chip.

I know each stage of the HC14 adds some delay to the signal. Is it possible to introduce a fixed amount of phase lead using the HC14?

Quote
Another strange observation - when connecting my probe ground lead to the negative leg of the AC input, the coil stopped skipping and missing beats and its output increased. Almost like when there is interference or when you use an antenna the coil can have a intermittent output. There was even a little spark to the alligator clip. The performance change happened when I connected the GND clip to the source of the MOSFETs as well. Do you know why that might be?
Quote
Your coil (especially bottom of secondary winding) needs a good ground connection.  A grounded counterpoise is ideal, but not critical for SSTC.  Grounding the primary (half-bridge - supply) also helps, but must be connected to ground through a small (2-10nF) capacitor to avoid shorting line-to-ground.

I was not familiar with what a counterpoise is and google images did not offer much help. Are you suggesting a large flat ground plane under the coil vs just connecting the coil to ground? Does the surface area of the ground plane improve performance?


Quote
Lastly, while trying to scope both gates/sources of the MOSFETs at the same time, the output of the coil was drastically decreased when connecting the second GND clip to the source of the MOSFET. Is this normally the case? I have scene scope pictures before showing both MOSFETs signals.
Quote
Again, you are lucky you didn't fry your circuit.  Connecting scope ground clips to both sources is shorting them together.  You must have been running at low power (low voltage and/or low duty cycle) for the upper FET to have survived.

Even scoping only the high-side FET alone can cause artifacts.  The high-side FET source is the half-bridge output, so switching at relatively-high frequency with rapid transitions.  Grounding that node induces this high-frequency signal across the transformer supplying power (or from DC supply to ground).  Parasitic capacitance in the supply or transformer loads the half-bridge output and can couple into your drive electronics.

Silly mistake indeed. I was thinking of measuring the gate and drain signals of a single MOSFET at the same time, and should not have measured the gates of both at the same time. Glad nothing exploded.

Well I am satisfied knowing why the transformer that I was using was not working initially when trying to power the bridge. Looks like I have a lot of room for improvement regarding my layout and design. I will also need to brush up on using my scope so I do not destroy it.

Thanks for all of the info. I will take a look at the gate signal to see if it improves after rewinding the GDT.

Here is a nice arc at 120V.



Offline davekni

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #5 on: December 20, 2021, 05:29:39 AM »
Quote
Here is the link to the probe I was using. It is set to 100x in the oscilloscope so I am not sure why the voltage was so high.
I'd meant to say 10x probe with scope set to 100x, not 1x.  Anyway, the voltage appears to be real.  It is another issue of grounding two points (TP1 and TP3) with scope probes simultaneously.  That is shorting out C2.  Thank you for the schematics to make this clear.

Quote
I will rewind the GDT as per Steve's suggestion and measure the gate/source signal again.
BTW, you can do even a bit better using two twisted pairs as in this tutorial I wrote recently:
https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949

Quote
Can you recommend any links or resources for topology/layout best practices?
Here's my example.  Initial post is half-bridge.  Second is extending it to full-bridge.
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
BTW, make sure the heat sink doesn't short FET drains (cases).  Either separate electrically-isolated heat sinks or insulating thermal pads between FETs and heat sink.  I presume you already did this for your existing half-bridge (or it wouldn't be working).

Quote
I know each stage of the HC14 adds some delay to the signal. Is it possible to introduce a fixed amount of phase lead using the HC14?
SSTCs usually do not run in ZCS mode (bridge voltage switching just before zero current) as DRSSTCs usually do.  Proper phasing on SSTCs does help performance.  It is hard to know actual current phase.  The CT secondary is feeding an C + R + clamp-diode load, so the voltage signal on the CT secondary is not in phase with actual current.  You'll need to measure current separately.  Changing the CT load may be the easiest way to adjust phase if you determine such would be helpful.  BTW, your schematic shows appropriate place for 0.1uF capacitor C10.  Some often-copied schematics have it in the wrong place after clamp diodes.

Quote
I was not familiar with what a counterpoise is and google images did not offer much help. Are you suggesting a large flat ground plane under the coil vs just connecting the coil to ground? Does the surface area of the ground plane improve performance?
Yes, although "large" is relative.  Also needs to be a few inches below bottom of coil in order to avoid blocking magnetic field lines from the primary and secondary.  Counterpoise are more common in DRSSTCs, often directly on the floor (or ground when outside).  It probably isn't necessary.  However, it can also double as shielding between coil and drive electronics.

Quote
Here is a nice arc at 120V.
Yes, nice arc.  In the picture it looks like primary coil leads may be long and separated.  It is best to have the primary leads leave the coil as a pair (twisted or taped together) and stay paired to the bridge.  That reduces parasitic inductance, which both increases effective primary-to-secondary coupling factor and reduces unwanted coupling into driver circuitry.
« Last Edit: December 20, 2021, 05:36:06 AM by davekni »
David Knierim

Offline ZakW

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #6 on: December 20, 2021, 07:42:22 AM »
Quote
I'd meant to say 10x probe with scope set to 100x, not 1x.  Anyway, the voltage appears to be real.  It is another issue of grounding two points (TP1 and TP3) with scope probes simultaneously.  That is shorting out C2.  Thank you for the schematics to make this clear.
Good to know, thanks for taking a look. I am glad the schematics helped.

Quote
BTW, you can do even a bit better using two twisted pairs as in this tutorial I wrote recently:
https://highvoltageforum.net/index.php?topic=1854.msg13949#msg13949
Thanks for documenting and sharing this! I will definitely give this a try.

Quote
Here's my example.  Initial post is half-bridge.  Second is extending it to full-bridge.
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795
BTW, make sure the heat sink doesn't short FET drains (cases).  Either separate electrically-isolated heat sinks or insulating thermal pads between FETs and heat sink.  I presume you already did this for your existing half-bridge (or it wouldn't be working).
Very interesting, I took a look at your examples and I think I follow the construction. I know that connections that cross over each other is not ideal but I was not aware that planes were more effective. Thanks for the tip!

 
Quote
SSTCs usually do not run in ZCS mode (bridge voltage switching just before zero current) as DRSSTCs usually do.  Proper phasing on SSTCs does help performance.  It is hard to know actual current phase.  The CT secondary is feeding an C + R + clamp-diode load, so the voltage signal on the CT secondary is not in phase with actual current.  You'll need to measure current separately.  Changing the CT load may be the easiest way to adjust phase if you determine such would be helpful.  BTW, your schematic shows appropriate place for 0.1uF capacitor C10.  Some often-copied schematics have it in the wrong place after clamp diodes.
At least I got this one right!  ;D Actually, I cant take credit for it because it is what you and Mads recommended in this thread https://highvoltageforum.net/index.php?topic=840.0.msg5651#msg5651. Thanks to that advice (including the resistor between pin 1&2 or the SN74hc) I was able to get my class e coil running. https://highvoltageforum.net/index.php?topic=1014.0

Quote
Yes, although "large" is relative.  Also needs to be a few inches below bottom of coil in order to avoid blocking magnetic field lines from the primary and secondary.  Counterpoise are more common in DRSSTCs, often directly on the floor (or ground when outside).  It probably isn't necessary.  However, it can also double as shielding between coil and drive electronics.
I can try to include something in the final build. I like the idea of shielding the driver from the EMI.

Quote
Yes, nice arc.  In the picture it looks like primary coil leads may be long and separated.  It is best to have the primary leads leave the coil as a pair (twisted or taped together) and stay paired to the bridge.  That reduces parasitic inductance, which both increases effective primary-to-secondary coupling factor and reduces unwanted coupling into driver circuitry.
Thanks! longest arcs I have made since I started making SSTC's. You can't tell in the photo since the primary is facing away but it is plugged directly into the bridge with very short leads. The wire you see is the mains input covered with some tape (super safe). I am more concerned with high velocity silicon from the MOSFETs. Surprisingly with all of my probing and shorting mishaps I have not blown up any MOSFETs during this project. A couple silent deaths with some salvaged FETs so no real losses.

 

Offline Magneticitist

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #7 on: December 24, 2021, 05:00:27 AM »
Awesome!!  Ever since I saw the PLL ramped setup on SciTubeHD's channel I knew I had to make one and it's a really cool SSTC to have. I still haven't figured out what causes all the kinks but a foot or so of arcs from 120VAC is a great deal to me. I'm still not sure why I got longer arcs from the half bridge instead of the full bridge though. I noticed depending on where I actually plug in my half bridge setup it will run differently.. something about the environment and the way the cord is sitting I guess. Using the PLL feedback addition everything seems more robust.

It doesn't surprise me there is an issue with a random step-down because a lot of my first tries at the circuit using different coils seemed to not work properly at certain voltage ranges using the variac. The addition of the CD4046 seemed to help this though.

Just for the sake of trying maybe you could play around with the breakout point using the step-down to see if maybe you can persuade the arcs to form a little quicker than when the breakout point is fairly short and pointing straight out the top. Maybe try a longer point hanging out to the side or something.

Offline ZakW

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #8 on: December 24, 2021, 11:46:09 PM »
Quote
Awesome!!  Ever since I saw the PLL ramped setup on SciTubeHD's channel I knew I had to make one and it's a really cool SSTC to have. I still haven't figured out what causes all the kinks but a foot or so of arcs from 120VAC is a great deal to me. I'm still not sure why I got longer arcs from the half bridge instead of the full bridge though. I noticed depending on where I actually plug in my half bridge setup it will run differently.. something about the environment and the way the cord is sitting I guess. Using the PLL feedback addition everything seems more robust.

Hey! Awesome that you are on this forum as well. I have always wanted those VTTC like arcs and after seeing what you were able to achieve I figured I'd give it a try. Cant believe you got 16in strike! They are so much more quiet than a regular interrupted SSTC. I have to usually wear ear protection with my micro class E coil but this new one is so quiet.

I am currently working on a full bridge version. I just took everything apart and am working on the driver now, my last one was poorly routed and had a lot of noise. I did not include the PLL chip since in one of your videos you mentioned that you had a hard time really noticing a difference in performance once the coil was fully powered up. I did order some so I have the option. I will start a new thread (if it works ;D) but here is a picture of the new full bridge. Followed Dave's tips for lower inductance.

wont let me attach the picture of the bridge here for some reason - its at the bottom.
 [ You are not allowed to view attachments ]

Quote
It doesn't surprise me there is an issue with a random step-down because a lot of my first tries at the circuit using different coils seemed to not work properly at certain voltage ranges using the variac. The addition of the CD4046 seemed to help this though.

Once I powered it up at full voltage with the Variac I was able to get the step down to work just fine. I had start up issues when starting the variac at 0v and slowing turning the coil up. I have nice toroidal transformer that is semi permanently attached to my class E coil but I plan on using that for testing the full bridge so I can scope everything at a decent input voltage.

Quote
Just for the sake of trying maybe you could play around with the breakout point using the step-down to see if maybe you can persuade the arcs to form a little quicker than when the breakout point is fairly short and pointing straight out the top. Maybe try a longer point hanging out to the side or something.

You're right, the break out point needed to be extended with my new top load. It's 3D printed and wrapped with aluminum tape. Works great and looks awesome too.

Like I said, I will be sure to post a new thread detailing my success hopefully and showing how the final build turned out.
« Last Edit: December 24, 2021, 11:49:20 PM by ZakW »

Offline davekni

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #9 on: December 25, 2021, 05:36:53 AM »
Quote
but here is a picture of the new full bridge. Followed Dave's tips for lower inductance.

wont let me attach the picture of the bridge here for some reason - its at the bottom.
 [ You are not allowed to view attachments ]
Not sure what is making an attachment issue, but glad it worked at the end.  Looks great, except that I think the snubber cap is across bridge outputs rather than Vbus input.  (Q3 connections are hidden, but I think I am inferring correctly from the other three.)  Cleanest fix would be to drill two small holes matched to the cap pin spacing.  Clear top copper foil for ~2mm radius around the holes, then mount the cap vertically as a standard through-hole part, soldering on the bottom where Vbus+ and Vbus- planes are.  (I often bend pins over on the back side to get more solder contact area, since holes are not plated through as they would be on a real ECB.)

Also, be careful about touching the gate pins until you have touched the bridge (to avoid ESD).  I like to add small TVS diodes gate-source soldered directly to FET pins before starting construction.  That way accidentally reaching for the bridge and touching a gate pin first is unlikely to damage anything.  Not necessary as long as care is taken to avoid ESD.
David Knierim

Offline ZakW

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #10 on: December 25, 2021, 09:31:48 AM »
Quote
Not sure what is making an attachment issue, but glad it worked at the end.  Looks great, except that I think the snubber cap is across bridge outputs rather than Vbus input.  (Q3 connections are hidden, but I think I am inferring correctly from the other three.)  Cleanest fix would be to drill two small holes matched to the cap pin spacing.  Clear top copper foil for ~2mm radius around the holes, then mount the cap vertically as a standard through-hole part, soldering on the bottom where Vbus+ and Vbus- planes are.  (I often bend pins over on the back side to get more solder contact area, since holes are not plated through as they would be on a real ECB.)


I think the photos exceed the size limit. See below.

Thanks! I'm really happy with how it turned out. Nice and compact. I used Steve's full bridge schematic. The DC blocking cap is not across vbus. I only used a single sheet piece of plastic so vbus and vbus- is on the back side. It was tricky to solder.

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #11 on: December 28, 2021, 12:13:01 AM »
OK, I see that the cap is bridge output coupling, connected at only one side of the bridge output in your initial picture.  That part is fine.

In order to get the advantage of low parasitic inductance, a capacitor is needed from VBus+ to VBus- at the bridge with low lead inductance (short leads), C1 in the schematic you shared.  Low parasitic inductance is less important for the bridge output coupling capacitor C2.  C2 can thus be mounted a bit farther from the bridge.  C1 is the capacitor needing to be at the bridge.
David Knierim

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #12 on: December 28, 2021, 02:25:24 AM »
In order to get the advantage of low parasitic inductance, a capacitor is needed from VBus+ to VBus- at the bridge with low lead inductance (short leads), C1 in the schematic you shared.  Low parasitic inductance is less important for the bridge output coupling capacitor C2.  C2 can thus be mounted a bit farther from the bridge.  C1 is the capacitor needing to be at the bridge.

Thanks for pointing that out. By not including C1 could that be causing the strange Vds output?

What is C1's function to the full bridge? I am supplying half wave rectified mains so its not really a smoothing capacitor, right? Are there any specifications for the capacitance?

Not to cause confusion but I did start a new thread specifically for the full bridge build. Since this thread was centered around my old half bridge and driver layout. I have since rebuilt both. 

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #13 on: December 28, 2021, 06:47:26 AM »
Quote
Thanks for pointing that out. By not including C1 could that be causing the strange Vds output?
Probably.  See answer to other thread.

Quote
What is C1's function to the full bridge? I am supplying half wave rectified mains so its not really a smoothing capacitor, right? Are there any specifications for the capacitance?
Yes, C1 should be small compared to line frequency (high impedance at line frequency).  It should be low impedance at your SSTC operating frequency.  1uF should be plenty at your ~400kHz.  C1 keeps Vbus voltage roughly constant at the microsecond time scale, while allowing it to track line voltage at the millisecond time scale.
David Knierim

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Re: Mains Ramped SSTC - Power Supply Issue
« Reply #13 on: December 28, 2021, 06:47:26 AM »

 


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