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I am planning on using a very nice 1MHz secondary I have and want to do some experimentation at this frequency, particularly because it is smack dab in the middle of a common AM radio's reception. I've worked out three circuit variants, all modified versions of schematics I've seen others use online. I've been building SSTCs (singly resonant) for years now with no problems, I've even done a 1.25MHz CW SSTC, which had poor output due to the large primary reactance at such a high frequency. This time I would like to delve into Class-E and doubly resonant solid state work, and I need your (collective) help on choosing which design would work best.
UPDATE: I HAVE UPLOADED AN UPDATED SCHEMATIC, SEE MY NEXT POST FOR PERTINENT FILES AND CIRCUITRY DESCRIPTIONS.
Here is a schematic containing all three variants:
The first (top) circuit is based off of the most common design for Class-E SSTC's that I've seen online; the primary is in parallel with the MOSFET. I have added a resonant primary capacitor in series with the primary, but as I've never seen or heard of a single-ended DRSSTC, and certainly not heard of one where it is blended with a Class-E topology, I have no idea if it will work!
My understanding of Class-E implementation is that the damped oscillation that allows for ZVS is formed by the Drain-Source capacitance and the Inductance of the DC Choke. I've chosen values for these that would cause oscillation just below that of the secondary resonance. However, I don't know what effect having a resonant series capacitor in place of the normally non-resonant DC blocking capacitor will have. My idea was that its capacitive reactance would cancel out the primary's inductive reactance, same as in a DRSSTC. Obviously it cannot run CW with a resonant series capacitor. If the primary inductance and the DC choke inductance is used to form the damped oscillation for Class-E then I'll need to choose a different value for the Drain-Source capacitance as this was not included in the calculation. The primary cap and DC choke should resonate at roughly half the secondary resonance, if they resonate at all, and again that is if the primary inductance is irrelevant there.
One thing that confuses me for this implementation of Class-E is that the load is in parallel with the switch. I know that due to the reactance of the DC choke at the secondary resonant frequency very little current will be shunted to ground through the switch, but this orientation seems reminiscent of a basic Boost Converter, which makes me think that suddenly stopping current flowing through the L1 choke will cause a large voltage to develop; perhaps this is the source of current flowing through the primary to ground? In which case I've added an ultra-fast freewheeling diode in anti-parallel with the MOSFET to protect it. Do you think a body-diode-isolation-schottky is called for then? It would be in series with the MOSFET, with the freewheeling diode then in anti-parallel to both the MOSFET and diode. I've never seen a Class-E schematic where either diode is added in, so I am hesitant to think that it is needed. Any enlightenment is always appreciated.
The Second (middle) circuit is based on the Class-E implementation variant I've seen where the load is in series with the switch. This seems the most straight forward design to me, but again I don't know if the Xlpri plays a role in forming the damped oscillation frequency that allows for Class-E. To me, this seems like it has the highest chance of working as I have a better grasp of how it will behave. What makes me hesitant is that I've never seen one of these series-primary Class-E configurations actually work! UPDATE: SERIES CAPACITOR "Cpri" HAS BEEN REMOVED TO RESTORE A DC CURRENT PATH.
The Third (bottom) circuit is just a distilled single-ended DRSSTC. Never heard of it being done or attempted, but I'd like to try it (unless it is blaringly stupid and has no chance of working). This is kind of my fail-safe last resort. If I can't get Class-E working at all I can simply remove the Drain-Source capacitance and DC Choke and give this a whirl. I expect that I'll have to play around with the ON time to make sure too many cycles aren't back to back, which would cause current to ring up and exceed the MOSFET's current capacity. UPDATE: SERIES CAPACITOR "Cpri" HAS BEEN REMOVED TO RESTORE A DC CURRENT PATH.
So; 1) When designing a Class-E SSTC, is the primary reactance part of the LC circuit that forms the damped oscillation allowing for ZVS? Or is it just a product of the Drain-Source capacitance and the DC choke inductance? 1a) Does having a resonant sized DC-block capacitor in series with the primary effectively remove these two component reactances out of the Class-E/ZVS equation as I envisioned? UPDATE: MOOT POINT; NO DC CURRENT PATH.
2) The first two circuits have a resonant DC-Block capacitor, assuming the answer to question #1 is implemented (i.e. the D-S capacitance is set correctly), would making this capacitor non-resonant enhance or detract from the operation of the circuit? If it would enhance it, which is better, more or less capacitance? UPDATE: MOOT.
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Joined: Thu Feb 02 2006, 01:11PM
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Hi Sigurthr,
Looks great. Exciting to see what you guys are doing. Very impressive work and its a pleasure to see high voltage products other than DRSSTCs.
My one big comment on this is the use of the UCC37322. You're going to find that these are going to get extremely hot and not work too well, especially at 1MHz and running CW, which i believe you intending to do.
I would definately recommend the TO-220 version of the IXDD614CI. This is an RF gate driver and works well up to at least 8MHz. Its a bit more expensive, but you won't see the heating issues with the UCC device as the output impedance is less, less parasitic capacitance in the internal MOSFETs, and a better thermally conductive path as well for dealing with heat dissipation.
Keep up the great work. Look forward to seeing your results!!!
Also, be sure to check out Richie Burnett's website on Class-E operation. He has some great write-ups on the subject as well as graphical represenatations of how Class-E works.
Looks great. Exciting to see what you guys are doing. Very impressive work and its a pleasure to see high voltage products other than DRSSTCs.
My one big comment on this is the use of the UCC37322. You're going to find that these are going to get extremely hot and not work too well, especially at 1MHz and running CW, which i believe you intending to do.
I would definately recommend the TO-220 version of the IXDD614CI. This is an RF gate driver and works well up to at least 8MHz. Its a bit more expensive, but you won't see the heating issues with the UCC device as the output impedance is less, less parasitic capacitance in the internal MOSFETs, and a better thermally conductive path as well for dealing with heat dissipation.
Keep up the great work. Look forward to seeing your results!!!
Also, be sure to check out Richie Burnett's website on Class-E operation. He has some great write-ups on the subject as well as graphical represenatations of how Class-E works.
Ah, wow, thank you very much! That gate drive chip is JUST what I was looking for! It never ceases to amaze me how much my projects and experiments have been held back by not knowing a certain device exists. Yeah, I've had heating issues with the UCC chips at frequencies above about 700KHz, my solution has been to thermally bond heatsinks to the tops of the 8Pdip packages, which is never fun. I'm looking forward to getting my hands on some IXDD614 CI / YI now! I'm familiar with Richie's work and it has been influential in my experiments!
@Antonio; Thanks for pointing out that glaring error; you're right, with no DC path it will just fizzle out once the cap is fully charged.
Here is an updated schematic trio: Circuit 1 is identical as no errors were pointed out. Gate drive chip has been swapped, but operation is identical. Circuit 2 has had the series capacitor "Cpri" removed, this is now the same as the "second Class-E variant" I've come across in my searches. Circuit 3 has been re-envisioned. It is now a solid state take on the classic SGTC, in keeping with the goal of finding a single ended DRSSTC topology.
Other than imageshack going paid and taking down all my photos from 2014, not much has happened.
I still have the amazing IXYS gate drivers from EVR (thank you again!).
Initial tests showed most of the oddball design variants I was going to experiment with (like the single ended DR) would simply not work, at least with the implementations I was going to try.
I'd still like to experiment with the two variants for Class-E regarding switch placement (series with primary or parallel as is usually done), but I've suspended the project as my old analog scope just isn't fast enough to accurately do class-E at the frequency I had set up for. I can't see >4f ringing on my scope when the f0 is above 1MHz. I plan on coming back to it when I have the equipment needed, or better components/tools that will rule out the need for it (I can't source copper clad or etchants here, and am stuck using solder-trace construction on pad-per-hole board).
Sig, you might be interested in these Pulse GDTs, which probably will work at the frequencies you want. I'm using them for my Class E work
this is an old thread
Says the man who necro-revives a 2yr+ old thread.
@Chris, yeah they look pretty decent. It's been a long time since I've done the math on my own GDTs so I don't remember their electrical characteristics anymore, I just know they work, hehe. I've tested them up to 1.25MHz (50% duty) with 18Apk current and no saturation. I'd have to run the numbers again and compare to see if the ones you linked could be suitable.
The main problem with the ideas for a DR coil as referenced in this thread was/is that the peak currents and voltages that such a prolonged ring-up (very long burst times) resulted in were incompatible with the gate drive needs of a transistor that could handle such Drain-Source stress. In addition, the idea was that by using a fixed frequency oscillator and class-E for minimal switching losses one could make a dual resonant coil in such a way as to circumvent the need for primary current sensing and feedback. The few experiments and maths I did do showed that this really wasn't feasible. The slightest detuning would throw the unit into runaway and result very quickly in silicon death.
When I get time, tools, and opportunity to work on the (regular) class-E coil I'll be able to test the two variants referenced here and report back. It isn't a high priority project though.
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Joined: Sat Jan 09 2016, 06:48AM
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Posts: 43
Hey sig, on the subject of class e parts, have you seen these new st mosfets? They're about $2 and have faster switching times and less gate charge than irf630
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Help requested; Class-E and DRSSTC finer points
