How to alter the duty cycle of this SMPS? - Page 1
Forum section: electronics / projects

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 09, 2018, 11:24:07 pm
content: Anyone know what controls the Q1 MOSFET on and off time of this self oscillating SMPS? I've tried putting various capacitors and resistors in parallel with the ones already on there and that just seems to alter the frequency but not duty cycle.It currently has 54% on time and this is causing some hard switching losses as its not enough time for the resonant capacitor across the primary to reach zero, its currently running at 40khz. I just need to extend the dead time a bit or delay the turning on of the MOSFET again for about 10% of the cycle.Sorry the image is a bit low resolution, I didn't design the circuit as it came from ebay but it does work (just a switching cycle that begins a bit too early).Thanks.

author: eblc1388
author_rank: Frequent Contributor
timestamp: February 10, 2018, 02:47:29 am
content: On every switching cycle, Q4 produces a steep pulse to enable the MOSFET's gate voltage to rise. This charges Capacitor C7 and controls how long the period the gate voltage is allow to rise, thus affects the duty cycle. Increasing the C7 value would increase the duty cycle.Try reducing its value from 1nF to 270pF.

author: xavier60
author_rank: Super Contributor
timestamp: February 10, 2018, 11:13:39 am
content: Q4 would turn on just after the peak of the flyback pulse. C8 charging through R10 delays the turn of of Q5. Maybe more delay is needed by increasing R10.FYI, changing the frequency will also affect the flyback voltage.Extra: Also, the MOSFET does not need to be turned on immediately after the flyback pulse has finished. Current reversal occurs during the flyback pulse which makes Drain want to go negative, but is clamped by the body diode for some time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2018, 01:05:15 pm
content: Gosh, they could've at least done it right...I've been making these circuits for years, it's not like they're hard to find. Black's discrete circuits have been around even longer, though they have poorer performance.Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 10, 2018, 01:36:35 pm
content: Quote from: eblc1388 on February 10, 2018, 02:47:29 amWell I have some good news and bad news, C7 does indeed have something to do with duty cycle which is the good news. The bad news is how I found out, it was getting pretty late and I was getting a bit tired and did something stupid in holding a 2.2nF capacitor across C7. Within a split second I saw around 90% on time on the scope followed by anoise and flash. Found Q4 partially shorted so maybe that flash was C5 arcing over and sending HV DC into the base of Q4.Thankfully I was able to replace Q4 with a spare and I also swapped C5 just in case (although it tested fine capacitance wise). It just goes to show that messing around with things after bedtime is a terrible idea and leads to all sorts of silly cock-ups. Luckily the main fuse blew and saved the MOSFET from silicon heaven.Its working again although the switching frequency is about 10khz higher so maybe something else has been partially damaged. Will try asmallerC7 next time!Quote from: xavier60 on February 10, 2018, 11:13:39 amThanks, I did experiment with frequency by changing the number of turns but there was always a bit of hard switching overlap between the end of the flyback pulse and next cycle. I'll try increasing R10 and see what I get, by C8 did you mean C6? I know its hard to see with my low resolution schematic.Are you saying it needs a bit of extra off time even after the flyback pulse has finished? I've drawn a terrible paint drawing to show what I saw on the scope, MOSFET gate at turn off actually looked very good with no ringing and once it was off it stayed off. Turn off was pretty fast so I don't think there is much switching loss there. At turn on you can see the overlap, from memory the FET was turning on with about 10v across its linear region.Quote from: T3sl4co1l on February 10, 2018, 01:05:15 pmWhat adjustments would you make to the circuit in my OP? I should have mentioned that its driving one of those CRT LOPT transformers so maybe they did it differently because they couldn't take feedback from the secondary side, at-least not without increasing the cost.It's from China so every penny counts. This leads me to another point, I swear when I was soldering everything down a few components measured a bit off in spec. Maybe they just shipped out some out of spec passives with it and this is causing the duty cycle overlap thing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2018, 02:23:08 pm
content: I wouldn't make any adjustments, I'd start over...Example, can't get much cheaper than a 555, probably cheaper than inserting four transistors even with Chinese labor at work. (The filtering stuff really isn't necessary.)If a MOSFET is too expensive, it's easily adapted to BJT:Even better, something with current protection. The circuit I posted above shows one scheme to do that. It can be done with a 555, but requires more parts:https://www.seventransistorlabs.com/Images/555%20Boost.pdfThis being a proper regulated supply, but the op-amp output can be replaced with a resistor divider to set output turn-off current. The MOSFET voltage clamp circuitry is still required.Tim

author: xavier60
author_rank: Super Contributor
timestamp: February 10, 2018, 05:41:24 pm
content: Quote from: theleakydiode on February 10, 2018, 01:36:35 pmThe capacitor on the Base of Q5 is C6 with a closer look.No, it is ok for the MOSFET to turn on directly after the flyback pulse ends, just not necessary.You should avoid doing things that change the frequency if the pulse is already at the desired level. Lowering the frequency increases the pulse voltage and can cause things to go wrong.The whole thing would work better with controlled frequency drive from a 555 oscillator.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 11, 2018, 02:12:52 am
content: Quote from: T3sl4co1l on February 10, 2018, 02:23:08 pmThanks but this is already a complete PCB and I didn't design it in the first place, it does actually work apart from the overlapping problem. I know there are better circuits like the ones you posted (and I appreciate you sharing them) but I would like to continue with this one and hopefully fix the issue that I describe below.Quote from: xavier60 on February 10, 2018, 05:41:24 pmWell reducing C7 did reduce the duty cycle by terminating the gate pulse earlier, but does't solve the overlapping hard switching at the start of the cycle problem. I also tried increasing R10 but that didn't have any effect and the circuit failed to oscillate when I set it above 12k, 10k was the highest I went with it still starting up.Seems to me I need some way of delaying the gate pulse, I'm guessing that'll involve modifying R1 and C5 but I'm not really sure. I also played with C3 by reducing and increasing its capacitance but that just changed the peak drain and output voltage, the overlap was still always there. I also did a number of other things including: removing D2, putting a 2.2k in parallel with R7, putting a diode in series with R6, connecting R6 and its diode directly to +12v so its not connected to the gate of Q1, checked the base of Q3 and it was a nice on/off affair with no messing around in between.Putting a 470 ohm resistor in parallel with R5 did help to reduce the overlapping time, but it instead took place when the drain was at a higher voltage so the switching losses are probably about the same.I've included some scope shots to show what the gate and drain look like,this is using a PP3 9v batteryin case of anymore high current disasters whilst I probe about. One thing I notice is the place where the gate is hovering at turn on is 1/2 of the supply voltage. The transformer is home wound with a gapped core, it puts out around 15v and the load is 390R. I labeled what everything is in the scope shots.As you can see I just need some way to delay the initiation of the gate pulse, any pointers to achieve this would be good. Lowering C7 does decrease on time but only when the pulse terminates. I guess I need to modify C5 and R1 but I'm not really sure.tl;drHow to remove overlapping area in the gate and drain waveform? Decreasing the value of C7 only changes when the gate pulse terminates. Are C5 and R1 the key?Apologies for the inconsistency between trace colors, wasn't my intention to mix the probes around.

author: xavier60
author_rank: Super Contributor
timestamp: February 11, 2018, 07:46:03 am
content: Try increasing C6 by a small amount at first, for effect.

author: xavier60
author_rank: Super Contributor
timestamp: February 11, 2018, 08:20:32 am
content: If that doesn't work, compare the flyback pulse to the waveform on C7.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 11, 2018, 01:54:52 pm
content: Quote from: xavier60 on February 11, 2018, 07:46:03 amHow much should I increase it by? Does 2.2nF sound good? Secondly does this cap also set the frequency too? Last time I put a capacitor there it adjusted the frequency. Seems this circuit is a delicate balance.Quote from: xavier60 on February 11, 2018, 08:20:32 amWhat should I be looking for whilst comparing them?Also what does D2 do? Is it some kind of baker clamp? When I took it out I didn't see any noticeable differences (not sure about turn off speed).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2018, 02:15:36 pm
content: Quote from: theleakydiode on February 11, 2018, 01:54:52 pmI don't see that it will ever conduct. Just more evidence of it being an evolved circuit by someone with no understanding.Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 11, 2018, 02:33:45 pm
content: Quote from: T3sl4co1l on February 11, 2018, 02:15:36 pmWell I've read that it can help prevent the transistor from going too deep into saturation and allows it to turn off quicker because of less charge it has to get rid of.Any suggestions for delaying the turn on pulse? I'll post the schematic again and see if its easier to see this time.

author: xavier60
author_rank: Super Contributor
timestamp: February 11, 2018, 02:50:25 pm
content: Yes, try adding 2.2nF to the 10nF, C3. This capacitor looks like it is there to delay the turn on of Q5.Q4 turns on sometime after the pulse peaks, charging C4. It's difficult to tell when Q4 turns off. Comparing the timing of the waveform on C4 to the flyback pulse might give us a better idea of what's happening.Extra: If adding 2.2nF has a small effect on the delay without causing other problems, try adding more.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 11, 2018, 03:13:38 pm
content: Quote from: xavier60 on February 11, 2018, 02:50:25 pmAlrighty then I'll give that a try, I'll grab the waveforms you asked for too so we can compare them. Half the fun with these Chinese circuits isthem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 11, 2018, 03:56:56 pm
content: Quote from: theleakydiode on February 11, 2018, 02:33:45 pmYeah, but it has to be a schottky diode. 1N4148 turns on at about the same voltage as Vbe, so it doesn't do anything.R6 doesn't do anything either, at least, it's much too large as shown, compared to R7, to get Q3/Q5 to act as a schmitt trigger.I think it's intended as a free running oscillator, that is to say, as Q1 drain voltage starts swinging down (after a pulse), Q4 turns on, later turning Q1 on. C3 and C4 only seem to delay this action. Q1 gate capacitance, with R5, also slows turn-on considerably (low microseconds).More or less, C2 sets on time, nothing really sets off time. Increasing C3 will slow things down, but also increase switching loss.It would've had more straightforward multivibrator behavior if R1 were connected to Q1 gate instead of drain, and Q4 collector went to Q3 base instead of Q5 base (through resistors). Then you would be able to control pulse widths more obviously. It would still be a shit circuit, but further improvements could be made, like resetting the multivibrator with a current sense transistor.Which could probably be done, with only a little gymnastics. Remove Q2 because you don't need it anyway (who wastes BOM cost on superfluous transistors? This idiot..) Just switch the supply voltage instead. Replace Q2 with a small resistor (say 0.1 ohm), from D to S. Tie D and G (so R3 is in parallel with the 0.1, doesn't matter). Replace the switch with a SS8050 (or any general purpose NPN), E = GND, B = (to R4), C = (to +V), and cut the +V trace and move the collector connection to Q5 base. This way, C2 can be much larger, or replaced with a resistor (maybe 22k or thereabouts).C1 should be made smaller, and Q1 switching, faster. Gate drive can be enhanced by cutting the Q1 gate trace and inserting a complementary emitter follower inline (NPN collector to +V, PNP collector to GND, bases to R5, emitters to Q1 gate).Reducing C1 increases the peak drain voltage, which must be clamped. A TVS diode rated for somewhat less than the transistor will do.Also, let me actually look up the IRFP2907... holy mother of fuckballs that's an enormous transistor. What the hell do they need 4.5 milliohms and 200A rating for, on a 5 amp circuit?That one fucking transistor costs more than the entire assembly, I guarantee. What is this shit even?Yeah, replace that with an IRFZ34N (or any higher IRFZxxN will do), don't need that loading things down. 400nC gate charge, that's 40nF equivalent, or a gate rising time constant of TWENTY MICROSECONDS!Yet, the waveform shows under 5 microseconds... that's very interesting.They wouldn't splurge for a legitimate IRFP2907, of course; I'm absolutely certain they've used a counterfeit. So there you go, unknown transistors to add to the mix!Even so, the counterfeit probably still costs about as much as the assembly!If you don't know how to rework a circuit, it's quite easy. Just tedious. To cut traces, use a utility blade to score a shallow V-groove across the trace. This removes some length from the trace, ensuring a cut. To add connections, scrape through the solder mask to expose clean copper, and tin it. To add new nodes, you can wire things through space, supported by leads, or glue down bits of PCB with exposed copper on them (having copper clad PCB stock handy is nice here).Obviously, to cut one pad out of a chain of connections, you need to make two cuts and jumper the ends together. Say the switch's +V connection is chain-routed, so V1 goes to C7 and so on, then to SW1, and on to Q4 and stuff. Cut the trace on either side of that pin, scrape off the soldermask, and add a jumper. Or just jumper between the nearest pads on that node, even easier.Tim

author: xavier60
author_rank: Super Contributor
timestamp: February 11, 2018, 04:58:07 pm
content: T1 looks like a modified CRT Line Output Transformer. The deflection waveform in a CRT monitor has a short duration pulse compared to the total line period. This flyback pulse is actually a bit over a half cycle of resonance between the flyback tuning capacitor and the combined transformer and deflection yoke inductance. This half cycle of resonance causes current reversal in the circuit inductance that makes the voltage at the transistor to want to go negative but is clamped by a diode at near zero volts. The transistor typically doesn't get turned on until almost half way along the next line scan, then forward current starts flowing again, storing energy in the circuit inductance for the next flyback pulse. The rapid current reversal during the flyback pulse also cause the electron beam to be quickly deflected or fly back to its start position.In the current state of problem circuit we have here, the duration of the flyback pulse is actually a bit longer than the flat part of the waveform. This means that not much energy is being stored in the inductance.It would't need much load on the output to cause the pulse to not make is back to ground voltage. So the MOSFET might need to be turned on early anyway.I also noticed that it is one monster of a MOSFET.What does the circuit do? Make sparks maybe?

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 11, 2018, 08:39:13 pm
content: Quote from: T3sl4co1l on February 11, 2018, 03:56:56 pmWow, lots to take in! I have read it all a few times even if it doesn't seem like it in my replies, like to focus on one thing at a time. Turns out Q1 is actually the smaller IRF2907Z version with lower gate capacitance (my bad). It doesn't have the "Z" on the package so probably is a fake.Q2 is actually needed to get the oscillator running, just switching the supply voltage doesn't give it the kick required to get going.Quote from: xavier60 on February 11, 2018, 04:58:07 pmCorrect, it is a LOPT designed to give sparks.Well I'm stumped, I increased C6 and all it did was increase the duty cycle by extending the on time and decrease the frequency a little bit. So I guess that means if I decrease the capacitance of C6 it would decrease duty cycle but only by terminating the pulse earlier.Does this mean that Q4 is the key to all this? Seems that Q4 turning on initiates the next cycle regardless.Below are the requested wave forms and remember this is using a 9v PP3 battery for power so the gate voltage looks a bit low.Thanks.Update: Increased C5 by a few 100 pF and that also just prolonged the "on" pulse, same with decreasing R6 to 12k.I wonder if putting a resistor in series between Q4 and C7,R10 might delay the charging of C7 and C6 from the inevitable turning on of Q4?

author: xavier60
author_rank: Super Contributor
timestamp: February 11, 2018, 11:49:16 pm
content: Ill use the references from the first diagram.That first waveform does show the MOSFET turning on a bit later if that is with C6 increased? But is causing Q5 to stick on for longer. This could be partly due to BJTs being slower to turn off from saturation. BJTs classed as switching transistors are designed to be quicker, or a speed up diode could be add also.Making C7 discharge quicker with some parallel resistance is another option.The early turn on of Q4 is the main problem. I'm not certain what to do about this that won't lead to other problems.I made a circuit years ago for testing LOPT transformers. I made its Q4 turn on only when the flyback voltage dropped below 9V to turn on the MOSFET. The MOSFET turn off was triggered by when the current ramped up to some set value.

author: xavier60
author_rank: Super Contributor
timestamp: February 12, 2018, 12:09:23 am
content: Quote from: theleakydiode on February 11, 2018, 08:39:13 pmThat could add some useful delay.The circuit is too crude. I think it's not possible to change one aspect of operation with affecting others.Really need to consider the 555 circuit that gives independent frequency and duty cycle control.

author: eblc1388
author_rank: Frequent Contributor
timestamp: February 12, 2018, 01:04:12 am
content: @theleakydiodeMany of the component annotations of your second "clearer" schematic does not correspond to the original one. This would cause confusion as the discussion refers to the parts.The original schematic is sufficiently clear for the discussion. I have no trouble reading it.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 12, 2018, 01:41:55 am
content: Quote from: eblc1388 on February 12, 2018, 01:04:12 amOh noI didn't realize that until you mentioned it. I'm like a real life Mr Bean aren't I, one disaster after another.Quote from: xavier60 on February 12, 2018, 12:09:23 amGuess I'll try that, will probably have to cut a trace though.Quote from: xavier60 on February 11, 2018, 11:49:16 pmShould I try using a baker clamp on the driver transistors? Also putting a resistor in parallel with the 1nF does indeed reduce the duty cycle but only by reducing the end of the pulse. The start is always in the same overlapping area.Mind posting that circuit if you've got a drawing?

author: xavier60
author_rank: Super Contributor
timestamp: February 12, 2018, 02:30:55 am
content: Yes, I'm curious to know what effect the baker clamp has on Q5, Schottky diodes are better for this. What value had you increased C6 to?

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 12, 2018, 02:05:27 pm
content: Quote from: xavier60 on February 12, 2018, 02:30:55 amWhich schematics C6 and Q5 are we talking aboutI deleted the second one as eblc1388 alerted me to the messed up component labels, I think I tried to make the second drawing a bit more compact and ended mixing some parts around.If you mean C6 in the original schematic then I first put a 2.2nF in parallel and all I could detect was a bit of frequency drop, then I put a 33nF in parallel and that increased the duration of the on time and decreased frequency to about 30khZ.I've got some BAT48's in the parts box, should be good enough right? And would it be safe to try one on Q4?BAT48 base to collector across Q4: duty cycle decreases from 50% to 45%, frequency increases by a few khz. No change on the cycle starting point.Double update: I think I'm on to something!

author: xavier60
author_rank: Super Contributor
timestamp: February 12, 2018, 06:45:36 pm
content: I had gone back to the references in the first diagram.Increasing C6 delays the MOSFET turn on. It is expected to also increase the on duration which then needs to be decreased back to normal. I would try the BAT48 as a Baker clamp on Q5 first then quicken the discharge of C7 also if necessary.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 12, 2018, 10:28:43 pm
content: Well, looks like I've had a bit of success for once. I inserted a 1k resistor between Q4 and R10, C7. That seems to "chop" off the dithering about at the start of the gate drive waveform and shift it back a little.It does increase the switching frequency by a few khz and plays with the duty cycle a little (can't remember which way) but it could be worse I guess, I made sure it was below 50% just in case.I also decreased the gate resistor to 300 ohm to speed up turn on, don't want to reduce it too much or else the losses in the pull up resistor will get silly.Maybe if I reduce R7 and R10 I can get the frequency down a bit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2018, 10:50:30 pm
content: Oh hey, you've managed to get class E operation. Don't touch it!Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 12, 2018, 11:02:48 pm
content: Quote from: T3sl4co1l on February 12, 2018, 10:50:30 pmI know but I kind of need a lower frequency, the LOPT won't jump gap with it that high.

author: xavier60
author_rank: Super Contributor
timestamp: February 12, 2018, 11:41:24 pm
content: That looks good. Increasing C7 should lengthen the on time. It will also delay the turn on time.

author: xavier60
author_rank: Super Contributor
timestamp: February 12, 2018, 11:58:55 pm
content: Also if you look at the Gate waveform now, you will notice that the Miller Plateau at turn on has gone. MOSFETs turn on quicker when the Drain voltage is near Zero.https://electronics.stackexchange.com/questions/66660/why-is-the-gate-charge-curve-miller-plateau-of-mosfets-dependent-on-vds

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2018, 12:00:19 am
content: How many turns are on that primary?Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 12:20:03 am
content: Quote from: xavier60 on February 12, 2018, 11:41:24 pmAnything after the new 1k resistor (now 910 ohm) has no effect on delaying the turn on cycle, adding a 470pF across C7 now results in a lower frequency (47khz) but a duty cycle of about 60% which is too high I think.What would be the maximum duty cycle I could safely get away with? I blew Q4 a few days ago by accidentally making the duty cycle too high.Quote from: T3sl4co1l on February 13, 2018, 12:00:19 am8 wound around an exposed leg of the core of a CRT LOPT.Hmm anyone know how I could reduce the frequency without upsetting the duty cycle? Also I'm guessing this is technically phase locked loop now as its the falling of the flyback pulse what triggers the turn off of the MOSFET.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 12:57:35 am
content: Quote from: xavier60 on February 12, 2018, 11:58:55 pmAh I was wondering about that! Thinking it had something to do with a capacitive divider somewhere as the middle part was always half the supply voltage.Still need to get the frequency down really, its just too high for the LOPT.

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 12:57:47 am
content: Quote from: theleakydiode on February 13, 2018, 12:20:03 amWhen you say that something has no effect on the delaying the turn on time, how are you observing this? After the flyback pulse has finished, the voltage will stay near zero, actually about 0.5V negative for some time on its own. The MOSFET can be turned on at any point during this time. There is some efficiency advantage in turning on the MOSFET just after the pulse is finished.Because the period on the flyback pulse is mostly fixed, you only have control of the ON part of the waveform, where the voltage is at zero. When you change this ON time, you can't help but alter the duty cycle and the repetition rate/frequency.The duration of the flyback pulse can be altered by changing the LC resonant frequency, either changing the value of the flyback tuning capacitor, C3. Or by changing the number of turns. Reducing the value of C3 will shorten the pulse period and increase the peak voltage. Decreasing the turns will have the same affect plus the change in turns ratio to other windings which will increase the EHT voltage even further, but this risks saturating the core.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 01:10:41 am
content: Quote from: xavier60 on February 13, 2018, 12:57:47 amI mean the only thing I found that delays the point where the gate turns back on is the addition of the resistor, other things below it all change duty cycle and frequency but it seems I can't change one thing without messing up something else. I think the new resistor changes the divider on the base of Q5, hence why it seems a bit backwards at times.Do you think 8 turns might be pushing it for 12v? I guess I could try a smaller LC capacitor but its only a 75v mosfet, avalanching may have saved it during the 90% duty cycle incident but I don't think its a good idea to run it near the maximum breakdown voltage. The required arc gap is only about half a centimeter so its not like I need 30kV like it originally gave in the CRT TV.Does it act like a constant power load with the short (arc) on the output and fixed drive frequency and duty cycle?

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 01:15:46 am
content: What does the transformer normally belong to?

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 01:24:52 am
content: When a LOPT/EHT transformer is used in a TV there is a big difference. Most of the energy storage happens in the deflection yoke inductance. I could do some experiments here if it is a colour TV type EHT transformer.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 01:34:17 am
content: Quote from: xavier60 on February 13, 2018, 01:24:52 amOh yes please, it is a colour TV type LOPT that's black and has the big red lead coming out the top.So a solution could be adding a big inductor across the transistor? Any idea what kind of inductance the deflection yoke would have?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2018, 02:53:15 am
content: What is your basis for wanting a lower frequency?The FBT does not need to operate at 15.7kHz. As mentioned before, the secondary is usually resonant at some harmonic thereof, convenient for refresh purposes.If it's from a multisync computer monitor, the resonance will be even higher (thanks to some clever internal design!).Lowering the frequency will only give you a thinner, weaker spark, assuming the coupling factor is good (which it isn't, in this configuration -- so it's hard to say which direction is best, aside from finding the nearest resonant harmonic).I think it more likely you merely need to reduce the primary turns and/or increase supply voltage. To keep the same waveform, of course, you need to increase the capacitor in parallel with the transistor, proportionally with the square of the ratio of turns (so, going from 8 to 4 --> ratio of 2^2 = 4, increase C by 4x) to keep the primary resonant frequency the same.Tim

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 03:36:27 am
content: There is no need for the extra inductor, the EHT transformer can store enough energy on its own, any more would destroy the EHT transformer sooner.I did a mock up with 10 turns and a 0.22uF flyback tuning capacitor. I drove an IRFP460 MOSFET with a signal generator which has frequency and duty cycle control.The flyback pulse width was 10us, ideally should be 12.5us, but it's not critical. At 15Khz, the flyback peaked at 110V. I estimate the EHT at about 20KV.Although varying the duty cycle over some range made no difference with the EHT unloaded, it makes a huge difference to the EHT output current and power draw when it's arcing.Because the EHT voltage is stored in capacitors built into the EHT transformer, the arcing is actually a series of high energy discharges. I don't think it would last long doing this.Be careful with test equipment when working with EHT. I used a X100 probe with my DS0 for extra protection. High voltage spikes can end up anywhere.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 03:08:18 pm
content: Quote from: T3sl4co1l on February 13, 2018, 02:53:15 amI've noticed the opposite, using a different driver with adjustable frequency and duty cycle I found that for a fixed duty cycle going lower in frequency allowed the secondary voltage to be much higher. With just 12v and a frequency in the hearing range I could get a continues arc that jumped a 7cm+ gap whilst at 24v it was well over 10cm but just made the LOPT flash over. The MOSFET was a 650v .03mohm type that could take some ridiculously high current, I made sure to keep the peak drain voltage below 500v as they were about £12 each!I've also driven them in half-bridge too where resonance was used as a form of FM slope detection in combination with an audio signal injected into the RC timing section of the drive IC, it actually sounded pretty good and loud as a plasma speaker!This was using LOPTs from CRT TV's, the PC monitor ones didn't work nearly as good and the massive internal capacitors would "snap" over if the voltage got too high. Internally what makes the monitor LOPTS different from the TV versions? Are they driven differently inside a monitor?Quote from: xavier60 on February 13, 2018, 03:36:27 amVery nice! Was that LOPT out of a monitor or TV? What duty cycle would that 12.5us be at 15.7khz? Low I'm guessing.Yeah I need to get some better probes, the ones that came with my DS1052E are falling apart and the 1x/10x switch is in a stupid place where you can easily switch it by mistake. I think mosfet breakdown voltages and avalanche capability's have saved my ass a couple times in the pastWhat would be the maximum safe duty cycle in this circuit? At some point I guess the core is going to stop resetting.

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 04:03:43 pm
content: Quote from: theleakydiode on February 13, 2018, 03:08:18 pmThe one in the photo is from a small colour TV. It has built in rectifier diodes and filter capacitors. It would never be able to produce a continuous arc, only a rapid series of high energy discharges, sounds like a loud buzz.The duty cycle thing is getting tricky to explain. It will make more sense if you research how CRT horizontal deflection works. In TVs, the duty cycle of the drive to the Horizontal Output Transistor was simply set to 50% because the Collector voltage stays low for the first half of the scan line anyway. The current flow gets reversed during the flyback pulse and actually flows back into the supply rail during the first half of the scan line.Ill take a photo of this happening in my mock up tomorrow.Older TVs had LOPTs that needed a separate rectifier block to rectify and filter the HV AC from the HV winding on the transformer. The HV AC would produce a continuous arc with a hissing sound. I'm wondering was sort of LOPT you are using?Extra: I'm starting to suspect that my transformer might be from a computer monitor after all. It was new in its box. Ill try to find something else.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 04:13:31 pm
content: It looks like this one (pic related). I did find the datasheet at one point and it had no internal capacitors, it came out of a colour TV.I've also got a PC monitor LOPT and that has a big cylinder on the side containing a capacitor, that one doesn't work as well. Still haven't found an AC LOPT yet and CRTs are getting very hard to find around here. I think most people got rid of them years ago and the local recycling place doesn't let people take stuff.Quote from: xavier60 on February 13, 2018, 04:03:43 pmCan I see the front side where the dials are and bottom pin arrangement? As I can normally tell where it came from, if its got a sticky out bit on the side and lots of extra pins on the bottom along the one side then its most likely from a monitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2018, 07:01:04 pm
content: Quote from: theleakydiode on February 13, 2018, 03:08:18 pmOK, so what you're seeing is most likely normal flyback behavior.More on time --> more average current draw. At constant D (and assuming saturation never occurs in the transformer), power output is inversely proportional to frequency.The waveform likewise shows increasing peak voltage following turn-off (when current peaks), but since it's less frequent, it's not as powerful as it would be simply increasing D instead.In class E, the peak voltage typically isn't too much greater than the supply voltage. Ideally, you'd have fixed off-time and adjustable on-time, which has the effect of increasing D and decreasing F at the same time -- giving better control over the output.Primarily, though, you'd want to control a class E driver by varying its supply voltage.Maybe you don't need/want an output that hot -- that's fine. Then you need some way to adjust off-time much longer. Which, you really need a different circuit for, it seems.QuoteQuotePossibly a combination of lower voltage ratings, with high frequency construction. Instead of one big winding (as the old CRT FBTs had), it's wound one layer at a time, with a HV diode per layer. This is done so that each layer can have the same AC voltage along its length -- so that inter-layer capacitance doesn't pile up, severely limiting the self resonant frequency.HV tracks tube size, usually, so a big (30"+) TV might use 30kV, while a 17" monitor might use only 20kV.Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 07:35:43 pm
content: Quote from: T3sl4co1l on February 13, 2018, 07:01:04 pmYeah in this application a less intense arc is preferred as the electrodes are less than a centimeter apart and I don't want them overheating, I've been there done that got the tee-shirt with massive arcs and whilst its impressive there are limited applications for them. Biggest arc I ever drew was over 1ft long and just pure white hot plasma around 1 inch thick (ZVS driver 48v).I guess in the case of the other circuit it would have been regular flyback mode since the capacitor across the primary wasn't fully resonant, it was just to keep the peak drain voltage down. First pic is the schematic if you want to try it, I just replaced the MOSFET with a higher voltage one and decreased the capacitor on the primary to allow moar output voltage (use high impedance RCD snubber to monitor peak drain voltage with an inexpensive voltmeter). I had to put a resistor in series with the gate resistor diode or else it wasn't stable.The capacitor across the +35v needs to be low esr and very close to the top of the primary and low end of the current sense resistors since the peak current demands can get high, keep loop areas small.Second one is the plasma speaker, use the "better output stage", put the output electrodes close so it doesn't flash-over unloaded and tune it using the frequency pot and at some point you'll hit resonance with the biggest output voltage (if your power supply can handle it). Tune it again with music playing to get the best sound (use an old phone/audio source just in case!).I didn't design any of these circuits btw but have tested them both. Its easy to turn a DC LOPT into an AC one with this if the output voltage rises high enough, the internal diodes just short and you can end up measuring the secondary winding resistance with an ohm meter. Keep the gap in the core as it prevents saturation and stops the output diode from biasing the core. MOSFETs will run a bit hotter like this but its the price to pay for an open loop design.The second one can have clear sound with careful electrode design, I found vertical with a point for the bottom and larger rounder surface for the top worked well and the arc was silent.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2018, 09:37:09 pm
content: Ooh, that first one is really nice: peak current mode operation, quasi-resonant snubber even. Transistor is inappropriate for the driver (UC3844 can only deliver about 1A, IRFP260 has a fuckoff big gate; a modern equivalent with 1/3 or less Qg and the same Rds(on) would have much lower switching loss), probably hence the heatsink. Then a smaller gate resistor (maybe 10 instead of 27 ohms) can be used.The other one is pretty typical, but it doesn't have any protection, it's just a full wave AC source.Tim

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 09:42:18 pm
content: Its the miller capacitance that messes with the first one once the drain starts swinging to a decent level, I've been experimenting with miller clamp gate drive IC's recently to go between the fet and PWM IC.Problem is you need both a high voltage and low rdson fet which drives the parasitic capacitances of the device up.What kind of protection would you put on the second one?

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 11:12:18 pm
content: What do the ultrafast diodes do in the UC3844 driven design?I tried a few more of my transformers and all had capacitors inside. I took a photo of the voltage and current waveforms anyway with no load.At the end of the line, the current reaches 10A. After flyback, the current flow is reversed and is a bit under 10A. The current crosses zero a bit before half way along the line.I didn't want to make any sparks while the DSO was connected. I figure that at 50% duty cycle, the final current will stay about the same at 10A when the EHT is loaded. With a higher duty cycle, the current would increase with load, and the possibility of saturation.No wonder why the rail bypass eletros have to be low ESR with +/- 10A flowing. I had to use 2 capacitors. One on its own got hot.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 13, 2018, 11:35:17 pm
content: Did you build the UC3844 circuit or is that using the signal generator? The diodes are part of a non dissipative LCD snubber and I don't think they actually see more than the supply voltage so no idea why the designer spec'd them at such a high voltage.See method 3https://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtmlIs it safe to view the gate whilst arcs are being drawn? I've done it in the past and haven't have any problems yet.Edit: I think these open loop drivers are constant power in a way with the energy being dissipated somehow or returned to the supply rails via the capacitor and mosfet body diode, the peak current limit on the UC3844 only kicks in once when the frequency is turned low enough as is (I personally think the sense resistor is too low). People used to run these things without any capacitor and that would avalanche the mosfet until it popped.

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2018, 11:54:18 pm
content: Quote from: theleakydiode on February 13, 2018, 11:35:17 pmNo, I'm still using the signal generator.It possibly is safe to use a DSO while arcing. The main precautions would be to ground the probe close to where the measurement is being taken. The arc current path should be as direct as possible and avoid being shared with other circuitry. You are likely to be already taking these precautions if you have had no trouble.I always put a 15V zenner between Gate and Source of MOSFETs when experimenting.

author: theleakydiode
author_rank: Regular Contributor
timestamp: February 14, 2018, 12:05:19 am
content: Same with the zeners and protection, I always go clamp crazy with zeners on the gates and TVS on the drain to source. Destroying silicon gets old fast and becomes expensive when its a half decent mosfet.I've had a near miss in the past where I probed across the primary capacitor by accident and had the full drain voltage (about 550vdc in my case) across the 300vac probe. Thankfully the scope and probe survived but I had to change my underwear.Did you find any performance increases in the arc since adding the low ESR capacitors across the supply rail?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2018, 03:24:39 am
content: Quote from: theleakydiode on February 13, 2018, 09:42:18 pmYes, you can throw in a complementary emitter follower like a PBSS303NX/PX pair, or a driver like TC4420, for driving bigger gates. Helps a lot.QuoteOr a higher voltage to start with.QuoteHmm...bleh... maybe a current transformer in series with the primary, and a 1N4148 FWB off that, burden resistor and RC filter, and use that to limit duty cycle if load current shoots up.Getting the error amp involved also opens up other protection features, like overtemp, UVLO and so on.Main overall effect of current limiting, you can connect pretty much any primary and see how it deals with it. Too few turns, not much voltage, stays in current limit. Too many turns, not much voltage, never gets close to current limit. Somewhere inbetween, the load impedance matches up good enough and you get maximum power output.To go with that, the output (AC or DC) becomes current limited, so instead of going from light sparks to a burning hot arc, it stays modestly sparky/burning no matter the gap length (except where it's too long to break down, of course).Tim

author: xavier60
author_rank: Super Contributor
timestamp: February 14, 2018, 08:32:20 am
content: This is the FBT tester circuit I used years ago for doing quick tests. I couldn't find the actual gadget and had to guess some of the component values.It might need some changes to work at high current. Resistive shunts become wasteful at high current. Some pre-bias could be applied to the base of Q1 so that the lower value shunt resistance can be use.It has momentary switches to start and stop it. I haven't given much thought to making it self start.