Flyback controller for driving voltage multiplier, Scope clock PSU - Page 1
Forum section: electronics / projects

author: Martinn
author_rank: Frequent Contributor
timestamp: July 15, 2022, 06:19:43 pm
content: Hi,I am trying to rebuild Tom's excellent super compact scope clockhttps://www.eevblog.com/forum/projects/maybe-risk-a-guess-what-thats-gonna-be/msg1810052/#msg1810052using JLCPCB PCB assembly, which means finding LCSC parts or equivalents for the parts Tom chose.Unsolved problem still is the HV supply. Tom used a current mode, integrated switch (NCP1032) flyback regulator. As he explains in his post, this did not really work well as with the voltage multiplier on the transformer secondary, there is a current pulse on the primary (when primary switches on) as the multiplier cascade conducts "backwards" (that's how the multiplication works). Unfortunately, a current mode regulator detects this current and terminates the switch cycle prematurely.So what would be a good alternative driving a flyback transformer with voltage multiplier output? Some kind of blanking for the reverse pulse would be nice. Maybe even better, a voltage mode controller?Unfortunately, I have not found a suitable replacement. Off-line converters often have rather high UVlo lockouts (I'd like to run the circuit from 12 V). Voltage mode converters are very rare. Suitable converters with integrated switch as well: The NCP1032 has a 200V switch, good for telecom or maybe POE.Does anybody have a good idea? Flyback converter, integrated switch preferred (>100 V), 12 V supply, voltage mode, if possible available at JLCPCB (or not QFN so it can easily be hand soldered).I have even thought of putting together a 555 and a MOSFET (don't laugh). Load is near constant I guess so a slow regulation (or maybe even fixed PWM) might work. Still there are millions of PWM controllers out there, there has to be a better way!BTW: What ratio would you ideally use for a 12V to 250V flyback? For keeping charge/discharge times equal, it would need to be 12:250 or about 1:21, but I am not sure if leakage inductance would get too high. Anyway I'd prefer some off the shelf transformer like the WE MID-FLYhttps://www.we-online.com/katalog/de/MID-FLYseries and they have a 1:6 to 1:8 max ratio.Thanks! Martin

author: Marco
author_rank: Super Contributor
timestamp: July 15, 2022, 08:24:16 pm
content: It's not really flyback, it's hybrid forward flyback. In DCM the MOSFET current is the sum of a semi-sinusoidal current, from the forward transformer action pushing the leakage inductance and output capacitance, plus a linear increasing magnetizing current.In theory in DCM the drain voltage will start oscillating and the MOSFET can be switched in near ZVS, but no controller will do that for you. So you probably just want a fixed duty cycle fixed frequency drive, gated by a feedback optocoupler. A really basic PWM controller. Most the modern simple switching controllers with integrated switches will have slope compensation unfortunately, I don't know if there's something modern as far as integrated switch is concerned but old fashioned as far as the controller is concerned.

author: David Hess
author_rank: Super Contributor
timestamp: July 15, 2022, 11:58:35 pm
content: Quote from: Martinn on July 15, 2022, 06:19:43 pmMany suitable current mode controllers use blanking to prevent that exact problem.I would also consider using a constant-off time controller.

author: Marco
author_rank: Super Contributor
timestamp: July 16, 2022, 04:58:16 pm
content: Quote from: David Hess on July 15, 2022, 11:58:35 pmDepending on the inductances and capacitances involved it's not necessarily a short pulse, the time constant of forward mode current can be relatively large.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 16, 2022, 05:10:05 pm
content: Take a look at the 2 transistor "Royer converter".

author: David Hess
author_rank: Super Contributor
timestamp: July 16, 2022, 07:25:53 pm
content: Quote from: Marco on July 16, 2022, 04:58:16 pmAn alternative might be to get rid of the flyback transformer and add a common base output transistor to raise the voltage capability of the output switch. That gets to the cathode voltage in a reasonable number of stages.

author: Martinn
author_rank: Frequent Contributor
timestamp: July 17, 2022, 04:30:46 pm
content: Quote from: David Hess on July 16, 2022, 07:25:53 pmSounds cascode style? Do you mean just using a boost converter with high voltage switching FET?Quote from: NiHaoMike on July 16, 2022, 05:10:05 pmInteresting thought. I have mentally connected royer converters with ultra cheap consumer stuff only, like electric toothbrush chargers.Maybe one note regarding the output voltages: Tubes for scope clocks seem to need around 1.5 kV of total voltage. The arrangement differs: Tom's tube needs 250 V for the deflection amps, -500 for cathode, focusing and +1 kV post acceleration.On my desk I have a D10-160 which does not have a post acceleration terminal, so deflection +250 V and cathode (with focusing and blanking) at -1.25 kV.Interestingly the voltage multiplier cascade can generate both positive and negative (which I don't find obvious, as the flyback pulse is unipolar) voltages together.Blanking: I also would suspect that the standard leading edge blanking is too short for the capacitive part of the multiplier.I think I will try my luck with an UC3845. This dinosaur has none of the modern fancy extras (QR, ZVS, PSR etc) that might fail in this special case.Out of curiosity I tried to find out if there are any older switchmode controllers that are still available. I found the TL497, but not much more. UC3524 maybe?Interesting:http://www.bitsavers.org/components/unitrode/_dataBooks/1987_Unitrode_Applications_Handbook.pdfUnitrode applications handbook from 1987

author: David Hess
author_rank: Super Contributor
timestamp: July 17, 2022, 09:01:49 pm
content: Quote from: Martinn on July 17, 2022, 04:30:46 pmIf you are using a controller which has an integrated switch, then a discrete cascode can be used to raise the switching voltage without changing how the current sensing works. In an application with high switching voltage, a two transistor cascode may be preferred anyway for higher performance.QuoteThe resonate version of a Royer converter is used for florescent lamp supplies, and early solid state CRT oscilloscope supplies but was replaced with a single transistor blocking oscillator which is cheaper.A Royer converter would certainly work but a special transformer is required. There likely is a suitable CFL transformer available for cheap. That leaves:QuoteOscilloscopes used multiple windings on the inverter transformer to get exactly the voltages they required, with DC feedback from the high cathode supply. That is not going to be found on anything you can buy. The much higher post deflection acceleration came from a voltage multiplier driven from the AC for the -2000 volt cathode supply.Winding your own custom transformer is feasible *if* you know what you are doing. Otherwise I would expect several failures getting it right.Where you planning on using a capacitive voltage multiplier and tapping the voltages off of different stages? I think that could work.

author: Martinn
author_rank: Frequent Contributor
timestamp: July 18, 2022, 03:30:44 pm
content: I think Jim Williams spent a good part of his life on CCFL drivers... I don't intend to follow his footstepsQuote from: David Hess on July 17, 2022, 09:01:49 pmYou find Tom's schematic attached here:https://www.eevblog.com/forum/projects/maybe-risk-a-guess-what-thats-gonna-be/msg1810052/#msg1810052He uses the voltage multiplier to generate three voltages (-500 V, +250 V, +1000 V).I don't intend to design/wind my own transformer here (not that I wouldn't find it interesting - but this particular type is probably dominated by parasitics, fiddly manufacturing and unobtainium specialized materials).The idea would be to ideally have everything assembled by JLCPCB, including transformer. Best candidates seem PoE transformers: Flyback, 250 kHz, 40-70 V input, 5 or 12 V output. Reversed they seem a decent starting point for the multiplier cascade.I did a quick simulation of a flyback/multiplier combination, the waveforms look truly strange to me.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2022, 06:20:46 pm
content: Quote from: Martinn on July 18, 2022, 03:30:44 pmThe transformer does not require anything special, but construction is finicky and it has to be vacuum potted. The parasitics create an upper boundary on the frequency of operation. I might wind my own transformer for a single unit because I think I could get it right in one try, but I don't recommend it if there is an alternative.QuoteI am not sure I would have thought of using a PoE flyback transformer backwards. If I did not find a suitable flyback transformer, I would try using a boost converter with an inductor and cascode switch like I described, or trying using a commercially available CCFL transformer with a Royer converter but at a lower output voltage.The Jim Williams implementations of the flyback transformer circuit give part numbers for the flyback transformer which might be worth checking out.

author: niconiconi
author_rank: Frequent Contributor
timestamp: July 22, 2022, 03:39:01 am
content: Quote from: David Hess on July 18, 2022, 06:20:46 pmI was also looking for a standard, name brand, and in-production flyback transformer for a 1.5 kV, 1 mA power supply in a project. Unfortunately my conclusion is that they're a dying breed these days. CCFL lighting transformers used to be a good source in the last decade, but most are already discontinued in this LED age. I think ALL of the HV flyback transformers mentioned in Jim Williams' articles are obsolete.Most HV experimenters seem to get them either from old CRTs or random eBay surplus sources, or random AliExpress parts. Of course they're still made by the millions in China, but the project is meant to be fully replicable, so it should use standard parts on the product catalog from a big vendor. Obsolete or random components are unacceptable (obscure vendors from Chinaper seis okay, as long as it's on a big catalog, but...).The closest transformer I was able to find was the one in 1000 V Output, No-Opto, Isolated Flyback Converter by Analog Devices (https://www.analog.com/en/technical-articles/1000-v-output-no-opto-isolated-flyback-converter.html), Sumida 0399-T208, but it's only high-pot tested to only 1.5 kV, exactly at my operating voltage, it would be a pretty bad idea to use it at the absolute maximum.My conclusion is to just stop wasting time and use a pre-made DCDC module instead, some are sold at $40. But I'd like to be proven wrong, if anyone knows a suitable transformer...

author: Marco
author_rank: Super Contributor
timestamp: July 22, 2022, 07:45:42 pm
content: Those "20 kV" transformers all over ebay and Aliexpress have been available for a long time and the manufacturer is on Alibaba. With every other possible part mentioned having gone extinct, how is this any less replicable?https://twgdsolar.en.alibaba.com/productgrouplist-818333128/High_Voltage_Ignition_Coil.html?scene=rts&spm=a2700.shop_plgr.41413.dtoptab&filter=all

author: Martinn
author_rank: Frequent Contributor
timestamp: July 23, 2022, 04:05:01 pm
content: Quote from: niconiconi on July 22, 2022, 03:39:01 amAlso my observation. Anyway there seem to be only few transformers available off-the-shelf - in 99.9% of the cases you need one, you have it custom made. If it is low volume, cost doesn't matter anyway and for large volumes cost will go down also.What about David's suggestion using a boost converter? In AN118https://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf, Jim Williams (after lots of royer circuits) shows (fig. 17) a boost converter based on LT1172+cascoded IRF840 for a 5 V -> 200 V conversion - impressive ratio. Not sure what the maximum realistic boost ratio is. Must be dominated by parasitics. Feed the boost output into a voltage multiplier to get into the kV range. Unfortunately a current mode converter (like the 1172) won't work with the multiplier so maybe use a voltage mode controller with external FET. Maybe you can start with 12, 15, 24 or 48 V instead of 5 V.Do you need galvanic isolation? Workaround might be using an isolated DC/DC module to power the boost converter, if you need on/off control only.Maybe look at the WE HV inductors:https://www.we-online.com/katalog/de/WE-PD-HV,they even have LTspice models with parasitics.Quote from: Marco on July 22, 2022, 07:45:42 pmInteresting... no data available though. Might be worth a try...

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2022, 04:45:55 pm
content: It seems to be this transformer, not much more info but it has the primary and secondary inductances at least.http://www.twgdsolar.com/productinfo/426651.html

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2022, 04:56:08 pm
content: To use a voltage multiplier one needs true AC with current drive in both directions. So one would be more in the range of a push-pull forward converter than a flyback. The push pull does not provide much regulation (if at all), but it can get away with a smaller transformer. The resonant royer is a bit in that direction, but with additional voltage gain from the resonance. The loading with the voltage doubler is a bit tricky though and it may only work with certain loads. So the non resonant push pull is the easier, more predictable way.Using both direction of a flyback and thus one direction forward and the other as flyback would kind of need the same power from both parts, which could be a bit tricky to balance. So the range for regulation is likely also limited.I have used a a kind of royer type converter for a PMT supply (some 900V, ~0.6 mA), but this one was a bit tricky and needed some tweeks. The transformer was acutally CM choke for the secondary and a few extra windings for the primary.For my DMM I use a push pull converter driven by SN6505 (currently out of stock) and a voltage douber at the secondary, though at a much lower voltage (+-20 V from +-5.5 V windings). The principle idea should also work with higher voltage and a different drive. I had a different version with a µC controlled drive, that also works and gives some freedom. For the higher voltage I would go more for a lower frequency like 20 kHz to reduce the switching loss and possible nasty capacitive effects and extra discharges.The transformers from CLF LCD backlight might be a suitable choice for a ready made part. Another option would be a 250-12 V (or 250-24 V) flyback transformer in reverse - lower frequency would mean less voltage and thus a larger transformer size.

author: Marco
author_rank: Super Contributor
timestamp: July 23, 2022, 05:05:49 pm
content: Using forward/flyback hybdrid with a multiplier has a single switch and no need for a split primary and reasonable duty cycles with relatively low winding ratios, those are some significant advantages.It can work, it's just a bit unconventional and maladapted to modern controllers.

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 24, 2022, 11:41:13 am
content: When I was looking into a similar problem, the most promising candidate seemed like a flyback or tapped-inductor boost made from off-the-shelf flyback transformers of the type made for 5V or 12V wall warts. These are available off-the-shelf for cheap from multiple manufacturers. Due to being designed for use with mains, primary-secondary insulation is very robust, and the primary is designed to have 500+ V across it in normal operation, so they would likely be good for 500 V output conservatively, maybe twice that if you feel brave. Multipliers can also be added, with the caveat of operating in charge-pump mode when the MOSFET turns on as discussed previously, giving trouble when operated from current mode controllers and introducing additional losses.Another option is to run the transformer from a half bridge, and control the frequency on the upper slope above the first series resonance (Lleak*Csec generally), giving low EMI, ZVS and a decent control range, while being controllable from off-the-shelf resonant control chips. It also works well for driving voltage multipliers. This is my preferred method for higher voltages, where transformer parasitics are significant at any practical operating frequency. If your particular transformer resonates way above the frequency you want to operate at, it can always be pulled down via some external capacitance across the secondary or inductance in series with the primary.For lower voltages like 200 - 300 V, tapped inductor boost with coupled 1:1 inductors like the SRF0703 work much better than plain boost, especially if the input voltage is low.

author: Martinn
author_rank: Frequent Contributor
timestamp: July 24, 2022, 12:16:10 pm
content: Quote from: Wolfram on July 24, 2022, 11:41:13 amSounds interesting...Having never used a resonant controller before, what would you recommend as starting point for such a design? Any simple controllers you would recommend?

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 24, 2022, 12:44:16 pm
content: The classic part is the MC34067, but there might be more modern parts worth looking into. I've mostly used the FAN7621 and the related FSFR2100 with integrated MOSFETs, but these are generally made for higher power levels and higher input voltage operation. I always thought the UCC25600 looked like an interesting part, and it has a powerful enough output stage to provide some useful output on its own, but like many parts from TI (and others) it has issues with availability at the moment.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2022, 01:21:47 pm
content: I would look for one of the current mode constant off-time or micropower burst mode parts.The boost or flyback converter based multipliers does not make the best use of the high voltage multiplier, but they work great at lower power levels.I think were you are likely to run into trouble is regulation problems from tapping the multiplier at multiple locations. A DC restorer circuit could be used for local high voltage regulation. Tektronix did this their CRT high voltage sources when the inverter operated without feedback from the high voltage supply. But it is probably easier to spend a little more money to make the impedance of the high voltage multiplier lower.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 25, 2022, 03:36:09 am
content: Quote from: David Hess on July 24, 2022, 01:21:47 pmHow does constant off-time or burst mode solve the problem of reflected current spike? I can understand pure hysteresis control based on Vsense feedback. But I just took a look at the offerings, and it seems most of the constant off time chips still employ peak current detection in the control loop, so I think they still have the same false tripping problem. What is an example of a suitable controller?Now I'm considering to abuse a current-mode controller with external Rsense, with a transistor for shorting it out at the beginning of each cycle for forced blanking.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2022, 12:40:16 pm
content: Quote from: niconiconi on August 25, 2022, 03:36:09 amOnce tripped the comparator ignores inputs during the off-time. Linear Technology published several application notes showing a constant-off time controller in boost and flyback mode driving a capacitor voltage multiplier.

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2022, 02:30:53 pm
content: The problem isn't during the off time.During the on time there is forward transformer action which causes a primary current pulse as a bunch of capacitors get charged with only the leakage inductance to slow it down. That then dies down and gets overtaken by the flyback mode primary current, but the controller will think the forward current has already become high enough for flyback purposes.If the controllers just have a fixed primary current limit for safety it would be fine, it's the slope compensation of the threshold current which makes them unsuitable ... and all modern controllers have slope compensation.

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2022, 08:55:38 pm
content: Quote from: Marco on August 25, 2022, 02:30:53 pmThat is how the constant off-time controllers work, and they have no slope compensation. One of their advantages is simplified frequency compensation. Although I do not know if that is why Linear Technology always showed them in use with voltage multipliers; it never occurred to me until now and they never mentioned the problem you describe.Although it is not quite the same type of switcher, check the end of the LT1073 datasheet for an example using a flyback transformer which produces 500 volts after voltage multiplication. It is a lower power application, but they have similar switching regulators which are more powerful.

author: Martinn
author_rank: Frequent Contributor
timestamp: August 26, 2022, 03:24:52 pm
content: Quote from: David Hess on August 25, 2022, 08:55:38 pmInteresting find! But the LT1073 is a "gated oscillator switcher" - which is quite different to more modern topologies and probably similar to the equally stone age MC34063. These are of course immune to the forward mode current spike.You write there are others that would be similar? I searched quite a bit and did not find any. The LT1073 does not even appear on the ADI web site (eg. in the selection tables). Only if you type it directly into the search box you are able to access this chip.So maybe one could use the MC34063 or LT1073, with external switch (or cascoded).Quote from: Marco on August 25, 2022, 02:30:53 pmSpeaking of forward: Actually a forward converter would suffer from the same problem (output capacitor short circuiting the transformer primary during switch on), if not for the output choke.So one countermeasure could be to add an inductor into the flyback output (similar to the forward converter output choke) in order to limit the current pulse (the amplitude of which is otherwise only limited by parasitics). I wonder if this could be sized to make current mode work.Reminds me of the horizontal drive circuit of CRT TVs: They are also resonant, but as I understand it the aim is just to recycle the energy in the deflection coil, the current in the HV transformer simply being a by-product.Another thing that I came across is a trick they mentioned in the UC3845 data sheet (https://www.ti.com/lit/ds/symlink/uc2845.pdffigure 8-13): By presenting a fixed sawtooth to the ISENSE input, you essentially make a voltage mode converter out of it. Of course you could use a voltage mode converter from the start, but these are extremely rare it seems.

author: Martinn
author_rank: Frequent Contributor
timestamp: August 27, 2022, 05:17:51 pm
content: Did a quick simulation: Would a forward mode inductor help?First simulation with inductor removed, second one with 100 uH. Transformer is 100 uH 1:1 100% coupling.Current spike is gone, as expected.I see the legend is missing...Green: Output voltage D1-cathodeRed: Current into switchBlue: "forward mode" inductor L1 current (D2 cathode)

author: Marco
author_rank: Super Contributor
timestamp: August 27, 2022, 07:09:20 pm
content: A coupling factor near 1 is probably not a reasonable assumption for a high ratio high voltage transformer.Poor coupling will already slow down the pulse (also makes a regenerative snubber more of a gain).

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2022, 07:46:34 pm
content: Quote from: Martinn on August 26, 2022, 03:24:52 pmThe LT1073 is a gated oscillator, but the constant off-time switchers like the MC34063 are very similar and have the same advantage. Linear Technology published a lot of examples of their constant off-time controllers being used like this, but the LT1073 application was the first one I came across and showed a transformer instead of boost inductor so I cited it first.

author: Martinn
author_rank: Frequent Contributor
timestamp: August 27, 2022, 08:00:20 pm
content: Quote from: Marco on August 27, 2022, 07:09:20 pmTried with 0.98 coupling and 1:5 turns ratio - primary current spike is gone (see simulation)In the meantime, I got a "20kV" transformer from Aliexpress - have to do a test setup with those. Coupling must be miserable and turns ratio huge.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 01, 2022, 05:03:58 am
content: Inspirited by David Hess's suggestion of using a gated-oscillator boost converter IC (which has a crude but modding-friendly control loop), I just tested the cascode configuration with an LM2623 chip. Just to see how high the output can go, the feedback was bypassed, forcing the chip to be free-running, and a roughly 90% duty cycle was set, using a 1600 V SiC MOSFET and a 33 uH inductor. The result was indeed quite impressive, with a ~10 V input, this "AA-battery type" boost converter charged the 1 uF output capacitor up to ~200 V in 5 seconds and to ~400 V after a minute.Though I noticed some problems with this approach. First, logic-level MOSFETs rarely go above 200 V, so this circuit cannot be powered from a 5 V supply without an additional auxiliary supply (possibly a charge pump) to bias the MOSFET gate. Another problem was specific to the LM2623 in this experiment, its duty cycle control was basically a hack added by its designers as an afterthought using a peculiar "non-linear effect" they've discovered when designing the LM2621. It's controlled by a capacitor between the switching node and the frequency pin, and depends on input and output voltage, it's determined by trial-and-error and "cannot be calculated" (datasheet's words). With such a huge boost ratio, the duty cycle varies wildly during start-up and cannot be controlled.Overall this is an interesting idea.For my specific problem, I think I've found another solution. I don't really need a high-voltage DC supply, but only a high-voltage capacitor charger. Using a series CLC resonant charging circuit, the voltage gain from the first to the second capacitor is approximate 1.5x to 2x when C1 >> C2. Thus, I should be able to use an off-the-shelf flash capacitor charger IC and flyback transformer to get up to 1000 V (since a flyback transformer designed for DC-DC rated higher than 1.5 kV is difficult to find as a standard product, though I do see some "off-line switching" type transformers with high ratings). Then one or two CLC charging stages can boost it to the target. Problem solved.Though there's still the problem of how to regulate all capacitor voltages to be freshly charged, and how to add a programmable output voltage. It also uses expensive high-voltage switches, which means a boost + voltage multiplier circuit may still make more sense.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 01, 2022, 07:40:11 pm
content: Did some simulations, due to ESR loss the voltage gain can be as low as 140%. The CLC idea is perhaps practical for a single stage voltage boost, but for 2+ stages it requires increasingly huge capacitors, which are bulky and expensive at high voltages. So it's still highly desirable to find a flyback transformer with a high voltage rating... Then I found the specs of some PoE transformers seem to have reasonable voltage and current ratings... Which brings me back to the very first post of the thread. I'll bite the bullet, get a PoE transformer and report back.

author: Martinn
author_rank: Frequent Contributor
timestamp: September 02, 2022, 06:19:25 pm
content: The POE transformers as I found out have rather low secondary (for us primary) inductance, causing uncomfortably short on times at 12 or 15 V. I initially thought of PoE because they are available at LCSC.These transformers by WE might be an alternativehttps://www.we-online.com/catalog/en/WE-FLEXHV- 1:5 ratio by combining the windings.Bourns also has some transformershttps://www.bourns.com/products/magnetic-products/transformers-power,I am particularly thinking of this onehttps://www.bourns.com/docs/product-datasheets/sm91047el.pdf?sfvrsn=de5466f6_13which has a 1:11.875 ratio. I did some simulations with it and it looks feasible. Not sure of the best controller though. In the simulation, some "forward mode" current when using with a cascade is clearly visible on the primary, making current mode control probably difficult. LT1073 seems rather exotic and has "EOL" all over it, MC34063 might be on the slow side (?) - maybe a UC3845, where the current sense signal could be filtered or altogether replaced with a voltage mode simulation.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 02, 2022, 08:21:42 pm
content: Quote from: Martinn on September 02, 2022, 06:19:25 pmYes, I noticed the same problem. I decided to use the LT3750 capacitor charger controller for this experiment. As mentioned before, I only needed to charge a HV capacitor, not a full power supply. Instead of using a voltage multiplier, if I can find a suitable transformer and get the voltage in one step, or add a CLC resonant charging stage, there will be no reflected primary current spike.Its datasheet warns that low primary inductance breaks its primary-side voltage sensing and causes the voltage to rise uncontrollably due to its limited bandwidth. I already added external secondary regulation so it may or may not work...Quote from: Martinn on September 02, 2022, 06:19:25 pmA great find. My only problem with these transformers is that they are only hi-pot tested to 1500 Vrms. But my output needs to be up to 1500 Vdc. Granted, technically there's still a 41.4% margin due to higher AC peak voltage, but this rating still makes me unconformable. So I want either a better transformer or add a voltage doubling stage. This is how I found those PoE transformers. From the Coilcraft catalog, after I filtered all transformers below 2000 Vrms, the only suitable flyback transformers I found are PoE transformers, with low secondary inductance.Another transformer I found is Coilcraft KA4823-CL, designed for 12 V to 220 V step-up in micro-inverters, 3000 Vrms Hi-Pot, 1:12 turns, primary inductance is 28 uH with a Isat of 10.5 A, inductance is still on the low side, but it's okay per LT3750 datasheet. But it's large and expensive. $12.59 each.https://www.coilcraft.com/en-us/products/transformers/power-transformers/power-converter-transformers/ja4635/I also found Coilcraft still has some in-production CCFL transformers. All are designed for Royer and full-bridge topologies.https://www.coilcraft.com/en-us/products/transformers/power-transformers/ccfl-transformers/fl/https://www.coilcraft.com/en-us/products/transformers/power-transformers/ccfl-transformers/fa2786/The FA2786 has a puzzling voltage rating: 1600 Vrms continuous, 2000 Vdc max at the secondary. This is a continuous working rating, not a Hi-Pot, which is good. But it also says "isolation is 500 Vrms between windings and 2000 Vrms from each winding to the core." This doesn't make any sense. I suspect it's a mistake. All the FL-series transformers have a 500 Vrms rating to aux windings, and they just got copied to FA2786, which doesn't have an aux winding.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 02, 2022, 08:31:08 pm
content: Wait, the SM91047-EL you found has a 2EL variant with Hi-Pot rating of 2000 Vrms to core and 3000 Vrms between windings. I think I can use that. But it's not a standard off-the-shelf part.

author: Martinn
author_rank: Frequent Contributor
timestamp: September 03, 2022, 08:34:19 am
content: Regarding the Hi-Pot-rating: Do you need a fully isolated output or are you just concerned about internal breakdown of the transformer?For isolation, you might simply add an isolated DC/DC converter to the supply (or make your own). Then you have a much wider selection of flyback transformers.https://www.mouser.ch/c/power/dc-dc-converters/isolated-dc-dc-converters/?q=dc%20dc%20converter&isolation%20voltage=3%20kV~~-&rp=power%2Fdc-dc-converters%2Fisolated-dc-dc-converters%7C~Isolation%20Voltage&sort=pricing

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 06, 2022, 09:25:09 am
content: Quote from: Martinn on September 03, 2022, 08:34:19 amI was just worried about transformer breakdown, not safety isolation. As far as I know, implementing robust safety isolation at a continuous high voltage with a margin required by regulatory standards is just so difficult, that none of these generic transformers is acceptable to begin with. For human safety, it needs to be either Reinforced Insulation or Double Insulation. For Reinforced Insulation, it needs to have a >8 mm creepage per 1 kV (200% spacing compared to Basic Insulation). The footprints and pinouts of most isolation devices ruled them out from the start for safety purposes (even if one's rated at a higher working voltage, that rating can only be treated as Functional or Basic Insulation). Or you can use Double Insulation, which means two layers of Basic Insulation in series.Thus, if I want isolation, it's going to be a separate design problem. Both an isolated DC-DC converter and a battery-only power supply are under my consideration.

author: Martinn
author_rank: Frequent Contributor
timestamp: September 06, 2022, 06:13:16 pm
content: But regarding transformer breakdown, the Hi-Pot test does not help you at all. You'd need a specification for dielectric strength in the winding - for a kV this proably looks like a multi chamber coil former like the "20kv" aliexpress iginition coils or the CCFL transformers.I checked again on CCFL transformers, but currently not a single one is available from Digikey. There are still a handful of "CCFL inverter" modules on Aliexpress, but of course without proper specifications.However, if you just need a non-isolated voltage of 1.5 kV, you could simply use any flyback (like SM91047EL) and add a 5x cascade. This is what I am going to do (I need +250 V and -1.25 kV for this tube). In that case, the transformer only needs to withstand a few hundred V or so (250-300), which I'd guess should be fine (although it is not specified).

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 10, 2022, 08:37:52 am
content: Quote from: niconiconi on September 01, 2022, 07:40:11 pmUsing the PoE transformer Coilcraft DA2257-ALD with two secondary windings in series, the highest achievable voltage was ~600 V at no load, LT3750 was switching at ~200 kHz. The extremely low secondary (primary when reversed) inductance prevented it from reaching a higher voltage. This experiment was done at near saturation (LT3750's peak current limit was set to 1.6 A via Rsense = 0.05 Ω, but the measured actual peak current was ~2 A, the datasheet says the MOSFET drive has a 100 ns propagation delay so it makes sense). The transformer was also becoming alarmingly hot (because the target voltage couldn't be reached, it was running continuously, which certainly didn't help). Attempting to reduce Rsense further to 0.03 Ω triggered overcurrent protection, likely due to saturation. Oscilloscope no longer showed a ramp, but instead a huge 2 V pulse across Rsense before it turned off.Conclusion: As predicted, due to low inductance, PoE transformers are not suitable for this task. The only advantage is that this model is locally available to me at low cost from a Shenzhen vendor, so using two transformers in series or parallel may not be completely unreasonable. But it's still a sloppy hack at best. I need to find a better transformer.SM91047EL is starting to be an attractive option, though it's not locally available here and the long lead time is problematic.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 11, 2022, 07:44:34 am
content: Quote from: Martinn on September 06, 2022, 06:13:16 pmAs far as I know, most transformers are never specified this way, often the only available rating is a Hi-Pot test, and nothing else. If the transformer is used for safety isolation, there will be a safe working voltage rating, but it's calculated by derating its functional working voltage, which is much higher and often is not given. Würth Elektronik seems to be the only manufacturer that always provides a dielectric strength rating for continuous operation. My observation is that if you only need functional isolation and the transformer is not rated, staying well below the Hi-Pot test voltage is the only way to proceed, which was why I was looking for a part with at least 2000 Vrms Hi-Pot.Update: An application note says a general rule in the industry by transformer manufactures is 200% functional working voltage + 1 kV. 1500 Vpk x 2 + 1000 V = 4000 Vpk = 2828 Vac, so a safer target is 3000 Vrms Hi-Pot.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 14, 2022, 02:24:02 am
content: I'm now trying to abuse this transformer - Coilcraft Y8848-AL. Originally designed for small AC/DC supplies, 1.8 mH primary, 1T:10T, so 18 uH secondary, 12 V / 0.83 A output, 2000 Vrms Hi-Pot. Not exactly the most ideal choice, but looks usable and samples are available locally at my location.https://www.coilcraft.com/getmedia/04e8f70d-a647-4b17-980a-cbfeb91a7699/y8844.pdfI implemented voltage regulation using a comparator as a bang-bang controller. The result is not bad, I was finally able to reach 1000 V (still need to adjust my set point and hysteresis). Another problem is that the voltage is only reached after pulsing it at several times the current rating of this transformer, but should be okay if the average current remains low.

author: MathWizard
author_rank: Super Contributor
timestamp: September 14, 2022, 09:17:07 am
content: So for HV switching PSU's, are voltage multipliers used just because it's cheaper, than using bigger inductors and mosfets ? What's safer and more reliable for a hobbyist ?I can't think of much of anything modern that still uses CRT screens, but when they do, how are they making HV these days, compared to the discrete transistors days ?I have a few HV circuits from old TVs, but I never tried to power them. Some day I'll measure the parts and calculate or run a sim to see what to expect.I''d love to make a Jaccob's ladder type thing.

author: Martinn
author_rank: Frequent Contributor
timestamp: September 14, 2022, 10:55:47 am
content: See this application note on resonant crt drivehttp://www.bdtic.com/datasheet/fairchild/AN-9009.pdfIt mainly seems optimized not to waste energy stored in the horizontal deflection coil, HV generation being a byproduct.Cascade vs straight boost: In a single stage boost with 25 kV output (color TV), you‘d also need a rectifier diode (and capacitor) with this voltage rating, while in a cascade required voltage is many times lower. I guess it comes to what parts are available at lowest total cost.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 14, 2022, 02:59:31 pm
content: Quote from: MathWizard on September 14, 2022, 09:17:07 amMy understanding is that multipliers are very effective for reducing voltage stress.Achieving high voltage gain in a single step using only inductors is often not practical. It makes the switching circuitry work many times harder than it needs to be, components will be expensive and less reliable. The voltage and current stresses on the components are enormous, and the duty cycle approaches 100%. Even then, voltage gain is limited by circuit parasitics. You can use multiplier stages to get additional voltage gain after the inductor instead.Alternatively, a flyback transformer (coupled inductor) solves the problem by boosting the voltage gain further by its turn ratio. But it also creates a huge potential difference across the windings, and requires a more heavy and expensive transformer, and it can be more difficult to find. Again, using multiplier stages reduces the stresses on the transformer and other components at the HV secondary.

author: Marco
author_rank: Super Contributor
timestamp: September 14, 2022, 03:05:31 pm
content: The hybrid forward-flyback with multiplier in this thread is almost never used by the way, Royer + multiplier is more common.

author: David Hess
author_rank: Super Contributor
timestamp: September 16, 2022, 03:23:43 am
content: Quote from: Marco on September 14, 2022, 03:05:31 pmI have only seen them with low current loads, like Geiger tubes and high voltage pulse generation circuits.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 19, 2022, 03:10:07 pm
content: Quote from: niconiconi on September 06, 2022, 09:25:09 amCorrection: For Pollution Degree 2, Material Group III, it's 10 kV/mm creepage for basic insulation (not 8 kV). For reinforced insulation, it's 20 kV/mm creepage. My previous conclusion still applies - off-the-shelf DC/DC modules and transformers are rarely designed or certified to meet this safety standard, no matter how high their Hi-Pot and peak transient ratings are. If there's no suitable choice, a possible workaround is using two Basic Insulation transformers in series to reach Double Insulation.But luckily there's a solution: Bourns HCTSM150102HL, 1.5 kV rated, reinforced insulation.https://www.bourns.com/docs/product-datasheets/hctsm150102hl.pdfThe creepage is 15 mm, not 20 mm, but it claims to have proper certification for 1.5 kV reinforced insulation per IEC/UL 62368-1. The datasheet doesn't say anything about Material Group, but I noticed another similar but lower-rated Bourns transformer uses Material Group I, not III, so this transformer must also be the case here.Indeed, using the table from IEC 60950-1 or IEC 62368-1, for Material Group 1, Pollution Degree 2. Apply the linear interpolation rules, the required creepage at 1500 V is 7.51 mm, round it up to 7.6 mm, and double that for Reinforced Insulation, it's 15.2 mm. This is exactly the distance across the transformer pins.

author: Martinn
author_rank: Frequent Contributor
timestamp: November 26, 2022, 05:41:54 pm
content: Quick update.In the meantime, I made a PCB with all kind of test circuits - boost power stages, gate driver, multiplier cascades and some unrelated stuff.I found that with a simple boost stage, it is very easy to generate a few hundred volts from 12 V input. Also, driving the multiplier with the boost switch node easily achieved near 1 kV. At 850 V, I realized I better not fry my new, expensive bench DMM! So next thing will be to build a 1000x 5kV probe (string of SMT resistors to get to 5kV voltage rating) and I'll revive one of my handheld DMMs for that. The scope tube I intend to use needs 1.5 kV total acceleration voltage, so I better have a HV probe for that.So next step will be building a plain current mode boost converter with attached multiplier cascade. I thought of an UC3843 for simplicity, or maybe something more contemporary (although finding switched mode regulators at JLC is a major pain). Or maybe finding something integrated and cascoding a HV transistor.A nice side effect of using a plain boost stage is that the reverse conduction from the cascade is mostly irrelevant - as the primary peak current is much higher than the "secondary" HV current going into the cascade.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 18, 2023, 11:15:36 pm
content: I had a chance to revisit this topic, as I decided to focus on finishing my own scope clock design. Looking at available magnetics, the Wurth 750310787 seemed like one of the better options.I investigated resonant variable frequency operation, but it did not seem like the best way going forward. The transformers have a pretty prominent secondary stray capacitance, around 50 pF. I tried different primary resonant circuits like (L+C)L and L(L||C), but all of these resulted in higher order resonances that could potentially interact badly with harmonics of the drive frequency, so the safest approach is just to exaggerate the existing parasitics to get the resonant frequency into the target operating range. 200 pF across the secondary and 20 uH in series with the primary is the right ballpark, but the primary current required to get 250 V peak across the secondary would be higher than the thermal rating of the transformer. It would probably still work, but the lack of simple driver chips with enough current capability makes this a less desirable approach in terms of complexity.My plan was to try to do the hybrid flyback forward/charge pump mode described in this thread, with something like a MC34063 or UC3842 operated for voltage mode control, but this has some potential issues as well. Since voltage multiplier stages see the sum of the charge pump and flyback voltage, line regulation would suffer in case regulation was taken from a plain flyback-derived rail. Which means that all rails have to be derived by doublers, and the 5:1 ratio between the target cathode and deflection voltages, a 5 stage multiplier would be needed instead of a 2 or 3 staged one, with the N cubed term in the output ripple suffering and requiring larger caps. I could always add a pre-regulator to deal with the cross-regulation, but it doesn't feel elegant if there are other options.Coilcraft still make CCFL transformers, so a self-oscillating approach could be viable, with advantages of simplicity and a nicely symmetric output voltage giving me the target +250 and -1000 with a simple half-wave rectifier and 2-stage doubler respectively, but all of the available parts have too high ratio to make this into a viable option. Too much voltage and too little current. These could be used to derive all the rails as a single 1250 V supply, but biasing the deflection drivers across such a voltage while still getting useful bandwidth would involve a lot of losses.On a hunch, I tried the classic self-oscillating ZVS "Royer-ish" oscillator with the Wurth transformer, and I can report that it works wonderfully. It takes around 70 mA at 7 V to develop 250 Vp across the secondary, a textbook perfect sinewave. If the gate bias resistors are powered from a separate rail, it oscillates down to 0 V on the input. I tested loading it with 50 k with a half-wave rectifier and cap across the whole secondary, and it delivered 250 V with 8.5 V on the input. Efficiency was not amazing, but a respectable 85 % when subtracting the half watt of idle draw. Nothing got much above room temperature. Next step is to feed it from a buck converter, taking feedback from the half-wave rectified 250 V rail, and hook up a two-stage multiplier to get the cathode voltage rail. The aux winding also develops a respectable 60 Vpp, which could potentially be used to feed a doubler to generate G1 drive bias. Ideally through a coupling capacitor, as I don't think it would be safe to float this winding at -1000 V relative to the main secondary.The circuit doesn't seem too critical, I just used parts I had on hand and it worked first try. BUK7Y153-100E MOSFETs and 2.2 mH DC link inductors, although much less would likely be fine. I tried 47 and 94 nF resonant capacitance. Efficiency was better at 47, and voltage peak compression was lower at 94. I'll see if I can draw up a schematic tomorrow if there is any interest.

author: Martinn
author_rank: Frequent Contributor
timestamp: September 19, 2023, 06:28:03 am
content: Quote from: Wolfram on September 18, 2023, 11:15:36 pmSure! Sounds interesting!