Build instruction of step-up converter 12V to 300V with high switching freq. ? - Page 1
Forum section: electronics / general

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 26, 2020, 02:43:17 pm
content: Hello, I hope I am posting here in the right place.I would like to develop a step-up converter, which should be as simple as possible.Input voltage 12V or 24V and output voltage 300V +- 50V adjustable / 100mA.Intended use: a tube amplifier.To save transformer weight, I want to use a ferrite transformer, so I think the switching frequency must be well above 50KHz.And I also absolutely need a galvanic isolation.I am thinking of PWM according to the attached schema, where I need only one secondary voltage:Does anyone of you know a manual that is well described ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2020, 04:29:46 pm
content: PWM alone, is too simple: if the load is shorted, even momentarily (such as during startup, charging the output capacitor), primary current is undefined and the transistor is likely to explode.The simplest possible solution, then, is to sense primary or switch current, and set PWM based on that.A better way to think of it is: turn on the transistor until current reaches some threshold (which takes some time dt = L dI / Vin), then turn it off. Wait some time (can be a delay triggered by turn-off, a clock oscillator, etc.) then turn it on again, the cycle restarts. You're still producing PWM (the pulse width varies with demand), but that's notwhyyou're doing it.Here's a discrete solution, which uses frequency modulation as well as peak current mode control:Operation is similar to the UC3843 family, minus some features. Relevant parts can be scaled up for the higher power, and the filament winding can simply be removed.The circuit design is pretty simple, however you will find the transformer design is much more critical. It is not a bundle of wire either side of a core, as many textbook depictions would mislead you to believe...Tim

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 26, 2020, 05:40:13 pm
content: Thanks for your scheme, Tim.What frequency does the PWM work with ?And the numbers beside the transformer "8t #24", "57t 37", ... That are the windings ?There is a little flaw in the scheme - there is no galvanic isolation.This could be fixed with an optocoupler ?And: exist there a scheme with a single chip for PWM ? e.g. NE555 or MC34063 ?br,Frank

author: trobbins
author_rank: Frequent Contributor
timestamp: October 26, 2020, 05:57:28 pm
content: Frank, have you looked at the few eBay inverter pcb modules that do what you are aiming for - they are typically set up for 12Vdc input, don't have feedback, and need the output rectified and filtered - so output regulation is dependant on the 12Vdc supply. They work fine for valve amps applications, and are based on a 3525A controller. They are typically found with a title like "150W Inverter Boost Board Transformer DC-AC Converter 12V to 110V 200V 220V 280V".Why do you specifically need an isolated output?

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 26, 2020, 06:40:21 pm
content: Quote from: trobbins on October 26, 2020, 05:57:28 pmNo trobbins, I have not yet looked at eBay.I also wanted to learn a bit about such a circuit, so I thought about building it myself.Once I understand the circuit, I can change it to my own wishes.This is my idea.On eBay there is unfortunately no schematic for a finished circuit ( I think)I need an isolated output because I want to heat the tubes with the input voltage (12.6V).

author: trobbins
author_rank: Frequent Contributor
timestamp: October 26, 2020, 11:49:23 pm
content: There are a few forum threads on reverse engineering the schematic of some typical ebay inverter modules - many pcbs are very easy to extract a schematic from, although I recall one pcb took some guessing as to the control IC being used (as the top of the IC had been buffed). I have generally found that the pcbs are nicely laid-out for smps, and are compact and use mainly smd, and are double sided - so difficult to improve on. Safety with respect to isolation of those modules is likely only functional, which is fine for valve amps that are powered from a 12Vdc supply. The main issue to manage is heatsinking the power FETs in the final environment.Tube heaters can be powered from the incoming 12Vdc, as well as that 12Vdc feeding a non-isolated dc/dc for B+. Do you have a particular circuit configuration where that won't work? Eg. perhaps you have heater-cathode voltage stress and need to raise the heater above the ground for B+ ?In the small amp I have on the bench at the moment, the dc/dc output is isolated, and so I can connect the 12Vdc input positive to the B+ negative, which allows the 12Vdc to not only power the heaters, but also provide a negative bias supply for the output stage for fixed bias operation. There are numerous small dc/dc available (especially from ebay) that can generate a bias in excess of 12Vdc for higher powered output stage tubes - I have done that for 6CM5 which need at least 65V bias.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2020, 12:20:36 am
content: Quote from: Frank_MV on October 26, 2020, 05:40:13 pmUhh, somewhere in the 100-200kHz range I think, and much lower at light loads (equivalent to a modern IC's "pulse skipping" mode).Again, it's not so much PWM, as peak current control. It produces a square wave, where the pulse width varies, but that's not the point, the point is inductor current.The numbers are windings, yes, t = turns, # = AWG used. And the core and air gap as specified. The windup I think was all single layers, which was fine for that power level and frequency. The cross-regulation between the dual outputs was poor, necessitating a change in the feedback circuit which is not shown.You'll probably want a multi-section primary, maybe two sections of secondary as well, to ensure good coupling (low leakage inductance). The EMI will be awful, without careful transformer design -- plan on needing two stages of common mode filtering, on both input and output, and build it inside a shield of tinned steel, or soldered PCB stock, etc.Isolation can be added in the usual way; it's even using a TL431 error amplifier already.QuoteYes, I just mentioned UC3843:https://www.ti.com/lit/ds/symlink/uc3843.pdf34063 does not provide current limiting; it uses a hack to approximate it. It's also quite slow, and leaves high ripple at the output.555 can be used, but it's something of a force-fit as you can see:https://www.seventransistorlabs.com/Images/555%20Boost.pdf(Only a boost, missing some things for a full isolated flyback solution.)I've linked my schematics, more for general knowledge; please investigate the 3843 family of parts, read the datasheets and appnotes, and follow the typical application circuits. It's a good way to go, one that I have used many other times.Tim

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 27, 2020, 06:54:42 pm
content: Quote from: trobbins on October 26, 2020, 11:49:23 pm"perhaps you have heater-cathode voltage stress and need to raise the heater above the ground for B+ ? ">> I have not yet decided, I do not have a concrete circuit as yet.I have done that for 6CM5 which need at least 65V bias.>> Thanks for the tip

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 27, 2020, 07:00:47 pm
content: Quote from: T3sl4co1l on October 27, 2020, 12:20:36 amYes, EMC could be a problem, a spectrum analyzer is usefull for that.Maybe it is better to try to modify a finished circuit or at least buy the transformer ready.Thanks for your links

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 27, 2020, 07:17:06 pm
content: Related to EMC Problem I once looked for it on eBay:"150W Inverter Boost Board Transformer DC-AC Converter 12V to 110V 200V 220V 280V":I found the following hint related to AC output:"1) We recommend connect to "DC Output", because normal electric units are using normal rectifier. If connected to the "High Frequency AC Output", rectifier will heat up fast and damaged, diode will be destroyed.">> Why that ?>> I think there are suitable double diodes for switching power supplies - they should withstand this.And now I write you a crazy idea.The ferrite transformer is normally easy to disassemble. I could unwind the secondary winding halfway and attach a center tap.Then I connect a rectifier tube to this secondary winding - what do you think about this idea ?That would fit better to a tube amp ;-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2020, 07:32:29 pm
content: Vacuum rectifier just wastes efficiency and space. You certainly can if you want to, but you aren't... doing anything with it, only taking away, y'know?Interesting upside, though: essentially no recovery losses, and capacitance doesn't vary with voltage (or at least, not nearly as much as semiconductor diodes do).Tim

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 27, 2020, 07:44:22 pm
content: Yes Tim, you are right.But a tube amp wastes efficiency and space tooBut the tube rectifier would fit better to a tube amplifier and not everybody has such a high frequency rectification.So you think that this could work ?I am itching to try thisI have found this converter:https://www.ebay.de/itm/324345542557?chn=ps&norover=1&mkevt=1&mkrid=707-134425-41852-0&mkcid=2&itemid=324345542557&targetid=301754823111&device=c&mktype=pla&googleloc=9043229&poi=&campaignid=10215338044&mkgroupid=105782582127&rlsatarget=pla-301754823111&abcId=1103836&merchantid=7364532&gclid=EAIaIQobChMIuu3cyvrU7AIVmrp3Ch2WOglKEAQYASABEgLjp_D_BwEBut which max AC amplitudes do I get at the output of the converter and which DC behind the tube rectifier ?The output wave form of the converter is square wave.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 28, 2020, 12:34:32 am
content: Those ebay 150W 'inverter' style converters need a fast ss rectifier placed on the pcb with care for layout, as the secondary is operating with a high frequency waveform above the audio frequency. Perhaps google search for some forum threads such as this one:https://www.diyaudio.com/forums/power-supplies/313234-150w-12vdc-isolated-25khz-step-smps.htmlPS. another relevant link:https://www.guitargear.net.au/discussion/index.php?topic=51615.0

author: Frank_MV
author_rank: Regular Contributor
timestamp: October 28, 2020, 05:43:45 pm
content: Many thanks for the links