Electronics > Beginners
MC34063 high voltage dc-dc boost converter
dazz:
Greetings everyone. I've been mulling over the idea of building a dc-dc boost converter that outputs some 300V to power tube amps. I reckon it should be easy to build one capable of powering a two 12AX7 preamp, but I wonder if I can build something with enough juice for an EL84 single ended amp, or even a push pull with two EL84's. This would be used for the plate and screen voltages, I would deal with the heaters separately.
I've already done some research and found a few chips that might do the trick, like the MT3608, the LM3478 or the MAX1771, but because I already have some MC34063 lying around, I decided to try this ratio extender circuit: https://www.changpuak.ch/electronics/High_Voltage_Power_Supply_MC34063.php
So far I have only simulated the circuit, and to my surprise, the simulation throws very promising results. (See attachment 1 for schematic)
The guy on the above link who built those boards to power nixie tubes got pretty terrible efficiency figures at 30-40% with this circuit, and the simulation agrees, but then I swapped the switching transistor (FJN13009) for a mosfet (IRF740) and the efficiency (again, in LTSpice) skyrocketed to a 85%. It also seems to make R7 unnecessary, I guess because the mosfet's gate current is negligible so it's not loading pin 2 of the IC, but is this a result that makes sense? Does the mosfet do a better job at staying away from the linear zone hence dissipating less power? Or is this simply an artifact of the simulation?
Perhaps even more impressive, or at least it was surprising to me, is that the simulation is perfectly fine outputing 200mA at 300V for 60W at 85% efficiency. According to my research, the MC34063 would exceed it's 1.5A peak current limit at some 50mA in the output at 300V, so 200mA seems way above spec. I will know for sure once I build it, but I thought I would ask in case some of the local experts can help me understand all this stuff.
I've read that the MC34063 is old and doesn't support PWM. I guess it works at a fixed frequency and it switches the pulsing signal on and off, and that makes it a worse option than a proper PWM chip, but I don't understand why that may be a problem. Is it because it generates noise as the switching signal turns on and off? I have added a capacitance multiplier to filter out most of the ripple and it seems to work great in the sim, with only 15mV peak-to-peak ripple at 300V. That ripple seems to be at the low frequency (200Hz) at which the mc34063 is controlling the switching signal. The switching frequency itself, at some 20KHz, is completely gone after the capacitance multiplier
I'll be using a toroidal inductor for this build, it's [ulr="https://www.ebay.com/itm/3A-6A-Toroid-inductor-Coil-LM2596-Magnetic-Ring-Inductance-22UH-to-100UH/223484117366?ssPageName=STRK%3AMEBIDX%3AIT&var=522239495543&_trksid=p2060353.m2749.l2649"]one of these 3A 470uH coilshttps://www.digikey.es/product-detail/en/aavid-thermal-division-of-boyd-corporation/513102B02500G/HS346-ND/1216353.]. Does that seem adequate?
And this heatsink for the mosfet, which according to the simulation, draws 8W with a 200mA load at the output: [url]https://www.digikey.es/product-detail/en/aavid-thermal-division-of-boyd-corporation/513102B02500G/HS346-ND/1216353. It's 11ÂșC/W so might be a bit too small, not sure.
Thoughts? Ideas? Criticism?
Thanks in advance
T3sl4co1l:
*Cracks knuckles*
https://imgur.com/gallery/M1S0DbI
Enjoy :P
But yeah, what you're really looking for is at least UC3843, probably a current transformer to improve efficiency, and definitely a transformer or autoformer so you aren't beating the shit out of the switching parts. Any transformer from a power supply of the same size will do -- they work just fine in forward or reverse. A 120/240V input, 12V output, 50W supply say, will easily get you 300V at 50W from 12V supply.
You should have no need of active filtering on a properly designed switcher. Passive filters are simple and effective. The 34063's hysteretic behavior makes a huge mess.
Tim
dazz:
--- Quote from: T3sl4co1l on December 30, 2019, 03:58:26 pm ---*Cracks knuckles*
https://imgur.com/gallery/M1S0DbI
Enjoy :P
But yeah, what you're really looking for is at least UC3843, probably a current transformer to improve efficiency, and definitely a transformer or autoformer so you aren't beating the shit out of the switching parts. Any transformer from a power supply of the same size will do -- they work just fine in forward or reverse. A 120/240V input, 12V output, 50W supply say, will easily get you 300V at 50W from 12V supply.
You should have no need of active filtering on a properly designed switcher. Passive filters are simple and effective. The 34063's hysteretic behavior makes a huge mess.
Tim
--- End quote ---
Thanks, Tim. I have a few things to google now, because I don't understand some of the things you say there.
I think it's time to crack open The Art of Electronics, chapter 9 and learn how these buggers work before I go on with the build. I should've done that already.
I was hoping to get away with not using a transformer, but if that's the right way and I won't need something specific, I'll do it
dazz:
--- Quote from: T3sl4co1l on December 30, 2019, 03:58:26 pm ---*Cracks knuckles*
https://imgur.com/gallery/M1S0DbI
Enjoy :P
But yeah, what you're really looking for is at least UC3843, probably a current transformer to improve efficiency, and definitely a transformer or autoformer so you aren't beating the shit out of the switching parts. Any transformer from a power supply of the same size will do -- they work just fine in forward or reverse. A 120/240V input, 12V output, 50W supply say, will easily get you 300V at 50W from 12V supply.
You should have no need of active filtering on a properly designed switcher. Passive filters are simple and effective. The 34063's hysteretic behavior makes a huge mess.
Tim
--- End quote ---
Wow, I simulated the same circuit at a lower output voltage, higher output current and the efficiency got up to 95% with much less power consumption in the mosfet, so I think I see what you mean now about using a transformer.
T3sl4co1l:
Yep. You don't usually see SMPS analysis in terms of reactance, but it generalizes just fine. By driving a higher voltage ratio, or higher current ripple ratio, you draw proportionally more reactive power from the inductor, and that power flows into the transistor and out the diode (or however you like to assign the signs, it's AC).
So going for a say 10:1 ratio at 50W might take 10 times more reactive power than 1:1 (flyback type), and if that's at 100% ripple (DCM or BCM), then that's about 500VA reactive power, and you better have one hell of an inductor to handle that with low dissipation (Q >> 100?). And the transistor and diode need to be sized for 500VA despite that you only get 50W from it.
Or more generally, for a given factor of range, you need not only components that handle the extra range, but to maintain efficiency you need factor squared better components. And that's just conduction loss alone.
So having a modest range, pays. A lot.
By using a transformer, you get all the ratio with none of the wasted capacity. You get an additional wrinkle of leakage inductance (and also stray capacitance, at high voltages), but this isn't new as such, it was always present (stray inductance between the transistor, diode and cap); it's only a matter of correct transformer design, and snubbing if applicable.
Some more study highlights: current mode control. Draw the switching waveforms (current ramps and voltage flats). Commutation and 1st order parasitics if you like. Derive the transfer function (voltage ratio in terms of D) using the inductor definition. You get the latter non-consecutive two here, http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html (i.e. not commutation), but I do suggest understanding how it works, before getting too accustomed to it. :-+
Tim
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