Miniature 175V SMPS thread

[Cliff Schecht]

I would be nice to see how the output looks on the scope. Also to perform some A/B tests with actual equipment.
I would like also to remind you about the discussion concerning the dead batteries, compression etc. I suppose a 4k7-10k resistor after first cap (or maybe didode?) will do the job.
I also have one question. If I need 350V for a pair of 12AX7s for example I guess it's not a problem to use 2 diodes in voltage doubling configuration?

As far as anybody will notice, for audio use the supply will look like 175V DC with about 100 mV of P-P ripple. The frequency of the switching is so high though (100 kHz) that the noise you will see on a scope is meaningless for audio use. The only time it could POTENTIALLY cause problems is if you have something else in the system that oscillates near 100 kHz, then this noise could "beat" down into the audible range and give you some grief. Chances are that if you are working with a circuit such as this then you know of this problem already and have enough bypass capacitance around everything to prevent SMPS noise from causing system noise.

This supply is NOT meant to emulate tube rectifiers in any way. Putting a resistor in series with the voltage output, whether before or after the caps, will just drop voltage and burn up power through the resistor. Sometimes a small resistance is necessary with solid state and tube recitifiers is used to limit in-rush current before the caps but isn't needed with this supply. The time it will take for the 175V supply to recover after an output power spike is somewhere in the microseconds..

The voltage doubliing configuration can't be used with DC, at least not without some sort of square wave generator that can quickl;y flip the polarity of the voltage feeding a typical voltage multiplier circuit (diode and cap variants). These need some sort of switching waveform to work properly. This supply isn't meant for voltages that high without a bit of redesign. Adjusting R3 slightly will allow for the voltage to be tweaked, but too much may cause instability or random white puffs of smoke to appear.

The power supply can be adjusted for a bit more headroom. I wouldn't exceed 200V as a relative max (if the switch and cap can handle this) but the output current will also be derated a bit.

[Cliff Schecht]

I finally got around to etching and building some prototype boards today. I made a few tweaks to the design and actual board, most of which was adding enough bypass capacitance to keep everything quiet and stable. I also added in a pot/trimpot that allows the output to be adjusted from 125V to 250V, although at elevated voltage levels the output current should be derated a bit. I will post a more detailed report later but as of now the supply fires up nicely and the output voltage control works as it should. At low voltage levels with too little current draw I get some oscillations but increasing the minimum load takes care of this problem. Here's a picture of the first build:



Here's a huge image link where you can see the SMD stuff in detail: http://imgur.com/zY6jE.jpg

I'm going to build the through-hole version soon and will post both versions when I get everything revisioned and verified. Something that always nagged me with the Nixie tube supply designs is that they weren't designed for use with audio, they were meant for use with Nixie tubes. This supply is actually meant to be used in audio applications and while I haven't tested other peoples designs, I think people will dig how this supply (doesn't) sound.

[davidallancole]

Hi Cliff,

Its great to see this working.  I am glad the trimpot works for adjusting the voltage.  That will be handy for dialing what ever voltages a guy needs.  For determining the supply current needed, would it be safe to assume (output voltage*max current)/supply voltage?

[head_spaz]

Hi Cliff,
Great job.
I can hardly wait to build this puppy.

[Cliff Schecht]

Hi Cliff,

Its great to see this working.  I am glad the trimpot works for adjusting the voltage.  That will be handy for dialing what ever voltages a guy needs.  For determining the supply current needed, would it be safe to assume (output voltage*max current)/supply voltage?

For determining the input supply current it's better to take the output power, multiply this by 1.25 (this assumes about 80% efficiency plus a good amount of breathing room) and divide by the input voltage. With a 10 W load (say 175V at 57 mA) and a 12V input, you do 10W/12V = 0.8 A, so I'd go with a 1 A supply minimum. With a 12 V supply, you should never need anything over 2 A (this is about 17 W, more than the switcher can handle safely!). I'm not sure how other people rate their supplies but my experience shows that it's always a good idea to overrate the input power supplies to non-isolated power supplies for various reasons (keeps system heat and noise down).

I've been using a 12.6V, 3.5 A supply to run this thing off of. Definitely overkill, but I've just spent too much time trying to figure out why a power supply was saturating just to figure out that the input power supply was too weak for the job.

[davidallancole]

Thanks Cliff.

On a side note, I understand the use of these switchers, and also desire them for my own projects, but doesn't it seem funny using a switching power supply (probably but might be linear) to reduce 120Vac to 12Vdc and then bump that back up to 175Vdc using another switching power supply?  Not to ask you to do more work, but since you know more about these supplies then me, would it be possible to build a switching supply that will give the same result as what you have made, but could be powered straight from a 120Vac source?

[Ice-9]

Thanks Cliff.

On a side note, I understand the use of these switchers, and also desire them for my own projects, but doesn't it seem funny using a switching power supply (probably but might be linear) to reduce 120Vac to 12Vdc and then bump that back up to 175Vdc using another switching power supply?  Not to ask you to do more work, but since you know more about these supplies then me, would it be possible to build a switching supply that will give the same result as what you have made, but could be powered straight from a 120Vac source?

I'm sure Cliff will give some very good reasons why this is a bad idea. It's my belief that if you were to rectify the mains voltage 110VAC or 240VAC in the UK, then you will have absolutely Zero isolation from the mains so if you were also connected to any other equipment that was wired incorrectly or faulty then you will probably die.

[Cliff Schecht]

Thanks Cliff.

On a side note, I understand the use of these switchers, and also desire them for my own projects, but doesn't it seem funny using a switching power supply (probably but might be linear) to reduce 120Vac to 12Vdc and then bump that back up to 175Vdc using another switching power supply?  Not to ask you to do more work, but since you know more about these supplies then me, would it be possible to build a switching supply that will give the same result as what you have made, but could be powered straight from a 120Vac source?

There are safety issues associated with making a homemade straight-from-the-wall powered SMPS. This type of SMPS, known as an "off-line" power supply (literally meaning off the mains line) requires some very special considerations and building techniques that isolate the dangerous 120V AC from the outputs using coupled inductors (special type of transformer). These transformers, if we were to use them for our application, would have to be wound by the builder and could potentially cause all sorts of safety issues. These special transformers, along with specialized opto-couplers, are what are used to place an isolation barrier between the input wall voltage and output sections. By instead using a SAFE (hopefully UL approved) power supply to power everything, you know that you are using safe power and I'm a lot more comfortable knowing that people are playing around with truly dangerous voltages (aside from the ones that the supply makes!).

Other reasons for not going straight off the wall include the cost would go up, the overall efficiency would go down and the board size would grow. Using already filtered and cleaned up DC from a wall wart makes my job a heck of a lot easier and worries me a lot less .

[davidallancole]

Thanks for the response Cliff.  So to sum it up, it is technically possible but would open up a whole different can of worms for the average builder?

[Cliff Schecht]

Yes because you wouldn't be able to use off the shelf inductors like this boost design does, you're instead stuck with paying out the ass for a commercial product (especially for a toroidal core transformers) or winding your own transformers. Winding your own sucks because in order to get decent efficiency, you need to interleave of the windings or you end up with a fairly inefficient core that will get hot quickly.

Also like I said, I was going for as small of a switcher as possible. Having a transformer with at least three separate windings (primary, secondary and "housekeeping" windings) doesn't allow for a very small board..

[juancra]

Cliff, there's something I wish to ask to you to fully understand this development: Could this smps with its 100ma power a small tube amp like a fender Champ? I know that some problem might rise with tube heaters so I thought i could use normal diode rectification and filtering for the heaters and this smps for b+. Im I right?

[armstrom]

Disclaimer: I may be completely wrong about this! However, I believe 100mA might be enough for a Champ (It may be close though). Voltage would be a bit low (champs typically use voltages in the 300 - 260V range) but you could still have a very "champish" sound. You won't have all of the mojo of using the 5Y3 tube rectifier (doesn't make much of a difference in SE tube amps anyway) but should be passable. As for the heater filaments, just run them from the same 12V DC supply you use for the HV supply. Keep in mind that the 6V6 power tube requires a 6.3V heater voltage so you'll have to regulate it down for that tube. The 12AX7 can use the 12V supply directly. Just make sure to increase the current rating of your 12V supply to account for the current requirements of the heaters.

[juancra]

Yes, looking at this proyect http://www.jjs.at/electronic/class_a_subminiature.html I understood that I could power the heathers from the same line i'm feeding the smps. What I'm not sure is what could happen if I feed 6v6 with less than 300+ volts... If I'm lucky I believe it would only sound quieter.. Is this correct?

[Cliff Schecht]

I'll post some more specific design guidelines once I can do some more extensive testing and figure out the real limits. I really designed this supply for submini tube projects that I've been working on (about 1W and below) and I doubt that it can really push the 100mA it was designed for. I did the layouts and everything to fit parts that can handle the full load power but I haven't gotten to even test this design out with a tube design yet (test week next week!). Hopefully I can find more time soon to do the testing that I need to..

With a tube like the 6V6, in order to get the distortion one wants from the tube you need to run it at higher voltages. The datasheet says 250V requires 45mA of quiescent plate current. With a preamp tube (or two), you'll be at about 13-15 watts of power draw (pentode screen currents and triode bias currents add up here too..) which is going to start heating up the power supply. Maybe it can handle this amount of power, I'm not so sure yet because I don't have a 250V cap on the output to test everything that high (mine has a 200V part on the output). Again, I will post more details as time allows.

[head_spaz]

Whatever became of this project?
I'm getting anxious to build this sucker. I'm ready for sparks and smoke!
Did you give up on it Cliff?

[Skruffyhound]

+1
Been watching this hopefully from the first post.

[Cliff Schecht]

Give up? No! I used this power supply in my little amp in the November Submission thread. As of now I just need to do one more build to verify that the design works as it should at higher voltages. I had some lockup problems with the supply when I ran it at higher voltages, essentially being when I plugged in the power the SMPS didn't boot up properly and the FET gets really hot (enough to permanently attach a nut to the drain tab!). I think this is because I used an older variant of the controller IC that I usually use and I'm thining that my build with all of the correct parts will work properly. The newer generation of the UC3842 controllers have built in soft-starts to prevent the lockup problems that I'm having. I'll put this on my to-do list but I've got another big amp on the bench that I need to finish before I get back to my miniature designs.

[robmdall]

Great stuff Cliff! I do have a question or two about the components used building this supply or similar.

How important is component selection with regard to the resistors and caps. The internet has me confused with one builder using x type of cap and another using z type.

Are Tantalum caps and metal film resistors a necessity or is it just a builders preference? I believe this has been covered in other topics but not specific to SPMS (I did a search here).

anyway, thanks for the great design and if this is a silly concern please chalk it up to another newb.

Bob

[Cliff Schecht]

There are certain places where you want to use the correct type of part. These parts are usually low value, high accuracy and high precision although there are other places (like the RC timing circuit) where you again don't want to use cheap resistors and caps. The caps are really critical on the output however, they need to be the special low esr types specifically designed for power supplies. A common trick for supplies like this is to put lower voltage caps in series to keep cost and size down while getting the voltage and esr ratings you need. And in certain parts of the circuit you want to use NP0/C0G ceramic caps. None of these parts are extraordinarily expensive, aside from the MOSFET switch, and the SMPS controller can be ordered for free from TI's website (up to 5 free samples per part).

[robmdall]

Thanks Cliff!