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-   -   Fully Discrete Switched PS (http://www.diyaudio.com/forums/power-supplies/58057-fully-discrete-switched-ps.html)

tguzella^TuX64 27th May 2005 11:21 PM

Fully Discrete Switched PS
 
Greetings everyone,

In http://www.diyaudio.com/forums/showt...hreadid=57281, i was looking for suggestions on an amplifier to be built. I ended up going for a modified A/B Class Citation 12 schematic (http://passdiy.com/pdf/citation.pdf).

Well, the original supply voltage is +-42V, but I have decided to go for 30-35V, saving me some money on the transfo :dodgy:. The "problem" is: i really don't want to use the originally proposed power supply (which is not regulated, using only filter capacitors).

When simulating, I realized that an ordinary linear regulator wouldn't handle it, as the circuit demands ~4A :bigeyes: on each rail when the output transistors are conducting. Power dissipation on the series pass transistor would be huge, and the ripple voltage is a little too high for me. BTW, I simulated using a 36Vrms transformer, using 6800uF filter capacitors.

So I wonder: what if I use a fully discrete (no specialized ICs) switched regulator in this case? Why discrete? Mostly for fun :D ... I am aware on the restrictions (/www.diyaudio.com/forums/showthread.php?s=&threadid=28028) on the discussion of mains operated SMPS :att'n:, but i would like to know: how feasible is the use of a switched regulator to supply 4-5A @ 35V out of a transfo+rectifier+filter combo...

As I can't afford another transfo, i am considering only transformer-less topologies, specially boost/buck... AFAIK, i can build the pwm controller using opamps + capacitors + transistors, but I haven't done it yet (in PSPICE; I may give it a try on this weekend)...

In case I haven't been clear, I am planning on using a sec center-tapped transformer (36Vrms), plus a bridge rectifier, and two filter capacitors before the switched regulator... The input dc voltage would be between 40 and 50 V (an estimate), for a 35V regulated output voltage. By using the boost circuit, I (if I'm not wrong) could go for a cheaper transformer (around 24-25Vrms)...

What do you guys think?

Kenshin 29th May 2005 04:22 AM

transformer-less: you can't! it's too dangerous.

fully discreet? there's a thread of self-oscillating one right now.

Dem 29th May 2005 04:24 PM

2 Attachment(s)
When I was young, I built a lot of idiotic things, one of them - relay-type buck regilator 30V to 24V 5A, The main idea - a comparator with small hysteresis, when output voltage drops a little - it opens the switch, when grows to much - closes it. It even worked :) ... (see picture) . You can adjust Vout vith R5, and output ripple with R3, of course You can't us LM393 when Vin more than 30V, just build another "full-discrete" comparator...

About boost - I don't think that it's a good idea, and transformer will not cheaper - it will lower voltage, but higher current, so it will have the same power, and tr-rs with same power cost the same...

Mr Evil 29th May 2005 06:15 PM

If power dissipation is your main concern, then you might consider a capacitance multiplier instead of a regulator. It would allow you to achieve low ripple and low power dissipation, at the expense of regulation.

tguzella^TuX64 29th May 2005 07:50 PM

Quote:

Originally posted by Kenshin
transformer-less: you can't! it's too dangerous.

Is it dangerous due to the non-isolation from the mains line? Or something else? So, in this case, I have to go for a topology using a transformer... Any recomendations? Flyback, Half-bridge?

BTW, Has anyone successfully used transfos from "old" PC power supplies (i'm thinking about ~200W AT ones, which should be easy to find in old PCs, for a good price) ???

tguzella^TuX64 29th May 2005 07:54 PM

Quote:

Originally posted by Mr Evil
If power dissipation is your main concern, then you might consider a capacitance multiplier instead of a regulator. It would allow you to achieve low ripple and low power dissipation, at the expense of regulation.
The thing that keeps on bothering me is: should I rely on the feedback to prevent hum due to supply voltage ripple??? If so, then I guess I could go for a capacitante multiplier...

Mr Evil 29th May 2005 08:16 PM

In the experiments I have done with capacitance multipliers, it's not hard to get ripple down to 4mV with 4A output current. Even if the amp has zero PSRR it's getting on for 80dB down, which isn't bad, plus it has a relatively benign, almost sinusoidal shape, as opposed to the higher frequencies from linear of switching regulators. Even lower ripple is quite achievable.

Sch3mat1c 30th May 2005 04:38 AM

You can certainly make a discrete SMPS, but why bother when chips are so much better? LOL

You can try anything from a variable-bias multivibrator to a full-blown expanded SG3524 or TL494 internal diagram, using darlington long tailed current-mirrored differential amps for the comparators, a current mirror to charge the timing cap and an open collector to discharge it, to form the triangle wave.

The former would look something like this:
http://webpages.charter.net/dawill/t..._SMPS_Junk.gif
While the latter would look like this, 50 years ago:
http://webpages.charter.net/dawill/I...0Generator.gif

Tim

Kenshin 30th May 2005 01:20 PM

Not idiotic things, but fairly good design.

I have also done something like this, then develop the thing into some class D audio amplifier.

A year ago I started a project of building a 24V to 5V 7.5A supply for a industrial PC on a robot. SSEEKER, my companion, suggest to use 3x LM2576 in parallel, and I think it's not reliable. So we decided to build one use discrete power stage.

Soon, I sliped into designing some advanced thing operating at 200KHz. Also, I considered too much problems such as speed of comparator or overshooting of loop responce. So I didn't really made anything until end of the robocon2004.

Then we, the whole team lost the game in Beijing -- since we spend too much time in PSU instead of sensor.

This year, for robocon2005 (my schoolmates of lower grade do it, :D ) I built one with IR2111 + IRF540NS power stage, it works at 100KHz. IR2111 has too much dead time and the MOSFETs gets overheated due to body diode reverse recovery (IMHO.) when the load is heavy.

I got angry of it. Instead of operating it at lower frequency, I give up the whole project and threw it to SSEEKER (companion of last year :D )

He build it with three LM2576 in a week. Now, it runs perfect on our young school brother 's robot.

Class D audio ampilfier technogy may be the biggest good thing left in this MEGA failure. I have bulit some class D amplifiers of 10w power level, it sounds fairly well.

Quote:

Originally posted by Dem
When I was young, I built a lot of idiotic things, one of them - relay-type buck regilator 30V to 24V 5A, The main idea - a comparator with small hysteresis, when output voltage drops a little - it opens the switch, when grows to much - closes it. It even worked :) ... (see picture) . You can adjust Vout vith R5, and output ripple with R3, of course You can't us LM393 when Vin more than 30V, just build another "full-discrete" comparator...


Kenshin 30th May 2005 01:52 PM

Of course, extremely dangerous.
How can you use an audio equipment without mains isolation? If you touch the audio input/output terminals, you will die.

For a isolated one -- the topic "Self-oscillating SMPS with saturable drive transformer" is discussing a simple self-oscillating one.
http://www.diyaudio.com/forums/showt...815#post652815

Quote:

Originally posted by tguzella^TuX64

Is it dangerous due to the non-isolation from the mains line? Or something else? So, in this case, I have to go for a topology using a transformer... Any recomendations? Flyback, Half-bridge?

BTW, Has anyone successfully used transfos from "old" PC power supplies (i'm thinking about ~200W AT ones, which should be easy to find in old PCs, for a good price) ???



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