Switching Regulator for Audio Amplifier

Several years ago I designed a class-D amp with an off-line SMPS. I used a common mode filter transformer at the output of the SMPS to get rid of the noise (windings arranged so that the AC currents cancel each other). It works great.

transistors,

controls

cinch AND XLR

So here are some pictures from the PA3000 inside, the transformer is 220VA.

Discrete is simple :-) and no, it is not MOSFETs, the power transistors seem to be

now 87.95 Euro...

transistors,

volume controls

cinch AND XLR

to be

are now 87.95 Euro...

Pictures, what pictures?

What is this equation called V * T = C * I, and what is is supposed to do?

I is supposed to make sure the output maintains a certain amp capacity?

I've been reading that calculations of ripple current a really just a technical excersise, because the ESR of both the filter caps and the transformer will only be know after build time. The rule of thumb that was presented was to size the filter caps with an ESR rating of about 3.5 to 4.5 the output current depending on the size of the transformer.

On the capacity side, were the sightly contradictory suggestions of aminimum capacity to reduce ripple voltage to

On a sunny day (Fri, 19 Sep 2008 13:46:29 -0700 (PDT)) it happened Fred wrote in :

to be

are now 87.95 Euro...

Oops, seems I forgot the link: ftp://panteltje.com/pub/PA3000/

Well, we say 'U' and you say 'V', but apart from that, it says that if you charge capacitor C with current i, then after t seconds the voltage on that cap will be U (or V ;-) ) volts. Actually it is Q = C .U = i . t, where Q is the charge.

I only used it to see how big the voltage drop would be between 2 consequentive mains tops in your rectifier for a given line frequency and a given cap, and a given current.

I dunno about rule of thumb, you can simply calculate it, or, if you want a visual, run it in spice, or stick a scope on it. That last thing rules. Electrolytic caps have tolerances that can be quite large anyways, say 20%.

It is up to you, again if you want to run 100% sine waves all the time, then you need more C, I would think, then for normal music.

Somebody suggested thermal protection in the transformer, I think that is a cool idea.

seem to be

they are now 87.95 Euro...

voltage on that

or

cool idea.

I've got the chip up and running again with the old transformer:

I resoldered the wiring and removed some caps. The voltage now is about +/-37 volts. it drops to about +/-35 under load.

A interesting thing I noticed is that it only puts out about 7.5V into a 5R power resistor before the positive side starts to clip. If I raise the input signal, the top rail will continue to clip, while the bottom rail will go to about -18V pk before the spike protection starts activating.

Also I reviewed the thermal considerations, and realized that my 3C/W heat sink is only half of what it should be. I can see on the scope the thermal protection activate as the heat sink gets hot.

The data sheet states at +/35V the max dissipation of the chip is

I have to deal with over a hundred watts of dissipation with two chips. I've been looking at heatsinks and fans. I found a neat control circuit that provides linear voltage response in regard to temperature with a floor that will just keep the fan turning with the heat sink is cool.

Fred

seem to be

they are now 87.95 Euro...

voltage on that

or

cool idea.

I forgot to mention, that's neat that capacitors hove linear relationship with voltage when charged with constant current. :)

Fred

On a sunny day (Fri, 19 Sep 2008 17:10:08 -0700 (PDT)) it happened Fred wrote in :

Same goes for inductors, the current rises linear with time for a fixed aplied voltage. i = U . t / L

So if you have 1 Henry, and apply 1 Volt, then after 1 second the current is 1 Ampere, after 2 seconds 2 Ampere, etc...

For a non-ideal coil the resistance will cause non-linearity, and the curve will begin to flatten, finally the current will be U / Rs.

voltage.

Ampere,

will begin

I must have damaged that chip too. I am glad I started with such a simple project to learn the basics with.

I replaced it with a LM3876 that I have two, and it sounds much better, makes about 14pk into 5 ohms before SPiKe activates. Now I just have to figure out how to keep it cool. Junction to heat sink is aout 1.5C/W at 65W dissipation at 50W that leaves very little room for the heat sink 0.5C/W max really should be less.

Maybe I will liquid cool them. Maybe I put the whole thing in a bucket of oil! :)

Fred

voltage.

Ampere,

will begin

I find it fasinating how the energy storage devices, capacitors and inductor inversely parallel there action in the electro-static and magnetic ways.

Fred

On a sunny day (Sat, 20 Sep 2008 07:49:24 -0700 (PDT)) it happened Fred wrote in :

Tjunction max is 150 C, so with 50C environment, you have 100 degrees play room.

You can make a simple temp sensor with a normal si diode, or old TO220 transistor (use be junction) that you can screw on the heatsink, and an opamp.

On a sunny day (Sat, 20 Sep 2008 15:56:42 GMT) it happened Jan Panteltje wrote in :

PS that cannot be right?

Since the unit was a proto type some of the connections use wire nuts. The chip is on a PC board. the output leads are kinda thin maybe 20-18 awg stranded. There might be high resistance some where. I was thinking that might have been the reason for the abnormally high voltage in the first place. When I just rewired the PS, that high voltage went away.

I will check into 10 ohms too.

I donno about forced air cooling, the heat sink would still have to be big and the air flow would have to be substancial, I have a half dozen of these heat sinks, and they would require 1000ft/min to get to

I'm still thinking liquid cooled heat sink. Especially if I use the LM4780, that make 165W to deal with. thats about 30W/cm^2

Fred

On a sunny day (Sat, 20 Sep 2008 13:20:20 -0700 (PDT)) it happened Fred wrote in :

Those processor coolers, that you can buy in any computer shop, cool a 90W thermal design power Pentium XX to safe temperature.

That chip is specified at max 125 W in any case, less if your heat sink environment dictates that (Note 3 page 4 of the data sheet).

I would not drive it to that extreme.

Get one of those big processor coolers perhaps.

That waa 14Vrms.

I checked the circuit and found the mute resistor was too high of a value. I replaced with lower value and now I get 17Vrms clean, then a little something at the top of the wave form from there to 19.2Vrms where clipping starts.

Amp sound Much better now. Thermal limitation of heat sink plain limits power, I can hear it start to crackle a bit after just a few second of high output.

I put fan on the heat sink, maybe 20-25cfm for a little improvement. Sound is very clear even through my cheap 93db/w drivers. Can hardly wait for better thermal management and good speakers!

Fred

:)

The came up with some thing new since I worked on PCs, heatpipes!

That might work if I can figure out how to mount it.

Fred

at a given supply voltage.

supply.

would be enough.

worse than 50% regulation? 42/24 = 1.75. Ridiculous. Use a better transformer.