car amp standard SMPS

[harvie256]

Hey all,

I was hoping to pick the brains of some of the knowledgeable on here about what the "industry standard" was for SMPS's in car amps.

I built this little prototype a couple of years back for my education, and now that I'm back to having a car (I'm an avid motorcyclist), I'm thinking about putting together a cleaned up design and having a PCB made.


PS the majority of the circuitry is on the other side...

Anyway, I've never seen inside a commercial car amp so these questions may seem a bit stupid :

- From what I've read on sites and posts around the place, it seems un-common to bother with current mode control for push-pull SMPS car amps. Would that be a fair assumption? (I like this idea, simplifies things greatly)

- I've read that it's common to have no output inductor on output of the transformer. Is that true? In theory at least, wouldn't that give the output caps a hiding?

- Have commercial amp SMPS's got any more advanced over the years, or are they still just a basic SMPS controller like the TL494 and run of the mill circuit?

Sorry for the n00b questions, just wanted to clear a few things up before I launch into the next board.

thanks

[harvie256]

Just found Perry's pages. Pretty much answered my questions

[luka]

- From what I've read on sites and posts around the place, it seems un-common to bother with current mode control for push-pull SMPS car amps. Would that be a fair assumption? (I like this idea, simplifies things greatly)

I don't know of one that is not voltage mode.

- I've read that it's common to have no output inductor on output of the transformer. Is that true? In theory at least, wouldn't that give the output caps a hiding?

Yes that is true. Since none of them have feedback, inductor only hurts the output

- Have commercial amp SMPS's got any more advanced over the years, or are they still just a basic SMPS controller like the TL494 and run of the mill circuit?

Still the same SG3525 and TL494 and will stay so for years...apart from few that have uC for all of that. No special components, most of the times, not even good fets, drivers,...



BTW your unit looks great, did you do any measurements on it, maybe have some scope pics?

[harvie256]

Hi Luka, thanks for the response!

Can you tell me, even with no regulation, why does the inductor hurts the output? Is it the resistive losses you're referring to?

At the time I certainly did a lot of measuring and fiddled with optimizing the snubber circuits and output filtering to minimize output noise. However I don't have any of the scope captures anymore. In saying that, if there's anything you're interested in, it wouldn't take me more than a few minutes to hook power up, a dummy load and get some DSO captures. I don't have a functioning amp that I could load it with though at the moment, only power resistors.

Basic specs were to run two LM3886s, so about +/- 26V and I can't remember what continuous power I tested it to. I started designing my own discrete amp, but then work took over my life and the project came to a halt.

I'm hoping to make the PSU small enough to fit on the 10x10cm prototyping service by iteadstudio, or the 10x15cm if I need to. Beyond that the price of low volume PCBs sky rocket.

[luka]

Hey

Can you tell me, even with no regulation, why does the inductor hurts the output? Is it the resistive losses you're referring to?

There is always that but that is not it, it it when you apply AC voltage to it, inductor has impedance of xx, depending of the freq and value of inductor used. now, ok you would say, I use this same inductor when I have regulation, why not use it when I don't. In regulated supply, you have for that reason higher transformer voltage then output. inductor stores energy. So would here, but that would create voltage drop. I am probably not the best to explain this. You could look at this as low pass filter, inductor helps filter out your switching voltage... and more current it has to pass, larger drop it makes.


Its regulated, right? to what power did you try it?

[Perry Babin]

It depends on the value of the inductor. High value inductors can do what you describe but lower values won't. PPI uses large inductors that cause the rail voltage to be lower than it would without it but most other amps and power supplies use a much lower value and the rail voltage isn't affected.

[harvie256]

It's had a few re-incarnations while I tried a few different things. Originally it was regulated, right at the moment it's not. And I think the future version wont be. I plan on using my own amp class AB amp design that's not that fussy about supply voltage. But then again maybe I'll put the space on the board for regulation in case I want it.

I just measured the inductance I've got on this one, 14uH per rail. When it was running I measured negible voltage drop across the inductor when unregulated. It just absorbed the voltage changes between switching. I'll get some scope captures to explain what I mean.

I can't remember exactly what power I tested to, somewhere between 50-100W continuous. But it had proper heat sinking at that stage which I don't have on me now. The new board will be a more practical shape for a car friendly heat sink.

[harvie256]

Right, it's just clicked in my head what you're saying. However I think Perry's right in that a small amount of inductance to help filtering is a positive, too large an amount and the inductor will not maintain a 0V difference between the crest of the squarewave and the output voltage.

[harvie256]

Well having now measured it, it becomes so obvious

Here's the wave across the inductor


Now if I zoom in on that


There's the drop!

The only thing I could find at hand to load it was a 240V light globe, so I only had a dismal 10W load on it. Anyway, here's the measure results:

With inductor:
Vout(total) - 52.5V
Efficency - 84.6%

Without the inductor (shorted it out):
Vout - 54.2V
Efficency - 87%

So clearly in this case I either need a much lower inductance or I've stuffed something along the way! I might try taking a few turns off the inductor and see what I get.

Anyway if you're interested in more of the wayforms, here's a link:
Pictures by harvie256 - Photobucket

[harvie256]

Here's a few more waveforms possibly of interest:

Noise with the inductor:


Noise without the inductor:


and zoomed in:


So without the inductor the output noise with about a 10W load increase by about 3x (~250mV to 750mV). Though from the zoomed in waveform, the noise I'm observing is around 80mHz. So the inductor is way to big and the wrong material for this sort of noise anyway. So I'd probably be better off with a more suitable inductor and some caps with a higher corner freq, as I'm pretty sure the 0.1uf caps in there have a corner freq of about 30mHz if I remember correctly. Or maybe I should be trying better to correct the source of the noise instead of bandaid the output.

Anyway, it's all a good learning thing which is why I play with this stuff.