Help designing car buck boost for amp.

[TheLaw]

Hey guys,

I guess I'm almost addressing this to Unique, but of course, it's a free world...

I've been going through Mouser trying to find a simple switching step up converter...but I'm not finding much.

I want to build my own sub-amp. Take a look at what JL is putting in their amps...to people who are unaware, it looks pretty nice. http://mobile.jlaudio.com/products_a...hp?page_id=250.

They look like maybe BH or YEC capacitors, which suck.


I'm just about done with my other amp + speakers, but I got myself interested in car audio now. Is there a simple-ish circuit that I could etch myself and get decent results. I'm looking at roughly 4-5A @ 25-30V. (Something like an LM2576 except Buck Boost)

I'm not sure if any of you know about this kind of stuff, but I'll give it a shot.

Thanks!

[etnietering]

Re: Help designing car buck boost for amp.

You might want to look into the M33063: https://cdn.badcaps-static.com/pdfs/...d54f50c1cb.pdf
Output voltage up to 40V. I've used them in several projects to kick 5v up to 12v to run a PC fan. It's only good for 1.5A though, but if you look at page 7 of the datasheet, there's a way to use external switch transistors to increase this current.
I can give you more info on the project I've used them for if you'd like, just let me know! Good luck!

[TheLaw]

Re: Help designing car buck boost for amp.

Hey thanks a lot. I know I probably could have found that if I looked harder, but I reasoned that maybe someone else had some favorites off the top of their heads.

It's still a long way out. I probably won't build it for another year...as I have too many projects on my hands, but it's never too early to start the design part. I'm not an electrical engineer, but for switching converters, the idea is to keep the components as close as possible...generally?

For the external transistor...Do you actually use a Darlignton NPN transistor? Or is Darlington just the topology?

Do you have a recommendation on whether to use 7a or 7b configuration?

Thanks!

[etnietering]

Re: Help designing car buck boost for amp.

Sure thing. You're correct, in general you do want to keep things close together. Shorter traces have lower resistance (and therefore the circuit is more efficient and stays cooler), and less parasitic inductance and capacitance. Neither of these should be a problem though. When I'm designing a circuit board for a power supply, I use a ground plane on the bottom for all of the ground connections. This keeps the ground resistance very low and can act as a heatsink. All the rest of the traces go on top. This also can make it easier to route things. I use huge traces, and sometimes even small planes for the high current traces on the top. Once I found an online calculator where you input the current, length, thickness, and maximum temperature rise of a trace and it'll tell you how wide it needs to be. I then try to have the trace at least 50% greater than what it says.
On the other hand, you don't want to jam everything in so tightly that there's no airflow. That also can make routing traces and assembling the components more difficult. I generally pack things in as close as I can while leaving breathing room for components that might get hot, and not needing to do any crazy routing.

[TheLaw]

Re: Help designing car buck boost for amp.

I'll be sure to keep all of that in mind. I'm no longer afraid of SMD work, so I'll give it a go. Thanks.

Sorry to bug you, but about those transistors in my above post...

Thanks.

[etnietering]

Re: Help designing car buck boost for amp.

Oh sorry, I'd started my reply before you made that edit. I've never actually used the external transistor circuit, 1.5A has always been enough for me. From the schematics, it looks like there's already a darlington configuration inside the chip, and the external transistor just adds an extra stage to it. By putting a darlington outside, you'd be effectively making a 4-stage darlington. I don't see why it wouldn't work, maybe get a few of each and test things out.

7b needs a tapped inductor, so if it were me I'd go with 7a.

[TheLaw]

Re: Help designing car buck boost for amp.

Oh sorry, I'd started my reply before you made that edit. I've never actually used the external transistor circuit, 1.5A has always been enough for me. From the schematics, it looks like there's already a darlington configuration inside the chip, and the external transistor just adds an extra stage to it. By putting a darlington outside, you'd be effectively making a 4-stage darlington. I don't see why it wouldn't work, maybe get a few of each and test things out.

7b needs a tapped inductor, so if it were me I'd go with 7a.

Thanks. Sorry for editing like that. I assumed I was quick enough.

What kind of transistor do you think is appropriate? Bipolar power? What specs should I look for?
Sorry I'm not good with theory.

Thanks.

[Th3_uN1Qu3]

Re: Help designing car buck boost for amp.

What kind of amp are you planning to use? Unless it's full bridge it won't run from a single rail. Well, it'll run... but will need huge output coupling caps. Totally unrealistic especially for a subwoofer application.

The push-pull topology is a common choice for low voltage input. It suffers from flux walking however, so you either use a current mode chip, or a slow error amp on a voltage mode chip, or no regulation at all. Boost type converters can't deliver high power AND high voltage at the same time, forget them. You need a transformer based supply.

Since the car's alternator and battery provide very stable power, there's little point for regulation in the step-up power supply. You just design for fixed duty cycle, wind an appropriate number of turns on the transformer, use a fat capacitor on the 12v input to remove any engine noise and that's it. Use the plain old TL494. Since the error amps will not be needed you can use them to switch the power supply off (for protection). So you get away without an extra chip beside the 494.

30v @ 5A isn't a lot of power. 30-0-30 @ 5A, maybe. What kind of amp are you planning to run on this power supply?

[TheLaw]

Re: Help designing car buck boost for amp.

What kind of amp are you planning to use? Unless it's full bridge it won't run from a single rail. Well, it'll run... but will need huge output coupling caps. Totally unrealistic especially for a subwoofer application.

The push-pull topology is a common choice for low voltage input. It suffers from flux walking however, so you either use a current mode chip, or a slow error amp on a voltage mode chip, or no regulation at all. Boost type converters can't deliver high power AND high voltage at the same time, forget them. You need a transformer based supply.

Since the car's alternator and battery provide very stable power, there's little point for regulation in the step-up power supply. You just design for fixed duty cycle, wind an appropriate number of turns on the transformer, use a fat capacitor on the 12v input to remove any engine noise and that's it. Use the plain old TL494. Since the error amps will not be needed you can use them to switch the power supply off (for protection). So you get away without an extra chip beside the 494.

30v @ 5A isn't a lot of power. 30-0-30 @ 5A, maybe. What kind of amp are you planning to run on this power supply?

I dunno really. It's seriously just a thought, nothing more.

I think maybe an LM3886 or something along those lines.

So...I'm confused. I'm achieving 30V via only a transformer? No chip?

I don't quite understand what the TL494 does?

And is it possible to get split supply from this?

[Th3_uN1Qu3]

Re: Help designing car buck boost for amp.

I dunno really. It's seriously just a thought, nothing more.

I think maybe an LM3886 or something along those lines.

The LM3886 will be good on +/-30v.

So...I'm confused. I'm achieving 30V via only a transformer? No chip?

I don't quite understand what the TL494 does?

Well, the TL494 is the "chip". It generates a fixed frequency pulsed output that is used to drive the power transistors, which in turn drive the transformer.

And is it possible to get split supply from this?

Yes, you can get split supply. You can get any output configuration from a transformer-based supply. Whereas it's not possible with buck-boost.

[TheLaw]

Re: Help designing car buck boost for amp.

Sounds simple, yet a bit complicated. Usually I just use unregulated supplies for my amps.

Thanks for the advice. Time to do my research.

[Th3_uN1Qu3]

Re: Help designing car buck boost for amp.

Usually I just use unregulated supplies for my amps.

An unregulated supply will work just fine in this case as well.

Time to do my research.

Look up "Switching Power Supplies, Third Edition". It's the best SMPS starter book IMO.