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Old 23rd November 2007, 07:23 PM   #1
mobyfab is offline mobyfab  France
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Default 12v -> 24v power supply

Hi all,

i've been looking for some nice T-class amps, but most of them need at least 24v to produce a bit of power

I would like to put one in my car, and I think the 41hz's Amp9 is a good choice. (its supply goes from 12v to 27v)

now, I need a 20A@24V supply for it.

i've been looking on the web but I didn't find that much content.

Does somebody have some schematics ?

Any ideas on how to make one ?

Thanks!
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Old 24th November 2007, 05:04 PM   #2
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This actually ends up being very simple if you don't need isolation. even if you need isolation its not too bad. Look up "push-pull" power supplies. In short, you can build a simple, and cheap voltage doubler with very high efficiency and low size by using a ferrite toroid, 4 quality MOSFETs, maybe a small inductor, and a low ESR output capacitor.

Snubbing components can be added as needed.

The ferrite is would with 2 high current windings in a 1:1 ratio.
The ferrite is also wound with 2 low current windings with a 1:1 ratio to each other, and arount a 2:3 to 3:3 ratio with the high current windings. going 1:1 is fine if the MOSFETs can handel a 15V Vgs,

This makes the power toroid easy to wind.

Connect the primaries as a normal push pull would be connected.
Now, at the drain terminal of each MOSFET, connect the other 2 mosfets' sources. Connect the additional FETs drains together (forming a diode bridge, but with FETS). The extra windings on the toroid are used to drive the gate of the FETs (add a small resistor in series, and a mid-sized on in parrallel with the gate to help with turnoff and ringing.

Use a tl494 for the FET driver on the push-pull FETS. when one FET turns on, it engages a primary of the transformer, placeing 12V across it. 12V appears across the other primary for 24V total. The "syncronous rectifier" FET turns on because 12V appears across its gate. a bit later, everything flips.

This design acheives high utilization of the semiconductors and the copper of the transformer, and presents only fully-on MOS devices in series with the high current.
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Old 29th November 2007, 04:07 PM   #3
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Chris,

Trying to visualize this layout. If it's what I think, it's similar to N8XJK's 11V-to-13.8V booster ckt that appeared in a back issue of QST.

Or, it could be a slight variation of the 4-switch Buck-Boost topology (not to be confused with "Inverting"), where V(out) lies within the range of V(in-min) & V(in-max).

Anyway, It's definitely worth a closer look.

Steve
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Old 11th January 2008, 07:54 PM   #4
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Hey, just found this thread - I want to do exactly the same thing, power a AMP9 in the car.

Would a boost circuit work? I'd need 24v at 10a really, I can't see me hitting 20a.

After reading here: http://en.wikipedia.org/wiki/Boost_converter
Is a boost circuit simply a switching semiconductor, a cap and an inductor?

I'm getting a but mixed up as to why you need a toroid with more than 1 winding you see.

FWIW, I've got about 3 or 4 old busted car amps, so I can easily use their toroid cores for my own use. I also have loads and loads of copper wire, I'm just not sure where to start as far as the circuit goes....

Also, with a boost circuit, would output voltage vary with input voltage? So when your battery drops in voltage, will the boost circuits output drop too? Is there any way to prevent this?
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Old 12th January 2008, 04:17 AM   #5
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N-Channel:

consider a push-pull, but ONLY the primary side. when one fet is active, Vcc is applied across both coils (by transformer action). thus, just getting 2x voltage can be accomplished without secondaries!

Since this is LV, and efficiency is key, syncronous rectification should be done, which is the rest of the design. the extra windings are for the sync-fets

@MikeHunt79:
yes/no. boost is a terrible choice for beginners*, and other designs can provide voltage gain without being based on the boost topology. a transformer, for instance, can provide a gain with a buck-derived design, giving a gain of D*(Ns/Np).

now comes the question -- do I _need_ regulation? line/load regulation means that the output voltage of the power supply does not change much when the input voltage or the output current change.

for many amplifiers, the answer is, not really. and the lack of regulation makes the design simple.

In general, you will see 1-2 toroids in the car amps -- 1 is a ferrite transformer, the other, when used, is an iron powder inductor. these are very different! The inductor is used in regulated designs. regulated designs also need feedback to work, which add complexity.

my suggestion is to use an unregulated push-pull design for your first attempt. these designs are highly efficient and simple.

the toroid with 4 two+ center-tapped windings is the push-pull transformer. basically, +12V attaches to the center. FETs switch each winding to ground, but alternating, and not at the same time. the other winding, but transformer action, sees Ns/Np times 12V, and using diodes, the correct secondary winding is attached to the correct output.


*boost and buck-boost have a bifurcation point giving two possible modes of operation with different dynamics. one of these modes introduces a right-plane zero requiring the controller bandwidth be reduced significantly. further, peak current mode control which is commonly used for highest line regulation is not unconditionally 1-cycle stable because there is not an isometic relationship between peak current and average current -- to acheive a high steady state duty ratio, the controller will send out a narrow pulse then a very long one, with the average duty ratio being correct!
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Old 12th January 2008, 04:50 AM   #6
star882 is offline star882  United States
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Quote:
Originally posted by theChris
This actually ends up being very simple if you don't need isolation. even if you need isolation its not too bad. Look up "push-pull" power supplies. In short, you can build a simple, and cheap voltage doubler with very high efficiency and low size by using a ferrite toroid, 4 quality MOSFETs, maybe a small inductor, and a low ESR output capacitor.

Snubbing components can be added as needed.

The ferrite is would with 2 high current windings in a 1:1 ratio.
The ferrite is also wound with 2 low current windings with a 1:1 ratio to each other, and arount a 2:3 to 3:3 ratio with the high current windings. going 1:1 is fine if the MOSFETs can handel a 15V Vgs,

This makes the power toroid easy to wind.

Connect the primaries as a normal push pull would be connected.
Now, at the drain terminal of each MOSFET, connect the other 2 mosfets' sources. Connect the additional FETs drains together (forming a diode bridge, but with FETS). The extra windings on the toroid are used to drive the gate of the FETs (add a small resistor in series, and a mid-sized on in parrallel with the gate to help with turnoff and ringing.

Use a tl494 for the FET driver on the push-pull FETS. when one FET turns on, it engages a primary of the transformer, placeing 12V across it. 12V appears across the other primary for 24V total. The "syncronous rectifier" FET turns on because 12V appears across its gate. a bit later, everything flips.

This design acheives high utilization of the semiconductors and the copper of the transformer, and presents only fully-on MOS devices in series with the high current.
Might as well just use Schottky diodes. At 24v, the efficiency loss due to diode drop is very small.
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Old 13th January 2008, 10:56 PM   #7
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Thanks for the info - I'll look into push-pull designs.

The only thing I am unsure of is how all the FET's and things work... If there a good introduction or sticky thread about push-pull?

I already know how full bridge rectification and bridge rectifiers work on normal mains amps.
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Old 14th January 2008, 02:47 AM   #8
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Quote:
Originally posted by MikeHunt79
Thanks for the info - I'll look into push-pull designs.

The only thing I am unsure of is how all the FET's and things work... If there a good introduction or sticky thread about push-pull?

I already know how full bridge rectification and bridge rectifiers work on normal mains amps.

EDIT: Found this awesome link:

http://sound.westhost.com/project89.htm#fig9

I now know how push pull SMPS units work.
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Old 18th January 2008, 06:17 AM   #9
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I looked on Wikipedia and found a link to this:

http://www.smps.com/Knowledge/Boost_PFC/Boost_PFC.shtml

Dear goodness those are some crazy equations!
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Old 18th January 2008, 02:08 PM   #10
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Quote:
Originally posted by theChris
N-Channel:

....consider a push-pull, but ONLY the primary side. when one fet is active, Vcc is applied across both coils (by transformer action). thus, just getting 2x voltage can be accomplished without secondaries!

Since this is LV, and efficiency is key, syncronous rectification should be done, which is the rest of the design. the extra windings are for the sync-fets.......

Chris,

If I understand you correctly, you propose a 2-F interleaved boost converter, with sync-FETs instead of Schottkys. I like this approach. Very interesting..... It's just like the 2-F boost I did, except I would replace the MBR2545CT with two low Rds(on) N-Ch MOSFETs (like an MTP75N06 or equiv.) and the appropriate gate windings. You have piqued my curiosity........
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