DC-to-DC converters

[dotneck335]

I just purchased a DC-to-DC converter off of eBay; (3A DC-DC Boost Step-up Converter Voltage Regulator 5V-35V to 9V 12V 24V 36V 48V | eBay)
but I have no idea how this thing works. Of course, they give no schematic. I guess it is a switcher of some sort, but the active component's markings are hidden by its heatsink, so no help there. Does anyone have a schematic of one of these things, or something similar? What frequency do they usually operate at? How big of a coil is needed on its output to calm the noise?

 

[nigelwright7557]

I suspect it is a fly back converter given all the different volts it works at.
They usually work around 100KHz.

 

[FauxFrench]

It is a boost converter (non-isolated). It is likely to be based on an LM2576 (52KHz) or an LM2596 (150KHz), most likely the latter. The circuit diagram is here:
http://www.siongboon.com/projects/2005-08-07_lm2576_dc-dc_converter/lm2576-adj%20circuit.gif
You can eventually increase the size of the output capacitor if you find the ripple to high.

 

[dotneck335]

Thanks for the replies! However, both the LM2576 and the LM2596 appear to be step-DOWN regulators; mine is a step-UP (boost) type. Is there an equivalent chip?

 

[FauxFrench]

My excuses, that I had overlooked.
Then, I arrive at LM2577 (52KHz): http://www.ti.com/lit/ds/symlink/lm2577.pdf

Alternatively, it may be based on an LM2585 (100KHz): http://www.ti.com/lit/ds/symlink/lm2585.pdf
Figure 19 in the datasheet.

Desolé, FF

It seems to be the LM2577: DC Boost LM2577 Converter Step up Module 3.5 35V to 5 56V 3A MAX with Light -in Voltage Regulators

 

[dotneck335]

That sure looks like the one I have---THANKS for finding it!! I'm a bit disappointed that the switching frequency (52KHz) is so low.....makes filtering harder (larger-value inductor needed). But I did find two 100 µH, 3 Amp chokes that I think I'll use in series with the output. If my TDA7492 Class-D amp is putting out 20 watts into 4Ω, that's about 2.2 amps of current. Figuring the Class-D at ~ 90% efficient, that's about 2.44 amps, 15 volts, and the chokes should provide ~-20db of attenuation at 50k Hz. I hope that's enough.

 

[basreflex]

that converter is rated for input current, so if you like to step up from 5V you can do max 10W input, say 8W out. from 12V input you can do 24W in, or 20W out. the continuous rating is 2A input. guessing the efficiency at 80%..

 

[basreflex]

the other limit is ripple current. the output cap has only a 400mA rms rating at best, if it is a renowned brand. it's a matter of time but the cap will die. best to replace it with a polymer type like the panasonic os-cap, these have ripple ratings 10x larger. the price may me more than the complete DCDC ...

 

[dotneck335]

The data sheet for the LM2577 says it will handle up to 3 amps of output current.

 

[basreflex]

it says switch current; so for example a 50% duty does a 2x stepup so with 3A switch current you can absorb 1.5 (50% of the time of 3A) times the input voltage of power , that then will be converted to the output by 80%. higher ratios gets only worse.

basreflex

 

[dotneck335]

Hmmmm....OK, how do I figure what I'll get? I'm stepping 9 volts up to, say, 15 volts for my TDA7492 Class-D amplifier. How much current can I expect from my DC-to-DC converter?

 

[dotneck335]

Well, not if you want to remain battery-powered, as in MY application. Power source is 6 x AA batteries.

 

[dotneck335]

Yeah, that'd work! Thanks for the suggestion. But I am currently trying to work within the constraints of my existing amp; trying to squeeze the most out of it. It's a little Roland MicroCube with 6xAA cells. So I just need to figure how much peak current I can get from the DC-to-DC converter.