Boost/Buck Converters for a Split Rail Supply

tl;dr No.

I like op amps and they are best served with a 0v-referenced split rail supply, say +/-15v.

I’ve got a bag of cheap, adjustable, switching boost converter modules [1] and a bag of cheap, adjustable, switching buck converter modules [2]. I have an old laptop power supply with 12v out. Can I get +/-15v?

First (mildly irritating) snag, the boost converter will only pump the 12v to 28v.

Second (killer) snag, they aren’t isolating. They have a common negative line.

Doing 12 -> 28 -> 14, then using the 14 as 0 ref nearly works. With balanced 680R resistors (ie. 20mA load on each rail), it stays very close to +/- 14. But loading just the upper segment does knock it out of balance (unsurprising, I guess). With 340R (40mA) it swung it to -20/+8. So it’s assymetrically sensitive.

That suggests that in practice it would probably be better to split the 28v with a resistive divider buffered by an op amp. (I have occasionally experienced instability doing this in the past, a little care and/or trial & error is needed with decoupling capacitors).

Incidentally, most of the electronics I do is audio/music-related. When I first encountered switch mode PSUs many years ago I avoided them like the plague, the switching frequency would often interfere with the signal. Until very recently I’d opt for a trad transformer with a linear regulator (78xx/79xx or the slightly noisier LM317/LM337) or a similar discrete circuit. But being tempted by the low prices and high efficiency the modern devices, I discovered they’re fine. At least those for low-voltage conversion use relatively high switching frequencies and so are entirely reasonable for audio applications (though shielding might still be desirable).

The modules I have are generic Chinese-manufactured things, even ordered from a European supplier via Amazon they come in less than €2 a piece, probably cents from a Chinese supplier. Specifically :

[1] Boost converters, based on B628/MT3608, module labelled HW-183.
2-24v in, ?-28v out, 2A. Switching freq : 1.2MHz.

[2] Buck converters, based on LM2596, module labelled HW-411.
3-30v in, 1.5-35v out, max out 3A. Switching freq : 150kHz.

The current ratings for these should be considered as absolute max. Personally I’d not even bother trying them if >1A was needed.

danny

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