30V from 24V

Well, COTS 24V to 5V isolated 5W DC-DC converters are plentiful, 6V less so, but still available.

Cheers

You could buy a cheap potted DC/DC converter, like 24-to-6. Either stack it on top of the 24 to get 30, or use -6.

If volume is high, making it from parts would be cheaper (and you'd get more consulting hours!) We have a nice quiet Cuk (or is that sepic? I can never remember...) circuit that we like...

In volume, I'd use some cheaper part than the LTC, $2.73 looks like. They are good, but their prices are insane.

(I'd love to use one of their SOT23 resistor-programmable clock oscillators in my hysterical switcher right now, but not for $3! I can get an XO for less.)

LTM8023 works nicely in invert mode, again not cheap, just under $10. They have internal Ls and Cs and are small and quiet.

Thanks. This design needs to have a long product life, though, so it would be nice if we could roll our own. I might even use an MC34063 for that reason.

Cheers

Phil Hobbs

This is cheap and fairly simple, but it's noisier than a Cuk-type switcher.

How about a boost for $1.63? Maybe cheaper in quantity.

Thanks.

I have to have a bunch of reservoir caps in the box anyway, because pulling 700 mA of 1-2 kHz square wave out of the 24V supply would probably make me unpopular with the other folks. So there's going to be at least 4700 uF on the rail. A fixed frequency would be sort of nice, because otherwise my boxcar lock-in back end might have some spurious response to the ripple.

Cheers

Phil Hobbs

The problem with the hard switching type inverters (or classic boost converters) is the high di/dt sloshing around between parts. That shoots awful spikes into ground planes, and makes interesting magnetic fields.

A Cuk is softer. I like the LTM8023 in places that are not super cost sensitive. It's tiny and very quiet, and you can control the frequency, with a resistor or an external clock.

how about something like an LT3517?

boost, current control and on/off all in one

-Lasse

Thanks. I've used those in protos, and they work fine if you derate the output current by a factor of 2 or 3.

This is an industrial application, though, so it needs good quality parts. The LM2577 isn't a bad one, but the distributor prices are ridiculous--$3 in thousands. For less than that, I can get a better chip that runs a lot faster than 50 kHz, e.g. the LT3580 for $2.30 @ 1k.

Cheers

Phil Hobbs

LT1071 would certainly boost 24V to 30V. Indeed, it could probably handle 12V to 30V. I've used LT1072 for 24V to 30V running 300mA, but

Steve

It needs a big inductor (100uH or 220uH) but the MC34063 is about the cheapest chip out there that can deal with relatively high supply voltages.

Runs at ~33kHz. You could make an inverting regulator or a boost regulator, the internal switch current of 1.5A may be enough.

I suppose if you didn't mind wasting 10 or 15W you could drive it push-pull through a capacitor. Or SMPS regulate the 24V down to 15V or so and do that?

Yeah, that inductor didn't look right for that sort of power. Or the thermals.

How can they sell that for $1.63? By US standards, the trimpot is worth that much.

LTC is always expensive.

A boost has to move all the power through the semis and the inductor; that's 21 watts. An inverter only has to make -6 volts, 4.2 watts.

That's not a "30V from 24V" topc, it's 700mA current pulse with 28V compliance topic. If ultra-miniaturization is not an issue, an auxiliary power supply is not needed.

ISTM that boost and inverting aren't that different in this case. Average inductor current is the same, and the volt-seconds are comparable (a bit less with the boost). PIV of the diode is slightly less, Vin rather than Vout. So it's really just the voltage rating of the output cap that's the issue, unless I'm missing something ultra-important (which I may well be).

Cheers

Phil Hobbs

Do tell. How would you drive a 28V LED from a 24V supply without doing some switching thing?

Cheers

Phil Hobbs

Errm no. I would say the boost (like the inverter) only has to make 6 volts 4.2 W which is added to the supply. Basically the same amount of energy in both setups.

piglet

On Tue, 4 Nov 2014 09:52:05 -0800 (PST), Lasse Langwadt Christensen Gave us:

A good, old fashioned UC3832 is always a good choice too.

On Tue, 04 Nov 2014 15:11:34 -0500, Phil Hobbs Gave us:

LED is very low consumption and if dirty DC is OK, then a simple diode/cap multiplier and storage cap will work from an oscillating pulse feed.