Better to boost 12v to 24v, or drop 24v to 12v?

[Tumerboy]

I am working on a project that will have both 12v dc and 24v dc components. I plan to have a primary power supply, and then step up/down as needed. Is it a better idea to start high, and drop down? Or start low and boost up? 

[DavidAlfa]

Usually a buck converter is more efficient than a boost one, as the switching current is lower in the drop converter.
Also buck converters are a lot more common, you'll have plenty of parts to choose.

I've rarely seen the boost approach else than in situations where no other options where possible (ex. battery-powered), or very specific circuits requiring high voltages like display backlighting.

[Tumerboy]

That was what my gut was saying, but I appreciate the confirmation. Thanks.

[jwet]

Agreed- unless the 24v supply is lightly loaded.  If you have heavy 12 usage and light 24v, a boost architecture makes more sense.  I would say it makes sense if I24 < 4x I12, otherwise start with 24 and buck down.  Bucking is almost always more efficient, peak currents are lower, etc. as the last poster said.  24 to 12 is an easy ratio for a buck converter.

[Mechatrommer]

Buck is more efficient but in case switcher failure, circuit exposed to higher voltage damaging all.. boost is less efficient but in case failure, power simply go to 0 or lower voltage. So it depends on design and economy.

[Tumerboy]

Thanks all.

[mariush]

I would also say it depends on how much power will be consumed from 12v and from 24v.  If the power consumption on 24v is low, a step-up conversion from 12v to 24v would make sense.

If you plan to power your project with an external power supply (ex have a DC In barrel jack connector on your project), you may also have to account for losses in the cables going to your device at high power consumption. For example you may get only 11.5v at the end of a long cable at high power consumption.

You may also want to make device safe just in case users pick a random power supply with same barrel jack connector - maybe they pick a 16.5v or 18v from a laptop and just plug it in.  In that case, if you want to make it wide input voltage safe, you would probably want to use a step-down converter to 12v for extra safety. 

[BeBuLamar]

Buck is more efficient but in case switcher failure, circuit exposed to higher voltage damaging all.. boost is less efficient but in case failure, power simply go to 0 or lower voltage. So it depends on design and economy.

I have used a lot of switching power supplies from 480VAC to 120VAC to 24VDC over the years. I have many power supply failures but none would output too high a voltage to damage the equipment connected to it.

[artag]

I have used a lot of switching power supplies from 480VAC to 120VAC to 24VDC over the years. I have many power supply failures but none would output too high a voltage to damage the equipment connected to it.

99% true - I think I've only ever had one cheap ebay buck converter fail in a way that took out other components - but there's always the one example that you never want to see.



That's a good example of an expensive item that should have had crowbar protection. Doubtless BeBuLamar's high power designs take more care, but cheap off-the-shelf modules don't. If the downstream parts are expensive, pay the extra for a buck converter that's well protected. Use the cheap ones where the cost of the extra parts are too high and you can tolerate a very occasional total failure.

I think I'd still regard buck as better than boost, but sometimes there are other factors. I have a design that uses an 18650 li-ion cell and boosts to 5v. I'd prefer to use 2 cells and a buck ... but using two cells pushes the charger out of the simple single-chip designs and into charge-balancing. So the overall cost is much higher.

Don't forget that boost converters can fail bad too - OK,  a very likely failure is that the switching transistor goes short or open and you get the input voltage at the output. But feedback failure can push output high, too. My favourite example is using a boost converter to power a long serial string of leds (24V worth) from a wind generator (nominal 6-12V) in constant-current mode. I found it necessary to add a zener to limit the worst-case voltage. If the leds went open circuit, the output would rise to 30+V. The leds didn't care : they were disconnected. But the boost converter couldn't withstand its own output voltage and blew up every time !

[Mechatrommer]

treat that as a kind reminder to beginner designing first dc-dc converter. offline smps is different, its isolated.

[CatalinaWOW]

Depending on your project the wiring cost or volume might influence the decision.  Smaller conductors are required to distribute the power at 24 volts. Efficiency is just one thing to think about. 

[james_s]

Buck is more efficient but in case switcher failure, circuit exposed to higher voltage damaging all.. boost is less efficient but in case failure, power simply go to 0 or lower voltage. So it depends on design and economy.

I have used a lot of switching power supplies from 480VAC to 120VAC to 24VDC over the years. I have many power supply failures but none would output too high a voltage to damage the equipment connected to it.

I'm betting most of those were isolated switching power supplies though right? With a buck converter in particular if the switching device fails shorted as transistors most often do when they fail, you will get the full input voltage on the output. Generally a properly designed converter will be very reliable but it is something worth considering. In almost all cases I would drop a higher voltage rather than boost a lower voltage if given a choice though.

[Zero999]

Is there a long cable between the 24V source and 12V device? If so, it's better to put the buck converter near the device and transmit the power most of the distance at 24V. P = I²R so doubling the voltage, will halve the current, given the same power, which will reduce losses by a factor of four.

[SmallCog]

As above it depends and I can't really give an opinion of your situation without knowing more of what you're trying to accomplish. Given that 12V and 24V are often used in my industry I'll share some perspective and experience of my own that may or may not be in any way relevant to what you're doing.

I work with PLC's to monitor/control equipment and 24VDC is a frequently used voltage in this industry both for signaling (DI/DO) and as a power source. The primary voltage that I use in the systems I put together is a nominal 12VDC (from batteries, fed from a mains or solar charger or both if redundancy is required) which I then step up to 24VDC using DCDC converters for things that need it.

The way I see it is that your DCDC is another point of failure, so don't rely upon it, and if you need to duplicate it.

So, my core equipment such as PLC's and Radios and whatnot are all fed from the 12V, whilst loop current sensors that need 24V are fed from DCDC converters stepping it up to 24VDC

Rather than have one converter take out all of my loop current sensors each sensor gets its own DCDC converter. 

If I'm for example monitoring motors then each motor will get its own 24V converter to monitor the status of things such as overloads, contactors, phase fails, etc. This way if a DCDC dies I'm only loosing the monitoring of one motor, the other motor (there's always redundancy!) is still monitored.

Likewise if a wiring fault shorts my 24V's out then the DCDC will sit there with it's overload light on and the monitoring of the other motors is just fine, with no potential to knock out power to the PLC.

[EPAIII]

Old school approach would be two power supplies. Or a single 24 V transformer with a center tap for the 12 Volts. Rectifiers, filter capacitors, and linear regulators (single chip) on each.

Perhaps a more modern approach to this would be two separate switching supplies on the one transformer. Each supply would be sized for the current needed at that Voltage. Again separate rectifiers and filter capacitors could be used. Or just one such group if cost is a big factor.

If there is a concern over component damage, the standard answer is over-current protection on both supplies. If that is not enough, staging can be set up to inhibit the higher Voltage until the lower one is up and in tolerance or to shut it down if the lower one should fail after start-up.

[EEVblog]

Without knowing anything else, buck is usually the first choice approach due to upstream I2R cable, trace, and component losses.