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2 * 24v outputs of 2A each: supply with one 12v 4A boost or two 12v 2A boosts?

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maker8122

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Supposing you make a box with 12v going in and 2 outputs each offering 2A at 24v.

You could use a single 4A boost converter.

Or you could use two 2A boost converters, one for each output.

It seems that 2A boost converters are MUCH smaller, lighter and CHEAPER than 4A converters. The inductor is much smaller, the capacitors much smaller and the switching components too.

So at it looks like 2 * 2A converters is the way to go.

However there are never free lunches (good ones, at least!).

So what am I missing? :-?
 

If the designs are well optimized in this power range I think you'll find that there isn't much difference in terms of cost or size between the two options. Cost and size scale fairly well with power.

So I wonder if system level pros and cons can make the choice for you. If you have 4A shared between two outputs that means they could share 3A/1A and aren't each limited to 2A. That may be a benefit. Or instead if you need 2A limits that would have to be a following stage to the 4A solution which would be a con there.
 
Instead of one large converter, consider building two small interleaved converters. By interleaving you stagger the two Ampere waveforms going to the converters, thus you get a smoother current waveform drawn from the 12V supply.
 
2 x boost converters ( 12 -> 24V) gives you two channels to control separately if you wish

remember if you short a boost converter - there is no current limit, just a diode drop and a choke - so a fuse or poly switch on each output is recommended

:eek:) EP.
 
@asdf44, good point. The outputs each go to stepper drivers and it's true that when driven by switching supplies, these motors benefit from some apparent overcapacity. Putting them both on a single 4A would mean that one motor might benefit from a little extra torque for acceleration (if the second motor is not drawing too much) than it might otherwise have.

@BradtheRad thanks for teaching me the term "interleaved". Found an excellent explanation of this topology https://www.edn.com/design/power-ma...dc-dc-converters-boost-efficiency-and-voltage

(If time-pressed, just look at Figure1 and Figure2 comparing a single large convertor with two smaller ones interleaved.)

@Easy peasy will put poly fuse on outputs as you suggest. If it's a 2A convertor, do you put a 2A fuse or less?
 

Yes there are real theoretical benefits to interleaving, particularly ripple cancellation and also the benefit of cutting inductor current by half (which, if the value is held equal means energy/size will be 1/4).

On the other hand the part count goes up. That's why I initially said I thought it was probably a wash at these power levels. At higher power levels the overhead of doubling the control and feedback circuitry becomes comparably smaller and interleaving is more likely to show a clear benefit.

PTC's are good to have but their properties are quite poor. It's a stretch to market them as 'fuses'. They're slow and have a significant derating curve over temperature which means you usually need to specify one rated for 2X or greater of load current to guarantee they can carry it at temperature. You'll get more reliable current limiting for 'free' in the boost converter controller.
 

as I stated above there is no current limiting in a boost converter
 

Ok yes I missed that and that's true except for synchronous controllers. Although I'm seeing less synchronous controllers on a quick look than I expected, so that might not be the way to go. Though it looks like TI has some:

https://www.ti.com/product/tps55340-q1
 

"except for synchronous [boost] converters" ? perhaps you had better look at the schematic a bit harder, pretty sure there is a diode inside those fets pointing the same way as a conventional boost diode => no current limit for low impedance loads...
 

Ok you’re completely and obviously right. I apologize.
 

no apology sought or needed, boost converters - although schematically simple, are tricky beasts to control well, the delayed action of the switching stage ( RHP zero ) being just one of the major hassles, peak current thru o/p diode at power up being another one, turn off volt spikes in the main fet... the list goes on.

They are quite susceptible to high Z filters on the input too ...
 

OK so in this context the choice of polyfuse is quite important.

Presumably, given the temperature derating, it's a good idea to place it as far as possible from the switching area, quite possibly off board?

How do we account for the "slowness" of the cutoff? By choosing a value a bit under that rated value for the convertor? Or...
 

Fuses are relatively cheap, can be had in different I2t curves and are your insurance when everything else fails.

For boost converter, they are a must
 
If your fet ever goes short you might want to consider a polyfuse on the input, as well as the output, for 2A out we would use a 4A polyswitch.

Its just that you don't need to go find another fuse once you have removed the fault, for circuits that need close protection a fast blow fuse is essential.
 
@Easy peasy a 4A on the 2A output is something I'd never have guessed.

Just to try to understand, are any of the following pertinent to your application (or somethings entirely different)?

- is it local heating that makes a 2A inappropriate?
- are there transients in the load that surpass 2A but are short and insignificant in the scheme of things?
- does this make the assumption that an overload will take the form of an outright short rather than, say, the load trying to draw 3A continuous (which might be accepted by the polyfuse and toast the convertor)?
- does this imply that the switching topology can handle currents in excess of 2A for a short time? Perhaps therefore external - rather than internal(which might be more sensitive) - FET ?

Ever come accross an app note advising choices such as this?
 

does this make the assumption that an overload will take the form of an outright short rather than, say, the load trying to draw 3A continuous (which might be accepted by the polyfuse and toast the convertor)?

Switching must continue in all situations. If the switch is stuck 'On', it drains current from the supply. That is the typical overload. A fuse is the normal solution.

If your load tries to draw overmuch current, then output V drops. A regulator tries to increase duty cycle. However if (for any reason) duty cycle is allowed to approach 100 percent, it fails to increase output V. The condition resembles switch stuck 'On'. Therefore oscillations should be designed to continue in all situations. Duty cycle should be made to increase to a maximum safe length, and no further.
 
simply that the volt drop is less for a 4A poly-switch than for a 2A device - and nuisance tripping is avoided...
 
OK. Looking at a specific example, 4A trip is indeed necessary to get a sufficiently high Ihold to avoid tripping.

https://www.littelfuse.com/~/media/...e_ptcs/littelfuse_ptc_1210l_datasheet.pdf.pdf

The terrible thing is that the 4A with an 8A current would take 1 second to trip. With a 4A current that would be, 4 seconds (if half current = quarter power). Horrible. All the meantime the resistance of the FET shoots up with temperature....

On the voltage drop, gather that lab power supplies wire Vsense to the output terminal to account for all the output protections. But then again that may not be for the switching part but the linear smoothing stage?
 

Hi,

The terrible thing is that the 4A with an 8A current would take 1 second to trip.
A fuse usually is defined by it's hold_current.
Thus, to be more exact: The 2A fuse will trip in about 1s with 8A applied.

With a 4A current that would be, 4 seconds (if half current = quarter power)
Following your rule down to 2A ... then this means 16s at 2A.... (because at a quarter current there is only 1/16 power)
This simp,y is wrong. The fuse is meant to never tril with 2A applied.
--> I recommend to use the "average time current curve" in the datasheet.
And read the additional informations given in this chapter of the darasheet.

In my eyes a polyfuse isn't an exact current limiting device. Thus it should be used to protect against malfunction.

A polyfuse in combination with some fast current limiting system sometimes is counter productive.

In your case... I assume it can be used to protect againt short circuit at the output. Expect a very high current before the fuse trips.
Take care to use thick and wide enough traces to withstand this high current.

Klaus
 

Gosh @Klaus, you are right and none of those fuses are suitable. This would be the appropriate range from the same constructor.

https://www.littelfuse.com/~/media/...cs/littelfuse_ptc_lowrhosmd_datasheet.pdf.pdf

Both of the 4A (Ihold) offerings have trip times of many seconds.

This is most instructive. Have been spending too much time with a power supply with programmable OCP and not worrying about this. Beginning to understand that it might make sense to:

a) either look for a controller IC that has current limiting built in, or add a feedback circuit oneself.

eg https://www.ti.com/lit/ds/symlink/lm5122.pdf on p24 has "Hiccup mode Protection".

b) failing that consider a means (mosfet, relay) of cutting the current at the input to the convertor. Have a microcontroller on the board so many options for monitoring and control, though perhaps analogue lets one sleep best.

c) fuses then in last defence...
 

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