buck converter + regulator , is it more efficient ?

[danieltronic]

so i have a power supply of 15v, i want to go down to 3.3v, i was thinking using a buck converter to go down to 5v and the output then connect to a regulator of 3.3v. its is safe and optimal?  yeah i could only use the regulator but wouldnt a lot of heat be wasted and would to use extra big heatsink? also im not sure if buck converter are good with spikes, so thats why i was thinking this configuration. as you know most 3.3v chipsets can be easily killed with overvoltage.

[ez24]

I plan to do the same thing.  Linear regs will take out some of the noise out of the buck.  In AoE the author says this is the only way to reduce the noise.   But some believe LC filters will work.

 

[ovnr]

It's not "safer" by much. If your load is not sensitive to noise, just use a buck converter directly.

Consider that the CPU of a modern PC runs at 0.9-1.3V or so, straight off of a 12V-input buck converter.

[T3sl4co1l]

I don't see that it's any more or less safe than any other method.  Is it cheaper and more efficient?  Hell yeah!

Solve safety by attacking root causes and their immediate symptoms: overvoltage, overheating, short circuits, etc.  In and of themselves, the converter and regulator will run ~forever.

Best would be a 5-3.3 converter also, but that's really not necessary for low current consumption (under an amp, say).  For that case, an LDO is a cheap and quick way to deal with it and be done!

Tim

[Rick Law]

so i have a power supply of 15v, i want to go down to 3.3v, i was thinking using a buck converter to go down to 5v and the output then connect to a regulator of 3.3v. its is safe and optimal?  yeah i could only use the regulator but wouldnt a lot of heat be wasted and would to use extra big heatsink? also im not sure if buck converter are good with spikes, so thats why i was thinking this configuration. as you know most 3.3v chipsets can be easily killed with overvoltage.

Buck converts the power from one voltage to another while consuming some of it to operate the buck.

Linear regulator burns away the extra power as it drops a higher voltage to a lower voltage.

So, say you must use a regulator, you would still be better off bucking the 15V down to around 5V and regulate that 5 down to your 3.3V.  Besides more efficient, you have less heat problem to deal with.

[jackbob]

I would not say it is more efficient. Any linear regulator throws away the voltage difference as heat. It will help with noise and is perhaps a bit simpler and far easier to implement than a proper LC filter. However, Buck converters can be amazingly stable, I mean just crack open your PC and take a look. It takes 12V and has tons of onboard switching regulation going straight to the CPU with extremely accurate voltage regulation running cores sometimes less than 1V. So it is possible to design a buck switching regulator without the issue of spikes. Because you say you are going down to 5v with the buck then from 5V to 3.3V with the linear, there won't be much loss at all, however, make sure whatever regulator you use has a dropout voltage or minimum voltage not too close to your 5v. Also if you plan to use this for more than several amps of current, the losses will begin to stack up and it will be easier and more practical to just implement the buck going from 12v to 3.3V.

[David Hess]

With a little bit of care, low noise conversion directly from 12 volts to 3.3 volts is feasible for all except the lowest noise applications.  If the 12 volts is regulated or at least bounded, then an inverter could produce the 5 volts to feed a linear regulator and it is easier to make a low noise inverter than a low noise switching regulator.

LC filters more easily reject high frequency noise than linear regulators.

A lot of switching regulator noise is a measurement artifact from using a singled ended probe.

[Zero999]

If you don't need 5V, then use a buck converter to go directly from 15V to 3.3V. It won't be as efficient as 15V to 5V but it'll be more efficient than 15V to 5V buck plus linear 5V to 3.3V.

If optimum efficiency is desired then you need a multi-phase buck or a converter with transformer, but I doubt that's the prime concern here.