# DC/DC Converter VS AC/DC Converter

#### KGlo_88

Joined Mar 10, 2021
2
If you are using several different DC voltages inside a unit, (say 5v, 12v, 48v) ignoring efficiency, would it be better to use separate AC/DC power supplies for each voltage or use 1 AC/DC power supply for the highest DC voltage and step down the voltage with DC/DC converters from there?

#### wayneh

Joined Sep 9, 2010
17,405
It depends on the current needed at each voltage level. Needing >90% of the total power at 5V means different choices than if 90% of the power is needed at 48V.

#### Papabravo

Joined Feb 24, 2006
20,383
There is one immutable guideline that you should learn and repeat with religious fervor:

The power out will always be less than the power in. Sometimes it will be a great deal less.

In your case you should run the numbers for each case and come to a conclusion.

#### KGlo_88

Joined Mar 10, 2021
2
It depends on the current needed at each voltage level. Needing >90% of the total power at 5V means different choices than if 90% of the power is needed at 48V.
Say the breakdown was 55% at 48V, 35% at 12V and 10% at 5V. what would be best?

#### Deleted member 115935

Joined Dec 31, 1969
0
Yes..

Question is about the basic difference in the units.

An ACDC, takes in AC voltages !! normally direct from the mains,
If it doesn't , you will add a transformer to the front, which is liable to be as expensive and big as the ACDC units.

If you have mains voltages, on the board, then you have a big lot of safety concerns / regulations to comprehend.

If you have a single ACDC unit, you set that to give an Intermittent DC voltage,
this ACDC is off board, and provides you with the isolation from the mains,

Typical intermediate voltage are 12,24 or 48 volts,
All are below the Low Voltage Directive,
so safe.

Then you have on board DCDC's , which provide your local voltages.
these can be simple POL ones or isolated if needed, most of the time, you use POL units.

#### BobaMosfet

Joined Jul 1, 2009
2,082
If you are using several different DC voltages inside a unit, (say 5v, 12v, 48v) ignoring efficiency, would it be better to use separate AC/DC power supplies for each voltage or use 1 AC/DC power supply for the highest DC voltage and step down the voltage with DC/DC converters from there?
There is a great mis-understanding about DCC converts for people not familiar with them. DCC converts convert Amps to Volts- that's how they work, via an inductor. As such, they require a LOT of amperage to work.

If you are going to 48V, 12V, 5V, it makes a lot more sense to use voltage circuitry and regulators to strip 12V and 5V off the 48V input. If you are trying to get 48V and 12V, for example, from a 5V input using a BOOST (DCC) converter, that is going to take a lot more current than you're likely expecting or maybe able to handle.

#### wayneh

Joined Sep 9, 2010
17,405
Say the breakdown was 55% at 48V, 35% at 12V and 10% at 5V. what would be best?
Are you starting with AC mains power?

I'd certainly look to take the 5V off of the 12V. Maybe a 7805 regulator would do the job. Or a DC-DC buck converter If the greater efficiency makes it worthwhile.

I'd probably go directly from mains to 12V because that's so common and there would be a lot of off-the-shelf solutions. A lot of those may offer the 5V option you need as well. Anything to take you from 48V to 12 or 5 is much less common. Doable of course, just not as common.

That leaves the 48V supply. Do you need a smooth and well regulated voltage or would fully-rectified AC be OK? A transformer plus rectifier, plus filter capacitor, is a pretty simple way to go.

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#### michael8

Joined Jan 11, 2015
355
It depends on the actual power needed at each voltage too which hasn't been stated. How about 55% at 48V but at 100 A (4800 watt) vs 10 mA (about 1/2 watt).

#### AnalogKid

Joined Aug 1, 2013
10,568
they require a LOT of amperage to work.
Actually, they require only enough current to meet the load's power requirement (plus conversion losses).

Example: DC/DC conversion system using a simple buck regulator. +50 Vin, +5 Vout, 1.0 A load current, 80% conversion efficiency.

At 100% conversion efficiency, the average input current would be only 100 mA. At 80%, this increases to 125 mA.

For a high step-down ratio like this, there are other converter topologies that do not have as high a peak input current, but they also are more complex.

ak

#### AnalogKid

Joined Aug 1, 2013
10,568
Say the breakdown was 55% at 48V, 35% at 12V and 10% at 5V. what would be best?
What would be best is not playing 20 questions just to get the most basic system information needed to discuss the question.

ak