Guide: Choosing the right DC-DC PSU
September 05, 2017
By Aleksandar Kostovic
With the increasing popularity of DC-DC power supplies, we have made a guide to help choose the right one for your needs.
In the context of computer motherboards, a DC-DC converter accepts a single DC input and converts it into the very stable voltages that a motherboard needs to operate (generally 3.3V, 5V and 12V, though some boards only require a single regulated voltage). A DC-DC converter will always be paired with either an AC Adapter (aka 'Power Brick') or a DC voltage from another system such as a battery.
The advantages over a standard ATX power supply are twofold: reduced size, and reduced noise as DC-DC converters are almost always fanless. ATX power supplies have the upper hand when larger amounts of power are required. We probably won't be mining any bitcoins using DC-DC converters any time soon.
How many Watts do you actually need?
We will often total the maximum power consumption of each component to calculate the power draw of a system under load. Thermal Design Power or TDP - though strictly speaking a measure of heat dissipation - is a useful measure for processors and GPUs. Other components such as your SSD and even your RAM will consume more power. And don't forgot those powered USB devices.
Several online calculators exist, though they tend to calculate from the perspective of a traditional ATX power supply. Figures given will often be much higher than real world figures. For example we've measured an H270 Mini-ITX system fitted with i3-7100T processor, SSD and Slim Optical Drive at 23W idle and 32W under medium load.
Some online calculators:
The other thing to note it that no power supply can ever be 100% efficient (DC-DC units are typically within the 90-95% range), so there will always be some Watts wasted in the form of heat.
Our rules of thumb: Any motherboard with an embedded Celeron, Atom etc. will be fine with a 80W or 90W DC converter. If you have a 35W TDP processor a 90W DC converter will be sufficient. A 45W Xeon or 54W Pentium or above will need at least 120W.
Types of DC-DC Converter
We can roughly break down typical DC-DC converters used with Mini-ITX motherboards into Fixed Input, Wide Input and Vehicle.
Fixed Input DC-DC converters will only accept a regulated input. If the PSU is rated to accept 12V it will only accept 12V as the input.
Wide Input DC-DC converters are more adaptive and will take a range of voltages. For example 12-24V, 6-34V, 12-72V etc.
Vehicle DC-DC converters will work with a wide range of voltages and include protection against the rapid changes in voltage cranking an engine can cause. They will often include battery-specific functionality - powering off a computer if a battery has discharged below a certain voltage for instance.
Our rules of thumb: If you are using a battery, get a vehicle capable DC converter. If you have a regulated power source that isn't 12V, get a wide input DC-DC converter. If you are powering from a standard 12V AC Adapter, use a standard DC-DC converter.
PCB-type DC-DC converters
Traditional DC-DC converters are built on a printed circuit board with an input and an output side. This allows them to be easily mounted within a case or sometimes an external enclosure. Many Mini-ITX specific cases use this type of DC-DC converter in conjunction with an external AC Adapter. Power output can be as high as 400W for a custom-designed model, though typically much lower to suit the task in hand.
picoPSUs are a category of DC-DC converter that connect directly to a motherboard's ATX power connector, removing the need for the ATX cable between the DC-DC converter and motherboard.
Fixed Input picoPSUs have yellow pins, the wide input have red and the vehicle capable picoPSUs have blue pins. Power output for official picoPSUs varies from 80W to 200W peak. For edge cases, higher wattage home-grown 'unofficial' picoPSUs also exist.
Our rules of thumb: A 20pin picoPSU will work just fine if your board has a 24pin ATX connector. The additional 4 pins aren't required except in very specific circumstances.
Choice of AC Adapter
If you are powering your Mini-ITX system from a wall socket you will require an AC Adapter. The AC Adapter converts the Alternating Current from your wall socket into a single Direct Current suitable for the DC-DC converter. You will need to pair your DC-DC converter with a suitable AC Adapter. No matter how much wattage your DC-DC converter can output, it will be limited by the power output of your AC adapter, e.g. a 200W DC-DC with an 80W AC Adapter will still only provide maximum 80W of power.
Our rules of thumb: A 80W AC Adapter pairs well with the picoPSU-90; A 150W pairs well with the picoPSU-150-XT. Your AC Adapter and DC-DC converter will need to have the same voltage, plug and socket type to connect. A re-purposed Laptop power brick will probably need a new plug and a wide input DC-DC converter to function.
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