Tiny, Silent and Efficient: The picoPSU

May 10, 2006 by Devon Cooke with Mike Chin

A 120 watt ATX power supply? Surely you jest. How can a 120 watt power supply hold its own in a world where there are power supplies that put out nearly ten times that amount of power?

Here's the secret: High powered systems may be as popular as ever, but the industry's growing interest in performance-per-watt has people thinking about power efficiency. Besides, just because a system is powerful doesn't mean it has to be power-hungry. We've known for ages that even powerful systems rarely draw more than 200 watts, and the proliferation of low power, high performancce Athlon 64 chips and even lower power Pentium M, Core Duo, and Turion 64 chips have made it easier than ever to build a system that peaks below 100 watts.

For a modest system, a 120W power supply like the picoPSU is an elegant alternative to a conventional power supply. What makes the picoPSU different from any other power supply? A wise man once said "A picture is worth a thousand words". Let's take his advice:

Where's the rest?

No, that's not just one of the cables. That's the whole darn thing. As its name suggests, the picoPSU is tiny — so tiny that 70 picoPSUs would fit inside the casing of a normal ATX power supply. The advantages of the picoPSU over a conventional power supply is obvious:

• It will fit in just about any case — even the smallest Small Form Factor systems with proprietary power supplies
• It's fanless, which means no noise
• Cable clutter is minimal

The picoPSU is so simple that it's amazing that someone hasn't done this before. Strictly speaking, that's not quite true, someone has done this before, but that someone is the same company. A year ago, Mini-box introduced a product called the PW-200M, which is more or less the same thing in a bulkier package and still available. Compared to its predecessor, the picoPSU is much smaller. Because its footprint is no bigger than the ATX connector itself, it's difficult to imagine any motherboard or system that the picoPSU would not fit into.

The secret to the picoPSU is that it's really only half a power supply. The bulkiest part of the power conversion, 120V or 240VAC to 12VDC, is taken care of by an external power brick which feeds the picoPSU. Because most of the power required by a modern computer is in 12V form, all the picoPSU has to do is pass most of the power straight through, taking only what it needs to power the +5V and +3.3V lines. And, as we noted in our article on power distribution, the total power requirements on these two lines is rarely more than 20~30 watts.

The engineers at Mini-box know this, and have taken the lesson to heart. The picoPSU can deliver only 6A on each of the +5V and +3.3V lines. But in most current systems that's enough.

FEATURE HIGHLIGHTS

SPECIFICATIONS

True to its name, the power output numbers are also tiny. Even so, neither the +3.3V nor the +5V line is ever likely to be overloaded. Only the +12V line looks like it might be a little on the weak side. But, keep in mind that the +12V power is supplied from the external power brick, so the 7A rating is probably only tentative. The "true" +12V capacity is probably the rating of the brick itself, minus whatever power the picoPSU uses to supply the other voltages.

Even if 7A is the maximum (a reasonable estimate for a 120W power brick), there's still 84 watts available for continuous output. Thanks to the ever-decreasing power consumption of AMD's processors, it's quite easy to build a fully-featured desktop system within an 84 watt envelope. With an especially cool processor, it may even be possible to jam in a mid-range graphics card. Even if it jumps above 84 watts every once and a while, there's enough headroom for peaks that it's unlikely to fail immediately.

Mini-box provides detailed efficiency specifications for the picoPSU. In the context of how efficiency is usually measured, the figures are quite unbelievable — 96% efficiency? Impossible!

It's important to understand just how efficiency is measured for the picoPSU. The picoPSU is a DC to DC power supply, so efficiency is calculated as DC watts in ÷ DC watts out × 100%. The power lost in the AC to DC conversion is not counted. Naturally, efficiency seems higher. On the other hand, a convention power supply converts AC to DC, and therefore the efficiency includes those losses, calculated as AC watts in ÷ DC watts out × 100%.

Although the power brick is sold separately, Mini-box supplied two power bricks to test alongside the picoPSU, one rated for 80W, the other for either 110W or 120W (depending on which numbers you believe). Aside from their ratings, the main differences between the two appeared to be size and cooling. The larger of the two power supplies had cooling vents at either end and a tiny cooling fan that turned on under heavy load.

Two external power bricks.

Cooling vents at the end of the 110W unit are evidence of active cooling.

Both bricks are from EDac. They feature active power factor correction and come with short-circuit, over-voltage, and over-current protection. In addition, the picoPSU itself has over-voltage protection if the input voltage rises above 13~13.5V. The 120W brick boasts of efficiency above 85%, so we hope to see it do well on our test bench. The bricks also feature numerous logos from various international certification agencies. There is no reason to expect that the use of an external brick is any less safe than a conventional power supply.

The power bricks let down our expectations in only one respect, voltage regulation. They are rated to maintain +12V ±10%, which is greater than the ±5% tolerance of the ATX12V specification. As the power bricks are likely to be used at close to full load, voltage stability could be a concern. Mini-box wisely rates the +12V voltage regulation as dependent on the "switched input".

Regardless of the potential for fluctuations on the +12V line, the picoPSU has a specified tolerance of just 1.5% for both the +5V and +3.3V lines.

Both bricks are made by EDac and are well certified.

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