Fig. 1.     XP Power’s CCB200, 200 W, AC-DC power supply.

XP Power recently unveiled its CCB200 series of ultra-efficient 200 W single output AC-DC power supplies aimed at medical and industrial applications. Using a detailed power analysis, designers were able to achieve a typical efficiency of 94% and a maximum of 95%, with a flat efficiency curve across the entire operating load range above 20% (Fig. 1).

This new supply follows evolutionary upgrades in previous 200 W supplies. In 2006, the company introduced its 200 W EMA supply with 88% efficiency. Next came the 200 W CCM supply in 2009 with 93% efficiency. Following these previous 200 W power supplies, XP started design of the CCB200 series as its next 200 W ac-dc supply with the goal of  94% efficiency. Requirements for this new supply included:

·     Convection cooling, no fan noise

·     Ease of integration

·     Improved system lifetime

·     No derating  at 200W full load

·     High efficiency / Very low loss

·     Compact footprint

Fig. 2 is a plot of the power supplies efficiency. Achieving efficiency gains above 90% at 90 VAC input were very challenging. Getting higher efficiency, like 95% in loss terms (W), requires the removal of about 55% of the remaining losses. To reach this goal meant that designers had to focus on each small area of loss within the product. Plus, new technologies might have to be required, such as digital control. Table 1 shows the power loss of each circuit and the percentage of their total loss in the new power supply.

Fig. 2.     Efficiency curve shows constant efficiency from 20%  for a 12 V output.

Meeting these goals also required careful attention to the power dissipation of every power supply component.

Often overlooked in reliability considerations is the effect of operating temperature on a power supply’s components, notably capacitors. Capacitor reliability, and thus their useful lifetime, depends on their operating temperature, which can be approximated by the Arrhenius' equation, a simple, but accurate formula that relates temperature with reliability, or useful lifetime. A generalization supported by Arrhenius' equation is that, for many common components, their useful lifetime doubles for every 10 °C decrease in operating temperature. Fig. 3 shows how a capacitor’s useful lifetime varies with applied temperature. This also means that a power supply’s operating temperature has a major impact on its capacitors’ reliability, which is also reflected in the supply’s overall reliability.

Fig. 3.     Effect of temperature on a capacitor’s useful lifetime.

Besides component power considerations,  power supply design also required attention to the circuits themselves. Fig. 4 is a block diagram of the power supply. Inrush protection uses traditional low cost negative temperature coefficient (NTC) thermistors. After the start-up phase they are removed from the inrush circuit.

Fig. 4.     CCB200 block diagram includes EMI Filtering & Inrush Protection, AC Rectification & Power Factor Correction, Main Converter, and Output Rectification & Filtering.

One of the biggest areas of loss in all switch-mode power supplies is the AC bridge rectifier, however they are used due to lack of a cost effective alternative. The current at 90 VAC is tough to deal with and usually results in the need for derating.  The bridge rectifier sees this current first hand along with the input filter. Probably 25% of remaining losses come from the bridge rectifier.

The technology employed in the CCB200 ensures the ultra-high efficiency rating is achieved from 90 VAC input, unlike current products in the market that have lower efficiencies at low line and therefore have to specify derating. The full operating input range of the CCB200 series is 80 to 300 VAC, with full power available from 90 to 264 VAC.

To condense the circuitry into the required 3 x 5-in. format and achieve a true 200 W convection-cooled rating it was necessary to increase the efficiency and the input was the obvious area to attack. Bridgeless circuits replacing the diode input bridge with MOSFET’s have become more common in higher power products where the input current is much more of a challenge, so this technique coupled with a discontinuous mode power factor correction (PFC) stage created a low power bridgeless PFC stage that cut the losses in this area from around 4.7 W to around 2 W (a 57% reduction).