Power Electronics

Power Converter Shrinks Offline Power Supplies

An isolated dc-dc converter developed by Vicor brings the benefits of the company's V-I Chip and sine amplitude converter technologies to offline power supply applications. Vicor's new offline bus converter module (BCM) enables offline power supplies to achieve significantly higher levels of power density and efficiency, along with fast transient response.

When Vicor first introduced its V-I Chips almost two years ago, the initial chips targeted the applications for distributed power in which a 48-V bus was being stepped-down to the low-voltage levels required by logic devices. However, the underlying power conversion engines used by the V-I Chips are also suited to operation over other input- and output-voltage ranges. With its development of the offline BCM, Vicor is exploiting the flexibility of its sine amplitude converter engine by creating a new use for it.

The offline BCM steps down a 384-Vdc input to a 12-Vdc output at 240 W, while providing the input-to-output safety isolation required in offline applications. When combined with a power factor corrected (PFC) front-end, one or more of the 1.26-in. × 0.85-in. × 0.25-in. BGA- or J-lead-packaged BCM modules can produce a complete offline power supply with more than double the power density of existing supplies.

Consider a typical silver box power supply that produces 12 V at 600 W. In this application, three of the BCMs could be used to perform the voltage stepdown and isolation required after the PFC boost converter. In such an application, power density might be on the order of 10 W/in3. According to Vicor, replacing the discrete circuitry found in the silver box with the BCMs could easily push power density to 25 W/in3. The actual power density associated with the BCM is much higher — 896 W/in3. However, the additional elements in the system — the thermal management components, filtering and the PFC boost converter — bring power density of the supply back into the double-digit range.

Although the BCM performs voltage conversion and provides isolation (4,242 Vdc), regulation is accomplished by the PFC boost converter. Though nominally 384 V, actual input to BCM can vary from 360 V to 400 V. Given the line and load variations, the BCM's output can range from 11.3 V to 12.5 V. In addition to its 240-W continuous rating, the BCM is capable of delivering 360 W for 1 ms.

The efficiency of the BCM exceeds 95% efficiency at high output power levels (see the figure). Noise performance is another distinguishing characteristic. Under full load and with an input of 384 V, output ripple has been measured at less than 200 mV p-p without any external bypass capacitor. When a 10-µF ceramic capacitor is added for bypassing, the output ripple falls to about 10 mV p-p. Another measure of noise performance is input reflected ripple at full load and 384-V input. Under these conditions, input reflected ripple measures about 700 mV p-p.

Because of its low-noise performance, a BCM by itself can achieve compliance with Class B requirements using a filter that is smaller than the BCM itself. However, in an actual offline power supply application, the noisiness of the PFC front-end probably will dictate the use of the typical bulky filter components. However, by reducing the noise associated with the stepdown and isolation functions, the BCM opens the door to further noise reductions in the PFC circuitry. These improvements, in turn, could dramatically shrink the size of the filter.

Like the low-voltage V-I Chips, the offline BCM specifies fast transient response. When subjected to a 0-A to 25-A load step with a 1-µF input capacitor and no output capacitor, the BCM typically responds in 200 ns with a 1-µs recovery time.

In OEM quantities, the offline BCM is priced under $25 each. For more details, see www.vicr.com.

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