Creating High Voltage Outputs with Multiple Converters

October 5, 2014
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Many power applications require a well-regulated, high-voltage DC supply. A typical requirement is for power rails of 80 V to 120 V for applications such as tunnel lighting installations that use a string of LEDs connected in series, high power amplifiers, ultrasound transducers and motor controllers. By connecting the outputs of converters such as the Vicor DCMs in series, these voltages can be generated using standard power components.

The output voltage of series-connected converters is the sum of all the individual outputs. Strings can be configured to deliver either a fixed or a variable voltage output. Variable output configurations can be in either single-voltage or, for applications such as amplifiers that require a symmetrical ± supply, split supply (center-tapped) versions. All configurations can be built quite easily using standard converters plus a few low-cost external components. .

Irrespective of the application, the total voltage of the series string should not exceed 200 V with respect to the baseplate, to avoid isolation barrier problems. This is driven by the FSC 5950 standard, which calls for an isolation barrier of 700 V and a recommended working voltage of 200 V. Most manufacturers, including Vicor, abide by this to allow an adequate margin of safety. If higher voltages are essential, then a ± 200 V split supply can be configured for a maximum differential voltage of 400 V.

Another practical consideration is that only isolated converter devices can be connected in series; without an isolation barrier, a short circuit path will be propagated between the ground of one device and the V+ output of the next.

When connected in series, the same current flows through all converters, so they share voltage drops and power dissipation equally, subject to the limitations of their manufacturing tolerances. Fig.1 shows a simple example of two Vicor DC-DC converters connected in series to provide a fixed voltage output. More converters can be added as required. The positive output of each module requires a series diode (shown as diodes D1 and D2) to prevent the converters being back-driven by load capacitance during turn-off, and to assure proper operation during start-up and dynamic loading. Diodes D3 and D4 prevent the output from going negative if only one converter is operational.

high voltage output

Fig.1: DC – DC converters connected in series for fixed voltage output

It is possible to develop configurations with adjustable voltage outputs, although they require some external components as each trimming channel requires its own ground because the –Out of each converter is at a different voltage. Split output supplies can also be generated by connecting the center point in the string to ground.

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