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Design IdeasJanuary 2, 1997 |
Designers whose applications demand increased
power or nonstandard voltages have traditionally needed to use custom supplies
or to compromise in the areas of component cost or pc-board space. However, the
flexibility you can gain by connecting isolated, compact dc/dc converters in
parallel or series allows low-cost, standard parts to meet system needs with
minimal cost and space penalties. As Figure
1 shows, you can
use multiple dc/dc converters to generate nonstandard positive- and
negative-voltage rails by simply connecting the devices in series, with the
positive output of one converter connecting to the negative input of another.
Although the connection in Figure 1 is relatively straightforward, it is important to ensure that the full load current does not exceed the current rating of the converter with the highest output voltage. Another important consideration is that the switching circuits in the series-connected converters are not synchronized. As a result, in addition to the additive ripple voltages, the output is likely to produce nonneglible beat-frequency signals. You can filter these frequencies by using a capacitor across the output of the tandem supply or, if necessary, by using a simple LC-filter circuit.
When it is not practical to use one large
dc/dc converter in an application, you can often increase the output rating by
connecting multiple converters in parallel (Figure
2). However,
because the output voltages of most converters are not well-enough matched, it
is not possible, for example, to guarantee that a 2W converter will supply
exactly twice the output of a 1W unit. Consequently, it is important that you
use only converters of the same rating. For example, you should use three
identical 1W converters for a 2.5W load, as opposed to two devices of 2W and 1W.
Even when you use identical converters from the same family, some uneven loading can occur; it is therefore critical that the devices do not operate at more than 90% of their combined power rating. Furthermore, because of nonuniform power sharing, you should connect no more than three converters in parallel. If the system requires more power than three devices in parallel can supply, the best solution is to use a combination of larger converters or a single, high-output device.
As with series-based designs, the switching
of the parallel converters is not synchronous, so you should use some form of
coupling at the outputs. One possible solution is to use a diode feed, as shown
in Figure 2.
However, although the diode drop (typically approximately 0.6V) does not
significantly affect functionality in 12 or 15V systems, it is generally too
large for 5 or 9V types connected in parallel. The optimum method to connect
converters in parallel is to use series inductors on the outputs, as in Figure
3. This
configuration exacts a lower voltage loss than the diode-feed method. And, by
the suitable choice of inductors and capacitors, you can greatly reduce both
beat-frequency signals and output ripple.
A common mistake when using dc/dc converters in parallel is to use regulated rather than unregulated devices. In general, you should not connect regulated converters in parallel, because their output voltages would require very accurate matching to ensure even loading within the tolerance of the internal linear regulator. One of the devices can thus be overloaded. In cases where you need a high-power, regulated supply, it is preferable to connect three unregulated converters and then add an external linear regulator to the circuit. Finally, take note of the safety and isolation requirements of the application. For example, 1-kV isolated devices can serve for rated working voltages up to 130V rms; devices offering 6-kV isolation can provide rated working voltages to 3050V. (DI #1969)
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