Minimum loads and cross-regulation on multiple output power supplies

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One topic that our Technical Assist group continuously will get requested about is minimal loads on multiple output power supplies, so I believed this may be a very good topic to write down about.

On a low value, low power, multiple output power provide, the datasheet will typically state that to take care of regulation, a minimal load needs to be utilized to a number of of the outputs.

To clarify why, here's a block diagram of 1 such easy triple output power provide.

On the center proper hand aspect of the diagram are the three output windings from the transformer.

On Channel 1 (+5V), the output from the transformer is rectified and filtered to supply a clean, DC output. If that output voltage just isn't on the set voltage, say resulting from a load change, the power provide will mechanically appropriate itself. It does this by sensing the output voltage, evaluating it to an internal reference, and feeding again a sign to the management circuit by way of the opto-coupler. The management circuit will then modify the heart beat width of the converter accordingly. The regulation on this output is often 1 to 2%.

On Channels 2 & 3 (+V and –V) although, it may be seen that there is no such thing as a feed again to the management circuit. These outputs are known as “semi-regulated”. If Channel 2’s load have been to extend for instance, the output would drop barely, however there wouldn't be any automated correction. That voltage drop is specified by the load regulation specification, usually 3 to 5%.

With respect to minimal loading, if there may be little or no load on Channel 1, the output will nonetheless be on the set voltage, however the switching converter pulse width will likely be very slender. The output voltage on Channels 2 & 3 drops pretty dramatically at these slender pulse widths, notably if the outputs are supplying their full rated load. An output voltage of 12V could drop to 8V.

Conversely, if the complete rated load is utilized to Channel 1, however Channels 2 & 3 aren't loaded, the voltages on 2 & 3 will rise, and a 12V output might ship over 14V.

The impact that various loads on Channel 1 has on the “semi-regulated,” Channels 2 & 3, is many instances known as the “cross regulation” specification.

Producers of power supplies specify a minimal load requirement on Channel 1, often 10%, to warn the consumer. Minimum loads may additionally be specified on Channels 2 & 3 to advertise a greater regulation specification.

Working with out a minimal load won't usually trigger a power provide to fail, however can stress the consumer’s tools.

Some merchandise like TDK-Lambda’s MTW sequence make use of two converters to enhance power provide regulation, one to produce Channel 1 & one to produce Channels 2 & 3. Word that each V2 and V3 are sensed by the management circuit.

Fig. 2: Two converters (click to enlarge image)

Here's a hyperlink for extra particulars concerning the MTW sequence: http://www.us.tdk-lambda.com/lp/products/mtw-series.htm

Different merchandise just like the NV175 sequence make use of submit regulators on every output, utterly eliminating the minimal load requirement. Though this does add value to the power provide, it removes any concern for the consumer and helps with system flexibility. Here's a hyperlink to the datasheet for NV175 sequence: http://www.us.tdk-lambda.com/lp/ftp/Specs/nv175.pdf