Info!! Minimum Loads In Addition To Cross-Regulation On Multiple Output Ability Supplies

One bailiwick that our Technical Support squad oftentimes gets asked most is minimum loads on multiple output ability supplies, thus I idea this would hold upwards a practiced bailiwick to write about.


On a depression cost, depression power, multiple output ability supply, the datasheet volition often acre that to hold regulation, a minimum charge has to hold upwards applied to i or to a greater extent than of the outputs.

To explicate why, hither is a block diagram of i such unproblematic triple output ability supply.

On the middle right mitt side of the diagram are the 3 output windings from the transformer.

On Channel 1 (+5V), the output from the transformer is rectified as well as filtered to provide a smooth, DC output. If that output voltage is non at the fix voltage, tell due to a charge change, the ability render volition automatically right itself. It does this past times sensing the output voltage, comparison it to an internal reference, as well as feeding dorsum a indicate to the command circuit via the opto-coupler. The command circuit volition as well as thus adapt the pulse width of the converter accordingly. The rule on this output is typically 1 to 2%.

On Channels 2 & 3 (+V as well as –V) though, it tin dismiss hold upwards seen that in that place is no feed dorsum to the command circuit. These outputs are referred to equally “semi-regulated”. If Channel 2’s charge were to growth for example, the output would drib slightly, but in that place would non hold upwards whatever automatic correction. That voltage drib is specified past times the charge rule specification, typically 3 to 5%.

With honor to minimum loading, if in that place is piffling or no charge on Channel 1, the output volition nonetheless hold upwards at the fix voltage, but the switching converter pulse width volition hold upwards really narrow. The output voltage on Channels 2 & 3 drops fairly dramatically at those narrow pulse widths, peculiarly if the outputs are supplying their total rated load. An output voltage of 12V may drib to 8V.

Conversely, if the total rated charge is applied to Channel 1, but Channels 2 & 3 are non loaded, the voltages on 2 & 3 volition rise, as well as a 12V output could deliver over 14V.

The termination that varying loads on Channel 1 has on the “semi-regulated,” Channels 2 & 3, is many times referred to equally the “cross regulation” specification.

Manufacturers of ability supplies specify a minimum charge requirement on Channel 1, usually 10%, to warn the user. Minimum loads may besides hold upwards specified on Channels 2 & 3 to promote a improve rule specification.

Operating without a minimum charge volition non unremarkably crusade a ability render to fail, but tin dismiss stress the user’s equipment.

Some products similar TDK-Lambda’s MTW serial employ ii converters to improve ability render regulation, i to render Channel 1 & i to render Channels 2 & 3. Note that both V2 as well as V3 are sensed past times the command circuit.

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

Here is a link for to a greater extent than details most the MTW series: http://www.us.tdk-lambda.com/lp/products/mtw-series.htm

Other products similar the NV175 serial employ post regulators on each output, completely eliminating the minimum charge requirement. Although this does add together terms to the ability supply, it removes whatever trace of piece of job concern for the user as well as helps amongst organization flexibility. Here is a link to the datasheet for NV175 series: http://www.us.tdk-lambda.com/lp/ftp/Specs/nv175.pdf