Minimum Load and Cross Regulation of Multiple Output Power Supplies

The data sheet for low-cost, low-power, multiple-output power supplies often indicates that a minimum load must be applied to one or more of the outputs in order to maintain regulation.

To explain why, here is a simple diagram of how a triple output power supply works.

Input——input; Output——output; Line filter——input filter circuit; Rectifier——rectifier circuit; Switching——switching circuit; Control——control circuit; Detect——output detection

To the right of the center of the figure are the three output windings of the transformer.

On output 1 (+5V), the transformer output is rectified and filtered to provide a stable DC output. If the output voltage is not at the set voltage level, for example due to load changes, the power supply will automatically correct itself. This is achieved by first detecting the output voltage, comparing it with the internal reference voltage, and then feeding the signal back to the control circuit through the optocoupler. The control circuit will adjust the pulse width of the converter accordingly. The regulation rate of this output is typically 1% to 2%.

However, on outputs 2 and 3 (+V and -V), it can be seen that there is no feedback to the control circuit. These outputs are called "semi-regulated" outputs. For example, if the load on output 2 increases, the output will drop slightly, but no automatic correction is provided. This voltage drop is expressed in the data sheet as load regulation, which is typically 3% to 5%.

Regarding the minimum load, if the load on output 1 is small or completely unloaded, the output will still remain at the set voltage, but the switching converter pulse width will become extremely small. The output voltage on outputs 2 and 3 will drop significantly due to the extremely narrow pulse width, especially when the output load is fully loaded, the output voltage of 12V may drop to 8V.

Conversely, if a full load is applied to output 1, but no load is applied to outputs 2 and 3, the voltage on 2 and 3 will rise so that the 12V output may exceed 14V.

The effect of a load change on output 1 on the "half-regulated" outputs 2 and 3 is referred to in many contexts as the "cross regulation" characteristic.

Manufacturers of power supply equipment specify a minimum load requirement on output 1 to alert the customer, usually 10%. Minimum loads may also be specified on outputs 2 and 3 to further achieve better regulation specifications.

Operation without specifying a minimum load will not normally result in a power failure, but will cause stress on the user's equipment.

Some products, such as TDK-Lambda's MTW series, use two regulation circuits to improve the power supply regulation specifications, one for powering output 1 and the other for powering outputs 2 and 3. Note that both V2 and V3 are sensed by the control circuit.

AC input——AC input; Inrush current prevention/input filter circuit——inrush current suppression/input filter circuit; Input rectification circuit——input rectification circuit; Current detection——current detection; MOSFET switch——MOSFET switch; Control circuit——control circuit; Auxiliary power supply——auxiliary power supply; Output current smoothing circuit——output current smoothing circuit; Over voltage protection circuit——overvoltage protection circuit; GND (G1)——ground (G1); V1: Output——V1: output

Some products, such as the NV175 series, have voltage regulation circuitry on each output, completely eliminating the minimum load requirement. Although this will increase the cost of the power supply, it will relieve the user of any concerns and help achieve system flexibility.