Research on Minimum Load and Cross Regulation of Multi-Output Power Supply

One question we often get asked about on our technical support team is the minimum load on multiple output power supplies , so I thought it would be a good topic to document. The datasheet for low cost, low power multiple output power supplies often states that a minimum load must be applied to one or more of the outputs in order to maintain regulation . To explain why this is the case, here is a simple diagram of how a three output power supply works.

In the middle right of the diagram are the three output windings of the transformer. At output 1 (+5V), the output of the transformer 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. This function 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 usually 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 be greatly reduced due to the extremely narrow pulse width, especially when the output load is fully loaded, the 12V output voltage may drop to 8V. On the contrary, 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. Power supply manufacturers specify a minimum load requirement on output 1 to alert the customer, usually 10%. A minimum load may also be specified on outputs 2 and 3 to further achieve a better regulation specification. Operating without specifying a minimum load will not usually result in a power supply failure, but will stress the user's equipment.

Some products, such as TDK-Lambda's MTW series, use two regulation circuits to improve the regulation specification of the power supply , one for output 1 and the other for outputs 2 and 3. Note that both V2 and V3 are detected by the control circuit. Some products, such as the NV175 series, have voltage regulation circuits on each output, completely eliminating the minimum load requirement. Although this will increase the cost of the power supply , it can relieve the user of any concerns and help achieve system flexibility.