One of the questions our tech support team is often asked is about the minimum load for multiple output power supplies, so I thought this would be a good topic to document.
For low-cost, low-power, multiple-output power supplies, the data sheets often show that in order to maintain regulation, a minimum load must be applied to one or more of the outputs.
To explain why, a simple schematic diagram of the operation of a three-output power supply is provided below.
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 middle of the figure are the three output windings of the transformer.
On output 1 (+5V), the output of the transformer is rectified and filtered to provide a regulated DC output. If the output voltage is not at the set voltage level, such as due to load changes, the power supply will automatically correct. This is accomplished by first detecting the output voltage, comparing it with an internal reference voltage, and then feeding the signal back to the control circuit through an optocoupler. The control circuit will adjust the pulse width of the converter accordingly. The regulation 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 “half-regulated” outputs. For example, if the load on output 2 increases, the output will decrease slightly, but no automatic correction is provided. This voltage drop is expressed in the datasheet as load regulation, 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 is greatly reduced due to the extremely narrow pulse width, especially when the output load is full, the output voltage of 12V can drop to 8V.
Conversely, if a full load is applied to output 1, but no load is applied to outputs 2 and 3, the voltages on 2 and 3 will rise so that a 12V output may exceed 14V.
The effect of a load change on output 1 on “half-regulated” outputs 2 and 3 is in many cases referred to as the “cross regulation” characteristic.
The manufacturer of the power supply specifies a minimum load requirement for output 1 to remind the customer, usually 10%. A minimum load can also be specified on outputs 2 and 3 to further achieve better regulation specifications.
Operating without specifying a minimum load will usually not cause a power 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 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)——Grounding (G1); V1: Output— —V1: output
Some products such as the NV175 series have voltage regulator circuits on each output, which completely eliminates the minimum load requirement. Although it increases the cost of the power supply, this relieves the user of any concerns and contributes to the flexibility of the system.
The Links: PM50CL1A120 LTM08C351S