Posted on: July 1, 2014

What is Minimum Load?

The minimum load is the minimum current or power that must be drawn from the power supply in order for the supply to meet its performance specifications. Less frequently, a minimum load is required to prevent the power from failing.

The minimum load is required to waken up the power supply. It refers to the amount of load required for a particular power supply to operate reliably, and it is usually a function of the output voltage. The amount of current is often specified by the manufacturer for each output voltage level provided by the supply. The value varies between manufacturers, form factors, and even between different designs from the same manufacturer.

A switching power supply requires some load to draw current from the supply for the supply to start functioning. If turned ON without a load, a high quality supply will not function. A low quality supply may powers up, however, it may end up providing unstable output or malfunctioning.

The better quality power supplies senses the no load situation and shuts down. However, the cheap models may not have this function and will remain in operation even at the no load condition, something that can damage the supply.

Some power supply designs such as the phase-shifted full-bridge may be unstable at light loads and even get damaged if operated at this state. When testing the output voltages for some single or multiple output supplies without the minimum load, the voltage levels may appear low or be unavailable.

Whereas some older computer supplies required the minimum load requirements to be met for the +5 and +12 voltage rails, the modern supplies have smaller load requirements on the +3.3 volts and the 5 Volts outputs. The lower loading requirements are easier for testing and troubleshooting purposes. In addition, the motherboard components provide enough loading for the supply.

Linear supplies are able to operate without a load or at very low loads; however the switching supplies need some current to flow for them to start operating. The minimum load is usually the highest resistance that draws the least current from the supply. When the minimum load conditions are not met, the power supply may experience some instability such as flickering and shutdowns.

The minimum load for resistive load can be calculated if the minimum current is known. By rearranging the Ohm’s law and making the resistance the subject:

R =V/I

R – Resistance in Ohms
I – Minimum load current
V – Output voltage

The current and voltage are supplied by the manufacturer.

To maintain the regulation, a minimum load has to be applied to one of more of the multiple outputs of the supply.
Manufacturers often specify minimum current which in essence will flow when the maximum load resistance is connected at the output. This is required to maintain reliability and the stability of the supply.

Typical minimum loads for switching supplies are 10% of the maximum load current and usually one ampere, or less. Some supplies have internal resistors to provide the minimum loading; however, they will only work when the loading is slightly above this internal load. Once the supply powers up, it transfers the load to the external one and disconnects the internal load so as to increase efficiency.