The peak current is the maximum amount of current which output is capable of sourcing for brief periods.

When a power supply or an electrical device is first turned on, high initial current flows into the load, starting at zero and rising until it reaches a peak value, known as the peak current. This is usually much higher compared to the steady-state current. The current will then decrease gradually from the peak current value to the steady state, where it stabilises. (Our products will state this amount on the data sheet that you will be able to download on our products pages or by request.)

The difference between the steady-state value and the peak current is what is referred to as the inrush current. This is a property that designers must pay attention to when selecting the components. The peak current should be within safe limits and only last for a short duration to avoid stressing the components or causing excessive heating or damage.

Why does the inrush current flow?

The main reason for the inrush current is to charge the large decoupling or smoothing capacitors in power supplies. Other causes include the current to energise inductors or could in the hard disk and fans, as well as the low resistance of components such as lamp filaments and others when cold.

During this time, as the capacitors charge or devices come out of the cold state, the current increases very fast from zero, rising all the way to the peak current and then starts decreasing gradually to the steady-state current.

The inrush current at the power supply input refers to the large initial current drawn from the mains. However, it also applies to the amount of current that some loads, such as hard disks, fans, and other devices, draw from the power supply. In either case, the power supply must be able to support the peak currents, whether at the input or at the output.

At the output, the power supply must supply the peak current without the output’s overcurrent protection circuit shutting down. In addition, the power supply circuit must have the ability to discriminate between the peak current and short circuit or overcurrent conditions.

Figure 1: The steady state, inrush and peak current when a device is turned on

Peak Current Power Supplies

There are some power supplies that are specifically designed to provide high peak currents. These will provide currents of between 200% and 300% of the supply’s maximum current without necessarily entering the current-limit state. The power supplies, which provide high current for a short period, are usually used for loads such as computer hard drives, electric motors, pumps, actuators, fans, etc.

When designing a power supply here at Sunpower, we understand and take into consideration the intended load requirements. Some devices, such as disk drives, pumps, motors, printer heads and others, will always draw high peak currents when first turned on. If you need support with your power supply for your project, feel free to get in touch with our specialists.