Posted on: July 1, 2014
The peak current is the maximum amount of current which an output is capable of sourcing for brief periods of time.
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 stabilizes.
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 do 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 energize 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 start decreasing gradually to the steady state current.
The inrush current at the power supply input and 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.
The steady state, inrush and peak current when a device is turned on – Image Credit
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 percent of the supply’s maximum current without necessarily entering the current-limit state. The power supplies, which provide the high current for a short period, are usually used for loads such as the computer hard drives, electric motors, pumps, actuators, fan, etc.
A person designing a power supply must understand 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.