What is inrush current?


Category: Noise suppression filter Room

  When electrical equipment is first turned on,a large current flows that exceeds the steady-state current value. This current is called an inrush current.
  Why does this inrush current occur? There are many factors that cause it, and the following are some examples: 

  - In equipment with large-capacity smooth capacitors or decoupling capacitors, 
    when the power is first turned on, a large current flows through to charge those 
    capacitors - a necessity when first powering up the equipment.
  - Immediately after the power is turned on, the filament and other parts have low 
    resistance, and a large current flows.
    (As they begin to generate heat and warm up, their resistance increases and the
     current drops to the steady-state current.)

  To provide a more easy-to-understand image of inrush current, Figure 1 shows the current waveform when the power is turned on. When the power is turned on, current begins to flow, and the initial current flow reaches the peak current value that is larger than the steady-state current value. Following this, the current value gradually decreases until it stabilizes at the steady-state current. The part during which a large current flows before reaching the steady-state current is the inrush current. If the size of the inrush current exceeds that allowed by the part in use, depending on the magnitude of the inrush current (difference between the peak current value and the steady-state current value) and length of its duration (the length of time until the peak current value converges with the steady-state current value, hereafter called the pulse width), the part used in the circuit may overheat, potentially causing the electrical device to malfunction or break down.
Figure 1. Current waveform when the device is powered up

  Next, I will give an example of common malfunctions in parts used for noise suppression in a power supply line. I will describe the case for chip ferrite beads (Murata's BLM series), which are relatively lower cost and user-friendly.
  If the inrush current exceeds the rated current value in chip ferrite beads that are used for noise suppression, they will overheat. In the worst-case scenario, an open circuit fault will occur. Figure 2 shows different waveforms obtained when changing the conditions for peak current value and pulse width of the inrush current waveform. Waveform (1) has a large peak current, waveform (2) has a large pulse width, and waveform (3) shows a case where malfunction does not occur.
In waveform (1), an excessive current flows instantaneously through the chip ferrite bead and the internal electrode melts, causing an open circuit fault. When the electrode melts, the chip itself may crack and break. In waveform (2), the chip ferrite bead continues to heat up, eventually causing the internal electrode to melt, similar to the case with waveform (1). The chip ferrite bead then becomes a source of heat and may cause the circuit board it is mounted on to burn out.

Figure 2. Inrush current waveforms

  As you can see, inrush current flowing through chip ferrite beads used for noise suppression can cause malfunctions. Factors such as peak current and pulse width contribute to inrush current, and it is important to consider rated current when selecting chip ferrite beads to be safely used in products. If you are concerned about noise suppression or inrush current in a power supply line, please contact Murata.

Person in charge: O.H., Component Business Unit Murata Manufacturing Co., Ltd.