I. Surge Definition and Classification
Surge (electrical surge), simply put, is a peak value that appears momentarily and exceeds the stable value. It is divided into surge voltage and surge current.
II. Inrush Current
Inrush current refers to the peak current or overload current that is much larger than the steady-state current and occurs at the moment when the power is turned on or in abnormal circuit conditions.
In the process of electronic design, it is often encountered. At the moment when our power supply is started, a very high peak current can be clearly seen with an oscilloscope. It is about more than twice the normal value and can be as high as about 30A. This is because when power is applied, the transformer, capacitor, and inductor in the power supply need to be charged and require a very high current, resulting in an instantaneous peak current.
III. Hazards of Inrush Current
The impacts of surges on sensitive electronic and electrical equipment are of the following types:
Destruction
Voltage breakdown of semiconductor devices.
Destruction of the metallized surface layer of components.
Destruction of printed circuit board printed lines or contact points.
Destruction of triac elements/thyristors.
Interference
Lock-up, loss of control of thyristors or triacs.
Partial destruction of data files.
Errors in data processing programs.
Errors and failures in receiving and transmitting data.
Premature aging
IV. How to Suppress Inrush Current
Since it has so many hazards, we need to suppress this “showy element” in the process of electronic design and control it within a reasonable range.
Series resistance
Connect a power resistor in series to keep the current (Vin/R) within a controllable range. However, it has advantages and disadvantages. As perceptive friends may have noticed, this design is simple and has a low cost, but it is only suitable for low-current applications, approximately <1A.
Add an RC circuit to slow down the switch startup speed
By controlling the startup speed of the switch, the magnitude of dv/dt can be reduced, thereby reducing the magnitude of inrush current. However, since the switch startup time is nonlinear, the inrush current is also nonlinear.
Select a switch with soft start function
To obtain a linear dV/dt, a switch with a soft start function can be used. This type of switch can dynamically adjust the rise time of the output waveform by changing the capacitance value. Taking TI's TPS22975 load switch chip as an example, the CT pin is externally connected to a Cr capacitor, and the output waveform rise time is different according to different capacitance values.
Since dv/dt is a constant, then the inrush current is also a constant. The magnitude of the inrush current can be dynamically controlled, as shown by the green waveform in the following figure.
