Speaking of 'current', I believe that everyone will not be unfamiliar. But what is the surge current? What is the relationship between it and current? Let's take a look at it below.

The inrush current refers to the peak current flowing into the power supply device caused by the high voltage in the short-term like 'wave' when the power is turned on or off. When some large-capacity electrical equipment is turned on or off, a surge voltage is generated in the power grid due to the presence of an inductance in the power grid, thereby generating a surge current.

Typical surge current waveform

The picture above is a typical Inrush current waveform. It has two spikes. The first surge tip current peak is generated when the input voltage source is activated. This peak current flows into the EMI filter capacitor and the input capacitor of the DC-DC converter and is charged to a steady state value. The second is that the current peak is generated when the DC-DC converter is started. This peak current flows through the DC-DC power transformer to the output capacitor and all load capacitors and is charged to a steady state value.

Since the output filter capacitor is quickly charged, the peak current is much larger than the steady state output current. Generally, regardless of the size of the equipment, there will be surge currents. Small-capacity equipment generates less surge current and does not cause major hazards, so it is often overlooked. However, large-capacity electrical equipment generates a large current, that is, a surge current. The surge current is several times or even several times larger than the normal current of the system, which may cause the voltage of the AC line to fall, thereby affecting all connected to the same AC line. The normal operation of the device sometimes blows off components such as fuses and rectifier diodes.

In order to avoid the impact of surge current on our daily life, in order to protect the normal use of electrical equipment. Therefore, the power supply usually limits the surge level that the rectifier bridge, fuse, AC switch, and EMI filter components can withstand, so that the switching voltage is repeated, and the AC input voltage should not damage the power supply or cause the fuse to blow.

How to measure the inrush current? At present, there are actually some mature test methods. For example, an inrush current can be measured using an R&S oscilloscope with a current probe. In addition, Chroma's AC power supply's own measurement function can be used to test the inrush current. In addition, power meters or power analyzers typically have a test function for inrush current.

In the previous section, we discussed the issues of 'What is surge current? How is surge current generated? How to measure surge current?' After learning from the previous chapter, we all know that the inrush current is generated when the power is turned on, due to the fast charging of the input filter capacitor. The surge current is much larger than the steady-state input current, so the surge current is bound to cause some harm to us. What are the hazards of surge current? How can we suppress the inrush current? Let us take a look at it below.

The main hazards of inrush current are: power input fuse blown, mechanical switch or circuit breaker stuck failure, sparking when plugged into the socket, rectifier bridge connected to the front end of the power supply is easily broken down, and the waveform of the power grid drops instantaneously. These phenomena will cause a lot of trouble for power supply manufacturers and users, especially in places where reliability and safety are required. In order to avoid such phenomena, some methods must be taken to suppress the inrush current.

Method of suppressing inrush current:

1. Series temperature coefficient thermistor (NTC). When the temperature coefficient is applied, the inrush current is limited. When the power is turned on, the NTC flows through the current to cause the resistance value to decrease. This method is relatively simple, but due to the limitations of self-heating and poor thermal start-up characteristics, this method is generally only applicable to applications below 500W and where the use of the power supply is not high.

Using NTC to suppress power surge current

2. Electromagnetic relay type, after the power supply is powered on, the contact of the electromagnetic relay will short-circuit the current limiting resistor after a certain delay. The shortcomings are large volume, large noise during suction, short relay contact life and poor shock and vibration resistance.

3,.solid state relay type, with thyristor instead of mechanical contacts, the disadvantage is that the peripheral circuit is more cumbersome, the bidirectional controllable leakage current is large, so it can not effectively control the inrush current.

                                             Mechanical contact short circuit                         Short circuit of electronic contacts

Surge current protection technology:

1. Transient diode (select the appropriate protection voltage, one-way or two-way, sink current), which is characterized by fast response and large current absorption. Usually connected between lines or lines and earth.

2. The self-recovering fuse is connected in series in front of the transient diode. When the transient diode absorbs current, the current through the self-recovery fuse increases the cut-off path. The characteristic is that the reaction is slow, but the current can be prevented from continuing to increase the application process. Should choose the appropriate voltage and current parameters

3, air discharge tube, used for high-voltage breakdown air discharge such as lightning, placed at the forefront, connected between the protection line and the earth or between the lines.

4, varistor, overvoltage protection line and the bottom line, the actual application is equivalent to a surge protector, in order to achieve the purpose of suppressing inrush current.

Resisting inrush current with resistor

Through the above study, we know that the generation of inrush current will cause various harms to us. In order to ensure the safety of our electrical appliances, we should learn to master the suppression method of inrush current and try to minimize this harm.