Surge impact immunity experiment (surge) -emc series Hardware Design Notes 6

Surge immunity in electronic equipment EMC It is a required test item during the test , Compared with other immunity experiments , Surge is characterized by high energy , The damage to the equipment is irreversible , The damage to electronic components is devastating . In circuit design , Surge protection has to be considered .

according to GB17626.5 Specified in the , For ports connected to symmetrical communication lines （ Such as RS485 RS422 etc. ）, You should use 10/700u Combined wave generator , In other cases , Especially the port connected to the power line and short-distance signal interconnection , You should use 1.2/50us Combined wave generator .

The above figure shows the output current and voltage waveforms of two combined generators .

Three commonly used surge protective devices are ： Varistor , Gas discharge tube , Transient suppression diode （TVS）.

1： Varistor

advantage ： The price is low , response time ns level , The maximum capacity is large .

shortcoming ： High parasitic capacitance , Clamping voltage is high .

Parasitic capacitance will cause great loss and leakage to AC circuit , It will also affect the service life of the varistor .

2： Gas discharge tube

advantage ： The maximum capacity is large , Small parasitic capacitance , As low as a few PF.

shortcoming ： The response time is 100 ns, Maintain low voltage after conduction .

3：TVS

advantage ： Fast response time PS level , Clamping voltage low

shortcoming ： Small maximum capacity .

Typical application circuits ：

1： Varistor and TVS Parallel use

Fast response time , The maximum capacity is large . The response time depends on TVS response time .

2： The varistor is connected in series with the gas discharge tube

The capacitance of the series branch is small , Under normal operation, the leakage current is small , Effectively improve the service life of varistors , The conduction voltage depends on the voltage of the gas discharge tube , The threshold value of the varistor can be appropriately reduced , The branch clamping voltage will decrease , The off voltage is determined by the varistor , Pressure sensitive current limiting , The gas discharge tube is turned off , Therefore, when opening, the gas discharge tube is opened first , The varistor is conducting , The response time is the sum of the two , Pressure sensitive current limiting during shutdown , The gas discharge tube is turned off .