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Protection of solid state relays

qianjia | 29/07/2021

(1) Overcurrent protection

Solid state relays use semiconductor switching devices as power output components, and are more sensitive to temperature changes. Overcurrent will cause semiconductor chips to overheat and cause quality degradation, reduced lifespan and even permanent damage. Although solid state relays can withstand 10 times the rated current in an instant The above surge current, but exceeding this value, it is easy to cause permanent damage. Therefore, the protection of overcurrent is very important. There are many protection methods for overcurrent, and the key is to respond quickly. For the use of thyristor as the output device For AC solid-state relays, due to the zero-crossing shutdown characteristics of SCRs, the general protection circuit is invalid for surge currents and short-circuit currents that exceed the SSR surge current withstand value within 10Ms (50Hz), and should be considered Use fast fuse dedicated for semiconductor devices. The nominal fusing current of the fuse should not exceed the nominal current value of the SSR. There are many types of fast fuse on the market, but the quality difference is large, please pay attention when choosing.

(2) Overvoltage protection

When the load is inductive or capacitive, it is likely to produce transient voltage (blocking voltage) and voltage rise rate (dv/dt) greater than the solid state relay can withstand. If the protection measures are improper or the response is not sensitive, it will not only cause the solid state relay to lose control, but also may burn the solid state relay or equipment in serious cases. Therefore, overvoltage protection is necessary. The common application is to add transient suppression (RC absorption) circuit and voltage clamp circuit (bidirectional Zener diode, varistor). When designing products, most of the products have been added with RC absorption circuits or varistors, which can play a certain protective role. It is recommended that users carefully calculate and experiment the selection of RC circuits according to the relevant parameters of the load and environmental conditions during use. value. If it cannot be satisfied, an RC circuit and a pressure-sensitive protection circuit should be connected in parallel. Under normal circumstances, the RC absorption circuit can effectively suppress the transient voltage and the voltage exponential rise rate (dv/dt) added to the solid state relay, and the varistor protection circuit can absorb wide pulse overvoltage. The RC value selection of the RC absorption circuit must be calculated and verified. The empirical choice (for reference only) is that the resistance is between 27-150 ohms, the power is between 2-5 watts, the capacitance is between 0.01-1uf, and the withstand voltage is between 250-500VAC, (for example: R=51 ohms/2W , C=0.2uF/500VAC) The exact value should be determined after the experiment. Be careful not to oscillate. The choice of varistor must also be carefully calculated and verified. Under normal circumstances (for reference only), use the standard in the 220VAC circuit It is called 470-680V, ​​ø12-16, and the nominal 780-1000V, ø12-16 is used in the 380VAC circuit.

(3) Overheat protection

If the solid state relay is overheated, it will run out of control at the worst, and cause permanent damage. It is recommended to install overheating protection measures. The usual practice is to ensure that the temperature at the bottom of the solid state relay does not exceed 75-80°C. The general temperature protection circuit can achieve the purpose. The more economical thing is to install a temperature control switch on the radiator near the SSR base plate, and cut off the SSR input signal when the temperature rise reaches a limited temperature.

When the solid state relay is working, there is a certain power loss on its internal chip. This power loss is mainly determined by the product of the solid state relay's output voltage drop and the load current, and is consumed in the form of heat. Therefore, the quality of heat dissipation directly affects the reliability of the solid state relay, and the excellent thermal design can avoid failure and damage caused by poor heat dissipation.

Generally speaking, solid state relays with load current less than 5A can use air convection to dissipate heat. It is required to be installed in a good convection environment and the distance between two solid state relays is greater than the width of one solid state relay. Solid state relays with current greater than 10A should be equipped with radiators. For solid state relays with load current greater than 40A, air cooling or water cooling should be used if necessary according to the design volume.

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