I. Advantages of Solid State Relays
(1) High lifespan and high reliability: Solid-state relays have no mechanical parts and the contact function is completed by solid devices. Since there are no moving parts, they can work in environments with high impact and vibration. Due to the inherent characteristics of the components that make up solid-state relays, solid-state relays have a long lifespan and high reliability.
(2) High sensitivity, low control power and good electromagnetic compatibility: Solid-state relays have a wide input voltage range, low drive power, and are compatible with most logic integrated circuits without the need for buffers or drivers.
(3) Fast switching: Solid-state relays use solid-state devices, so the switching speed can range from a few milliseconds to a few microseconds.
(4) Low electromagnetic interference: Solid-state relays have no input "coil", no contact arcing or back-jumping, thus reducing electromagnetic interference. Most AC output solid-state relays are zero-voltage switches, turning on at zero voltage and turning off at zero current, reducing sudden interruptions in the current waveform and thus reducing switching transient effects.
II. Disadvantages of Solid State Relays
(1) The voltage drop of the tube after conduction is large. The forward voltage drop of a thyristor or bidirectional thyristor can reach 1~2V. The saturation voltage drop of a high-power transistor is also between 1~2V. The on-resistance of a power MOSFET is also larger than the contact resistance of a mechanical contact.
(2) Semiconductor devices can still have leakage current of several microamps to several milliamps after being turned off, so ideal electrical isolation cannot be achieved.
(3) Due to the large voltage drop of the tube, the power consumption and heat generation after conduction are also large. The volume of high-power solid-state relays is much larger than that of electromagnetic relays of the same capacity, and the cost is also higher.
(4) The temperature characteristics of electronic components and the anti-interference ability of electronic circuits are poor, and the radiation resistance is also poor. If effective measures are not taken, the reliability of operation will be low.
(5) Solid-state relays are highly sensitive to overload and must be protected against overload using fast-acting fuses or RC damping circuits. The load of a solid-state relay is significantly related to the ambient temperature; as the temperature rises, its load capacity will decrease rapidly.
(6) The main drawbacks are the presence of on-state voltage drop (requiring corresponding heat dissipation measures), off-state leakage current, incompatibility between AC and DC, a small number of contact groups, and poor performance in terms of overcurrent, overvoltage, voltage rise rate, and current rise rate.
(7) The main disadvantages are on-state voltage drop, heat generation, heat dissipation countermeasures, off-state leakage current, incompatibility between AC and DC, and differences in indicators such as overcurrent, overvoltage, voltage rise rate, and current rise rate. Naturally, we also need to consider some issues. In actual working environments, products may be damaged due to overheating, requiring heat dissipation.
The main problems with solid-state relays also include:
When a solid-state relay is open-circuited and has voltage at the load terminal, there will be a certain leakage current at the output terminal. Care should be taken to prevent electric shock during use or design. When replacing a failed solid-state relay, try to select a product of the original model or with identical technical parameters to ensure compatibility with the original application circuit and guarantee reliable system operation.
1. Overheating
When an SSR is turned on, the component will dissipate power P = V (voltage drop) × I (load), where the effective values of V and I are the effective values of the saturation voltage drop and operating current, respectively. The load capacity of a solid-state relay is significantly affected by ambient temperature and its own temperature rise. Therefore, it is necessary to select the appropriate heatsink size or reduce the operating current based on the actual operating environment conditions and strictly refer to the allowable casing temperature rise (75℃) at the rated operating current. During installation and use, good heat dissipation conditions must be ensured; otherwise, overheating may cause malfunction or even product damage.
Generally, for currents below 10A, a well-ventilated instrument base plate is sufficient. Products with a rated operating current above 10A should be equipped with a heat sink. For currents below 30A, natural air cooling is sufficient. When the continuous load current exceeds 30A, forced air cooling with an instrument fan is required. Products above 100A should be equipped with a heat sink and forced cooling with a fan. During installation, ensure good contact between the relay base and the heat sink, and apply an appropriate amount of thermal grease to achieve optimal heat dissipation. If the relay operates at high temperatures (40℃~80℃) for extended periods, users can consider derating it based on the manufacturer's maximum output current versus ambient temperature curve to ensure normal operation.
Reasons for heat generation in solid-state relays:
Solid-state relays, during normal operation, experience power loss within their internal chip. This power loss is primarily determined by the product of the output voltage drop and the load current, and is dissipated as heat. Therefore, the effectiveness of heat dissipation directly impacts the reliability of solid-state relays; excellent thermal design can prevent failures and damage caused by poor heat dissipation.
3. Overcurrent and overvoltage
When using relays, overcurrent and load short circuits can permanently damage the internal output thyristor of the SSR solid-state relay. Consider adding fast-acting fuses and air switches to the control circuit for protection (when selecting a relay, choose one with output protection, a built-in varistor absorption circuit, and an RC buffer to absorb surge voltage and improve dv/dt withstand capacity). Fast-acting fuses and air switches are common overcurrent protection methods. Fast-acting fuses can be selected at 1.2 times the rated operating current; for small capacity applications, fuses are generally sufficient. Pay special attention to load short circuits, as this is a major cause of damage to SSR products.
For inductive and capacitive loads, in addition to internal RC circuit protection, it is recommended to use a varistor connected in parallel at the output terminal as a combined protection. The area of the metal oxide varistor (MOV) determines the power absorbed, and the thickness determines the protection voltage value. For AC 220V SSRs, select a MYH12-430V varistor; for 380V, select a MYH12-750V varistor; for larger capacity motors and transformers, select a MYH20 or MYH2024 varistor with a large current capacity. The selection principle is to use a 500V-600V varistor for 220V and an 800V-900V varistor for 380V.
