Introduction Hybrid relays are power switches consisting of static relays (also known as solid relays, electronic relays or semiconductor relays) and electromechanical relays. They combine the low voltage drop of electromechanical relays with the high reliability of solid state relays. The control switch of the home appliance motor starter or the household electric heater is a common application field of the relay. In view of the compliance with RoHS regulations, the reliability of electromechanical relay power switches may be reduced, and the market for hybrid relays is becoming more and more concerned.
However, proper control of hybrid relays is far less easy than it seems at first glance. For example, switching between electromechanical relays and solid state relays can produce spikes that radiate electromagnetic noise. This article provides several easy-to-implement control circuit design tips for reducing the peak voltage of hybrid relays.
1. Integrating solid technology and electromechanical technology When selecting an AC switch, designers are well versed in the advantages and disadvantages of electromechanical switches and solid state switches. The semiconductor switch, that is, the solid state switch, has a fast response speed, no voltage rebound when energized, no spark when power is off, does not radiate electromagnetic interference (EMI), and does not shorten the life of the relay product. The main advantage of the electromechanical switch is its low conduction loss, which eliminates the need for a heat sink for applications above 2 A RMS. The electrical isolation between the drive coil and the power supply terminals also saves the drive-controlled silicon (SCR) rectifier. Or optocoupler of a triac.
The third power switch solution is to connect a solid state relay and an electromechanical relay in parallel, and combine the two relay technologies to design a hybrid relay (HR). Figure 1 shows the hybrid relay topology used by the motor starter. The three-phase motor starter in the figure uses only two hybrid relays. If both relays are turned off, the motor will remain off as long as the load is not connected to the neutral.
If the load is connected to the neutral, a hybrid relay switch can also be connected in series on the L1 line.
Figure 1: Left picture) Motor starter based on hybrid relay; right picture) Relay/Bidirectional thyristor control sequence
Figure 1 also shows the control sequence of the hybrid relay:
- Conduction sequence:
- First, the triac is turned on (if the current is larger, two thyristors connected in parallel with reverse polarity should be used), which allows the load to be zero-voltage-conducting;
- The relay is then turned on after one or several AC cycles. The turn-on voltage of the relay is extremely low (usually between 1-2V, which is the voltage drop of the triac);
- Finally, the bidirectional thyristor gate current should be removed after the relay coil is powered up for at least 1 to 2 cycles, providing sufficient time for the relay to start operating before the triac is turned off. Therefore, the load current flows only through the electromechanical relay during steady state.
- Shutdown order:
- First, the triac is turned on. When the relay is in the on state, the load current is mainly sent to the motor through the electromechanical relay. .
- Then, the relay turns off after a few milliseconds. The shutdown voltage of the relay is extremely low, similar to the relay conduction operation. Therefore, the spark period is shortened.
- Finally, the bidirectional thyristor gate current should be removed after the relay coil is powered down for at least 1 to 2 cycles, the triac is turned off, and the hybrid relay is turned off at zero current.
The design method of shutting down the electromechanical relay at near zero pressure can extend the life of the relay by a factor of 10. If the switching operation is DC current or voltage, the life of the relay is not only 10 times longer, and may be higher.
Most importantly, because the RoHS industry regulations (2002/95/EC) will ban cadmium in July 2016, silver cadmium oxide used in contact rust and contact soldering processes may be Ag-ZnO or Ag- SnO2 replacement, in which case the contact life will be shortened unless larger contacts are used.
Zero-voltage conduction also permits the use of capacitive loads to reduce inrush current, such as electronic ballasts and fluorescent tubes with built-in compensation capacitors or inverters. Zero-voltage conduction also helps to extend the life cycle of the capacitor and avoid AC voltage fluctuations.
In addition, the solid state relay allows the motor to implement progressive software startup or start-stop. Smooth acceleration or deceleration will reduce the wear of the mechanical system and avoid damage to equipment such as electric pumps, fans, power tools, and air compressors. For example, the water hammer phenomenon in the transport pipeline will disappear and the cargo conveyor belt will prevent the V-belt from slipping and shaking.
Hybrid relays are common in 4-15 kW motor applications, but can also be used in motor applications up to 250 kW.
Hybrid relays are also used in heating products such as electric heaters. The setting of heating power or room temperature/water temperature is usually done by a burst controller. The essence of the pulse train or cycle skip control principle is to keep the load in the on state of N cycles and the off state of K cycles. The "N/K" ratio is responsible for defining the heating power, similar to the duty cycle in pulse modulation control technology. . The control frequency here is less than 25-30 Hz, but this speed is sufficient compared to the time constant of the heating system.
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