Driving Triacs with Phototriacs
Why Phototriacs are Replacing Electromechanical Relays in Modern Applications.
Modern applications use complex controls to enhance safety, implement convenient features, and save energy. Control units use switches to control the sensors and actuators in a system. Since most applications are powered from the AC mains network, several AC voltage loads have to be controlled (For example, heaters, lamps, motors, fans or valves). Switching used to be realized with electromechanical relays but have been recently replaced with triacs because of their smaller size, longer lifetime, better switching speed, and lower power consumption.
If galvanic isolation of control and load circuit should be realized with triacs, optically isolated triac drivers can be used. These phototriac devices consist of an LED and a triac device detector chip. If current flows through the LED, it emits infrared light which is detected by the detector chip. The detector chip triggers the small triac device making, the driver’s output conductible.
Output Triggered 2 Ways: Zero-Crossing & Non Zero-Crossing
The output is triggered in one of two ways: zero-crossing and non zero-crossing.
|Zero-Crossing:||When the input signal is activated, the internal zero-crossing detector circuit triggers the output as the AC load voltage crosses zero. To be more precise, the internal zero-crossing detector circuit monitors the output voltage and allows turn on only if its value is below a certain level, which is close to zero. Since the output is only activated at low load voltages, zero-crossing limits high inrush currents, consequently minimizing EMC effects and stress for the electrical load and the SSR.|
|Non Zero-Crossing:||When the input signal is activated, the output immediately turns on, since there is no zero-crossing detector circuit. As the output turns on immediately, a phase angle control circuit can be realized by controlling the effective voltage for a load.|
In both operation modes, the output can be used to drive a larger triac’s gate as shown in Figure 1 (Simple Triac Driving Circuit). Since the output of the phototriac introduces a gate current to the main triac, it will proceed to on state and carry the load current. As soon as the main triac is triggered, the voltage across the driver drops and therefore load current of the driver drops. Even though a typical forward current IFT of 10 mA is still applied to the input side, the triac driver will proceed to the off state when the load current drops below the holding current IH (typically 0.3 mA). This will happen every half cycle of the load voltage as long as a forward current IFT is applied. As long as the output of the phototriac coupler APT1221 is conductible, it can carry a continuous current of up to 100 mA with a voltage drop of VTM = 2.5 V across its output.
Figure 1 shows a simple circuit for driving a triac with a phototriac, e.g. APT1221. The maximum surge current through the phototriac is determined by the maximum load voltage and the value of the resistor R1. If the maximum surge current of the phototriac IFP is 1 A and we assume a 230 VAC line, the value of R1 can be determined as follows:
Since the main triac requires a gate current and voltage, a certain load voltage value results, which is necessary to trigger the triac. If the main triac’s electrical characteristics are IGT = 50 mA and VGT = 1.5 V, then:
But the phototriac may also be triggered to on state accidentally. This can happen by exceeding the maximum blocking voltage VDRM of 600 V or by applying very steep rising signals to the output. Such transient signals or noise may exceed the dV/dt rating of the triac driver and hence cause the device to proceed into on state. The dV/dt ratings of the triac and its driver are very important when switching inductive loads since load voltage and current are not necessary in phase. Since a triac turns off when the load current is zero, load voltage is not necessarily zero. Due to this fact, the triac may produce a sudden rise in load voltage to its own output, which may exceed its dV/dt rating. In order to increase voltage rise time, a snubber circuit can be used. In most cases, one snubber circuit will protect the main triac and the phototriac.
Designing a Snubber Circuit for Non Zero-Crossing Phototriac
We will take a look at designing a snubber circuit for a non zero-crossing phototriac (e.g. APT1221), which also protects the main triac in most cases.
Figure 2: Triac Driving Circuit with RC Snubber Network
When designing the RC snubber network for triac drivers, detailed knowledge about the load is necessary. By knowing the power factor PF, one can easily calculate the maximum turn off voltage that may appear across the output:
Assuming Vtmax = 200 V we will choose R1 to limit the current peak at maximum voltage
Since the peak current is limited by the resistor R1, the time constant for limiting dV/dt has to be set with R2 and C:
In the next step, the value of R2 is set by determining the smallest trigger voltage requirement. Assuming a triac gate trigger current IGT = 50 mA and a required load voltage of 30 V:
The snubber circuit in this example is designed to meet the dV/dt rating of the phototriac. If the dV/dt rating of the main triac is different, the worst case value has to be chosen for designing the snubber network. As can be seen above, there is no easy method for selecting the parts and their values for a snubber network. In particular detailed knowledge about the load circuit and the power factor is required. These facts make snubber design empirical and result in detailed measurements to verify the parameters calculated. If the user wants to save work when designing the circuit or have fewer parts and more space on his PCB board, he can choose an SSR (solid state relay). Besides the phototriac and a main triac, these relays may have an input protection circuit, integrated snubber circuits, or a varistor inside. The customer can choose among various alternatives based on his needs, e.g. space, number of parts, costs, input / output conditions.
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Select the Proper Solid State Relay for Your Application
Panasonic offers various products to provide the customer the freedom to choose the optimum part for their application.
Figure 3: Phototriac Models - Panasonic Industrial Devices
Figure 3 Gloassary: ZC: Zero-cross type | LZC: Low zero-cross type | NZC: Non zero-cross type | SOPx: Small outline package, x pins | DIPx: Dual In-line package, x pins | THT: Through hole technology | SMT: Surface mount technology
To learn more about Panasonic's Phototriac Solid State Relays, Click Here.