Design of Proportional Control Bidirectional Thyristor Trigger Circuit

Zero voltage switch controller integrated circuits can be used to control resistive heater loads. This type of controller generates an output pulse when the mains passes zero to trigger the bidirectional thyristor connected in series with the heater, thereby minimizing RF interference and mains voltage transients. There are many varieties and models of temperature or power zero voltage switch controllers. This article only takes the TDA1023 produced by Philips as an example to illustrate the working principle and application of this type of IC.

1. Basic structure, pin functions and characteristics of TDA1023

TDA1023 adopts 16-pin DIL package, and the pin arrangement is shown in Figure 1.

The TDA1023 chip circuit is mainly composed of a power supply circuit, an input buffer, a timing circuit, a fail-safe circuit, a zero-crossing detector, and an output pulse amplifier, as shown in Figure 2.

The pin functions of TDA1023 are listed in the attached table.

1. The power supply voltage Vcc is directly obtained from the mains circuit, with a typical value of 13.7V, an average power supply current (IRX) of 10mA, and provides a stable voltage (Vz) of 8V for the external temperature sensing bridge circuit;

2. The trigger pulse width tw (typical value is 200μs), the trigger pulse train repetition time Tb (typical value is 41s under CT=68μF) and the proportional interval width can be adjusted, and the output current is not more than 150mA (the average current is not more than 30mA);

3. The hysteresis voltage and the corresponding hysteresis temperature can be adjusted.

2. Working Principle

1. Power supply and its operation

The power supply circuit and external component connection of TDA1023 are shown in Figure 3. 220V AC power is added between IC pin 16 and pin 13 through diode D1 and resistor RD, where IC pin 13 (VEE) is connected to the neutral line of the AC power. In the positive half cycle of the AC power, the current through RD charges the external smoothing capacitor CS until IC pin 16 obtains a stable voltage from the internal voltage regulator diode. The selection of RD should be able to provide current for the normal operation of the IC. In the negative half cycle of the AC power, the charge stored on the capacitor Cs is discharged to provide the circuit with operating current. The selection of Cs capacity must be able to maintain the minimum voltage value of Vcc (12V).

2. Temperature control

The average load power is varied by changing the duration of the trigger pulse train, which is determined by the potential difference between the control input C1 (pin ⑥) and the reference input UR (pin ⑦) or BR (pin ⑨). Optimized room temperature control for 5°C to 30°C can be achieved using an IC conversion circuit. When the QR (⑧) pin of the IC is connected to URC (pin ⑦), the BR (⑨) pin is used as the reference input, ensuring an accurate linear room temperature setting. If the conversion circuit is not required, UR (pin ⑦) is used as the reference input and BR (pin ⑨) must be connected to Vz (pin {11}). For proportional power control, UR (pin {7}) must be connected to TB (pin {12}) and BR (pin {9}) must be connected to Vz (pin {11}).

3. Proportional range control

If the proportional range control input PR (pin {5}) is open, a change of 80mV signal on C1 (pin {6}) can change the duty cycle from 0% to 100%, corresponding to a temperature difference of 1K. If pin {5} is grounded, the proportional range increases to 400mV (i.e. 5K). If a resistor is connected between pin {5} and ground, when it is 3.3kΩ, the proportional range is 160mV; if the resistor value is 430Ω, the proportional range is 320mV.

4. Hysteresis control

When the hysteresis control input HYS ({4} pin) is open, the device has a 20mV hysteresis inside, and the corresponding temperature control is 0.25K. If the {4} pin is grounded, the hysteresis increases to 320mV (the corresponding temperature increases to 4K). If a 1.8kΩ to 9.1kΩ resistor is connected between the {4} pin and ground, the hysteresis changes from 100mV to 40mV.

5. Trigger pulse width modulation

The output trigger pulse width tw of TDA1023 can be adjusted by the external synchronous resistor Rs at the {10} pin. When the mains voltage (Vs) is constant, tw increases with the increase of Rs and is inversely proportional to the input current at the {10} pin. Figure 4 shows the relationship curve of tw changing with the change of Rs and Vs.

6. Output current regulation

The output stage of TDA1023 is an open emitter output, which generates a positive trigger pulse. The output stage has maximum current limiting and short-circuit protection functions. When the output pulse is stable at 10V, the maximum output current is 150mA. In order to reduce the total power supply current and power dissipation, a resistor RG must be connected between the {3} pin of IC and the gate of the thyristor to limit the output current. Under the mains voltage of 220V, 50Hz, the relationship curve between the average output current I3 (AV) on the {3} pin (Q) of IC and RG and RS is shown in Figure 5.

3. Application Circuit Example

Figure 6 shows a 1.2kW~2kW heater control circuit. Vs is 220V (50Hz), and the temperature range is 5℃~30℃. The NTC thermistor RT1 connected between the {6} and {13} pins of TDA1023 has a resistance of 22kΩ at 25℃ and is used for temperature sensing. The bidirectional thyristor uses BT139, and at 25℃, Vft＜1.5V, Ift＞70mA.

The application circuit of TDA1023 is relatively simple, low cost and simple to design.