High-voltage-ac-switcher

Description: A basic circuit to trigger a silicon-controlled rectifier (SCR) is illustrated in Figure 67-1A. This circuit has the limitation that the blocking voltage of the photonic coupler output device dictates the circuit's blocking voltage, despite the SCR's higher voltage capability. By incorporating capacitor C1 into the circuit, as depicted in Figure 67-1B, the rate of voltage change (dV/dt) experienced by the photonic coupler output device is reduced. The energy stored in C1, when discharged into the gate of SCR1, enhances the dV/dt capability of the main SCR.

Using a separate power supply for the coupler enhances the flexibility of the trigger circuit, eliminating the constraints imposed by the blocking voltage capability of the photonic coupler output device. However, this flexibility may increase costs, and multiple power supplies might be required for several SCRs if common reference points are not available. In Figure 67-1C, resistor R1 can be connected to Point A, which will remove the voltage from the coupler after SCR1 is triggered, or to Point B, ensuring that the coupler output is always biased by the input voltage. The former connection is preferred as it minimizes power dissipation in R1.

A more practical SCR triggering configuration is shown in Figure 67-1F, where trigger energy is sourced from the anode supply and stored in C1. The coupler voltage is regulated by a zener diode. This design allows for switching of higher voltages than the blocking voltage capability of the photonic coupler output device. To decrease power losses in R1 and achieve shorter time constants for charging C1, a zener diode is utilized instead of a resistor.

A guideline for selecting component values includes the following steps: Choose C1 within a range of 0.05 to 1 µF. The maximum value may be constrained by the recharging time constant (RL + R1)C1, while the minimum value is determined by the minimum pulse width necessary to ensure SCR1 latching. Resistor R2 is selected based on peak gate current limits, if applicable, and minimum pulse width requirements. A zener diode, such as a 25-V zener, is a practical choice, as it meets the typical gate requirement of 20 V and 200 mA. This diode also prevents unintended triggering due to voltage transients. Photonic coupler triggering is particularly suitable for SCRs driving inductive loads, as it guarantees that SCR1 remains latched, providing gate current until it remains in the on state.

The circuit design effectively balances the need for reliable SCR triggering with the practical considerations of component selection and power management, ensuring robust operation in various applications.A basic circuit to trigger an SCR is shown in Fig. 67 -lA. This circuit has the disadvantage that the blocking voltage of the photon-coupler output device determines the circuit-blocking voltage, irrespective of higher main SCR capability. Adding capacitor Cl to the circuit, as shown in Fig. 67-lB, will reduce the dV!dt seen by the photoncoupler output device. The energy stored in Cl, when discharged into the gate of SCRl, will improve the dildt capability of the main SCR.

Using a separate power supply for the coupler adds flexibility to the trigger circuit; it removes the limitation of the blocking voltage capability of the photon-coupler output device. The flexibility adds cost and more than one power supply might be necessary for multiple SCRs if no common reference points are available.

67-lC, Rl can be connected to Point A. which will remove the voltage from the coupler after SCRl is triggered. "or to Point B so that the coupler output will always be biased by input voltage. The former is preferred since it decreases the power dissipation in Rl. A more practical form of SCR triggering is shown in Fig. 67 -IF. Trigger energy is obtained from the anode su]Jply and stored in Cl. Coupler voltage is limited by the zener voltage. This approach permits switching of higher voltages than the blocking voltage capability of the output device of the photon coupler. To reduce the power losses in Rl and to obtain shorter time constants for charging Cl, the zener diode is used instead of a resistor.

A guide for selecting the component values would consist of the following steps: Choose Cl in a range of 0.05 to 1 p.F. The maximum value might be limited by the recharging time constant (RL + R1) C1 while the minimum value will be set by the minimum pulse width required to ensure SCR latching.

R2 is determined from peak gate current limits, if applicable, and minimum pulse width requirements. Select a zener diode. A 25-V zener is a practical value, since this will meet the usual gate requirement of 20 V and 20 0. This diode will also eliminate spurious triggering because of voltage transients. Photon coupler triggering is ideal for the SCR"s driving inductive loads. By ensuring that the LASCR latches on, it can supply gate current to SCRl until it stays on.