Pulse train generation for thyristor firing

Pulse train firing applies a burst of high-frequency, short pulses to the thyristor gate for the whole interval in which it should conduct (from $\alpha$ to about $180^\circ$), instead of a single long pulse. It gives reliable firing with inductive loads and needs only a small pulse transformer.

Circuit (block form)

 +-----------+   +------------+
 | zero-cross|-->| ramp &     |-- gating signal
 | detector  |   | comparator |   (alpha to 180 deg)
 +-----------+   +------------+        |
                    ^ Vc (control)     v
 +-----------+                     +------+
 | 555 timer |--- 5-10 kHz pulses->| AND  |
 | astable   |                     | gate |
 +-----------+                     +--+---+
                                      |
      +Vcc                            v
       |   pulse transformer    +-----------+
      PT  primary  <------------| driver Q  |
       | secondary --D--R--> G  | (transistor)
       |                  --> K +-----------+

Operation

1. Synchronisation: a step-down transformer and zero-crossing detector produce a signal at the start of each half cycle; it resets a ramp (sawtooth) generator.
2. Firing angle: a comparator compares the ramp with a d.c. control voltage $V_c$. When the ramp exceeds $V_c$ the comparator output goes high, at angle $\alpha$, and stays high until the end of the half cycle. Changing $V_c$ changes $\alpha$.
3. High-frequency carrier: a 555 astable oscillator produces a pulse train at about 5 to 10 kHz.
4. AND gate: combines the two, so carrier pulses appear only from $\alpha$ to $180^\circ$.
5. Driver and pulse transformer: a transistor amplifies the pulses and drives the pulse transformer. The secondary supplies isolated gate current through a diode (blocks negative pulses) and a current-limiting resistor. A freewheeling diode across the primary resets the transformer core.

Waveforms

 vs     /\        /\
       /  \      /  \
 -----/----\----/----\-----
 gate      ______     ______
 window   |      |   |
 ---------+      +---+
 pulse     ||||||     ||||||
 train  ---++++++-----++++++--
           ^ alpha

Advantages

• Reliable turn-on even if the anode current rises slowly (inductive loads) or the SCR is momentarily reverse biased.
• Low gate dissipation because each pulse is short.
• A small pulse transformer suffices, since it never saturates.