Block diagram of a thyristor gating circuit

Block diagram of a thyristor gating circuit

Waveforms for the circuit of block diagram of a thyristor gating circuit

• The general block diagram of the gate trigger circuit includes a synchronizing transformer, diode rectifier, zero crossing detector, firing-angle delay block, pulse amplifier, gate-pulse isolation transformer, and power circuit for the converter.
• The synchronizing mid-tapped transformer steps down the supply voltage for the zero crossing detector and provides a DC supply for the gate trigger circuit.
• The zero crossing detector converts the AC synchronizing input voltage into a ramp voltage synchronized with the zero crossing of the AC supply voltage.
• In the firing-angle delay block, a constant amplitude ramp voltage is compared with the control voltage (E_c). When the rising ramp voltage equals the control voltage, a pulse signal of controlled duration is generated.
• If control voltage is lowered, firing angle decreases and in case control voltage is raised, firing angle increases. This shows that firing-delay angle is directly proportional to the control signal voltage. 
• The pulse output from the firing-angle delay block is fed to a pulse amplifier circuit to amplify the pulses before triggering the thyristors.The amplified pulses are then used for triggering thyristors 1, 2, 3 and 4 through gate-pulse isolation transformers as shown in figure.
• The gate trigger circuit for thyristors in phase-controlled rectifiers should have the following features:

1. Detection of zero crossing of the input voltage.
2. Generation of trigger pulses with the required wave shape.
3. DC power supply for the pulse amplifier.
4. Isolation of the gate trigger circuit from the line potential using pulse transformers or optocouplers.

Gate Pulse Amplifiers

• Pulse output from integrated circuits (ICs) may be directly fed to gate-cathode circuit of a low-power thyristor to turn it on. But in high-power thyristors, trigger-current requirement is high. Therefore, pulses derived from ICs must be amplified and then fed to thyristor for its reliable turn on. In a thyristor, anode circuit is subjected to high voltage whereas gate circuit works at a low voltage. Therefore, an isolation is essential between a thyristor and the gate-pulso generator. As stated before, this isolation is provided by an optocoupler or a pulse transformer.
• A pulse-amplifier circuit for amplifying the input pulses is consists of a MOSFET (or a transistor), a pulse transformer for isolation and diodes D1, D2. When a voltage of appropriate level is applied to the gate of MOSFET, it gets turned on.
• As a result, most of the dc voltage Vcc appears across transformer primary and corresponding pulse voltage is induced in the transformer secondary. This amplified pulse on the secondary side is applied to gate and cathode of a thyristor to turn it on. When pulse signal applied to the gate of MOSFET goes to zero, MOSFET turns off.The primary current due to vcc tends to fall and likewise flux in core also tends to decrease. Due to this tendency, a voltage of opposite polarity is induced in both primary and secondary windings of pulse transformer. Diode D1 on the secondary side of pulse transformer prevents the flow of negative gate current due to the reverse secondary voltage, when MOSFET is off. Reverse voltage in primary, however, forward biases diode D2 when MOSFET is off. Current flow is thus established in the circuit consisting of primary, R and D2. As a consequence, energy in the transformer magnetic core gets dissipated in R and the core flux gets reset. In case pulse width at the secondary terminals is to be increased, then a capacitor C is connected across R as shown in figure long-pulse output.
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  Pulse amplifier circuit using a MOSFET for a thyristor trigger circuits:short-pulse output & long-pulse output

  
  

Pulse Train Gating

• Pulse gating is not suitable for inductive, ie. RL loads, because initiation of thyristor conduction is not well defined for these types of loads. This difficulty for such situations can be overcome by triggering the thyristor continuously. Continuous gating, however, suffers from some disadvantages like increased thyristor losses and distortion of output pulse due to saturation of pulse transformer by continuous pulse. In order to overcome these shortcomings of continuous gate signal, a train of firing pulses is used to turn on a thyristor. A pulse train of gating signal is also called high-frequency carrier gating. 
• A pulse train is generated by modulating the pulse width at a high frequency (10 to 30 kHz).
• The circuit for generating a pulse train includes an AND-logic gate, a 555 timer, MOSFET, isolation pulse transformer, and diodes.
• The pulse signal from the thyristor trigger circuit and the output from the timer are processed in the AND gate to obtain the pulse train waveform. This pulse train is then amplified and applied across the gate-cathode terminals of the thyristor to turn it on.
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  Pulse train gating circuit & waveforms

  
  

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