Pulse Transformer in Firing Circuit

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A pulse transformer is a type of transformer that is specifically designed to produce electrical pulses with high velocity and stable amplitude. It is commonly used in power electronic devices such as silicon-controlled rectifiers (SCRs) for gate terminal firing circuits.

The primary purpose of a pulse transformer is to provide isolation between the low-power gate firing circuit and the high-power circuit in which the SCR is being used. This isolation is important for safety reasons and to prevent disturbances or damage to the control circuitry.

The pulse transformer is often referred to as a "Gate Transformer" or "Gate Drive Transformer" because it is responsible for delivering the necessary gate pulses to the SCR to control its conduction.

In a pulse transformer, the two circuits are electrically isolated but magnetically coupled. The transformer consists of two separate coils of wire wound on a common iron core. The iron core serves as a pathway to provide magnetic flux and retain high magnetic fields without saturating the iron, ensuring efficient coupling between the coils.

By controlling the pulse characteristics and providing isolation, the pulse transformer enables effective and reliable control of power devices like SCRs in various applications.

 pulse transformers are commonly used in firing circuits for devices like SCRs (Silicon Controlled Rectifiers) and GTOs (Gate Turn-Off Thyristors).This transformer has usually two secondaries. The turn ratio from primary to two secondaries is 2:1:1 or 1:1: 1. These transformers are designed to have low winding resistance, low leakage reactance and low inter-winding capacitance. The advantages of using pulse transformers in triggering semiconductor devices are :

  1. The isolation of low-voltage gate circuit from high-voltage anode circuit and
  2. The triggering of two or more devices from the same trigger source.
When a square pulse is applied to the primary terminals of a pulse transformer, it can be transmitted at the secondary terminals in two different functional modes:
  1. Square Wave Transmission: In this mode, the pulse transformer faithfully transmits the square wave shape of the input waveform. This is useful when a precise square pulse is required for triggering the semiconductor devices.
  2. Derivative Transmission: In this mode, the pulse transformer transmits a derivative or modified version of the input waveform. This can be useful in certain applications where the exact shape of the pulse is not critical, and the derivative waveform can still effectively trigger the devices.
In the trigger circuit using a pulse transformer as shown in Figure, the diode allows the flow of current after the pulse period, when the transistor is off. This ensures that the energy stored in the primary winding of the pulse transformer is dissipated.
The transistor in the circuit acts as a simple switch, turning on when the pulse applied to its base is at its high level. This connects the DC bias voltage (VB) to the transformer primary.The advantage of this arrangement are : 
  1. There need not be a variable strength pulse generator since the pulses may be of the same amplitude and the strength of the generated pulses may be increased simply by varying the de bias voltage.
  2. The operation of the circuit becomes independent of the pulse characteristics since the only role the pulse plays is to turn-on or turn-off, the transistor. Therefore, there is no effect of pulse distortion (e.g. pulse edges or any spike superimposed on the pulse) on the working of this circuit.
Show in fig, RL limits the current in the primary circuit of pulse transformer.Its equivalent circuit is drawn in show Fig, where L is the magnetic inductance of the pulse transformer and Rg is the resistance of gate-cathode circuit of an SCR.Fig(c) shows the transfer of Rg to pulse transformer primary as R1 = (N1/N2)2Rg. This circuit can be analysed by applying Thevenin's theorem at the terminals a b. Fig(d) is the Thevenin's equivalent circuit, where,
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In practice, exponentially decaying trigger pulses, as shown in Figure, are often preferred in firing circuits for several reasons:
  1. This pulse waveform is suitable for injecting a large charge in the gate circuit for reliable turn on. 
  2. The duration of this pulse is small, therefore no significant heating of the gate circuit is observed.
  3. For the same gate-cathode power, it is permissible to raise VB to a suitable high value so that a hard-drive of SCR is obtained. A device with a hard-drive can withstand high di/dt at the anode circuit which is desirable. 
  4. The size of the pulse transformer is reduced. For an extended pulse, large L (with iron-core) is required which increases size and cost of the pulse transformer.