21 August 2017

Pulse Transformer ( Isolation Transformer )

  • The function of pulse transformer is to generate pulse for semi conductor device and provide electrical isolation.
  • The figure A shows toroidal shaped pulse transformer. 
pulse transformer
  • It consists of one primary winding and one secondary winding
  • Each winding consists of same number of turns therefore any winding works as primary or secondary winding. 
  • The pulse to SCR is given by 1:1 or 1:1:1 pulse transformer. 
  • The pulse to the back to back SCR is given by the three winding pulse transformer. 
  • The firing circuit of gate pulse by 1:1 pulse transformer is given in the figure B. 
  • The function of the series resistor R is to limit the holding current of the SCR
  • The function of diode D is to prevent reversing gate current. The 1:1:1 pulse transformer is used to generate pulse for back to back SCR.
three winding pulse transformer

The design of pulse transformer is made such that
  • Its efficiency should be high.
  • The inductance of primary winding should be high in order to reduce magnetizing current.
  • The direct current flows through primary winding of pulse transformer in order to prevent saturation of the core.
  • There must be insulation between windings in order to protect winding from saturation.
  • Tight coupling between primary and secondary winding must be necessary.
  • The stray signal provides path through inter phase capacitance at high frequency.

Effect of Frequency on Pulse

  • The frequency and shape of the output pulse and input pulse same for high pulse frequency.
  • The output is directly proportional to integration of input at low pulse frequency.
input and output of the pulse transformer
Advantages
  • Small size
  • Low cost
  • It can operate at high frequency.
  • High isolation voltage
Disadvantages
  • The secondary waveform should be different from primary waveform at low frequency.
  • The DC current flows through primary winding in order to reduce saturation of the core.
Applications
  • Power electronics
  • Communication
  • Digital electronics
  • Radars
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