The firing pulse control unit constitutes the heart of any thyristor power converter. The Gate Firing Converters required by the thyristors are often derived from the digital pulses using a simple buffer unit. Analog firing controllers are used in systems with dedicated hardware. They are recommended for constant low frequency operation where the firing precision is not critical. They are simple.
However, in variable frequency applications, especially at high frequencies, the implementation of firing angle control using analog means is not appropriate. The firing pulses would depend highly on the precision and stability of analog components used, such as resistors, capacitors, linear and nonlinear devices responsible for time lags, etc.
The firing pulses cannot be realised with adequate symmetry. The all digital controller is free from these drawbacks. Further, a digital controller can be made programmable to generate accurate Gate Firing Converters in various configurations and to perform triggering of thyristors at accurately repeated intervals, independent of frequency with desired symmetry. It is also possible to alter the firing angle on line without affecting the symmetry of pulse train.
The requirements of a firing angle controller are:
- A synchronised triggering output
- Thyristor bridge must provide the desired output. Suitable techniques such as linear ramp or cosine wave crossing may be employed.
- The transients and distortions of the supply voltage waveform should not have any effect on the firing angle delay.
- The firing angle must be varied in the range 0 to 180°
- The firing angle control should be smooth and precise. The resolution of firing angle decides the sampling frequency when generating firing pulses.
- The scheme should have enough flexibility to accommodate variation in circuit parameters in order to obtain the best performance.
- The response of the controller must be very fast.
- The controller must have self regulating property when the amplitude of supply voltage varies.
- A unified or generalised hardware circuit which can be adaptable to any type of converter or cycloconverter operating under large variations of
- The controller must be developed with low hardware content so that it can be integrated into a chip. This would reduce the cost and increase the potential field of application.
Depending upon the application, suitable software may be written to obtain the firing pulses.