Operation of single phase fully controlled bridge converter in the continuous conduction mode
Operation in the continuous conduction mode:
Fig: Waveforms in single phase fully controlled bridge converter in CCM
- In the continuous conduction mode of operation i0 never becomes zero, therefore, either T1T2 or T3T4 conducts.
- The firing angle of the converter is α. The angle θ is given by
- It is assumed that at t = 0- T3T4 was conducting. As T1T2 are fired at ωt = α they turn on commutating T3T4 immediately. T3T4 are again fired at ωt = π α. Till this point T1T2 conducts.
- The emf source E is greater than the dc link voltage till ωt = α. Therefore, the load current i0 continues to fall till this point. However, as T1T2 are fired at this point v0 becomes greater than E and i0 starts increasing through R-L and E. At ωt = π – θ v0 again equals E.
- Depending upon the load circuit parameters io reaches its maximum at around this point and starts falling afterwards. Continuous conduction mode will be possible only if i0 remains greater than zero till T3T4 are fired at ωt = π α where upon the same process repeats. The resulting i0 waveform is shown below v0.
- The input ac current waveform ii is obtained from i0 by noting that whenever T1T2 conducts ii = i0 and ii = - i0 whenever T3T4 conducts. The last waveform shows the typical voltage waveform across the thyristor T1.
- When the thyristor turns off at ωt = π α a negative voltage is applied across it for a duration of π – α. The thyristor must turn off during this interval for successful operation of the converter.
- The dc voltage waveform is periodic over half the input cycle. Therefore, it can be expressed in a Fourier series as follows.