Operating principle of a single phase half controlled bridge converter
Operating principle of a single phase half controlled bridge converter:
- For any load current to flow one device from the top group (T1 or T3) and one device from the bottom group must conduct. However, T1 T3 or D2 D4 cannot conduct simultaneously. On the other hand T1 D4 and T3 D2 conducts simultaneously whenever T1 or T3 are on and the output voltage tends to go negative.
- Therefore, there are four operating modes of this converter when current flows through the load. Of course it is always possible that none of the four devices conduct. The load current during such periods will be zero.
- It is observed that whenever D2 conducts the voltage across D4 is -vi and whenever D4 conducts the voltage across D2 is vi. Since diodes can block only negative voltage it can be concluded that D2 and D4 conducts in the positive and the negative half cycle of the input supply respectively.
- Similar conclusions can be drawn regarding the conduction of T1 and T3. The operation of the converter can be explained as follows when T1 is fired in the positive half cycle of the input voltage. Load current flows through T1 and D2.
- If at the negative going zero crossing of the input voltage load current is still positive it commutates from D2 to D4 and the load voltage becomes zero. If the load current further continuous till T3 is fired current commutates from T1 to T3. This mode of conduction when the load current always remains above zero is called the continuous conduction mode. Otherwise the mode of conduction becomes discontinuous.