Power Electronics Circuits
For inverters that make single-phase AC from a DC voltage E, which of the following correctly compares the output voltage of a half bridge (two switches) and a full bridge (four switches)?
- AThe half-bridge output swings between +E/2 and −E/2 and the full-bridge output between +E and −E, so the output voltage can be doubled
- BWith twice as many switches, the full bridge only has more loss, and the output voltage it can deliver is exactly the same as that of the half bridge
- CA half bridge can output only a unidirectional DC voltage, so a full bridge is needed to apply an AC voltage to a load
Show answer and explanation
Answer: A. The half-bridge output swings between +E/2 and −E/2 and the full-bridge output between +E and −E, so the output voltage can be doubled
In a half bridge the load is connected between the midpoint of the DC supply, split by two capacitors, and the junction of the two switches, so the load sees either +E/2 or −E/2. In a full bridge the load is connected between the junctions of two legs, so besides +E and −E it can also output 0 by connecting both ends to the same side. The same DC voltage gives twice the output voltage, and the three levels can also be used to reduce PWM harmonics. A three-phase inverter is built from three legs (six switches).
- A. Correct. A full bridge outputs the difference between two legs, so the output voltage is doubled.
- B. For the same DC voltage, the output voltage of a full bridge is twice that of a half bridge.
- C. A half bridge can also output AC by turning its two switches on alternately.