Switch-Mode DC-AC Inverter: Basic concepts

1. Inverters with single- and three-phase ac outputs will be focused
   
2. Input is dc voltage source
   
3. Such inverters are called voltage-source inverters (VSI)
   
4. The other type of inverter is a current source inverter (CSI) where the input is a dc current source
   
5. Discussion will be limited to VSI
   
6. Four-quadrant operation
   
7. Rectifier mode: quadrants 2 and 4
   
8. Inverter mode: quadrants 1 and 3
   
9. v_o can be assumed to be sinusoidal
   
10. i_o will lag v_o since the inverter will drive an inductive load such as a motor
11. In interval 1, both v_o and i_o are positive and interval 3, both v_o and i_o are negative
12. Therefore during intervals, 1 and 3, p_o=v_oi_o will be positive, and power will flow from the dc to ac side which is the inverter mode of operation
13. During the intervals 2 and 4, v_o and i_o will be opposite signs, and power will flow from the ac side to the dc side which is the rectifier mode of operation
14. Thus, switch-mode inverters are capable of operating in all four quadrants

Figure 3: Single-phase switch-mode inverter.

Freewheeling Diodes in Inverters

1. The inverter circuits normally use IGBT or MOSFET switches shunted by an anti-parallel connected diode.
   
2. The diode allows current flow in the opposite direction when the switch is open.
   
3. These freewheeling diodes prevent inductive current interruption
   
4. This provides protection against transient over voltage, which may cause reverse breakdown of the IGBT and MOSFET switches.
   

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