Inverter: DC to AC
Half-bridges and full bridges make AC from DC. Motor drives, solar and UPS units use the same principle.
One half-bridge
Turn the upper and lower switches on alternately and the output node moves between +Vdc and 0. Vary the duty cycle in the shape of a sine wave (SPWM) and, on an inductive load, the average voltage is a sine wave.
Full bridge (H-bridge)
Two half-bridges drive the two ends of the load. The load can see +Vdc, −Vdc and 0, so it is used for forward/reverse DC motor drive and single-phase AC output.
Three-phase inverter
Three half-bridges make three voltages 120° apart in phase. The standard for driving BLDC, PMSM and induction motors.
| Configuration | Switches | Use |
|---|---|---|
| Half-bridge | 2 | Buck converter, one phase |
| H-bridge | 4 | DC motor, single-phase inverter |
| Three-phase bridge | 6 | Three-phase motor |
Modulation methods
- SPWM: compare a sine wave with a triangle wave. Simple
- SVPWM: choose switching combinations by space vector. About 15% more output from the same DC voltage; used with FOC
Design points
Dead time, bootstrap supply, phase current sensing, and the ripple current of the DC-link capacitor. These continue in the three-phase inverter note of the motor control subject.