Full bridge inverter; dc-dc output power control

Single-phase full-bridge inverter

A full-bridge inverter converts DC into AC using four switches (S1–S4, with feedback diodes D1–D4) and a single DC supply Vs. The load is connected between the midpoints A and B of the two legs.

       +Vs
   +----+------------+
   |    |            |
   |   S1 D1        S3 D3
   |    |   LOAD     |
   |    A---[RL]-----B
   |    |            |
   |   S4 D4        S2 D2
   |    |            |
   +----+------------+
        0 (−)

Operation:

1. 0 < t < T/2: S1 and S2 ON → vAB = +Vs.
2. T/2 < t < T: S3 and S4 ON → vAB = −Vs.
3. S1 and S4 (or S3 and S2), which are in the same leg, must never be ON together.
4. With RL load, current lags. When S1, S2 turn OFF, the load current continues through D3, D4, returning energy to the supply, until it reverses and S3, S4 take over (and similarly for the next half).

Waveforms:

 S1,S2 |‾‾‾‾‾|_____|‾‾‾‾‾|____
 S3,S4 |_____|‾‾‾‾‾|_____|‾‾‾‾
 vo +Vs+-----+     +-----+
       |     |     |     |
     0-+-----+-----+-----+---> t
       |     |     |     |
   −Vs       +-----+     +---
       0    T/2    T
 io      /‾‾‾\       /‾‾‾\
 (RL) __/     \_____/     \_

• Output RMS voltage = Vs; fundamental RMS = 4Vs/(√2π) = 0.9Vs.
• vo = Σ (4Vs/nπ) sin nωt, n = 1, 3, 5, …
• Output is twice that of a half-bridge with the same supply, and no centre tap is needed.

Methods of controlling output power in DC-to-DC conversion

The average output Vo = (Ton/T)·Vs = k·Vs is varied by changing the duty cycle:

1. Constant frequency (PWM) control: T fixed, Ton varied. Most common; easy filtering.
2. Variable frequency (FM) control: Ton (or Toff) fixed, T varied. Harmonics vary with frequency, so filtering is harder.
3. Current-limit (hysteresis) control: switch is turned OFF when load current reaches a maximum and ON when it falls to a minimum, keeping current within a band.