Choppers; single-phase full bridge inverter

Choppers

A chopper is a static DC-to-DC converter that converts a fixed DC voltage into a variable DC voltage by switching a semiconductor device ON and OFF at high frequency. The average output is Vo = k·Vs (step-down), where k = Ton/T is the duty cycle. Choppers are used in DC motor drives, battery vehicles and SMPS.

Single-phase full-bridge inverter

A full-bridge inverter uses four switches (S1–S4, SCRs/IGBTs with antiparallel diodes D1–D4) and one DC source Vs to produce an AC square wave of ±Vs across the load.

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

Working:

1. 0 < t < T/2: S1 and S2 are ON. Current flows +Vs → S1 → A → load → B → S2 → negative. vAB = +Vs.
2. T/2 < t < T: S3 and S4 are ON. Current flows +Vs → S3 → B → load → A → S4 → negative. vAB = −Vs.
3. Switches in the same leg (S1–S4 or S3–S2) are never ON together, or the supply would be short-circuited.
4. With an RL load, current lags; when switches change over, the stored energy returns to the source through the diodes (D3, D4 or D1, D2) before the current reverses. These are feedback diodes.

Waveforms (resistive load):

gate S1,S2 |‾‾‾‾‾|_____|‾‾‾‾‾|_____
gate S3,S4 |_____|‾‾‾‾‾|_____|‾‾‾‾‾
           0   T/2    T
 vo   +Vs  +-----+     +-----+
           |     |     |     |
      0 ---+-----+-----+-----+---> t
           |     |     |     |
     −Vs         +-----+     +---

Output relations:

• Output is a square wave of amplitude Vs (twice that of a half-bridge with the same supply).
• RMS output voltage Vo = Vs.
• Fourier series: vo = Σ (4Vs/nπ) sin nωt for n = 1, 3, 5, …; fundamental RMS = 4Vs/(√2·π) = 0.9·Vs.
• Frequency is set by the switching rate; output voltage can be varied by PWM.