--- title: "Full bridge inverter; dc-dc output power control" subject: "Advanced Electronics" chapter: 7 key: "semester_4/advanced_electronics:full-bridge-inverter-dc-dc-control" source: https://www.bctengineer.com/q/semester_4/advanced_electronics/full-bridge-inverter-dc-dc-control site: BCT Engineering --- # Full bridge inverter; dc-dc output power control Advanced Electronics · Chapter 7: Introduction to Power Electronics - Asked in: 2070 Asar, 5+2 marks - Video: [Full-Bridge Inverter with MOSFET Switches](https://www.youtube.com/watch?v=39WwbQM1Hsc) — Engineering with Prof. Kim - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/full-bridge-inverter-dc-dc-control - Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-7 ## Question Explain the operation of single phase full bridge inverter with necessary figures. What are the methods of controlling output power in dc-to-dc conversion? ## Short answer A single-phase full-bridge inverter uses four switches to apply +Vs then -Vs across the load; with RL load, diodes provide freewheeling. Output power in dc-dc conversion is controlled by varying duty cycle k via PWM (constant frequency), variable frequency, or current-limit (hysteresis) control. ## Answer #### 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. ```text +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:** ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the full-bridge inverter output with the same supply compared with a half-bridge? A. Twice the amplitude, and no centre tap needed B. Half the amplitude C. Same amplitude, but centre tap needed D. DC output Answer: A. vAB = ±Vs versus ±Vs/2. (Key point: Full-bridge output) 2. What is the fundamental RMS output? A. Vs B. 4Vs/(√2π) = 0.9Vs C. 0.45Vs D. Vs/2 Answer: B. From vo = Σ(4Vs/nπ) sin nωt. (Key point: Fundamental) 3. In constant-frequency (PWM) control of a DC chopper, what is varied? A. T with Ton fixed B. Supply voltage only C. On time Ton with T fixed D. Load inductance Answer: C. Vo = (Ton/T)Vs. (Key point: PWM control) 4. Which is a disadvantage of variable-frequency control? A. The duty cycle cannot change B. It needs no switch C. Output is AC D. Harmonics vary with frequency, so filtering is harder Answer: D. Fixed frequency allows easy filter design. (Key point: FM control) ## Related in Advanced Electronics - [Choppers; single-phase full bridge inverter](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Achoppers-full-bridge-inverter) — Advanced Electronics, chapter 7, asked 2× - [Chopper definition; single-phase half bridge inverter](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Achopper-half-bridge-inverter) — Advanced Electronics, chapter 7 - [Variable inverter output; SCR full-wave average](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Ainverter-variable-output-scr-fwr) — Advanced Electronics, chapter 7 ## Related in other subjects - [Single-phase full-wave inverter with 5 Ω load](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Afull-wave-inverter-5ohm) — Power Electronics, chapter 5 - [Single-phase inverter fundamental output voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Asingle-phase-inverter-r-fundamental) — Power Electronics, chapter 5 - [Full-bridge inverter RLC load to fifth harmonic](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Afull-bridge-inverter-rlc-155uf) — Power Electronics, chapter 5 - [Full-bridge square-wave inverter, 400 V, RL load](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Afull-bridge-inverter-400v-rl) — Power Electronics, chapter 5