---
title: "Short note: SMPS"
subject: "Advanced Electronics"
chapter: 8
key: "semester_4/advanced_electronics:short-note-smps"
source: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-smps
site: BCT Engineering
---

# Short note: SMPS

Advanced Electronics · Chapter 8: Switched Power Supplies

- Asked in: 2071 Chaitra, 3 marks
- Video: [Understanding Switching Mode Power Supplies](https://www.youtube.com/watch?v=MS5vd3LomWg) — Rohde & Schwarz
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/advanced_electronics/short-note-smps
- Whole chapter: https://www.bctengineer.com/md/semester_4/advanced_electronics/chapter-8

## Question

Write a short note on SMPS (switched mode power supply).

## Short answer

An SMPS regulates output by high-frequency switching and PWM control. It rectifies AC input, chops it via a MOSFET, passes through a small HF transformer, then rectifies and filters. Advantages: high efficiency (70 – 95 %), small size; disadvantages: switching noise, complexity.

## Answer

A **Switched Mode Power Supply (SMPS)** is a power supply that converts AC or DC input into a regulated DC output by switching a power transistor ON and OFF at high frequency (20 kHz – 1 MHz) and controlling the duty cycle with PWM.

```text
AC->[RECTIFIER]->[HF SWITCH]->[HF  ]->[RECTIFIER]->DC
    [+ FILTER ]  [ (MOSFET)]  [TRAN]  [+ FILTER ]  out
                      ^                    |
                      |                    |
                 [PWM CONTROL]<-[ISOLATION]<+
                                [+ERROR AMP]
```

**Working:**
1. AC mains is rectified and filtered to unregulated DC.
2. A MOSFET chops this DC at high frequency.
3. A small high-frequency transformer steps the voltage up/down and gives isolation.
4. The secondary is rectified and filtered to DC.
5. The output is compared with a reference; the PWM controller adjusts the duty cycle to keep the output constant (feedback through an opto-coupler).

**Common topologies:** flyback, forward, push-pull, half-bridge, full-bridge (and non-isolated buck, boost, buck-boost).

**Advantages:** high efficiency (70–95 %), small size and weight (high-frequency transformer), wide input range.

**Disadvantages:** complex circuit, switching noise/EMI, output ripple, needs careful design.

**Uses:** computers, TV, mobile chargers, LED drivers, telecom equipment.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does an SMPS regulate its output?
   A. By dissipating excess voltage in a series transistor
   B. By switching a transistor at high frequency and adjusting the PWM duty cycle
   C. By using a large 50 Hz transformer
   D. By adjusting the load
   Answer: B. The output is compared with a reference and the PWM controller adjusts the duty cycle. (Key point: SMPS principle)

2. What is the role of the high-frequency transformer in an SMPS?
   A. It steps voltage up or down and gives isolation, and is small and light
   B. It rectifies the AC
   C. It filters ripple
   D. It provides feedback gain
   Answer: A. Operating at high frequency allows a small transformer that also isolates the output. (Key point: HF transformer)

3. How is feedback usually passed to the primary side control?
   A. Through an opto-coupler
   B. Through a resistor divider only to ground
   C. Through the mains
   D. It is not fed back
   Answer: A. An opto-coupler keeps isolation between secondary and primary while carrying the error signal. (Key point: Feedback isolation)

## Related in Advanced Electronics

- [Boost regulator relation; SMPS block diagram](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Aboost-derivation-smps-block) — Advanced Electronics, chapter 8

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