Describe the operation of switch mode power supply with circuit diagram. Give its merits also. (ii) Common mode rejection ratio (CMRR): (i) Discuss fixed and adjustable switching regulators in detail.

Switch Mode Power Supply (SMPS) Operation with Circuit Diagram

A Switch Mode Power Supply (SMPS) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Unlike a linear power supply, the pass transistor of a switch mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions, which minimizes wasted energy.

Step-by-Step Operation:

1. AC Input: The SMPS takes in AC power from the mains and rectifies it to DC using a rectifier.
2. Filtering: The rectified DC is then filtered using a capacitor to reduce ripple.
3. Inverter: The filtered DC is then fed into an inverter circuit that switches it on and off at a high frequency (20 kHz to 1 MHz), using a transistor or MOSFET.
4. Transformer: The high-frequency AC is then passed through a transformer to step down or step up the voltage as required.
5. Rectification: The transformed AC is rectified back to DC.
6. Output Filtering: The rectified DC is filtered again to smooth out the ripple.
7. Regulation: The output voltage is regulated by varying the duty cycle of the switching transistor, often controlled by a feedback mechanism to maintain a constant output voltage despite variations in input voltage or load conditions.

Circuit Diagram:

  AC Input -> [Rectifier] -> [Filter] -> [Inverter] -> [Transformer] -> [Rectifier] -> [Output Filter] -> DC Output

Merits of SMPS:

Merit	Description
Efficiency	High efficiency due to low power dissipation in switching components.
Size and Weight	Smaller and lighter due to high-frequency operation and smaller transformer size.
Cost	Lower cost due to smaller transformer and heat sink requirements.
Versatility	Can be designed for a wide range of input voltages and output power.
Regulation	Good regulation and can be easily adjusted to provide a stable output voltage.

Common Mode Rejection Ratio (CMRR)

CMRR is a measure of how well a differential amplifier or operational amplifier can reject common-mode signals, which are the signals that appear simultaneously and in-phase on both inputs.

Formula:

CMRR is defined as the ratio of the differential gain (Ad) to the common-mode gain (Acm):

$$ CMRR = \frac{Ad}{Acm} $$

In practice, CMRR is often expressed in decibels (dB):

$$ CMRR_{dB} = 20 \log_{10} \left( \frac{Ad}{Acm} \right) $$

Example:

If an amplifier has a differential gain of 1000 and a common-mode gain of 0.1, the CMRR would be:

$$ CMRR = \frac{1000}{0.1} = 10000 $$

$$ CMRR_{dB} = 20 \log_{10} (10000) = 80 \text{ dB} $$

Fixed and Adjustable Switching Regulators

Fixed Switching Regulators:

Fixed switching regulators provide a constant output voltage that is preset by the manufacturer. They are designed to operate at a specific voltage and are not adjustable.

Adjustable Switching Regulators:

Adjustable switching regulators allow the user to set the output voltage within a certain range. This is typically done using external resistors or a potentiometer.

Detailed Discussion:

Feature	Fixed Switching Regulators	Adjustable Switching Regulators
Output Voltage	Predetermined and non-adjustable	User can adjust within a specified range
Complexity	Simpler design, fewer external components	More complex, requires external components for voltage adjustment
Flexibility	Limited to the specified output voltage	Flexible, can be used for various applications requiring different voltages
Cost	Generally cheaper due to simplicity	Slightly more expensive due to additional components
Use Case	Ideal for mass-produced products with a standard voltage requirement	Suitable for prototypes, custom designs, or products needing variable power supply

Example:

A fixed 5V switching regulator would always provide a 5V output, whereas an adjustable switching regulator could be set to provide any voltage within its operating range, for example, from 1.25V to 35V, depending on the specific regulator.

In conclusion, SMPS is a highly efficient power supply system with numerous merits, CMRR is a critical parameter for differential amplifiers, and both fixed and adjustable switching regulators have their own unique applications and advantages.