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Explanation of SMPS Circuit Diagram
This image shows a Switched Mode Power Supply (SMPS) schematic. SMPS is widely used in power supplies because of its high efficiency, compact size, and ability to convert different voltage levels. Let’s break down the circuit and explain the working principles along with a detailed description of each component.
1. Input Section:
220V AC Input:
The circuit receives 220V AC from the mains. This is the standard alternating current used in households and is represented on the left side of the diagram.
A fuse is placed in series with the AC input to protect the circuit from overcurrent or short circuits. In case of excessive current, the fuse will blow and disconnect the circuit from the power source, preventing any damage.
Bridge Rectifier (4 Diodes or Bridge Rectifier IC):
After the fuse, the AC supply is fed into a bridge rectifier. The bridge rectifier converts the 220V AC into DC voltage.
A bridge rectifier consists of 4 diodes arranged in a way that converts both the positive and negative half cycles of the AC waveform into a pulsating DC.
The output of the bridge rectifier is DC voltage with significant ripples.
2. Smoothing Filter:
Electrolytic Capacitor:
The pulsating DC output from the bridge rectifier is then filtered using a large electrolytic capacitor. This capacitor smooths out the ripple by storing energy during peaks and releasing it during troughs, creating a more constant DC voltage.
The capacitor smooths the DC voltage to a level of around 300V DC. This high-voltage DC is necessary for the operation of the switching part of the SMPS.
3. Switching Transformer and Oscillator:
High-Frequency Transformer:
The heart of any SMPS is the switching transformer, which steps down the high DC voltage to the desired lower DC voltage. Unlike traditional transformers operating at 50/60Hz, this transformer operates at high frequencies (typically tens to hundreds of kHz).
The transformer in this diagram has multiple windings: the primary side is connected to the high-voltage DC and the secondary side provides the stepped-down voltage (e.g., 5V DC in this diagram).
PWM Controller IC (Oscillator/Control Circuit):
The transformer is driven by a PWM (Pulse Width Modulation) Controller IC. This IC operates as an oscillator, generating high-frequency pulses that are used to switch a transistor (usually a MOSFET) ON and OFF rapidly.
These rapid switching actions create alternating current at high frequency on the primary side of the transformer, allowing for efficient energy transfer.
Soft Start:
A soft start mechanism, often implemented using a resistor or a dedicated IC, is included in the circuit. The soft start gradually increases the power supply voltage to the circuit components to avoid large inrush currents when the SMPS is first powered on. This protects the circuit components from stress caused by sudden current surges.
4. MOSFET Switch:
Power MOSFET/Transistor:
The MOSFET is the main switching device in the SMPS circuit. It is controlled by the PWM controller, which sends high-frequency pulses to the gate of the MOSFET, turning it ON and OFF rapidly.
When the MOSFET is ON, current flows through the primary winding of the transformer, storing energy in the magnetic field of the transformer.
When the MOSFET is OFF, the magnetic field collapses, and the stored energy is transferred to the secondary winding of the transformer, stepping down the voltage.
5. Feedback and Regulation:
Feedback Circuit (FB):
For an SMPS to deliver a stable output voltage (e.g., 5V DC in this case), it requires feedback. The output voltage is monitored and compared with a reference voltage.
A feedback mechanism using an optocoupler or other components is employed to send information back to the PWM controller IC, which adjusts the duty cycle of the pulses to regulate the output voltage.
If the output voltage increases above the desired level, the duty cycle is reduced (MOSFET stays ON for a shorter time), and vice versa, keeping the voltage stable.
6. Output Section:
Rectification and Filtering:
The stepped-down voltage at the secondary side of the transformer is AC, so it needs to be rectified and filtered to produce a clean DC output.
Schottky diodes are used on the secondary side because they have low forward voltage drop and fast switching characteristics, making them ideal for high-frequency applications.
Another capacitor is placed across the output to filter the remaining ripple and provide smooth 5V DC output.
Vcc Capacitor:
The Vcc capacitor shown near the transformer is part of the power supply for the control circuitry, providing the necessary voltage to the PWM controller and other supporting components.
7. Protection and Additional Components:
VDR (Voltage Dependent Resistor):
A VDR or MOV (Metal Oxide Varistor) is used for surge protection. It is designed to absorb high-voltage spikes that may occur due to switching transients or lightning strikes, protecting the circuit components from overvoltage damage.
Snubber Circuit:
A snubber circuit (usually a combination of a resistor and capacitor) is often placed across the MOSFET to suppress voltage spikes caused by inductive loads, such as the transformer. These spikes can damage the MOSFET if not properly managed.
8. Working Principle of SMPS:
AC to DC Conversion:
The 220V AC input is rectified and smoothed to produce a high-voltage DC supply (around 300V DC) using a bridge rectifier and a capacitor.
High-Frequency Switching:
The high-voltage DC is then switched ON and OFF at high frequency using a MOSFET controlled by a PWM controller IC. The IC adjusts the switching frequency and pulse width based on the feedback from the output to regulate the voltage.
Energy Transfer via Transformer:
The rapid switching of the MOSFET induces high-frequency alternating current in the primary winding of the transformer, which steps down the voltage according to the winding ratio.
Energy stored in the transformer’s magnetic field during the ON state of the MOSFET is transferred to the secondary side when the MOSFET is OFF.
Output Rectification and Filtering:
The AC voltage on the secondary side of the transformer is rectified by fast-recovery diodes (like Schottky diodes) and filtered by capacitors to produce a clean, stable DC output (in this case, 5V DC).
Feedback Regulation:
The output voltage is monitored, and a feedback signal is sent to the PWM controller IC, which adjusts the duty cycle to maintain a constant output voltage regardless of variations in input voltage or load conditions.
Protection Mechanisms:
The circuit includes protections like surge suppression (using VDR), over-current protection (via the fuse), and soft-start to prevent the inrush current.
Component Details:
Bridge Rectifier: Converts AC to DC.
Capacitor: Filters and smooths the DC voltage.
PWM Controller IC: Controls the MOSFET switching and regulates the output voltage.
MOSFET: High-speed switch that drives the transformer.
Transformer: Steps down high DC voltage to the required output voltage.
Diodes: Rectifies the stepped-down AC voltage to DC.
Capacitors (Output): Smooth the rectified DC voltage.
VDR/MOV: Provides protection from high-voltage spikes.
Feedback Circuit: Ensures stable output voltage by controlling the switching of the MOSFET.
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