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Understanding the Basics of a Switch Mode PSU Circuit


Switch Mode PSU Circuit (2)

The SMPS circuits (Switch Mode Power Supply) are an essential element in modern electronic devices, they are used to convert the electrical power from a power source into the most efficient form. As opposed to the constant linear power supplies, SMPS circuits control output voltage by fast switching a series of power transistors on and off. These fast-paced switching mechanisms give SMPS circuits the ability to achieve high efficiency, small size, and low heat dissipation, which make them suitable for many applications, for example, mobile phones and industrial equipment.

Basic Components of SMPS

The main parts of the SMPS circuit are the rectifier, filter capacitor, switching transistor, inductor, and the control circuit. The rectifier changes AC input voltage from the mains or a transformer to DC. The filter capacitor then smoothes out any voltage ripples. The switching transistor (MOSFET) is the main element of the circuit that controls current flow and the inductor stores and releases energy during the switching process. The chopper circuit, which is also called a high-frequency converter, is an electronic circuit that switches the switching transistor on and off at several kHz. The control circuit regulates switching frequency and duty cycle to keep the output voltage steady.

How Does SMPS Work?

SMPS circuits operate through the use of fast switching of the transistor that is done at a high frequency usually in the range of kHz. In the on state, energy is kept in the inductor and in the off state, this energy is transferred to the output load. The duty cycle of the switching signal can be adjusted, which in turn allows the regulation of the output voltage. This process lets SMPS circuits achieve the high-efficiency conversion of input power (PIN) to the desired output voltage with the lowest possible power loss and current characteristics.

Advantages of Using a Switch Mode PSU

High EfficiencySMPS typically achieve over 80% efficiency, meaning less energy is consumed and operating costs are lower.
Compact and LightweightSMPS circuits are small and lightweight, making them ideal for portable devices.
Better Voltage RegulationSMPS ensure better voltage regulation and can operate across a wide range of input voltages.
Adaptable for Multiple ApplicationsDue to their ability to operate across a wide range of input voltages, SMPS are adaptable for multiple applications.
Simplified Control CircuitsIntegrated modules allow designers to simplify control circuits, enhancing the performance of their SMPS.

Switch Mode PSU Circuit

Working Process

The working process of an SMPS circuit involves several key stages. A detailed circuit diagram illustrating these stages will be provided.

Working principle of SMPS
  1. Input Filtering and Rectification
  • AC Mains Voltage and EMI Filtering: The first step of the Switch Mode Power Supply (SMPS) circuit is the AC main voltage (110V or 230V AC) that is filtered by an Electromagnetic Interference (EMI) filter. This filter consists of inductors (coils) and capacitors that remove high-frequency noise from the AC mains supply, thus prevents SMPS from sending back electrical noise to the grid.
  • BridgeRectifier and Smoothing Capacitor: The filtered AC voltage then goes through a bridge rectifier, which is a four-diode configuration that transforms the AC voltage to a pulsating DC voltage. For example, a 230V AC input will be somewhere in the range of 325V DC after rectification. The main function of the big electrolytic capacitance is to deliver a steady DC voltage by smoothing out the pulsating DC voltage which is used to power the next stage of the SMPS circuit.
  1. Switching and Transformer Isolation
  • PWM Controller: The PWM controller IC, for example the TL494, generates high-frequency PWM signals, typically between 20 kHz and 100 kHz, which are delivered to the gate of the switching transistor, most commonly a MOSFET.
  • Switching Transistor (MOSFET): The MOSFET is then used to switch the DC voltage that has been rectified, thereby producing AC pulses at the high frequency. The frequency and duration of the on/off switching are regulated by the PWM from the controller, which determines the flow of power to the output.
  • High-Frequency Transformer: The primary winding of a high-frequency transformer is fed with high-frequency AC pulses which perform two crucial functions: isolation between the high-voltage input side and the low-voltage output side and the voltage up or down conversion to match the required output level.
  1. Output Rectification and Filtering
  • SecondaryRectifier Diodes: At the secondary side of the transformer, high-frequency rectifier diodes are responsible for converting the high-frequency AC pulses back to DC voltage. These diodes must be able to handle high reverse voltage and have fast recovery time in order to operate efficiently at high frequencies.
  • Filter Inductor and Capacitor: The rectifier diodes provide the DC voltage which is then smoothed using a filter inductor and capacitor. During the on state of the MOSFET, the inductor stores energy and discharges it during the off state, allowing the current to flow continuously to the load. The filter capacitor then smooths the DC voltage, resulting in a stable output.
  • Output DC Voltage: The final output voltage is simply the product of turns ratio of the transformer and the duty cycle of the PWM signal. To illustrate, if the necessary output voltage is 12 DC V, the feedback loop will guarantee that this voltage is as stable as possible under different load conditions.
  1. Feedback and Voltage Regulation
  • Optocoupler Feedback: The regulated output voltage is given through an optocoupler feedback loop, which is used for ensuring the desired stability and accuracy. Regulation is done by the TL431 shunt regulator as the output voltage is sampled by the voltage divider network on the output side and then compared with a reference voltage. The TL431 optocoupler in the circuit regulates the LED’s brightness; if the output voltage is too high, the LED shines brighter and the stronger signal travels through the phototransistor on the primary side.
  • PWMDuty Cycle Control: The PWM controller IC works with the optocoupler feedback and it determines the duty cycle of the switching signal as per the feedback and it in turn, determines the amount of energy delivered to the secondary side, keeping the output voltage stable.
  1. Protection Circuits
  • Overcurrent Protection: Overcurrent protection is provided by a current sense resistor which is placed in series with the MOSFET drain and if the current exceeds the set limit, the PWM controller turns off the MOSFET to stop any further damage.
  • Overvoltage Protection: Overvoltage protection is achieved through a feedback loop that is to say if the output voltage is above the desired level, the controller decreases the duty cycle and then the output voltage.

Summary of Working Process

  • Input Stage: AC mains voltage → EMI filter → Bridge rectifier → Filter capacitor → DC voltage.
  • Switching Stage: PWM controller → MOSFET → Transformer → High-frequency AC impulses.
  • Output Stage: Rectifier diodes → Filter capacitors → DC output voltage.
  • Feedback Loop: Voltage divider → TL431 regulator → Optocoupler → PWM controller.

Design Considerations

psu (1)

Design considerations for switched mode power supplies (SMPS) include several key aspects that ensure their efficient and stable operation:

  • Topology Selection: The type of converter topology (buck, boost, flyback, etc. ) is the most important choice based on the output voltage and power needs. This results in a change of the complexity and efficiency of the circuit.
  • Component Choice: The choice of components such as MOSFETs, diodes, capacitors, and inductors is very crucial. The selection of these elements should be done in a way to handle the expected load and to minimize losses and electromagnetic interference (EMI).
  • Control Circuits and PWM: The control circuits and PWM controllers need to be used appropriately to ensure the voltage regulation and frequency control are accurate. These two are crucial for the power supply to be able to adapt to varying loads.
  • Protection Mechanisms: Putting the overvoltage, overcurrent, and thermal protection in place ensures the security of the SMPS and the connected devices from potential damage, and this in turn improves the reliability of the power supply.
  • Efficiency and Power Factor: The power factor and efficiency of conversion of energy are the crucial factors to reduce energy waste and the compliance to the regulations, especially when converting from AC mains to a stable DC output.
  • EMI Management: Ensuring that the design is optimized to minimize EMI by using proper layout, shielding, and component selection is the key to avoid interference from other electronic devices and meet international standards.

Common Problems and Solutions of SMPS Design

SMPS designs are often confronted with several common problems. A common problem is a bad voltage regulation, which is usually caused by an unstable feedback loop or an improper selection of control circuits. This can be done using high-quality optocouplers and precision resistors in the feedback network. Then,electromagnetic interference (EMI) is another problem which is caused by high-frequency switching, so the proper PCB layout techniques, EMI filters, and shielding are needed to minimize it. Besides,thermal management is very important as high frequency switching generates heat that may lead to shutdowns. Heat sinks, thermal pads, and ventilation are the ways to resolve this problem. Finally, component failure due to overvoltage and overcurrent is prevented with protection circuits and components that are rated appropriately.


In the future, SMPS designs will focus on higher efficiency, compactness, and the inclusion of new functions. The emergence of GaN (Gallium Nitride) and SiC (Silicon Carbide) semiconductors is a major step forward, which allows for faster switching speeds and less heat generation. This gives the rise to the production of small and highly efficient power supplies with low electromagnetic interference (EMI).


What Role Does the Boost Converter Play in SMPS design?

In SMPS, the boost converter is used to build up the DC input voltage to a higher output voltage by storing energy in the magnetic field of the inductor when it is on and then releasing it to the output when it is off. This circuit is ideal for applications that require increased output voltage so that it can power analog devices and inverters.

How Does a Buck Converter Function in an SMPS Circuit, and What are its Benefits?

The buck converter in the SMPS circuitry is responsible for regulating the DC input voltage to a lower voltage by repeatedly switching the MOSFET on and off, creating a square wave that charges the output inductor and capacitor. The regulator ensures a stable voltage with high efficiency. It is a very useful element in power electronics because of its simplicity and its stable current characteristics.

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