SMPS Circuit Essentials: What You Need to Know


Switch Mode Power Supply (SMPS) units are integral to the functionality of modern electronic devices. They stand out in the electronics market because of their superior efficiency and compact design. Unlike traditional power supplies, SMPS units don’t waste much energy as heat. They convert power at an efficiency rate often exceeding 85%. This is a stark contrast to the older linear power supplies, which typically operate at 40-60% efficiency.

Imagine if we still used those old supplies in today’s high-demand electronics market. With global electronic sales reaching nearly $2 trillion by 2021 (Source: Statista), excess energy usage would be astronomical. SMPS technology not only saves this energy but also leads to less heat dissipation, reducing the need for bulky cooling systems.

Market trends indicate a significant shift toward the adoption of SMPS. By the early 2020s, a substantial number of new electronic devices were predicted to feature SMPS circuits, reflecting their increasing importance in product design (Source: MarketsandMarkets).

What is SMPS

An SMPS, or Switch Mode Power Supply, is a highly efficient power converter. It’s designed to provide devices with the power they need without wasting energy. Let’s dive into what makes an SMPS so valuable.

SMPS epitomize efficiency in power conversion, markedly outperforming traditional linear regulators by minimizing energy losses. Utilizing rapid switching mechanisms, operating at high frequencies, SMPS units achieve efficiencies upwards of 85% — a stark contrast to the 40-60% typical of linear alternatives. Consequently, for every 100 watts drawn, an SMPS proficiently delivers 85 watts, with a mere 15-watt dissipation of excess power. This not only translates to significant energy and cost savings but also mitigates thermal management challenges.

Compactness is another hallmark of SMPS design, dispensing with the bulk of large transformers and heat sinks, a critical advantage in the manufacturing of lightweight, portable electronics such as laptops and smartphones.

Additionally, the consistent voltage output of SMPS technology safeguards sensitive electronic components against power irregularities, thereby bolstering device reliability and prolonging lifespan.

How SMPS Circuit Works

The image depicts the basic block diagram of a Switched-Mode Power Supply (SMPS). Here is how each component works in the process of converting AC mains power to regulated DC output:

  1. AC Mains: This is the alternating current (AC) input from the power grid.

  2. AC Rectifier and Filter: The AC input is first rectified by a set of diodes, which convert the AC to unidirectional current. The rectifier is typically a full-wave bridge rectifier. The filter, often a capacitor, then smooths out the rectified AC to reduce ripple voltage resulting in a pulsating DC.

  3. Switch (Chopper): The smoothed DC is then fed to a switch, commonly a transistor that chops the DC at a high frequency. The high switching frequency allows for a smaller size of the passive components like transformers and filters that follow in the circuit.

  4. Step Down Transformer: The rapidly switching DC is passed through a transformer, which steps down the voltage level. Since the power is being switched on and off quickly, the transformer can be much smaller than one needed at the mains frequency.

  5. DC Rectifier and Filter: The transformer’s secondary winding outputs AC, which needs to be rectified again. This is usually done with diodes or synchronous rectification. The output is then filtered to produce a smooth DC signal.

  6. Regulator (Control Circuit): Finally, a regulator (often implemented with a feedback control system) maintains a constant output voltage despite variations in input voltage or load conditions. It adjusts the operation of the switch to control the energy sent to the transformer and thus the output voltage and current.

The resulting DC output is a regulated DC voltage suitable for powering electronic devices. SMPS designs can be very efficient and provide a stable power source with much less weight and bulk compared to traditional linear power supply designs.

SMPS Topologies

  • Buck Converter

A buck converter is a form of DC-DC converter designed to regulate and step down voltage from its input (VIN) to a lower output voltage (VOUT). This is achieved through the operation of a semiconductor switch, typically a transistor labeled Q1, which is modulated at a high frequency.

During the “on” phase, Q1 conducts, allowing current to pass through the inductor (L), which accumulates energy within its magnetic field, simultaneously ensuring the continuity of inductor current. Upon deactivation of Q1, the magnetic field within the inductor collapses, which perpetuates the flow of current towards the load, now via diode D1.

A capacitor is incorporated into the design to stabilize the output voltage and sustain the correct polarity. The precise output voltage is controlled by modulating the duty cycle of Q1 — the proportion of the “on” time relative to the “off” time. By adjusting this duty cycle, the buck converter can consistently deliver a regulated output voltage that is less than the input voltage, aligning with the requirements of the load.

  • Boost Converter

The image illustrates a basic schematic of a boost converter, a type of DC-DC power converter that steps up (boosts) a lower input voltage to a higher output voltage. It operates by storing energy in an inductor (L) when a switch (Q1, typically a transistor) is closed, causing the current (IL) to build up. When the switch is opened, the inductor releases its stored energy through a diode (D1) to the load, resulting in an output voltage (VOUT) that is higher than the input voltage (VIN). A capacitor (CO) is used to smooth out fluctuations in the output voltage to ensure a stable supply. Boost converters are widely used in electronic devices where the power supply voltage is below the level required by the device, and the capacitor discharges through the load.

buck converter

  • Buck-Boost Converter

Buck-boost converter circuitry is designed to convert an AC input to a regulated DC output at a voltage level that can be above or below the input voltage level.

The bridge rectifier at the input converts AC to DC. The DC voltage is then processed by a switch (typically a transistor), controlled by a driver IC, which switches the current flow through an inductor. Depending on the duty cycle of the switch, the inductor either boosts the voltage (when the switch is open) or bucks it (when the switch is closed), hence the name buck-boost converter. The output capacitor (Cout) smooths the output voltage to provide a stable DC output.

This configuration is often used in applications where the input voltage can vary above and below the output voltage level, requiring both step-up and step-down conversion.

  • Flyback Converter

Flyback converter is used to step up or down voltages efficiently. It operates by first storing energy in the magnetic field of a transformer when the switch Q1 is closed, and then releasing that energy to the load when the switch is open. The transformer’s secondary winding (Ns) and primary winding (Np) have a turns ratio that determines the output voltage. Diode D1 rectifies the current, providing a unidirectional output, while capacitor Cs and the output capacitor (not labeled) smooth out the voltage ripples. Diode D2 prevents reverse currents during the switch-off phase. This topology is commonly used in applications where electrical isolation is necessary between the input and the output.

  • Forward Converter

The forward converter is used in power supply regulation where direct current from one voltage level is changed to another. It operates by storing energy in a magnetic field within a transformer when the input voltage is applied to the primary winding, and then transferring this energy to the secondary winding to provide a different voltage level. A key feature is the use of a transistor, which acts as a switch, turning on and off to control the power flow through the transformer. When the transistor is on, energy is transferred; when off, the transformer’s magnetic field collapses, transferring the remaining energy to the output. Diodes are used to direct the flow of current, ensuring it remains unidirectional. Forward converters are known for good efficiency and are widely used in moderate power applications, especially for lower voltage levels.

Overview Table

Buck Converter
Boost Converter
Buck-Boost Converter
Flyback Converter
Forward Converter
Basic Function
Converts a higher input voltage to a lower, stable output voltage
Converts a lower input voltage to a higher output voltage
Combines the functions of buck and boost converters; can increase or decrease voltage
Provides electrical isolation and voltage conversion, can step-up or step-down
Provides electrical isolation, commonly used for stepping down voltage
Single switch, single inductor, no isolation
Single switch, single inductor, no isolation
Single switch, single inductor, no isolation
Transformer-based, provides isolation, single or multiple outputs
Transformer-based, provides isolation, usually single output
High (typically over 90%)
High (typically over 90%)
High but usually less than separate buck or boost converters
Medium to high but lower than non-isolated topologies
High, but generally lower than flyback converters
Output Ripple
Medium to high (depends on design)
Low to medium
High (requires isolation transformer)
Medium to high
Power modules, battery chargers
LED drivers, battery-powered devices
Battery charging and discharging systems, renewable energy systems
Switched-mode power supplies, isolated converters
Low power AC/DC supplies, electronic transformers
Regulation Range
Limited, only below input voltage
Wide, output voltage can be much higher than input
Wide, output can be above or below input
Wide, adjustable via turns ratio of transformer
Wide but limited by transformer turns ratio and switch stress
Main Advantages
Simple, high efficiency, low cost
High voltage can be achieved without complex control
High flexibility for various voltage conversions
Provides isolation, suitable for multiple outputs
Higher efficiency and lower output ripple compared to flyback topology
Main Disadvantages
Output voltage is limited to below input voltage
Cannot step down voltage
Higher output ripple and less intuitive
More complex control, higher cost
Relatively complex, may require auxiliary winding

Designing an SMPS Circuit

When designing a Switched-Mode Power Supply (SMPS) circuit, attention to detail is crucial for both performance and safety. Here are specific guidelines to consider:

  1. Component Selection:

    • Transformers: Choose a transformer that matches your circuit’s needs—for instance, a 20W power supply operating at 85-265VAC input would benefit from a transformer rated at 24V/1A output. Inadequate rating may cause losses or thermal issues.

    • Switching Elements: Opt for a MOSFET like the IRF540N for its high current capacity (33A) and fast switching (77ns on-time) to ensure your SMPS operates efficiently without switching losses.

    • Capacitors: Select low-ESR capacitors, which can handle ripple currents and have high endurance, rated at 35V when your circuit operates at 24V, to provide a margin for voltage spikes and improve longevity.

  2. Topology Choice:

    • Flyback, Forward, Push-Pull, Half-Bridge, and Full-Bridge converters each present a distinct set of advantages and drawbacks, contingent upon the required output power level and the degree of complexity that can be managed effectively. For applications that necessitate low to medium power output, the Flyback converter is frequently selected owing to its inherent simplicity and ease of implementation.

  3. Control Scheme:

    • When determining the appropriate control methodology for your application, it is crucial to assess the necessity for a basic fixed-frequency controller versus an advanced solution incorporating synchronous rectification for enhanced efficiency. Controllers utilizing Pulse-Width Modulation (PWM) are standard in the industry for modulating output; however, they necessitate meticulous calibration of both frequency and duty cycle to ensure optimal performance.

  4. Thermal Management:

    • Determine the requisite thermal management strategy and engineer a suitable thermal dissipation system. Incorporate the utilization of heat sinks on power components, taking into account the necessity for sufficient space to facilitate optimal airflow. It is imperative to mitigate the risk of overheating which can diminish the longevity of the components and may precipitate the malfunction of the Switched-Mode Power Supply (SMPS).

  5. Protection Features:

    • Design specifications should encompass robust protection strategies to shield the circuit from overvoltage, overcurrent, and overtemperature anomalies, thus preserving its operational integrity. It is also recommended to embed a soft-start feature within the system architecture. This will serve to regulate the inrush current at the commencement of the power-up cycle, providing a safeguard against the risk of damage to electronic elements.

  6. PCB Layout:

    • Ensure that traces carrying high current are both wide and succinct to mitigate voltage drops and circumvent potential interference. It is imperative to maintain a distinct separation between high voltage components and low voltage control circuitry to preclude noise coupling. Furthermore, the positioning of the switching element should be in close proximity to the transformer, which is essential to minimize parasitic effects and reduce electromagnetic interference (EMI) for optimal performance.

Advantages of Using SMPS

Switch Mode Power Supplies (SMPS) are the heart of modern electronics, giving them the power they need but with benefits that are clear and measurable. Let’s break down these advantages:

First, we talk efficiency. SMPS are champions here, often reaching 80-90% efficiency. That’s a big jump from the 40-60% of linear supplies. Imagine a scenario where two power supplies consume 100 watts from the wall. The SMPS might only waste 10-20 watts as heat, while a linear might lose 40-60 watts, resulting in less power loss. Over time, this adds up, saving energy and money, especially in high-power applications.

Now, size and weight – crucial for portable devices. High-frequency operation of SMPS means smaller transformers, resulting in lighter, smaller units. For example, a laptop’s power supply is small enough to fit in your hand, largely thanks to SMPS technology. In contrast, an old-fashioned linear power supply could be as big as a brick.

Then there’s load and line regulation. With SMPS, the output stays stable, no matter if the input voltage changes or if different amounts of power are drawn. This means your devices work smoothly without interruption and reach a steady state. A typical SMPS might maintain a voltage regulation within +/- 0.5% despite changes, whereas a linear supply might fluctuate up to +/- 5% volts.

Lastly, less heat equals less cooling. Because SMPS are more efficient, they turn less power into wasted heat. This means smaller heat sinks and fans, making the whole unit cheaper and more reliable. Think of a buzzing, hot power adapter versus a cool, silent one—that’s the difference SMPS can make.


In summary, the realm of switch mode power supply (SMPS) circuits is intricate and engrossing. These circuits ensure a stable voltage supply, deftly managing the nuances of voltage regulation and the conversion mechanism. They play a pivotal, though often underappreciated, role in the functionality of modern electronic equipment. Understanding the fundamentals of SMPS, from the block diagram to the actual implementation of switch mode psu circuit diagrams, sheds light on the incredible technology that powers our daily lives. As we embrace high frequencies and seek out low ripple and high efficiency, the SMPS stands as a testament to human ingenuity in the face of ever-evolving electronic needs.
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