Switching Power Supply Design with Active Power Factor Correction for Harmonic Reduction in Embedded Systems

The Hidden Power Challenge in Embedded Systems

When designing embedded systems, attention usually focuses on the processor, memory capacity, and software efficiency. However, none of these intelligent circuits work without a suitable power supply. In practice, converting electrical energy from the wall outlet down to the low DC voltages required by microcontrollers encounters an invisible physical problem: the quality of the consumed power. Traditional switching power supplies, those using rectifiers and capacitors right at the input, draw current from the electrical grid in short, intense bursts rather than drawing it smoothly and continuously.

This behavior generates a phenomenon called harmonic distortion, which overloads the electrical grid with frequencies that are multiples of the fundamental fifty or sixty hertz frequency. To the average user, the light turns on and the equipment starts normally, but for power utilities and local transformers, these deformed currents mean high thermal losses and premature equipment aging. In industrial or high-volume embedded systems, ignoring this issue results in regulatory fines and electromagnetic compatibility failures.

Understanding Power Factor and Harmonic Impact

Power factor is the measure of efficiency with which electrical energy is used by a system. In an ideal scenario, voltage and current are perfectly synchronized and follow a pure sinusoidal curve, resulting in a power factor of one. However, when we insert a rectifier bridge followed by a large electrolytic capacitor at a power supply input, current only flows to recharge the capacitor during a brief instant at the peak of the voltage wave. In practice, this means useful energy is lower than the apparent energy circulating through the cables.

The non-sinusoidal currents generated by this process can be mathematically decomposed into multiple sine waves of higher frequencies, known as harmonics. These unwanted frequencies do not perform useful work, but they circulate through conductors causing excessive heating and electromagnetic interference in neighboring communication systems. To mitigate this scenario, electronic engineering developed correction techniques, divided essentially between passive approaches using heavy coils and capacitors, and active approaches employing semiconductors controlled by dedicated integrated circuits.

Boost Converter Architecture for Active Correction

The most widespread and efficient solution for power factor correction is the use of a boost converter placed before the main stage of the switching power supply. In practice, this circuit acts as an intelligent modulator that forces the input current to strictly follow the shape of the grid voltage. When the grid voltage rises or falls, the controller quickly adjusts the semiconductor switch, typically a MOSFET transistor, ensuring that energy is absorbed uniformly throughout the entire cycle.

The operation of this intermediate stage relies on high-frequency switching, in the range of tens to hundreds of kilohertz, which allows the use of magnetic components and capacitors considerably smaller and lighter than those required in low-frequency systems. The circuit continuously monitors input voltage and drained current, adjusting the transistor duty cycle to keep the voltage on the intermediate DC bus stable and high, typically four hundred volts, regardless of outlet fluctuations.

// Conceptual example of sensor reading and digital PFC control loop in firmware
uint16_t read_ac_voltage(void) {
    // Reads the ADC channel connected to the AC voltage divider
    return ADC_Read(CHANNEL_AC_VOLTAGE);
}

void adjust_pfc_pwm(uint16_t target_voltage, uint16_t current_voltage) {
    if (current_voltage < target_voltage) {
        // Increase duty cycle to raise energy stored in the inductor
        Increase_Duty_Cycle_PWM();
    } else {
        // Decrease duty cycle to stabilize the bus
        Decrease_Duty_Cycle_PWM();
    } 
}

Conduction Modes and Component Selection

When designing an active power factor correction circuit, the engineer must decide between continuous conduction mode and discontinuous conduction mode for the main inductor. In continuous conduction mode, current flowing through the inductor never drops to zero during the switching cycle. This characteristic reduces peak current in semiconductors, making the system ideal for medium and high power levels, though it requires strict attention to diode reverse recovery to prevent excessive losses.

On the other hand, in discontinuous conduction mode, inductor current drops completely to zero before each new switching cycle. This simplifies control and eliminates output diode reverse recovery losses, but introduces higher current peaks that require components with greater instantaneous handling capacity and more robust input noise filters. Proper selection depends directly on the power range of the embedded application and physical space constraints inside the product enclosure.

Thermal Challenges and Electromagnetic Compatibility

Introducing an additional energy conversion stage brings expressive advantages in power quality, but imposes new thermal and noise challenges. Because energy passes through two consecutive conversion stages, overall supply efficiency may suffer minor reductions if semiconductors are not chosen carefully. In practice, using silicon carbide transistors has revolutionized this design, as these devices drastically reduce switching losses compared to traditional silicon transistors.

Furthermore, the high switching frequency required to shape current generates conducted and radiated electromagnetic noises that must be strictly filtered. Including filter networks at the supply input is mandatory to ensure equipment complies with limits established by international compatibility standards. Careful printed circuit board layout design, separating high-current paths from sensitive control signals, is the determining factor between a product passing laboratory tests and an unstable prototype.

Final Considerations

The design of switching power supplies with active power factor correction is no longer a luxury restricted to large industrial equipment, having become a fundamental requirement for modern embedded systems. The constant pursuit of energy efficiency and compliance with increasingly stringent environmental regulations impose new demands on hardware design engineers. Mastering boost converter principles, harmonic dynamics, and proper conduction mode selection ensures the development of robust, reliable, and environmentally sustainable products.

In practice, investing time in correct input stage modeling avoids costly redesigns during the certification phase and ensures the embedded system operates with maximum stability throughout its lifecycle. Harmonious integration between power electronics, digital control, and thermal management is the optimal path for creating efficient and compact energy converters.