Choosing Rectifier & Capacitor for AC-DC Conversion

SMPS topologies are classified as Isolated (Flyback, Push-Pull, Half-Bridge, Full-Bridge using transformers) and Non-Isolated (Buck, Boost, Buck-Boost, Cuk using inductors). High-frequency operation (via E = 4.44fΦN) enables smaller transformers compared to 50/60 Hz designs. Pulse Width Modulation controls output voltage through duty cycle (D = on-time/period): Buck converters produce Vout = D×Vin (step-down); Boost converters produce Vout = Vin/(1-D) (step-up); Buck-Boost converters allow Vout = Vin×D/(1-D) (both step-up and step-down). Advantages include lighter weight and higher efficiency; disadvantages include switching losses, RFI generation, and higher output ripple (~100mV).

Switch-mode power supplies (SMPS) are preferred over traditional linear regulators and 50/60 Hz transformers because they offer both high efficiency and compact size. Unlike linear regulators that dissipate excess power as heat, SMPS uses switches (MOSFETs or BJTs) and energy storage components (inductors and capacitors) to transfer power through controlled switching at specific duty ratios and frequencies. The three basic unisolated topologies are: (1) Buck/step-down converter, which reduces output voltage below input voltage using a single inductor; (2) Boost/step-up converter, which increases output voltage above input voltage; and (3) Buck-boost converter, which can produce output voltage either lower or higher than input voltage but with reversed polarity. For isolated applications, the flyback converter is the most popular topology, operating in discontinuous mode where energy is stored in the magnetic element during the primary stroke and released during the secondary stroke. Output regulation is achieved through feedback mechanisms, with secondary sensing providing better accuracy while primary sensing offers cost savings. Resonant converters like LLC topologies use resonant phenomena for efficient power transfer by varying the operating frequency relative to the resonant frequency.

Switch mode power supplies convert AC line voltage to DC through bridge rectification and filtering, then use a switching transistor operating at high frequencies (e.g., 100 kHz) to drive a transformer. This high-frequency operation enables much smaller transformers compared to linear supplies because core size decreases with increasing frequency. The design includes current limiting via sense resistors and optoisolators for primary-secondary isolation. However, this topology generates significant electromagnetic interference requiring elaborate filtering, making linear supplies preferable for sensitive applications like radio equipment.

This section covers DC-DC conversion applications and comprehensive comparison between SMPS and linear power supplies. SMPS can perform pure DC-to-DC conversion using three main topologies: buck converters (step-down), boost converters (step-up), and buck-boost converters (either direction). Without isolation transformers, these converters lack electrical isolation between input and output. The voltage regulation mechanism uses a feedback loop where the regulator monitors output voltage against a reference and adjusts the MOSFET duty cycle accordingly—this is pulse width modulation. Feedback transmission typically uses opto-couplers for isolation. Comparative analysis shows SMPS advantages: smaller/lighter design (high-frequency reduces component sizes), higher efficiency (90%+ vs. linear), and greater input flexibility. Disadvantages include increased complexity, higher costs, and potential EMI issues. Despite these challenges, SMPS dominate modern electronics due to their substantial performance benefits over traditional linear designs.

Switch mode power supplies (SMPS) represent a revolutionary approach to power conversion that offers superior efficiency, compact size, and lightweight construction compared to traditional linear power supplies. SMPS achieve this by operating at very high frequencies (above 20 kHz) using square wave signals, enabling much smaller transformers with ferrite cores. MOSFET transistors switch between saturation and cut-off modes, minimizing power dissipation during switching. A basic SMPS consists of five main blocks: (A) Input protection and filter with fuse and EMC components, (B) Full bridge rectifier with primary filtering to produce ~320V DC, (C) Switching transistors controlled by PWM that chop DC into high-frequency pulses, (D) Transformer and secondary rectifier diodes, and (E) Output filter with feedback. This architecture enables efficient power conversion with minimal energy loss, making SMPS the preferred choice for modern electronic devices.

Practical thermal design involves calculating required heatsink specifications and selecting appropriate components. The required heatsink thermal resistance equals calculated JA minus JC. For example, with JC = 2°C/W and calculated JA = 31.6°C/W, the required heatsink thermal resistance is 29.6°C/W. Higher input voltages increase power dissipation: with 18V input and 0.5A output, power dissipation becomes 3W, requiring a heatsink. Practical demonstration shows the 7812 regulator maintaining approximately 11.4V output when unloaded, dropping to 9V under load. Power supply connections include AC transformer, rectifier, and filter capacitors. Operating at higher currents (e.g., 1A) increases junction temperature, potentially requiring heatsinking. Proper thermal management ensures reliable operation and extends component lifespan.

This video explains how to calculate the steady-state junction temperature of power electronic devices using thermal resistance concepts analogous to electrical circuits, where power losses act as current sources and thermal resistances (measured in °C/W) represent conduction barriers between the device junction, case, heatsink, and ambient; the thermal resistance values are obtained from component datasheets and heatsink specifications, and proper heatsink selection involves analyzing thermal dissipation characteristics under natural or forced cooling conditions while applying safety factors to account for real-world uncertainties.

The average power dissipated in a diode is calculated as the sum of power from the internal voltage source (P = Vt × Imean) and power from the internal resistance (P = R × Irms²), where Vt is the internal voltage, Imean is the average current, R is the internal resistance, and Irms is the RMS current. This power generates heat at the semiconductor junction, requiring thermal management through a thermal circuit model consisting of three resistances: junction-to-case (Rjc), case-to-sink (Rcd), and sink-to-ambient (Rds). The thermal resistance is defined as the temperature difference divided by power dissipation (°C/W). To select an appropriate heat sink, calculate the total thermal resistance from junction to ambient (Rja = (Tjmax - Tamb)/Pavg), then determine the required sink resistance (Rds = Rja - Rjc - Rcd), ensuring the junction temperature does not exceed maximum limits.

Thermal resistance (Rth) measures opposition to heat flow, measured in Kelvin per Watt (K/W). For TO-220 packages without heatsinks, RthJC is approximately 90 K/W, meaning 1W causes 90°C temperature rise. The total thermal path includes: junction-to-case (RthJC), case-to-heatsink (RthCS), and heatsink-to-ambient (RthSA). To select a heatsink, calculate required RthSA using: RthSA = (Tj_max - Tamb) / P - RthJC - RthCS. For example, with Tj_max=150°C, Tamb=30°C, P=25W, RthJC=2.5 K/W, and RthCS=1 K/W, the required RthSA is 1.3 K/W.

In voltage regulator circuits, power dissipation is calculated as P = (Vin - Vout) × Iload. For a TIP122 regulator with Vin = 9.1V, Vout = 3.3V, and Iload = 1.1A, power dissipation reaches 7.04W. The maximum allowable power without a heatsink is determined by Pmax = (Tjmax - Tambient) / RθJC. For Tjmax = 50°C, Tambient = 25°C, and RθJC = 62.5°C/W, Pmax = 0.4W. Since 7.04W exceeds this threshold, a heatsink is required.

PFC circuits require multi-stage electromagnetic interference (EMI) filtering to protect against noise. Differential mode noise is filtered using capacitors C1 (2.2 microfarad) connected across the input lines. Common mode noise is filtered using capacitors C8 and C9 connected between the line and earth. Additionally, a tuned network consisting of an inductor and resistor protects against high-frequency spikes generated by the 45kHz switching operation. These filters create barriers that prevent external noise from entering the circuit and prevent internal noise from returning to the power lines, ensuring clean power delivery.

An EMI (Electromagnetic Interference) filter is used to prevent 50 Hz AC line noise from entering the SMPS circuit. It typically consists of an inductor (coil) and capacitors. The PFC (Power Factor Correction) capacitor is a non-polarized AC capacitor (120uF, 275V) used because AC lines have no polarity. The inductor can be salvaged from old televisions or similar devices.

Harmonics and electromagnetic interference (EMI/RFI) are distinct issues: harmonics are low-frequency distortions while EMI is high-frequency noise from switching transistors. EMC filters ground this noise but must be enabled only on balanced, grounded systems. Utilities increasingly require high power factor thresholds (often 0.95 minimum). Standard drive claims of 0.98 displacement PF mask true PF of approximately 0.75 when harmonics are considered. Multi-pulse drives represent outdated technology. Matrix drives provide better overall efficiency than active front-end solutions while achieving compliance.

A PFC circuit typically includes: a coil (inductor), a MOSFET switch operating at high frequency, and a diode. The MOSFET creates a switching action that shapes the current to be sinusoidal and in phase with the voltage. This switching occurs at high frequencies (typically 20-100 kHz), which is why EMI filters are necessary to block these switching frequencies. The circuit ensures the current absorbed from the 230V source is in phase with the voltage, achieving a power factor of 1.

Active PFC (Power Factor Correction) circuits use MOSFETs in parallel with a diode and inductance to improve power efficiency. The presence of EMI filters before the bridge rectifier and a radiator with additional semiconductors indicates an active PFC design.