Voltage Regulators & SMPS

Cheatsheet Content

1. Power Supply Block Diagram Transformer Rectifier Filter Regulator Output Function: Converts AC mains to stable DC voltage. Stages: Transformer $\rightarrow$ Rectifier $\rightarrow$ Filter $\rightarrow$ Regulator. 2. Transistor Series Voltage Regulator Principle: A series pass transistor controls output voltage by varying its resistance. Components: Pass Transistor: $Q_1$ (e.g., NPN BJT) in series with the load. Error Amplifier: Compares $V_{out}$ with a reference. Voltage Reference: Zener diode or bandgap reference. Operation: If $V_{out}$ increases, error amplifier reduces $Q_1$'s base current, increasing $Q_1$'s $V_{CE}$ and decreasing $V_{out}$ back to desired level. Types: Fixed and Variable Fixed: Output voltage is set by internal resistor divider. (e.g., 78xx, 79xx series). Variable: Output voltage can be adjusted externally (e.g., LM317). 3. Linear Voltage Regulator Block Diagram Input $V_{in}$ Pass Transistor Error Amplifier Output $V_{out}$ Voltage Reference Ref FB Key Feature: Operates in linear region, dissipates excess power as heat. Advantages: Low noise, fast transient response. Disadvantages: Low efficiency (especially with large $V_{in}-V_{out}$ difference), requires heat sink. 4. IC 317 (Positive Adjustable Regulator) Pins: ADJ (Adjust), OUT (Output), IN (Input). Output Voltage: $V_{out} = V_{ref} \left(1 + \frac{R_2}{R_1}\right) + I_{ADJ} R_2$ $V_{ref} \approx 1.25V$ (internal reference). $I_{ADJ}$ is very small ($50-100\mu A$), often neglected. Typical Circuit: Connect $R_1$ from OUT to ADJ, and $R_2$ from ADJ to ground. $V_{in}$ to IN. Features: Overload protection, thermal shutdown. 5. IC 337 (Negative Adjustable Regulator) Pins: ADJ (Adjust), OUT (Output), IN (Input). Output Voltage: $V_{out} = -|V_{ref}| \left(1 + \frac{R_2}{R_1}\right) - I_{ADJ} R_2$ $|V_{ref}| \approx 1.25V$. Typical Circuit: Similar to LM317 but for negative voltages. $R_1$ from OUT to ADJ, $R_2$ from ADJ to the most negative voltage rail. 6. Current Boosting for Linear Regulators Purpose: To increase the output current capacity beyond the IC's rating. Method: Add an external pass transistor (e.g., NPN BJT for positive regulators, PNP for negative) in parallel with or driven by the regulator's output. Vin --+-- R_sense --+-- Load | | | E | / +---- R_boost --- B Q_boost | \ | C | | Regulator_Out--+ Operation: When load current exceeds a threshold, the external transistor turns on, shunting current around the regulator. 7. Low Dropout Regulator (LDO) Definition: A linear voltage regulator that can operate with a very small difference between input and output voltage ($V_{in} - V_{out}$, called dropout voltage). Key Feature: Uses a P-channel MOSFET or PNP BJT as the pass element. PNP/PMOS can be fully saturated (or in triode region for PMOS) with a small $V_{CE(sat)}$ or $V_{DS(on)}$. Advantages: High efficiency when $V_{in}$ is close to $V_{out}$, smaller heat sink, suitable for battery-powered devices. Disadvantages: Pass element current flows even if regulator is off, susceptible to load transients if output capacitor is not chosen carefully. 8. Switched Mode Power Supplies (SMPS) Principle: Uses a switching element (MOSFET/BJT) to rapidly turn the input voltage on and off, followed by an LC filter to smooth the output. Advantages: High efficiency (typically 70-90%+), smaller size/weight (no bulky transformer for mains), generates less heat. Disadvantages: Higher ripple voltage, more complex design, generates EMI, slower transient response. SMPS Block Diagram Input $V_{in}$ Switching Element Filter (L,C) Output $V_{out}$ Control Circuit Feedback Types of SMPS (DC-DC Converters) Buck Converter (Step-Down): $V_{out} Operation: Switch rapidly connects $V_{in}$ to an inductor. When switch opens, inductor current flows through diode to load/capacitor. Average $V_{out} = D \cdot V_{in}$, where $D$ is duty cycle. Boost Converter (Step-Up): $V_{out} > V_{in}$ Operation: Switch connects inductor to ground, storing energy. When switch opens, inductor voltage adds to $V_{in}$, delivering higher voltage to load/capacitor. Average $V_{out} = V_{in} / (1 - D)$. Buck-Boost Converter: $V_{out}$ can be higher or lower than $V_{in}$, and is inverted. Flyback Converter: Isolated buck-boost, often used in AC-DC adapters.