--- title: Power Supply Circuits url: https://doc.liz6.com/en/hardware/04-analog-circuits/03-power-supply-circuits locale: en area: hardware tags: - hardware - analog-circuits date: 2026-06-30 modified: 2026-07-16 description: 'Power Supply Circuits Power System Architecture Power System Architecture: Two Input Paths Converge to Regulated DC Output Path 1: AC Input AC Mains Transformer…' --- # Power Supply Circuits ## Power System Architecture Power System Architecture: Two Input Paths Converge to Regulated DC Output Path 1: AC Input AC Mains Transformer Rectification Filtering Regulation DC Output or Path 2: DC Input DC Input DC-DC Conversion Regulated Output Typical System Example Mains 220V AC AC-DC 12V DC POL 5V / 3.3V 1.8V / 1.2V… (Point of Load) Both input paths (AC or DC) ultimately converge to a regulated DC output. In actual systems, a 12V main supply is often generated first, which is then converted nearby by POL (Point of Load) into multiple voltages such as 5V / 3.3V / 1.8V / 1.2V. --- ## Linear Regulator ### Basic Principle Basic Principle of Linear Regulator: Pass Transistor Dropout Voltage = Vin − Vout Vin (Unregulated) Pass Transistor (BJT/MOS) Operates in Linear Region ≈ Variable Resistor Vout (Regulated) Error Amplifier Compares Vout with Vref Controls Pass Transistor Sense Vout (Feedback) Control Signal Vref (Reference) Essence: Pass Transistor Dropout Voltage = Vin − Vout; Power Dissipation = (Vin−Vout) × Iout. The larger the dropout voltage and current, the more severe the heating—this is the root cause of low efficiency in linear regulators. ### Pros and Cons ``` Pros: ✓ Low noise, low ripple ✓ Fast transient response ✓ Simple circuit (few external components) ✓ No EMI issues Cons: ✗ Low efficiency: η ≈ Vout/Vin Example: 12V→5V η=42% (58% becomes heat!) ✗ Can only step down ✗ Severe heating with large dropout/current ``` ### LDO (Low Dropout) ``` Standard linear regulators require Vin - Vout > 2~3V LDOs can achieve Vin - Vout < 0.5V (even 100mV!) Pass transistor uses PNP or P-MOSFET → Dropout is only Vce(sat) or I×Rds(on) Selection: Dropout requirement > 1V → 78xx (7805, 7812... cheap) Dropout requirement < 0.5V → LDO (AMS1117, LP2985...) ``` ### Typical Chips | Model | Vout | Imax | Dropout | Features | |------|------|------|------|------| | 7805 | 5V | 1.5A | 2V | Classic 3-terminal regulator | | LM317 | Adjustable | 1.5A | 2V | Adjustable output: Vout=1.25(1+R2/R1) | | AMS1117-3.3 | 3.3V | 1A | 1.1V | Common LDO | | LP2985-3.3 | 3.3V | 150mA | 280mV | Ultra-low dropout, low noise | --- ## Switching Regulator ### Basic Principle ``` Switch turns ON/OFF continuously → Energy storage elements (L, C) smooth energy → Output Vin ──[Switch]──[L]─── Vout │ │ [PWM] [C] │ GND PWM: Pulse Width Modulation → Regulate duty cycle D to stabilize output Frequency: Typically 100kHz ~ 2MHz ``` ### Comparison with Linear Power Supplies | Feature | Linear | Switching | |------|------|------| | Efficiency | Low (30-60%) | High (80-95%) | | Noise | Very low | Switching ripple + EMI | | Complexity | Simple | Complex (requires L, C, compensation) | | Size | Large (needs heat sinking) | Small | | Step-down only? | Yes | No (can step up or down) | --- ## Three Basic Topologies ### Buck (Step-Down) ``` ┌── L ──┬── Vout │ │ Vin ──[SW]───┤ ┌┴┐ │ │C│ └──▸├──┴┬┘ │ GND SW ON: L charges, C supplies load SW OFF: L freewheels through diode Vout = D × Vin (D = Ton/T) D < 1 → Vout < Vin ``` ### Boost (Step-Up) ``` ┌── L ──┬──▸├── Vout │ │ │ Vin ──┤ [SW] ┌┴┐ │ │ │C│ └───────┴───────┴┬┘ │ GND SW ON: L stores energy SW OFF: Induced voltage of L adds to Vin → Vout > Vin Vout = Vin / (1-D) ``` ### Buck-Boost (Step-Up/Step-Down) ``` Vout = -Vin × D/(1-D) (Inverting output) Can output voltage higher or lower than input (inverted polarity) ``` --- ## Practical Design Considerations ### Input/Output Capacitors ``` Input: Large electrolytic + high-frequency ceramic (0.1μF) Electrolytic handles low-frequency ripple, ceramic handles high-frequency spikes Output: Selected based on ripple requirements ΔV = ΔI/(8×f×C) + ΔI×ESR (Buck) ``` ### Inductor Selection ``` Buck: L = (Vin-Vout)×D / (ΔI×f) ΔI recommended ≈ 0.2~0.4 × Iout Isat > Iout + ΔI/2 (Peak current) ``` ### Layout Considerations ``` 1. Minimize switch loop area! (Smaller loop: Vin → SW → L → Cout → GND results in lower EMI) 2. Keep feedback traces away from inductors and switching nodes 3. Place input capacitor close to IC pins 4. Use GND pour, single-point grounding, or solid ground plane ``` --- ## Protection Features | Protection | Description | |------|------| | OCP (Over-Current) | Limits or shuts off when output current exceeds limit | | OVP (Over-Voltage) | Shuts off when output voltage is too high | | OTP (Over-Temperature) | Shuts off when chip temperature exceeds limit (thermal shutdown) | | UVLO | Shuts off when input voltage is too low, preventing abnormal operation | | Soft Start | Limits inrush current during startup | --- ## Common Chips Quick Reference | Chip | Type | Vin | Vout | Iout | Frequency | |------|------|-----|------|------|------| | LM2596 | Buck | 4.5-40V | Adjustable | 3A | 150kHz | | MP1584 | Buck | 4.5-28V | Adjustable | 3A | 1.5MHz | | MT3608 | Boost | 2-24V | Adjustable | 2A | 1.2MHz | | XL6009 | Boost | 5-32V | Adjustable | 4A | 400kHz | | TPS5430 | Buck | 5.5-36V | Adjustable | 3A | 500kHz | --- *Keywords: Linear Regulation, LDO, Switching Power Supply, Buck, Boost, PWM, Efficiency, Ripple, EMI*