Power Supply Circuits

Power System Architecture

Linear Regulator

Basic Principle

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

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

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

Common Chips Quick Reference

Keywords: Linear Regulation, LDO, Switching Power Supply, Buck, Boost, PWM, Efficiency, Ripple, EMI