Protection Circuits

The difference between engineering and experimentation: Protection circuits keep the board alive through the first power-on

ESD Protection

Problem

Human body static electricity: Up to 15kV (discharge you can't feel)
Device tolerance: MOSFET gate ~20V, CMOS IC ~2kV (HBM)

Dry winter, walking on carpet in slippers → touch the board → IC dies
Symptoms: "It worked yesterday, but not today."

Protection Components

TVS Diode (Most Common)

Selection:
  Operating Voltage Vrwm > Maximum normal voltage of the protected signal
  Clamping Voltage Vcl < Maximum withstand voltage of the protected device
  Power: Based on ESD rating (IEC 61000-4-2)

Layout Golden Rule:
  TVS must be placed next to the connector, not next to the IC!

  Trace: Connector → TVS (as close as possible) → Series Resistor/Ferrite Bead → IC
  Keep the TVS to GND loop as short as possible

USB Signal Line ESD Protection

USB 2.0 (D+/D-): Use dedicated USB ESD chips
  Example: USBLC6-2, SRV05-4
  Capacitance < 2.5pF (otherwise it affects high-speed signals)

USB 3.x (SuperSpeed): Must use ultra-low capacitance TVS
  Example: TPD4E05U06 (< 0.5pF)

VBUS: Use 5V~24V TVS (depending on PD voltage)

GPIO / Low-Speed Signals

Simple Solution: Series Resistor + Shunt TVS
  GPIO ── 100Ω ──┬──  TVS → GND
                  └──  To External

Resistor limits ESD current, TVS clamps voltage
100Ω ~ 1kΩ, depending on signal speed

Reverse Polarity Protection

Problem

DC barrel jack / terminal block → Polarity reversed → Board burns out
Especially: Late at night, under deadline, when wiring for others

Solution 1: Series Diode (Simplest)

Vin ──▸├── System
         │
      (Voltage Drop Vf)

Pros: Simplest
Cons: Voltage drop 0.3~0.7V, severe heating at high currents
      P_loss = Vf × I → 3A×0.7V=2.1W pure waste

Applicable: <1A low current / voltage drop insensitive

Solution 2: PMOS Reverse Polarity Protection ★ Recommended

         S   D
Vin ────┬── P-MOSFET ──── System
        │   G
        │   │
        └───┤├── GND
            Rgs (10k~100k)

Normal: Vin+ connected to S, G pulled to GND via R
      → Vgs = -Vin → PMOS turns on → Almost no voltage drop

Reverse: G pulled to Vin+ via R → Vgs ≈ 0 → PMOS turns off
      → Body diode reverse biased → System has no power

Voltage Drop = I × Rds(on) → Typically < 0.1V @ 3A (Rds(on)=30mΩ)
Loss = I² × Rds(on) = 0.27W @ 3A (vs 2.1W for diode!)

Select MOSFET: Lower Rds(on) is better, Vgs(th) must be lower than Vin
       Vds > Vin (with margin)

Solution 3: Bridge Rectifier (Fully Automatic)

Works regardless of polarity, but voltage drop is 2×Vf (two diodes in series)
Applicable: Cost-insensitive, low current

Fuse Selection

Types

Selection Parameters

Rated Current In: Normal operating current × 1.25 (with margin)
        Example: Operating at 2A → Select 2.5A

Rated Voltage: Must be ≥ Maximum circuit voltage
Breaking Capacity: Maximum current the fuse can interrupt during a fault
        Battery powered: At least 50A (short-circuit current is huge!)

I²t: Energy integral, energy required to blow
    When selecting a fuse, ensure I²t(fuse) < I²t(MOSFET withstand)
    Otherwise, the MOSFET burns out before the fuse blows

Resettable Fuse (PTC)

Principle: Overcurrent → Heating → Resistance increases sharply → Current limiting
Reset: Automatically recovers after power-off and cooling

Pros: Reusable, no need to replace
Cons: Slow action (ms~s), high leakage current, resistance sensitive to temperature

Typical Applications: USB port protection (500mA/1A/2A ratings)
        Battery pack output
Not suitable for: Precision circuits requiring fast cutoff

Inrush Current Suppression

Power-on Inrush

Capacitive load at power-on:
  Capacitor initial voltage 0V → Equivalent short circuit → Current spike

       ┌── R ──┬──
  Vin ─┘       ┌┴┐
               │C│ (Large Capacitor)
               └┬┘
                │
               GND

I_peak = Vin / (R_trace + ESR)
Can be several times the normal current → Blows fuse / triggers overcurrent protection

Solutions

1. NTC Thermistor (Simplest):
   High resistance when cold → Limits current → Self-heating → Resistance drops → Normal conduction
   Cons: Residual resistance remains when hot, ineffective with frequent switching (hasn't cooled down)

2. Soft Start (Active):
   MOSFET gate charged slowly → Gradual turn-on → Limits current slew rate
   Requires additional RC + MOSFET

3. Current Limiting IC:
   Dedicated Hot Swap Controller
   e.g., TPS2490, LM5069
   Used in servers/telecom cards (hot-swappable)

Overvoltage Protection

TVS clamping is transient (μs level)
Sustained overvoltage requires different solutions:

1. Zener + Fuse (Crowbar):
   Vin ── Fuse ──┬── System
                 ├── Zener → GND
   Vin > Vz → Zener conducts → High current → Fuse blows
   Simple and crude, but requires replacing the fuse

2. OVP IC:
   Detects voltage > Threshold → Cuts off MOSFET
   e.g., NCP346, AP9101C
   Recoverable, no need to replace fuse

3. OVP at LDO Input:
   Many LDOs have built-in input overvoltage protection
   Automatically shuts down when out of range

Practical Checklist

□ Every external connector has ESD protection (TVS close to connector)
□ DC power input has reverse polarity protection (PMOS or diode)
□ Every power rail has an appropriate fuse
□ VBUS/USB power has PTC resettable fuse
□ Large capacitor input has inrush suppression strategy
□ Power loop area is small (reduces spikes caused by parasitic inductance)
□ Inductive loads like relays/motors have flyback diodes
□ MOSFET gates have protection (TVS or Zener + series resistor)

Keywords: ESD, TVS, Reverse Polarity Protection, PMOS, Fuse, PTC, Inrush, Soft Start, Overvoltage Protection, Flyback Diode