--- title: Protection Circuits url: https://doc.liz6.com/en/hardware/08-power-and-protection/03-protection-circuits locale: en area: hardware tags: - hardware - power-and-protection date: 2026-06-30 modified: 2026-07-16 description: 'The difference between engineering and experimentation: Protection circuits keep the board alive through the first power-on' --- # 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! ``` ```text 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 | Type | Blow Speed | Typical Application | |------|------------|---------------------| | Fast-acting (Fast) | ms level | Semiconductor protection | | Slow-acting (Slow/T) | Hundreds of ms to s | Motors/Transformers/Capacitive loads | | Resettable (PTC) | Does not blow, limits current | USB output/Battery packs | | SMD Fuse | ms level | Low current on PCB | ### 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*