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Diodes & Circuit Protection

1. The Basics​

What they are​

Diodes are non-linear devices. They conduct once a forward threshold is reached (≈0.7V for silicon. Material dependent) and block current from the other direction.

Typical Silicon Diode I-V Characteristic

VIVf (≈0.7V)PIV / BreakdownForward BiasReverse Bias

They are a 2-terminal device:

  • Anode (+): Current enters here.
  • Cathode (-): Current leaves here (usually marked with a stripe on the package).

Current Flow Visualization

Anode (+)Cathode (-)

Forward Biased: Current flows Anode to Cathode

Current flows from Anode to Cathode under normal forward-biased operation.

  • PIV (Peak Inverse Voltage): The maximum reverse voltage rating before reverse breakdown. If you apply a voltage greater than the diode's PIV onto the cathode with respect to the anode, the diode enters the breakdown region, and current forces its way backwards from Cathode to Anode (often destroying the diode unless it's a Zener).

2. Common Circuit Uses​

A. Rectification​

Diodes are the core of half-wave and full-wave rectifiers, converting AC (Alternating Current) into DC (Direct Current).

Full-Wave Bridge Rectifier Circuit

Standard Full-Wave Bridge Rectifier configuration.

How the Current Flows (Full-Wave Bridge): In a standard 4-diode bridge rectifier:

  1. Positive Half-Cycle: Diodes D2 and D3 become forward biased. Current flows through D2 → Load → D3 → Ground.
  2. Negative Half-Cycle: The AC polarity flips. Diodes D1 and D4 become forward biased. Current flows through D4 → Load → D1 → Ground.
  3. Result: The load sees current flowing in the same direction during both halves of the AC cycle.

Ripple & Filtering

  • Filter capacitors are placed across the load to smooth out the "bumps" (voltage ripple) after rectification.
  • Voltage ripple is the leftover variation—the output isn’t perfectly flat DC yet.
Learn More

For in-depth detail on converting AC to cleaner DC, check out the Regulation Section.

B. Clamping (Simple Protection)​

Diodes can act as voltage clamps. Taking the output across a diode can limit a node to roughly the forward voltage drop above the reference rail.

Diode Voltage Clamp (Limiter)

InputR_limitOutputMax V_out ≈ 0.7V

ESD Steering Diodes: You can protect slow control lines with steering diodes to the rails: one diode to GND, one to VCC.

  • If a high-voltage spike hits the line, the top diode conducts and dumps the energy into VCC.
  • If a negative spike hits, the bottom diode conducts and pulls current from GND.

ESD Steering Protection

VCC (+5V)GNDInputConnectorTo IC PinD1 (Clamps to VCC)D2 (Clamps to GND)
Layout Note

This assumes your rails + decoupling capacitors can absorb the energy hit! Keep the traces short and the return to ground tight. Not ideal for high-speed signals as diodes add capacitance.

C. RC Snubbers & RCD Clamps​

When you switch inductive loads (motors, transformers) quickly, you get ringing and voltage spikes.

1. RC Snubber (Damping) An RC snubber (Resistor + Capacitor in series) damps the ringing (LC oscillation) and reduces the speed of the voltage rise (dV/dt).

  • Placement: Across the switch (Mosfet D-S) or across the Load.

2. RCD Clamp (Peak Limiting) Common in Flyback converters. It uses a Diode to act as a "check valve," allowing high-voltage spikes to bleed into a capacitor and burn off slowly through a resistor.

  • Components: Diode (routes energy), Capacitor (holds voltage), Resistor (dissipates heat).

Dissipative RCD Snubber

+
Bench Tuning Snubbers
  1. Measure Ringing Frequency (fr).
  2. Calculate Parasitics: L × C ≈ 1 / (2π × fr)²
  3. Pick Snubber Cap: Csnub ≈ 3 × Cparasitic
  4. Pick Snubber Resistor: Rsnub ≈ √(L / Csnub)

3. Diode Types​

Schottky Diodes​

Schottky diodes use a Metal-Semiconductor junction (usually platinum or tungsten on N-type silicon) rather than a P-N junction.

Circuit Symbol

Internal Structure

N-Type SiliconMetal (Au/Pt/W)AnodeCathode

Why use them? Because they don't have a P-N junction, they don't have "minority carrier injection." This means they turn off instantly (zero reverse recovery time), making them perfect for high-speed switching regulators.

  • Pros:
    • Lower Forward Drop: Typically 0.2V – 0.4V (vs 0.7V for Silicon). Less power lost as heat.
    • Fast Switching: Ideal for Buck/Boost converters operating at high frequencies (kHz to MHz).
  • Cons:
    • High Reverse Leakage: They leak more current when off, especially at high temps.
    • Low Breakdown: Hard to find high-voltage (>100V) Schottkys.

TVS Diodes (Transient Voltage Suppressors)​

TVS diodes are specialized avalanche diodes built to turn on fast and absorb high-energy, short events (ESD, Lightning, inductive kickback).

  • Unidirectional: Behaves like a Zener to ground. Blocks positive voltage, clamps if it gets too high. Conducts freely for negative voltage. (Good for DC power lines).
  • Bidirectional: Two diodes back-to-back. Clamps symmetrically for positive or negative spikes. (Good for AC or Data lines like RS-485).
Unidirectional
Bidirectional
High-Speed Design

For interfaces like USB, HDMI, or Ethernet, look for "Low Capacitance" TVS arrays. Place them at the connector with a short path to ground (single via right at the pad) to minimize stub length.