SD05 - 5V Unidirectional TVS Diode
Unidirectional TVS diode for overvoltage protection on the +5V power rail.
Overview
The SD05 is a unidirectional TVS diode designed for 5V DC power rail protection. Its unidirectional design is ideal for DC power rails where only positive overvoltage protection is needed, providing lower clamping voltage and higher current handling compared to bidirectional alternatives.
Part Information
| Parameter | Value |
|---|---|
| JLCPCB Part Number | C502527 |
| Manufacturer | MDD (Microdiode Electronics) |
| Manufacturer Part Number | SD05 |
| Package | SOD-323 |
| Stock | 402,350 units (excellent availability) |
| Estimated Price | ~$0.02 |
| Direction | Unidirectional (positive direction only) |
Electrical Specifications
Voltage Characteristics
| Parameter | Symbol | Min | Typ | Max | Unit |
|---|---|---|---|---|---|
| Standoff Voltage | V_RWM | - | 5 | - | V |
| Breakdown Voltage | V_BR | 6.0 | - | 6.6 | V |
| Clamping Voltage @ 1A | V_C | - | - | 9.2 | V |
| Reverse Leakage Current | I_R | - | - | 1 | µA |
Power and Current Ratings
| Parameter | Symbol | Value | Unit | Conditions |
|---|---|---|---|---|
| Peak Pulse Power | P_PP | 200 | W | 8/20µs waveform |
| Peak Pulse Current | I_PP | 21.7 | A | 8/20µs waveform |
| Maximum Reverse Voltage | V_RWM | 5 | V | Continuous |
| Maximum Clamping Voltage | V_C | 9.2 | V | @ I_PP = 1A |
Dynamic Characteristics
| Parameter | Value | Unit |
|---|---|---|
| Response Time | <1 | ns |
| Junction Capacitance | ~50 | pF |
| Operating Temperature | -55 to +150 | °C |
Pin Configuration
SOD-323 Package (2-pin)
Side View:
┌──────────┐
│ ▐█ │ ← Cathode band (marked end)
│ SD05 │
│ │
└──────────┘
Pin 1 Pin 2
(Anode) (Cathode)
Pin Functions
| Pin | Name | Function |
|---|---|---|
| 1 | Anode (A) | Connect to GND |
| 2 | Cathode (K) | Connect to protected +5V rail |
How Unidirectional TVS Works
Normal Operation
During normal operation (voltage below standoff voltage):
Normal State (V < 5V):
+5V Rail ─────┬─────→ To Load
│
[TVS2] Cathode (marked end)
│
GND (Reverse leakage: <1µA)
The TVS diode acts as an open circuit with minimal leakage current.
Transient Protection
When a positive voltage spike occurs:
Transient Event (V > 6V):
+5V Rail ─────┬─────→ Protected Load
│ (Voltage clamped to 9.2V max)
[TVS2] CONDUCTING
│
GND (Shunting transient energy to ground)
- Detection: Voltage exceeds breakdown voltage (6.0V)
- Response: TVS conducts in <1ns
- Clamping: Voltage limited to 9.2V maximum
- Energy Dissipation: Transient energy safely dissipated to ground
- Recovery: Returns to high-impedance state when voltage normalizes
Circuit Placement
TVS2: +5V Rail Protection
Linear Regulator Output:
U7 (L7805ABD2T-TR)
│
├─→ C18 (100nF) ─→ GND
│
├─→ C23 (470µF) ─→ GND
│
├─→ [TVS2] SD05 ─→ GND
│ ↑
│ Cathode to +5V
│ Anode to GND
│
├─→ PTC2 (1.1A) ─→ LED3 (Blue) ─→ +5V OUTPUT
Why Unidirectional for +5V DC Rail?
Design Rationale
For a regulated DC power rail, unidirectional TVS is the correct choice:
| Characteristic | Unidirectional (SD05) | Bidirectional |
|---|---|---|
| Protection | Positive spikes only | Both positive and negative |
| Best for | DC power rails | Signal lines, AC circuits |
| Clamping voltage | Lower (better) | Higher |
| Current handling | Higher | Lower |
| Complexity | 2-pin, simple | Often 4-pin |
Why unidirectional is sufficient:
- DC output: The LM7805 output is always positive (+5V)
- No negative transients expected: Linear regulator output with bulk capacitors
- Better performance: Lower clamping voltage provides better protection margin
- Simpler: 2-pin package, easier PCB layout
Voltage Safety Analysis
Normal Operation: 5.0V Below standoff voltage
Regulation Range: 4.8V - 5.2V Safe margin
TVS Activates: >6.0V Above regulation range
Maximum Clamp: 9.2V Below IC damage threshold (typically >10V for 5V logic)
Application Notes
1. Clamping Voltage Selection
The SD05 is chosen for the 5V rail because:
| Rail Voltage | TVS Standoff | TVS Breakdown | Max Clamp | Safety Margin |
|---|---|---|---|---|
| +5V nominal | 5V | 6.0V | 9.2V | 1V above nominal |
Design Criteria:
- Standoff voltage (5V) = Normal operating voltage ✓
- Breakdown voltage (6.0V) above normal + tolerance ✓
- Clamping voltage (9.2V) below component damage threshold ✓
2. Comparison with Bidirectional Alternative
| Parameter | SD05 (Unidirectional) | PRTR5V0U2X (Bidirectional) |
|---|---|---|
| Direction | Unidirectional | Bidirectional |
| Package | SOD-323 (2-pin) | SOT-143 (4-pin) |
| Clamping @ 1A | 9.2V | 20V |
| Stock (JLCPCB) | 402,350 | ~15,000 |
| Best application | DC power rails | Signal lines |
3. PCB Layout Guidelines
Optimal Placement:
✓ GOOD: Close to load, short traces
Regulator → [Cap] → [TVS] → Load
↓ ↓
GND GND
(Short traces minimize inductance)
✗ POOR: Far from load, long traces
Regulator → [Cap] → ─── (long trace) ─── → Load
↓ ↓
GND [TVS]
↓
GND
Best Practices:
- Place TVS close to protected circuitry
- Use short, wide traces to minimize inductance
- Connect anode directly to ground plane via
- Keep ground return path short and low-impedance
4. Thermal Considerations
Power Dissipation:
- Normal operation: ~5µW (5V × 1µA leakage)
- Transient events: Up to 200W for 8/20µs pulses
No heat sink required for this application because:
- Continuous power dissipation is negligible
- Transient events are brief and infrequent
- SOD-323 package provides adequate thermal mass
ESD Protection Compliance
The SD05 provides protection compliant with:
- IEC 61000-4-2 (ESD immunity)
- IEC 61000-4-4 (Electrical fast transient)
- IEC 61000-4-5 (Surge immunity)
Application in This Project
Usage Summary
| Component | Rail | Function | Quantity |
|---|---|---|---|
| TVS2 | +5V | Positive spike protection | 1 |
Protection Chain
Complete +5V Protection:
DC-DC → Linear Reg → [PTC2] → [TVS2] → +5V Output
❶ ❷ ❸
❶ Linear Regulator: Clean 5V output with current limiting
❷ PTC: Overload protection (1.1A hold, 2.2A trip, resettable)
❸ TVS: Transient/ESD protection (automatic, <1ns response)
Stock Availability and Alternatives
Primary Part
- C502527: SD05, SOD-323
- Stock: 402,350 units (excellent availability)
Alternatives
If C502527 is out of stock:
| Part Number | Device | Package | Notes |
|---|---|---|---|
| C193402 | SMF5.0A | SOD-123FL | Slightly larger package |
| C113952 | SMAJ5.0A | SMA | Higher power handling |
Related Components
- SMAJ15A - 15V Unidirectional TVS Diode (for ±12V rails)
- mSMD110-33V (PTC2) - PTC Resettable Fuse for +5V rail
References
Last updated: 2025-01-08