SMAJ15A - 15V Unidirectional TVS Diode

High-performance Transient Voltage Suppressor (TVS) diode for overvoltage protection on +12V and -12V power rails.

• View on JLCPCB: C571368

• Download Datasheet (PDF)

Overview

The SMAJ15A is a unidirectional TVS diode that provides fast-response overvoltage protection for sensitive electronics. In this power supply design, it protects the +12V and -12V output rails from voltage transients and ESD events.

Part Information

Electrical Specifications

Voltage Characteristics

Power and Current Ratings

Dynamic Characteristics

How TVS Diodes Work

Normal Operation

During normal operation (voltage below standoff voltage):

Normal State (V < 15V):
    +12V Rail ─────┬─────→ To Load
                   │
                 [TVS1] Cathode (marked end)
                   │
                  GND    (Reverse leakage: &lt;5µA)

The TVS diode acts as an open circuit with only minimal leakage current (<5µA).

Transient Protection

When a voltage spike occurs:

Transient Event (V > 16.7V):
    +12V Rail ─────┬─────→ Protected Load
                   │         (Voltage clamped to 24.4V max)
                 [TVS1] ⚡ CONDUCTING
                   │
                  GND    (Shunting transient energy to ground)

1. Detection: Voltage exceeds breakdown voltage (16.7V)

2. Response: TVS conducts in <1ns

3. Clamping: Voltage limited to 24.4V maximum

4. Energy Dissipation: Transient energy safely dissipated to ground

5. Recovery: Returns to high-impedance state when voltage normalizes

Circuit Placement

TVS1: +12V Rail Protection

Linear Regulator Output:
    U6 (L7812CV-DG)
        │
        ├─→ C17 (470µF) ─→ GND
        │
        ├─→ C14 (100nF) ─→ GND
        │
        ├─→ [TVS1] SMAJ15A ─→ GND
        │      ↑
        │   Cathode to +12V
        │   Anode to GND
        │
        ├─→ PTC1 (1.1A) ─→ LED2 (Green) ─→ +12V OUTPUT

TVS3: -12V Rail Protection

Negative Rail Protection:
    U8 (CJ7912)
        │
        ├─→ C21 (470µF) ─→ GND
        │
        ├─→ C16 (100nF) ─→ GND
        │
        ├─→ [TVS3] SMAJ15A (reversed)
        │      ↑
        │   Cathode to GND
        │   Anode to -12V
        │
        ├─→ PTC3 (1.1A) ─→ LED4 (Red) ─→ -12V OUTPUT

Special Note: For negative rail protection, TVS3 is reversed:
- Cathode connects to GROUND
- Anode connects to -12V rail
This protects against negative voltage spikes below -16.7V

Pin Configuration

SMA Package (DO-214AC)

    ┌─────────────┐
    │             │  Cathode (marked end with band)
    │   SMAJ15A   │  Connect to protected +rail
    │             │
    └─────────────┘  Anode (unmarked end)
                     Connect to ground

Polarity Identification

• Cathode: End with marking band (white or colored stripe)

• Anode: Unmarked end

Connection Rules

Application Notes

1. Unidirectional vs Bidirectional

Unidirectional (SMAJ15A):

• Protects against voltage spikes in ONE direction only

• Used for DC power rails with known polarity

• Lower capacitance than bidirectional types

• TVS1: Protects +12V from positive spikes above +16.7V

• TVS3: Protects -12V from negative spikes below -16.7V (when reversed)

Note: For the +5V rail in this project, we use the SD05 unidirectional TVS diode, which is optimized for DC power rail protection.

2. Clamping Voltage Selection

The SMAJ15A is chosen for 12V rails because:

Design Criteria:

1. Standoff voltage (15V) > Normal operating voltage (12V) ✓

2. Breakdown voltage (16.7V) protects against transients ✓

3. Clamping voltage (24.4V) below component damage threshold ✓

4. Adequate margin for voltage regulation tolerance ✓

3. Peak Pulse Power Rating

The 400W rating is specified for a 10/1000µs waveform:

Voltage Pulse Profile:
    24.4V ┤      ╱╲
          │     ╱  ╲___________
          │    ╱               ╲
    15V   ├───╯                 ╲___
          │
          └────┴────┴───────────┴────→ Time
              10µs  1000µs
              rise  fall to 50%

Peak Current: 400W / 24.4V ≈ 16.4A

This waveform simulates:

• Lightning-induced surges

• Inductive switching transients

• Power line disturbances

4. Response Time Characteristics

Transient Response Timeline:
    Spike        TVS           Voltage
    Detected     Conducts      Clamped
       ↓            ↓             ↓
    ───┴────────────┴─────────────┴────→ Time
       0          <1ns         stable
                 (subnanosecond response)

The <1ns response time means the TVS diode begins conducting almost instantaneously, protecting downstream circuitry before the transient can cause damage.

5. Placement Guidelines

Optimal Placement:

✓ GOOD: Close to load
    Regulator → [Cap] → [TVS] → Load
                  ↓       ↓
                 GND     GND
    (Short traces minimize inductance)

✗ POOR: Far from load
    Regulator → [Cap] → ─────── (long trace) ─────── → Load
                  ↓                                      ↓
                 GND                                    [TVS]
                                                         ↓
                                                        GND
    (Trace inductance can allow voltage spikes)

Best Practices:

1. Place TVS diode as close to protected load as possible

2. Use short, wide traces to minimize parasitic inductance

3. Connect anode directly to ground plane

4. Keep ground return path short and low-impedance

5. Place after fuses and PTCs in protection chain

6. Thermal Considerations

Power Dissipation:

• Normal operation: ~75µW (15V × 5µA leakage)

• Transient events: Up to 400W for 10/1000µs pulses

• Repetitive pulse rating: Consult derating curves in datasheet

No heat sink required for this application because:

• Continuous power dissipation is negligible

• Transient events are brief and infrequent

• SMA package provides adequate thermal mass

7. Testing and Verification

Functional Test:

1. Measure standoff voltage: Should be >15V reverse breakdown

2. Verify leakage current: Should be <5µA at 15V

3. Optional: Use surge generator to verify clamping (requires specialized equipment)

Visual Inspection:

1. Verify correct polarity (cathode band orientation)

2. Check for proper soldering (no cold joints)

3. Ensure component is not cracked or damaged

ESD and Transient Protection Standards

Compliance

The SMAJ15A provides protection compliant with:

• IEC 61000-4-2 (ESD immunity)

• IEC 61000-4-4 (Electrical fast transient)

• IEC 61000-4-5 (Surge immunity)

Protection Levels

Comparison with Alternatives

SMAJ Series Comparison

When to Use Different TVS Ratings

Selection Guide:
    Working Voltage → Add 20% margin → Select TVS Standoff Voltage

Examples:
    5V rail   → 5V × 1.2 = 6V   → Use SMAJ5.0A or SMAJ6.0A
    12V rail  → 12V × 1.2 = 14.4V → Use SMAJ15A ✓
    15V rail  → 15V × 1.2 = 18V  → Use SMAJ18A
    24V rail  → 24V × 1.2 = 28.8V → Use SMAJ30A

Application in This Project

Usage Summary

Protection Chain

The SMAJ15A is part of a multi-stage protection system:

Complete Protection Architecture:
    Input → [Fuse] → [PTC] → [TVS] → Load
            ❶        ❷        ❸

❶ Fuse (F1): Short circuit protection (permanent, requires replacement)
❷ PTC: Overload protection (resettable, 30-second recovery)
❸ TVS: Transient voltage protection (automatic, sub-nanosecond response)

Protection Coordination

Related Components

• SD05 - 5V Unidirectional TVS Diode (for +5V rail)

• Protection Circuit Overview - Coming soon

• PCB Layout Guidelines - Coming soon

References

• JLCPCB Part Detail: C571368

• TVS Diode Application Guide

• IEC 61000-4-x Standards Documentation

Last updated: 2025-12-28