Created:2026/01/22
Updated:2026/01/22
Author:
Takeshi Takatsudo

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)

SMAJ15A Package Preview

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

Parameter	Value
JLCPCB Part Number	C571368
Manufacturer Part Number	SMAJ15A
Package	SMA (DO-214AC)
Stock	713 units
Estimated Price	$0.15
Direction	Unidirectional (one-way protection)

Electrical Specifications

Voltage Characteristics

Parameter	Symbol	Min	Typ	Max	Unit
Standoff Voltage	V_RWM	-	15	-	V
Breakdown Voltage	V_BR	16.7	-	18.5	V
Clamping Voltage @ 16.4A	V_C	-	-	24.4	V
Reverse Leakage Current	I_R	-	-	5	µA

Power and Current Ratings

Parameter	Symbol	Value	Unit	Conditions
Peak Pulse Power	P_PPM	400	W	10/1000µs waveform
Peak Pulse Current	I_PPM	16.4	A	10/1000µs waveform
Maximum Reverse Voltage	V_RWM	15	V	Continuous operation
Maximum Clamping Voltage	V_C	24.4	V	@ I_PP = 16.4A

Dynamic Characteristics

Parameter	Value	Unit
Response Time	<1	ns to picoseconds
Junction Capacitance	~500	pF @ 0V
Operating Temperature	-55 to +150	°C
Storage Temperature	-65 to +175	°C

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)

Detection: Voltage exceeds breakdown voltage (16.7V)
Response: TVS conducts in <1ns
Clamping: Voltage limited to 24.4V maximum
Energy Dissipation: Transient energy safely dissipated to ground
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	Cathode	Anode
+12V Rail (TVS1)	Connect to +12V	Connect to GND
-12V Rail (TVS3)	Connect to GND	Connect to -12V

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:

Rail Voltage	TVS Standoff	TVS Breakdown	Max Clamp	Safety Margin
+12V nominal	15V	16.7V	24.4V	4.7V above nominal
-12V nominal	15V	16.7V	24.4V	4.7V above nominal

Design Criteria:

Standoff voltage (15V) > Normal operating voltage (12V) ✓
Breakdown voltage (16.7V) protects against transients ✓
Clamping voltage (24.4V) below component damage threshold ✓
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          &lt;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:

Place TVS diode as close to protected load as possible
Use short, wide traces to minimize parasitic inductance
Connect anode directly to ground plane
Keep ground return path short and low-impedance
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:

Measure standoff voltage: Should be >15V reverse breakdown
Verify leakage current: Should be <5µA at 15V
Optional: Use surge generator to verify clamping (requires specialized equipment)

Visual Inspection:

Verify correct polarity (cathode band orientation)
Check for proper soldering (no cold joints)
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

Standard	Test	Level	SMAJ15A Capability
IEC 61000-4-2	ESD	±8kV contact	✓ Exceeds
IEC 61000-4-4	EFT	4kV	✓ Exceeds
IEC 61000-4-5	Surge	1kV	✓ Exceeds

Comparison with Alternatives

SMAJ Series Comparison

Part Number	Standoff	Breakdown	Clamp @ 16.4A	Application
SMAJ12A	12V	13.3V	19.9V	Lower voltage rails
SMAJ15A	15V	16.7V	24.4V	±12V rails (this project)
SMAJ18A	18V	20.0V	29.2V	Higher voltage rails
SMAJ24A	24V	26.7V	38.9V	24V systems

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

Component	Rail	Function	Quantity
TVS1	+12V	Positive spike protection	1
TVS3	-12V	Negative spike protection (reversed)	1

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

Fault Condition	Response	Recovery
Voltage spike	TVS clamps	Automatic
Overload	PTC trips	30 seconds
Short circuit	Fuse blows	Manual replacement

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

Overview​

Part Information​

Electrical Specifications​

Voltage Characteristics​

Power and Current Ratings​

Dynamic Characteristics​

How TVS Diodes Work​

Normal Operation​

Transient Protection​

Circuit Placement​

TVS1: +12V Rail Protection​

TVS3: -12V Rail Protection​

Pin Configuration​

SMA Package (DO-214AC)​

Polarity Identification​

Connection Rules​

Application Notes​

1. Unidirectional vs Bidirectional​

2. Clamping Voltage Selection​

3. Peak Pulse Power Rating​

4. Response Time Characteristics​

5. Placement Guidelines​

6. Thermal Considerations​

7. Testing and Verification​

ESD and Transient Protection Standards​

Compliance​

Protection Levels​

Comparison with Alternatives​

SMAJ Series Comparison​

When to Use Different TVS Ratings​

Application in This Project​

Usage Summary​

Protection Chain​

Protection Coordination​

Related Components​

References​