Trigger Diode Technical Analysis: Working Principles, Real Models and Applications

In-Depth Analysis and Engineering Practice of Trigger Diodes

In the diverse world of semiconductor devices, the trigger diode is far from a headline star, yet it occupies a niche that is difficult to replace. Most diodes conduct according to the forward voltage drop of a PN junction or avalanche breakdown characteristics. A trigger diode, however, behaves like a gatekeeper: it remains in a high-resistance state until a preset voltage threshold is reached, and then it switches on abruptly.

This “wait until the right moment, then fully conduct” behavior makes the trigger diode indispensable in certain applications. If you’ve ever worked with SCR phase control, surge triggers, or precision pulse ignition, you’ve probably spotted it on a schematic—often drawn as a symmetrical symbol without polarity, labeled with part numbers such as “BR100/03,113.”

Operating Principle of Trigger Diodes

A trigger diode is typically a type of diode with a symmetrical bidirectional structure, capable of triggering conduction in both forward and reverse directions—quite different from ordinary single-direction diodes. Its internal structure can be described using PNPN layers or an equivalent dual-transistor regenerative model.

Stages of Operation：

1.Blocking Stage

Below the trigger voltage, the device behaves like a high-value resistor with negligible leakage current.

2.Trigger Stage

When the voltage across the terminals reaches the trigger voltage (V_BO), avalanche breakdown begins, free carriers multiply rapidly, and internal positive feedback quickly drives the device into conduction.

3.Holding Stage

Once conduction starts, the voltage drops sharply to a lower holding voltage (V_H). As long as the current stays above the holding current (I_H), the diode remains in a low-resistance conducting state.

4.Turn-off Stage

When the current falls below I_H, the device reverts to a high-resistance state.

Classic Models with Verified Parameters

When selecting components for engineering, using a real, verifiable part number is essential. Below are two well-established trigger diode models whose datasheets are readily available.

BR100/03,113 (NXP)

Trigger Voltage (V_BO): 28V – 36V (bidirectional, symmetrical)

Holding Voltage (V_H): ~7V

Maximum Repetitive Peak On-state Current (I_TSM): 2A (pulse width ≤ 10 μs, 25°C)

Holding Current (I_H): ~150 μA

Response Time: ~1.5 μs at 0.5A pulse

Package: DO-35 glass encapsulation

Applications: Precision phase triggering, pulse generators, low-voltage SCR gate triggering, laboratory test equipment.

This device’s stable trigger voltage makes it ideal for AC phase control circuits—e.g., in dimmers or small motor speed controllers—where it controls the conduction angle of an SCR for smooth power regulation.

Comparison with Other Types of Diodes

To avoid confusion, here is a direct comparison between trigger diodes and other common types of diodes:

Engineering Summary

For high-frequency power circuits: Schottky or fast recovery diodes are preferred over trigger diodes.

For surge protection: Trigger diodes and TVS diodes can be combined to balance response time and energy absorption.

Bridge rectifiers and trigger diodes operate in completely different functional domains.

Typical Applications and Circuit Use Cases

1.AC Phase Control

In dimmers or motor controllers, trigger diodes work with RC delay networks to fire SCRs at a desired point in the AC cycle, controlling load power smoothly.

2.High-Voltage Pulse Generation

Used in xenon flash lamps and laser ignition, trigger diodes release stored capacitor energy in a sharp pulse when their voltage threshold is exceeded.

3.Telecommunication Surge Protection

Combined with TVS and GDTs, trigger diodes act as the second-stage fast switch, protecting modems, switches, and PLCs from lightning-induced surges.

Selection Guidelines for Trigger Diodes

Match Trigger Voltage to the circuit’s working voltage and desired threshold.

Pulse Capability: Choose a part rated for the specific surge waveform (e.g., 8/20 μs).

Temperature Effects: Higher temperatures slightly reduce trigger voltage—important for hot environments.

Package Size: Larger packages for higher energy handling.

Complementary Components: Often paired with resistors, capacitors, MOVs, TVS diodes, or SCRs.

Failure Modes and Maintenance Tips

Trigger Voltage Drift: Overload or heat can shift the trigger point.

Degraded Avalanche Capability: Multiple surge hits can weaken performance.

Thermal Breakdown: Sustained overcurrent may short the device permanently.

Diagnosis: Use an adjustable power supply to slowly ramp voltage and check the conduction point. If it’s far below nominal or the device never recovers high resistance, it’s likely failed.

Trigger diodes may not be the most common type of diode, but their precise threshold behavior is unmatched in certain designs. Whether it’s a low-voltage precision trigger like the BR100/03 LLD or a telecom surge protector like the P0300AA, these devices prove invaluable in phase control, pulse ignition, and surge switching roles.

For circuit designers, understanding the principles, specifications, and differences from other types of diodes is essential—not only to avoid incorrect substitutions, but also to fully exploit their unique capabilities when the design demands a precisely timed, abrupt conduction event.