Overview
The Gas Discharge Tube (GDT) is a commonly used primary surge protection device. Characterized by high surge withstand capability, low parasitic capacitance, and excellent isolation performance, it is widely applied in power lines, communication interfaces, industrial control equipment, PoE devices, and antenna interfaces.
Practical design requires comprehensive consideration of the following factors:
- DC Spark-over Voltage
- Impulse Spark-over Voltage
- Surge Current Capability
- Follow Current
- Voltage withstand capability of downstream protection devices
This article introduces methods for selecting key GDT parameters—using a 220Vac line surge protection case study—and shares practical selection insights.
1. Selection Methods for Key GDT Parameters
Assumed application scenario:
Input voltage: AC 220V
Surge requirements:
- Line-to-Line (Differential Mode): 2kV
- Line-to-Ground (Common Mode): 4kV
1.1 Follow Current
When a surge occurs, the gas inside the GDT breaks down, creating a low-impedance discharge path that shunts overvoltage energy to ground.
However, after the surge subsides, if the line's operating voltage remains higher than the voltage required for the GDT to sustain the discharge, the GDT may fail to immediately return to a non-conductive state, resulting in continuous conduction.
This phenomenon is known as "Follow Current."
Example: AC 220V line
The peak operating voltage is approximately 311V, whereas the GDT's arc-sustaining voltage is typically only a few tens of volts.
Therefore, a GDT cannot be connected directly in series with an AC power line as the sole protection device; it must be used in conjunction with components such as Metal Oxide Varistors (MOVs) and fuses.
1.2 DC Spark-over Voltage
The DC spark-over voltage determines whether the GDT will trigger erroneously during normal operation.
Selection principle:
The minimum DC spark-over voltage of the GDT generally needs to be higher than the peak operating voltage of the line, with a certain margin allowed. Taking an AC 220V line as an example:
Converting AC RMS voltage to peak voltage:
220V × 1.414 ≈ 311V
Based on experience, the recommended minimum DC breakdown voltage for a GDT is:
VDC ≥ 1.8 × Peak Operating Voltage
Therefore:
1.8 × 311V ≈ 560V
Thus, one can select:
A GDT with a 600V DC breakdown voltage rating.
This avoids false triggering during normal operation while meeting surge protection requirements.
1.3 Impulse Breakdown Voltage vs. Voltage Withstand Rating of Downstream Components
Design Principle:
The voltage withstand rating of downstream components must exceed the maximum residual voltage resulting from GDT activation.
If the GDT impulse breakdown voltage is too high: GDT activation voltage rises ➡️ residual voltage increases ➡️ stress on downstream components (such as power ICs and capacitors) increases.
Therefore, a balance must be struck between safe voltage withstand limits, surge protection effectiveness, and the tolerance of downstream components.
1.4 Surge Current Capability
Surge current capability refers to the maximum surge current a GDT can withstand.
Common test waveforms:
- 8/20μs
- 10/700μs
For example:
Lightning protection for power lines typically focuses on the 8/20μs waveform;
Communication lines (such as DSL and PoE) typically focus on the 10/700μs waveform.
2. Typical GDT Application Scenarios
2.1 PoE Network Lightning Protection
- PoE lightning protection
- RJ45 interface protection
- Industrial Ethernet protection
2.2 Communication Interface Protection
- RS485
- RS232
- CAN
- Telephone lines
- RF antenna interfaces
2.3 AC Power Surge Protection
- Chargers
- Industrial power supplies
- LED drivers
- Home appliances
GDTs are typically used as primary surge discharge devices in conjunction with components such as MOVs and TVS diodes.
3. Practical Case Study: Optimizing GDT Selection to Reduce Surge Residual Voltage
Initial Design：
- Project: AC 220V Input
- Solution: The customer required high insulation withstand voltage, so a 3600V DC breakdown voltage GDT was selected.
- Testing Issues: Actual testing revealed that the 3600V GDT operating voltage was too high, resulting in a residual voltage of approximately 5.1kV, exceeding the capacity of the downstream electrolytic capacitors and causing device malfunction.
Optimized Solution:
- Solution: A 2500V GDT was selected, with a residual voltage of approximately 2.8kV. A 10kV common-mode surge test was then performed again.
- Result: The secondary electrolytic capacitor returned to normal, and the test passed.
Design Experience Summary:
GDT selection requires comprehensive consideration of:
- Operating voltage
- Surge rating
- GDT residual voltage
- Voltage rating of downstream components
4. Recommended Semiware GDTs for Surge Protection
| Application | Recommended GDT | Key Features | Package |
| PoE / Gigabit Ethernet surge protection / RS485 / RS232 | SG3D05B090 | 90V, 5kA, 1.5PF | 3-electrode ⌀5 mm |
| RJ11 | SG3D05B470 | 470V, 5kA, 1.5PF | 3-electrode ⌀5 mm |
| GaN charger surge | SG4532B600 | 600V, 2kA, 1PF | 4.5*3.2*2.7mm |
| Fast Ethernet | SG4532B400 | 400V, 2kA, 1PF | 4.5*3.2*2.7mm |
| WiFi / NB-IoT antenna | SG4532B090 | 90V, 2kA, 1PF | 4.5*3.2*2.7mm |
| 10Gb Ethernet | SG5042B800 | 800V, 5kA, 0.8PF | 5*5*4.2mm |
| RF antenna | SG2R09B090 | 90V， 20kA, 1.5PF | 8.3*8.3*6.0mm |
| 220Vac / 380Vac power | SG2R08B800 | 800V, 10kA,1.5PF | ⌀8 mm |
Conclusion
As a critical component for high-energy surge protection, the GDT plays a vital role in power supplies, telecommunications, and industrial equipment.
If you are designing surge protection for power lines, PoE systems, communication interfaces, or industrial equipment, Semiware can provide tailored GDT selection recommendations based on your operating voltage, testing standards, and application environment.
