TDK Introduces ThermoFuse Varistors for High-Surge Protection

Surge protection components for industrial and energy systems
In power electronics, renewable energy systems, and telecommunications infrastructure, surge protection components play a critical role in safeguarding equipment from transient overvoltage events. TDK Corporation has introduced the MT40 series ThermoFuse varistors, a new generation of surge protection components designed to combine compact dimensions with enhanced electrical protection.

The components measure 38.0 × 15.2 × 40.9 mm (L × W × H) and incorporate patented overmolding technology together with an integrated thermal disconnect system. This design enables protection against surge currents of up to 50 kA while maintaining a reduced installation footprint.

Typical applications include photovoltaic systems, industrial power supplies, outdoor lighting installations, telecommunications infrastructure, and surge protection devices (SPDs).

Integrated thermal protection and compact construction
ThermoFuse varistors combine a metal oxide varistor (MOV) with a thermal disconnect mechanism that separates the component from the circuit if overheating occurs. This integrated protection mechanism helps prevent thermal runaway and enhances system safety during extreme electrical events.

The MT40 series uses overmolding technology to encapsulate the protection component and thermal disconnect system in a single compact assembly. Such integration allows system designers to reduce component count while maintaining reliable surge protection performance.

The devices use strap terminations and share a common electrode footprint across the series, allowing straightforward integration into printed circuit board designs.

Electrical performance under high surge conditions
The MT40 series is designed for high-energy surge environments. The components support a peak surge current of up to 50 kA (8/20 μs pulse) and a short-circuit current rating (SCCR) of up to 200 kA, according to UL 1449 requirements.

The varistors are suitable for systems operating with AC voltages from 150 V to 550 V and DC voltages from 200 V to 750 V, enabling use in a range of industrial power and energy applications.

The series is recognized as a UL 1449 Type 1CA component assembly, supporting integration into surge protection devices designed for both AC and DC circuits.

Monitoring and operational safety features
To support system monitoring, the MT40 series can be equipped with a galvanically insulated normally open micro-switch. This feature enables remote status indication and integration with monitoring circuits that track the operational state of the surge protection component.

An optional visual indicator is also available, allowing maintenance personnel to quickly identify whether the protection element has been activated or disconnected.

The components are designed to operate across a temperature range from –40 °C to +85 °C, supporting installation in outdoor or industrial environments where equipment may be exposed to wide temperature variations.

Materials and environmental compliance
The MT40 varistors are encapsulated in a flame-retardant epoxy coating, providing mechanical protection and improving safety under fault conditions. The components are RoHS, REACH, and PFAS compliant, and are manufactured using lead-free materials.

Through the combination of high surge current capability, integrated thermal protection, and compact packaging, the MT40 series provides system designers with surge protection components suited for modern industrial power systems, communication infrastructure, and renewable energy installations.

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