Bourns’ Thermal Cutoff Mini-Breakers Meet 240 W USB Power Delivery Standard

The evolution of the USB Power Delivery (USB PD) standard to its 240 W limit presents new challenges for circuit protection, particularly in managing thermal events. The higher power capacity requires components capable of handling greater voltage and current levels. Addressing this market demand, Bourns has developed a new Model SE Series of mini-breaker thermal cutoff (TCO) devices that provide a compact, surface-mount solution for overtemperature protection.

The Model SE series is specifically engineered to meet the safety requirements for USB Type-C connectors, which are becoming a universal standard for charging personal electronics. The devices are rated for a maximum voltage rating of 54 V, making them compatible with the 240 W USB PD specification, which requires a maximum thermal protection working voltage of 48 V.

The SE series features a compact footprint that meets the 240 W USB PD standard. Image used courtesy of Bourns

SE Series Mini-Breakers

Bourns’ Model SE series mini-breaker TCO devices operate by combining two circuit protection technologies: a bimetal switch and a positive temperature coefficient (PTC) device. Under normal conditions, current flows through the device with an impressively low resistance of 5 mΩ max, ensuring negligible power loss and heat generation.

When the device’s trip temperature is reached, either from high ambient temperature or excessive current, the bimetal disc flexes and opens the circuit. This action diverts the current through a parallel path containing the PTC device. The PTC then heats up and acts as a current-limiting element, which generates enough heat to keep the bimetal disc flexed and the circuit open. 

This bimetal-PTC combination prevents the oscillating opening and closing of the switch that can occur in some designs, ensuring the circuit remains in a safe, open state until power is cycled and the device has cooled down. The TCO will reset once the ambient temperature drops to a safe level and power to the TCO has been turned off and on.

When the mini-breaker is triggered, a bimetal disc and PTC device work together to keep the circuit open until it cools down, preventing rapid on-off cycling. Image used courtesy of Bourns

A “Mini” Miniature Thermal Cutoff Device

With a footprint of only 4.7 mm x 2.8 mm and a height of 1.23 mm, the SE series is marketed as the company’s smallest 54 V surface-mount mini-breaker. This compact size is a critical design factor for applications with limited board space, such as miniature electronics and integrated cables. The small form factor allows the device to be installed directly on a PCB or embedded within USB cables, potentially reducing the overall component count in a design. 

The series is available with four distinct temperature settings: 72 °C, 77 °C, 82 °C, and 85 °C, each with a tolerance of ±5 °C. This range allows engineers to select a device that precisely matches the thermal requirements of their application.

The devices are constructed with high corrosion resistance, which helps them withstand humid environments, and are RoHS compliant.

A look inside the SE series TCO mini-breaker device built for overtemperature and battery cell protection. Image used courtesy of Bourns

Overtemperature and Overcurrent Protection

The Bourns SE series is a high-voltage, compact, and resettable thermal protection solution that meets the demands of the 240 W USB PD standard. Its bimetal-PTC design ensures stable and reliable circuit interruption in the event of an overtemperature condition. Applications for the SE series are far-reaching, including overtemperature protection for USB Type-C cables, notebooks, tablets, smartphones, electronic cigarettes, and various heating elements.