Today’s USB Type-C® cables allow consumers to use USB Type-C cables for charging battery-operated portable products at power levels up to 100W. This high power, fast charging capability reduces the time the portable product has to be tethered to a power line.
A 100W capacity is a significant amount of power for a USB cable. As a result, the cable can be subject to high heat build-up due to a fault condition on the cable. The connector has an enclosed construction and narrow pin-to-pin spacing. The pins have a 0.5mm pitch which is five times less than USB Type-A connectors. Dirt and other contaminants can cause resistive faults between power and ground pins. Mechanical failures such as bent pins or worn insulation also can cause resistive faults and overcurrent conditions. These faults can lead to overheating and cause failure of the cable and potentially damage to the product being charged.
The challenge for the designer is to incorporate thermal protection in the limited space of the USB Type-C connector. The designer needs to ensure both that all components of the cable are rated for handling the full 100W of charging power and that including thermal protection does not reduce the available charging power below 100W.
A solution to these challenges is a digital temperature indicator (DTI) component designed for use in the connector of a USB Type-C cable. The use of this component avoids power loss in the cable by being located in the Configuration Channel (CC) of the cable rather than in the power supply line, the VBUS line. This DTI component is the PolySwitch® setP™ Digital Temperature Indicator manufactured by Littelfuse. Its small surface mount size, 2.0 x 1.2mm, is 30 percent smaller than other temperature sensing devices and fits easily into a USB plug. The device works by substantially increasing its resistance when the surrounding temperature reaches a specific value, and the circuit using the device shuts down power delivery. This follow-on article presents design recommendations for using the setP Digital Temperature Indicator in a USB Type-C plug.
Figure 1 shows the resistance vs temperature curve for two setP temperature indicator models. At room temperature, the device resistance is under 10Ω. At around 100°C, the resistance rises by at least three decades.
Circuit design considerations
Place a setP digital temperature indicator in each plug of the USB Type-C cable to sense potential faults on either or both connectors. Connect each DTI in the Configuration Channel (CC). The DTIs leverage the system parameters defined by the USB-IF standards1 to cause a power shutdown when the DTI’s trip temperature is reached. The setP DTI is unique since it is placed in the CC line and not in the path of the charging power. Thus, the setP DTI enables the charging circuit to be power independent; the DTI provides reliable protection regardless of the power level negotiated between the source, the charger, and the sink, the product being charged. The USB-IF standard defines a monitoring circuit shown in Figure 2. Based on the voltage level measured on the CC line, the source will know when a sink is connected or disconnected. If the CC voltage on the source side reaches a level defined in the standard, then power is removed from the VBUS pins. The increase in the resistance of the setP DTI will cause the voltage increase. Figure 3 shows the pin layout for the USB Type-C plug. Pin A5 is the CC line, and VBUS is on four pins, A9, A4, B4, and B9. Figure 4 shows the DTIs, one for each connector, installed in the CC line.
Recommended location of the temperature indicator
The location of the temperature indicator in the connector will determine how much of the temperature rise during a fault condition at the connector pins that the sensor will detect. It is recommended that the temperature indicator be located no further than 3mm from the connector pins. Test results show that the temperature in the connector decreases by 5°C for every millimeter that the temperature indicator is placed further from the connector pins. For example, if the temperature at the detector placed 3mm from the pins rises to 100°C, then the temperature 3mm further from the connector will be 15°C lower or 85°C. Thus, placing the temperature indicator as close as possible to the connector mounting pins provides the best protection against a high temperature rise due to a fault condition. The recommended layout for the location of the temperature indicator is shown in Figure 5.
Printed circuit board (PCB) layout and soldering recommendations
Figure 6 shows the recommended solder layout pad for the temperature indicator. Use a maximum thickness of 0.25mm for the solder paste. To avoid damage to the device, ensure that the maximum package temperature never exceeds 260°C during the solder process. Also allow the temperature ramp-up and ramp down rates to be no more than 3°C/s and 2°C/s respectively. Keep the temperature above 217°C for no more than 150s.
PCB molding considerations
Connectors that house a PCB typically use an inner mold to protect the PCB. Polypropylene material is a common molding material. The mold injection pressure when using polypropylene can reach 400 psi. The digital temperature indicator can withstand common mold injection pressures. If planning to use another molding material, give careful consideration to the molding pressure so that physical constraints are not imposed on the setP DTI.
Design verification test recommendation
The designer will want to verify that the protection circuit performs to protect the cable when the cable reaches a specific temperature level. One test configuration is shown in Figure 7. The thermal chamber will provide the varying temperatures. Use the oscilloscope to monitor both the chamber temperature from a thermocouple in the chamber and the VBUS voltage. Verify that the VBUS voltage falls to a low value when the temperature indicator’s transition temperature is reached. Cool the thermal chamber and verify that the VBUS voltage returns to its operating level when the temperature falls below the temperature indicator’s transition temperature. With this test and similar design verification tests, the design team will be able to confidently transition the cable to manufacturing.
With up to 100W flowing through a USB Type-C cable, thermal protection is essential for ensuring that the cable will be robust and safe. The setP Digital Temperature Indicator provides the necessary temperature protection while not limiting cable charging power or interfering with the USB communication protocol.
For more details on the use of the setP Digital Temperature Indicator, download the setP™ Digital Temperature Indicators for USB Type-C Cables Design and Installation Guide.
References
1Universal Serial Bus Type-C Cable and Connector Specification. Revision 2.0. August 2019. USB Implementers Forum (USB-IF), Inc.
About the author
Todd Phillips is the Global Strategic Marketing Manager for the Electronics Business Unit. He joined Littelfuse as a sales engineer in 2006 for the industrial business unit. Todd joined the electronics business unit in 2011 as a regional sales manager. His current responsibilities include the development of marketing collateral material, management of marketing activities for new product launches, and performing market studies and feasibility analyses for new product ideas. He received his BSEE from Milwaukee School of Engineering. Todd can be reached at tphillips@littelfuse.com.
“Five Design Considerations for Thermal Protection Of USB Type-C Cables” as published in Sensors Daily (July 2020)