Circuit protection devices may seem mundane to mostengineers, but Greg Evans, P.E., CEO of Welcomm marketing for TE CircuitProtection, gave an eye-opening overview of two new ESD and thermal protectiondevices that changed my concept of these types of solutions. I came to realizehow critical these devices are to protect a system, while at the same time, ifnot designed properly, could seriously degrade system performance and turn themanufacturing process into a nightmare.
TE Connectivity (formerly TYCO) is the mothercompany to TE Circuit Protection, which has a 30-year foundation in advancedmaterials science (originally as Raychem Corp). The company was the developerof the first commercial PolySwitch Polymeric Positive Temperature Coefficient(PPTC) devices which are used to help protect against harmful overcurrentsurges and over-temperature faults.
Like traditional fuses, these devices limit the flowof dangerously high current during fault conditions. The PolySwitch device,however, resets after the fault is cleared and power to the circuit is removed,thereby reducing warranty, service and repair costs. This is achieved by usinga polymeric PTC material, which is a matrix of a crystalline organic polymercontaining dispersed conductive particles, usually carbon black.
RTP140AC and DC devices providing thermal protection for power electronicsNow here is where manufacturing is simplified forthe first time with these types of devices. Not until the introduction of the RTP200 family in 2011 has surface-mount reflowable thermalprotection been available. The RTP140series is the newest entry to these solutions introduced at APEC 2012. They canhandle the re-flow process upto 3 times (260°C peak).
The RTP device uses an electronic arming mechanism. Electronic arming must be done after reflow, and can be done during final test.
The RTP device is aunique thermal protector. It can be reflowed at temperatures up to 260°Cwithout opening, yet in operation it will open at temperatures well below260°C. To achieve this functionality, the RTP device uses an electronic armingmechanism.
Electronic armingmust be done after reflow, and can be done during final test.
Plus these devices are fully surface mountable asopposed to traditional Thermal Cut-offs (TCO) which are typically radial leadedrequiring post reflow assembly.
Single- and multi-channel silicon ESD (SESD) devices These new devices are the industry's lowestcapacitance and smallest size package available in the market with higher ESDkV protection for low-capacitance devices.
The ESD protection is the easy part as compared tohow these devices can affect system performance. The designer will want thesetransparent to the rest of the circuit. High-speed data signals demand lowcapacitance with high voltage ESD protection to maintain signal integrity. Inaddition, design engineers want small form factors since board space is at apremium. Enter the SESD series of devices.
New TE Circuit Protection SESD series has lowest capacitance, highest ESD protection and smaller package than other devices in the industry.
These solutions should present designers with quitea different view of "ordinary" circuit protection device.
Armingthe RTP deviceThe device is armedby sending a specified arming current through the ARM pin of the device. Armingis a time- and current-dependent event. Arming times versus current areprovided in the "Arming Characteristics" section of the data sheet. Current canflow in either direction through the ARM pin.
Prior to arming,RARM should have resistance levels as specified in the "Arming Characteristics"section of the data sheet. Once armed, the ARM pin will be electrically openrelative to the P1 or PTH pins.
Arming has beensuccessful once RARM exceeds the post-arming minimum resistance specified inthe "Arming Characteristics" section. RTP devices must be armed individuallyand cannot be armed simultaneously in series.
Once "armed", the RTP device willpermanently open when the device junction achieves its specified openingtemperature.