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Protecting Industrial Interfaces using the TBU High Speed Superior Protection TBU – State-of-the-art electronic surge protection TBU products are high-speed, current-tripped switches, that can be used to provide superior protection against all forms of surge including AC Power Cross, power Induction, and Lightning. Using a TBU creates a barrier between the exposed line and the communication port. The resulting protection is simple, inexpensive, small, and very effective. Common Questions Protection Advantages: • • • • • • • Extremely high speed Blocks high voltages and currents Low let-through energy Self-resetting Very high bandwidth Small size RoHS Compliant Key Characteristics • TBU is used in series with line • TBU is triggered by current and resets on voltage • TBU block up to 850V • High blocking voltage means easy shunt device coordination (eg GDT) • TBU operates in ≈ 1µs (Top) • TBU allows for very low let through energy • TBU is a 2-pin linear resistor which becomes an effective open circuit upon trigger (see VI curve at right) • TBU is unpowered and floating wrt ground • Because it is floating, TBU places no capacitance on the line. It therefore does not effect system bandwidth. • Like PTC, TBU have resistance (typically 5 - 15 ohms dependent on device) • Most TBU are bi-directional (input/output pads interchangeable) • The TBU DFN package has a 3rd (center) pad that is N/C and used only for heatsinking. • TBU resets when the voltage across the device falls to below Vreset (typically 15V) • TBU is a semiconductor MOSFET based product with a long surge lifetime (>>108 operations). TBU VI Curve 8 4 C Series Package Example TBU protection circuits for serial interfaces commonly used in Industry (Examples have a 2kA Surge rating and include optional secondary zeners) C650-260 Vcc CD214B-T12A C650-100 C650-100 RS232 CD214B-T26A RS485 G5500AS C650-100 CD214B-T12A RS422 Bourns Inc. G5500AS C650-180 G5500AS CD214B-T12A G5500AS CANBUS Page 1 An Overview of TBU Protection Operation and Design High Speed Superior Protection Why is TBU protection is superior Figure 1 shows a comparison between the performance of a GDT/PTC protector and a GDT/TBU Protector (Fig. 2). During a lightning event a PTC’s resistance doesn’t change. It is to slow. The voltage on the output of the GDT/PTC protector is simply the GDT firing voltage as seen through the PTC resistance. Input V Output V However during the same surge TBU switches quickly to an open circuit, disconnecting output from input. The voltages at the output become extremely small. This ensures electronics on the output of the TBU is thoroughly protected. Other benefits include lifetime (>100’s Millions operations and no reduction in performance), insensitive to waveshape (TBU protection is not sensitive to surge duration or repetition rate), wide operational bandwidth (TBU protectors can be used DC to microwave), ease of design, and low cost. Output I GDT/PTC Protector GDT/TBU Protector Figure 1: Performance of a TBU based protector against conventional PTC based protection when used on a transmission line. This article provides a description of how TBU protection works so you can tailor your own solutions for improved protection reliability, lower operational costs and piece of mind. TBU TBU Input The TBU is a silicon based, solid-state, resettable fuse. TBU is placed in series with a signal path (just like a fuse or PTC - See Figure 2). TBU TBU The TBU operates in approximately 1 µs - once line current exceeds the TBU trip point. When operated TBU restricts line current to less than 1mA. The current level at which the TBU operates is always greater than the TBU’s hold rating (Iop), and always less than the TBU’s maximum output current (Iout). When operated TBU will block all system voltages and any other voltages including the surge. Output Basic TBU Operation The purpose of the TBU is to protect equipment from the let through voltages of simple high energy shunt devices like GDT and MOV as they conduct lightning current. In the operated state the TBU can block voltages up to the Vimp. Vimp typically is in the order of 500V to 850V dependent on the type of TBU. These voltage levels are selected to be compatible with most low voltage GDT and MOV. After the surge TBU resets. A TBU resets when the voltage across the TBU falls to the Vreset level. TBU will always reset on lines which have no DC bias or have DC bias below Vreset (such as unpowered signal lines). If the line has a normal DC bias above Vreset, the voltage across the TBU may not fall below Vreset after the surge. In such cases special care needs to be taken to ensure that the TBU will reset, otherwise an automatic or manual power down will be required. Bourn’s application engineers can provide further assistance. Figure 2: GDT/TBU Twisted Pair Protector DESIGNING TBU PROTECTION STEP 1: Determine the maximum system current To select the correct TBU a designer must know the peak system currents. If a TBU is used with a Itrip below that level it may interfere with normal system operation. STEP2: Select the TBU hold current Ihold STEP 3: Determine the peak lightning currents the protection circuit is intended to withstand STEP 4: Choose a GDT or MOV capable of handling the required lightning current. STEP 5: Choose a TBU with a maximum impulse voltage (Vimp) greater than the let-through voltage of the GDT or MOV selected and with the correct Ihold. Ideally Ihold should be100% above peak signal currents in analogue systems like DSL and more than 33% above for digital systems (RS232, RS485 etc). In order to select the correct shunt protector to pair with a TBU, the designer needs to understand the stresses it must withstand. The device should have a operating voltage above normal system voltages DESIGN COMPLETE – The interface is protected against any lighting strike up to the rating of the GDT or MOV and from any let-though currents above Iout OPTIONAL 6th STEP STEP 6: Choose secondary clamp or avalanche diode. Some interfaces can be damaged if the input voltage exceeds a specified level - even if let-through current is limited to low levels by a TBU. In such cases place a small avalanche diode to ground or clamping diodes to the power supply rails after the TBU. This will ensure that under no circumstances can the voltage at the interface rise to unsafe levels without the TBU tripping. Bourns Inc. Page 2