Download TBU Industrial App Note V3

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:
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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.
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