“The proliferation of Industry 4.0, Industrial Internet of Things (IIoT), and 5G telephony technologies has resulted in more and more complex Electronic devices being deployed in harsher, more difficult-to-access environments. This facilitates repeatable, deterministic electrostatic discharge (ESD) and Electrical Overstress (EOS) event protection.
Author: Jeff Shepard
The proliferation of Industry 4.0, Industrial Internet of Things (IIoT), and 5G telephony technologies has resulted in more and more complex electronic devices being deployed in harsher, more difficult-to-access environments. This facilitates repeatable, deterministic electrostatic discharge (ESD) and Electrical Overstress (EOS) event protection. These applications need to meet the transient protection requirements of the IEC61000 standard. While Transient Voltage Suppression (TVS) diodes work well for designers, more and more applications require more deterministic, linear, compact and reliable ESD and EOS protection.
To meet these ever-increasing performance and form factor requirements, transient shunt suppressor (TDS) devices are available. This device has excellent clamping, linearity and temperature stability at the same time for a more assured level of performance. TDS devices do not dissipate surge energy like TVS diodes do, but divert this energy to ground. Compared to TVS alternatives, TDS does not dissipate energy, so its size can be smaller, which helps reduce solution size. In addition, the clamping voltage of the TDS device is 30% lower than that of the TVS diode, thus reducing the electrical stress of the system and improving reliability.
This article will describe how TDS devices work and the benefits they bring to critical applications. Then, Semtech’s TDS devices are used as examples and PC board layout guidelines for successful application of these devices are given.
How TDS Surge Protectors Work
Surge grade field effect transistors (FETs) are the main protection elements in TDS devices. When an EOS event occurs and the transient voltage exceeds the breakdown voltage (VBR) of the integrated precision flip-flop circuit, the driver circuit is activated and the FET turns on, conducting the transient energy (IPP) to ground (Figure 1).
Figure 1: In a TDS device, when an EOS event is detected, a precision flip-flop circuit (left) activates a FET voltage-controlled switch (right), diverting the energy spike (IPP) directly to ground (Image credit: Semtech).
As the pulse current increases to IPP, the on-resistance (RDS(ON)) of the FET becomes several milliohms (mΩ) and the clamping voltage (VC) is almost the same as the VBR of the trigger circuit. Therefore, the VC of the TDS device is almost constant over the range of IPP. This is different from the clamping action in TVS devices, which is known to:
where Rdyn is the dynamic resistance.
In TVS devices, the value of Rdyn is fixed so that the clamping voltage increases linearly with IPP over the rated current range. For TDS devices, VC is stable over the operating temperature and IPP range, enabling decisive EOS protection (Figure 2).
Figure 2: For a TDS device such as the TDS2211P (solid line), the clamping voltage remains constant over temperature and Ipp, providing deterministic EOS protection. (Image credit: Semtech)
The VC of TDS devices is relatively low, so the protected device is not only subject to lower electrical stress, but also improves reliability (Figure 3).
Figure 3: The low VC of a TDS device (represented here by VClamp, green curve) improves reliability by reducing electrical stress on the protected device. (Image credit: Semtech)
The performance of TDS devices supports system designs that meet the requirements of several standards: ESD immunity requirements of IEC 61000-4-2, Burst/Electrical Fast Transient (EFT) immunity requirements of IEC 61000-4-4, and the surge immunity requirements of the IEC 61000-4-5 standard. This makes TDS devices suitable for many harsh environment applications. Examples of TDS applications are described below, including 22 V TDS devices used to protect load switches, 33 V TDS devices suitable for protecting IO-Link transceivers, and 58 V TDS devices that can be used to protect PoE devices.
Protective load switch
Use the 22 V TDS2211P to protect load switches, electronic fuse inputs in industrial equipment, robotics, remote meters, USB Power Delivery (PD) and IIoT devices from EOS events. The EOS protection levels for this TDS device include:
・ The ESD withstand voltage level of contact and air is ±30 kV, which meets the requirements of IEC61000-4-2 standard
・ 40 A peak pulse current rating (tp = 8/20 μs) per IEC 61000-4-5; ±1kV (tp = 1.2/50 μs, shunt resistance (RS) = 42 Ω) per IEC 61000-4-5 standard requirements for asymmetric lines
・EFT withstand voltage is ±4 kV (100 kHz and 5 kHz, 5/50 ns) according to IEC 61000-4-4
When used in this configuration, the TDS2211P protects downstream devices from lightning strikes, ESD, and other EOS events, and the device also keeps VC below the damage threshold of the switching FET in the load switch (Figure 4).
Figure 4: The TDS2211P can be used to protect the load switch (HS2950P) and downstream devices from lightning, ESD and other EOS events. (Image credit: Semtech)
In addition to the common ESD and EOS hazards that occur in industrial environments, IO-Link transceivers can experience voltage spikes of several thousand volts when plugged into or unplugged from IO-Link master devices. TVS diodes typically used to protect IO-Link transceivers can be supplemented with TDS devices for improved protection. In a typical circuit protection application, the device used is rated for at least 115% of the input supply, so for 24 V applications such as IO-Link, a 33 V protection device like the TDS3311P TDS is appropriate. The main specifications of the TDS3311P are as follows:
・ The ESD withstand voltage of contact and air is ±30kV, which meets the requirements of IEC61000-4-2 standard
・ Peak pulse current capability of 35A (tp = 8/20 μs), and 1 kV (tp = 1.2/50 μs, RS = 42Ω), in line with IEC61000-4-5 standard asymmetric line requirements
・ Burst/EFT immunity requirements according to IEC61000-4-4
There are two common IO-Link port configurations, 3-pin and 4-pin. These two configurations require slightly different protection schemes. In both cases, the TDS device can be supplemented with a μClamp3671P TVS diode on the VBUS (L+ (24V)) line to provide reverse polarity protection (Figure 5).
Figure 5: Comparison of ESD protection for a 3-pin IO-Link port (top) and a 4-pin IO-Link port (bottom) using a TDS device (green rectangle). (Image credit: Semtech)
In the 3-pin case, 3 TDS devices are required. If desired, bidirectional protection can be provided by two face-to-face TDS3311Ps. In the 4-pin case, all four pins of the IO-Link port should be able to withstand both positive and negative surges. The connector needs to be tested between each pair of pins to ensure the surge protection performance of the IO-Link transceiver and should be in accordance with IEC 61000-4-2 ESD, IEC 61000-4-4 Burst/EFT and IEC 61000- 4-5 surge required levels to be tested.
PoE protection schemes must take into account that EOS events may be common-mode (relative to ground) or differential (line-to-line). PoE supplies 48 V, so a 58 V TDS device like the TDS5801P can be used to provide EOS protection on the RJ-45 connector side. The specifications of the TDS5801P are as follows:
・ESD withstand voltage: ±15 kV (contact) and ±20 kV (air), in accordance with IEC61000-4-2
・ Peak pulse current capability: 20A (tp = 8/20 μs), 1kV (tp = 1.2/50 μs, RS = 42 Ω), in line with the requirements of IEC61000-4-5
・ EFT withstand voltage is ±4 kV (100 kHz and 5 kHz, 5/50 ns) according to IEC61000-4-4
Power in a PoE system is provided through the center-tapped connection of the transformer. The PD (RJ-45) side must be protected for both Mode A (power supplied via data pair 1 and 2, data pair 3 and 6) and Mode B (power supplied via pins 4 and 5 and pins 7 and 8 ), so two pairs of TDS5801P are required for bidirectional protection across the center-tapped connection (Figure 6).
Figure 6: A back-to-back TDS device (green, TDS5801P) provides bidirectional protection against EOS events in a PoE system. (Image credit: Semtech)
Transformers provide common-mode isolation, but not differential surge protection. In a differential EOS event, the transformer windings on the line side are charged and energy is transferred to the secondary side until the surge ends or the transformer saturates. The TDS devices on the PD side can be complemented by four RClamp3361P ESD protection devices located on the Ethernet physical layer (PHY) side of the transformer to protect against differential EOS events.
SurgeSwitch TDS devices offer designers a choice of operating voltages including 22 V (TDS2211P), 30 V (TDS3011P), 33 V (TDS3311P), 40 V (TDS4001P), 45 V (TDS4501P) and 58 V (TDS5801P) (Table 1) . These devices meet the requirements of the IEC61000 standard and can be used in systems operating in harsh 5G telephony and industrial environments.
Table 1: SurgeSwitch devices are rated from 22 V to 58 V to meet many application requirements. (Image credit: Semtech)
Because TDS devices are non-dissipative and instead transfer surge energy directly to ground through a low impedance path, they can be housed in a small 1.6 x 1.6 x 0.55 mm package, compared to the SMA and SMB package for significant board space savings. The 6-pin DFN package includes 3 input pins and 3 pins for transferring surge energy to ground (Figure 7).
Figure 7: TDS device in a 1.6 x 1.6 x 0.55 mm DFN package with 6 leads (right); pins 1, 2, and 3 are grounded, while pins 4, 5, and 6 are used as EOS/ESD protection inputs . (Image credit: Semtech)
Board Layout Guidelines
When mounting a SurgeSwitch TDS device on a circuit board, all its ground pins (1, 2, and 3) must be connected to the same trace, and all input pins (4, 5, and 6) must be connected to the same traces for maximum inrush current capability. If the ground traces are on different layers of the board, it is highly recommended to use multiple vias to connect to the ground plane (Figure 8). Follow these PC board layout guidelines to minimize parasitic inductance and optimize device performance. Additionally, the SurgeSwitch TDS device should be placed as close as possible to the protected connector or device. This minimizes the coupling of transient energy to the trace, which is especially important during fast rise time EOS events. Since TDS devices do not dissipate any energy, there is no need for thermal pads under the device to conduct thermal energy.
Figure 8: Multiple vias are recommended for best performance when the ground planes are on different layers of the board. (Image credit: Semtech)
For designers of industrial and 5G telephony equipment operating in harsh environments, TDS devices can be used to provide reliable, deterministic protection from ESD and EOS events. The relatively low VC of TDS devices improves system reliability by reducing electrical stress on components. These devices meet the transient protection requirements of the IEC61000 standard and are available in a 22 V to 58 V voltage range to meet specific application requirements. The small size of TDS devices helps reduce the size of the overall solution, but designers need to follow some simple PC board layout requirements to get the most out of TDS devices.