A transient voltage suppressor (TVS) as the name implies is a device designed to suppress momentary over-voltage conditions (transients) that might otherwise damage a circuit. Such transients can be caused by very high energy events such as lightning strikes or high voltage switching, or by lower energy (but still fatal to unprotected electronics) sources such as electrostatic discharges from the human body.
Typically, we would rely on metal oxide varistors and gas discharge tubes to manage the former and semiconductor devices to manage the latter. I want to concentrate on semiconductor solutions that might be used to protect high speed signal lines such as USB and HDMI against electrostatic discharge (ESD).
Most commonly we characterise ESD circuits using a “human body model” (HBM) like that shown in Figure 1. This is intended to mimic the level of electrostatic charge a person may accumulate and pass on to the device by touching it. The human body is represented by a 100pF capacitor which is charged to a high voltage and discharged via a 1k5 resistor into the device under test. The capacitor would typically be charged to 2kV, 4kV or 8kV depending on the level of immunity we wish to test for.
When charged to 4kV, the HBM can inject a transient of 4kV with a peak current of 2.7A Fortunately, the spike is narrow, typically just a few hundred nanoseconds and contains a total energy of 800µJ. How could we protect a signal line against a transient such as this?
The simplest approach might be to use a simple Zener diode. Consider the general purpose BZT52H family. These are low cost 375mW devices in a surface mount package. They can handle a non-repetitive peak reverse current of around 6A for 100µs so would certainly do the job. There is however a downside—these devices have a capacitance in the order of 300pF to 400pF in the voltage ranges we are considering. This would be fine on a power supply line but this much capacitance will certainly kill high-speed signals like USB or HDMI. We clearly need to look a bit further.
Before we move to specialised TVS diodes, let’s not forget that a bipolar junction transistor can be used as a zener diode since its base-emitter junction exhibits reverse breakdown at around 7V. Figure 2 shows how the transistor should be connected for best performance. A typical jelly-bean small signal transistor such as a 2N3904 can handle a peak reverse current of a bout 4A for 20µs and comes with a very respectable capacitance of just 7pF. Definitely worth considering.
So, what about specialist TVS diodes? They are available in unidirectional types that only clamp voltages in one direction, or bidirectional types that clamp voltage symmetrically. They are also available in pairs or arrays as we shall see. Figure 3 shows the V-I characteristic curves for typical TVS diodes. Under normal operation the voltage applied to the device should not exceed the “reverse stand-off” voltage VRM. This is the voltage level at which the device is guaranteed not to break down.
Once the transient reaches the breakdown voltage VBR the device begins to conduct to shunt the voltage transient. The voltage will be clamped to no more than VCL at the maximum current IPP. A typical device, like the Toshiba DF2S5M4CT, has a reverse stand-off voltage of 3.6V (therefore suitable for 3V3 signal lines) and will clamp the transient voltage to less than 8V at a peak current of up to 8A. It has a maximum capacitance of 0.5pF making it suitable for very high-speed lines.
Finally, Figure 4 shows a TVS diode array designed for protecting USB 2.0 lines, in this case the D5V0F3B6LP20 from Diodes Inc. This device has one unidirectional 20V stand-off TVS diode to protect the VBUS line, and three bidirectional 5.5V stand-off TVS diodes to protect the signal lines. The unidirectional diode can handle an IPP of 9.5A and the bidirectional diodes can handle and IPP of 3.5A each. The capacitance of the signal lines is less than 0.5pF. All this comes in a 2mm x 2mm package and costs less than 50 cents.
TVS diodes are cheap insurance against ESD damage and well worth considering for your next design.
Adamczyk, Bogdan. “Human-Body Model and Electrostatic Discharge (ESD) Tests.” In Compliance Magazine (blog). https://incompliancemag.com/article/human-body-model-and-electrostatic-discharge-esd-tests
“Transient Voltage Suppressors (TVS) – STMicroelectronics.” https://www.st.com/en/protections-and-emi-filters/transient-voltage-suppressors-tvs.html.
“DF2S5M4CT | TVS Diodes (ESD Protection Diodes) | Toshiba Electronic Devices & Storage Corporation | Asia-English.” https://toshiba.semicon-storage.com/ap-en/semiconductor/product/diodes/tvs-diodes-esd-protection-diodes/detail.DF2S5M4CT.html
“D5V0F3B6LP20 USB2.0 and VBUS TVS Diode Array” https://www.diodes.com/assets/Datasheets/D5V0F3B6LP20.pdf