Nonsense! Who taught you to use TVS like this?
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For engineers, surge protection is more than just choosing the right power strip or unplugging a few cables. It mainly involves placing transient protection components in the PCB layout and applying a clear grounding strategy.
TVS diodes are common components used to protect components in PCB layouts that are placed on data lines by diverting current away from the protected component once an ESD pulse is received in the circuit. Make sure the PCB layout is optimized for transient protection and will prevent board frying and ensure a well-functioning device.
1. What is a TVS diode and how does it work?
Transient voltage suppression (TVS) diodes are components commonly used to protect equipment from transient events related to electrostatic discharge (ESD). (Do not confuse TVS tubes with Zener diodes or Schottky diodes.)
It consists of a pn semiconductor junction that becomes conductive during transient voltage spikes. Under normal conditions, TVS diodes have high impedance and very low leakage current, effectively acting as an open circuit.
When the voltage across a transient voltage suppressor rises above its threshold voltage, an avalanche effect in the semiconductor causes the pn junction to begin conducting, providing a low-impedance path to conduct excess current away from the device being protected.
TVS diodes have very fast response times, typically measured in picoseconds, so these components can deflect strong ESD pulses very quickly, even if the ESD pulse has a relatively fast rise time.
2. Select the appropriate TVS diode for PCB design
All TVS diodes are essentially diodes: if you apply a large enough forward or reverse bias voltage, a TVS diode will start to conduct. Of course, not all TVS diodes are created equal.
If the wrong protection is selected, transient protection may fail from the outset. When choosing a TVS diode, you need to know some parameters:
1. Reverse bias breakdown voltage (VB)
This is the reverse bias voltage at which the TVS diode begins to conduct. Once the TVS diode begins to conduct, it diverts the ESD pulse away from the component being protected.
2. Clamping voltage (VC)
The clamping voltage is the minimum voltage at which a TVS diode will significantly conduct after exceeding reverse bias breakdown. This value is defined within the limits of the specified peak current.
Generally, a lower VC value will provide more protection to the component, so VC should be chosen such that it is less than the input voltage limit of the component being protected.
3. Rated shutdown voltage (VWM)
This represents the reverse bias voltage limit below which the TVS diode will remain insulated. Within the rated shutdown voltage, TVS diodes have high impedance and only a small amount of leakage current.
4. Peak pulse power dissipation (PPP)
TVS diodes need to be able to safely dissipate excess current caused by transient voltages, which is represented by peak pulse power dissipation.
3. How do TVS diodes work?
The operating principle of all TVS diodes is simple: when a circuit receives an ESD pulse, the pulse quickly exceeds the reverse bias breakdown voltage value of the diode.
Equipment that exposes any of its conductors to the outside environment (for example, through a connector) can receive ESD pulses on those conductors. If these conductors are part of the signal lines leading to the component, the received ESD pulse will transmit high voltage/high current pulses into the component, which may damage the component.
When ESD occurs on a signal line and a TVS diode is present on the signal line, the diode will begin to conduct and the pulse can pass through the diode. This allows the diode to divert ESD pulses away from the protected circuit.
A typical connection is to connect the anode to ground so that the ESD pulses are transmitted to the ground. As long as there is a low impedance path to ground, the pulse will be diverted away from the protected component.
TVS diode
4. Bidirectional TVS diode or unidirectional TVS diode?
TVS diodes come in two types: bidirectional and unidirectional. These two types of TVS diodes have different symbols as follows:
Bidirectional and Unidirectional TVS Diodes
When purchasing TVS diodes, please note that generally speaking, TVS diodes refer to unidirectional TVS diodes. If you need bidirectional TVS diodes, you need to explain.
So which type of TVS diode should you choose? The main reason for using bidirectional TVS diodes is to provide protection when the circuit carries both positive and negative polarity signals. This is why you sometimes see dual Pioneer TVS diodes on differential pairs or analog lines that oscillate between positive and negative polarity.
Most people would prefer to use bidirectional TVS diodes to provide complete fault protection and ESD protection, because the ground area can accept ESD pulses, just like the signal lines to be protected.
If a ground fault results in a high impedance path to ground, then the path of lowest impedance may be through the unidirectional TVS diode and the component you want to protect. But if the TVS diode is bidirectional, there will be a chance to protect the component even if there is a ground fault.
5. PCB layout skills for TVS diodes
In addition to selecting the appropriate TVS diode, the effectiveness of protection also depends on the PCB layout itself. The example below shows 2 bidirectional TVS diodes connected in parallel to the circuit being protected in the schematic below. The schematic shows the typical connection of the TVS diode to the MAX3485 transceiver:
Typical schematic diagram of TVS diode connections
In this example, if ESD occurs where the D+ and D- lines are exposed to the external environment, and a positive voltage is developed with respect to GND, the TVS diode will begin to conduct as soon as the ESD voltage exceeds the TVS reverse bias breakdown voltage.
If an ESD occurs that causes current to start flowing in the GND plane, the current should be completely diverted away from the component as long as a low-impedance ground path exists in the system.
In situations where the grounded conductor receives ESD, it is better to use a bidirectional TVS diode because it will still provide some protection, whereas if the TVS diode is unidirectional, the transceiver may still be exposed to some voltages.
The preferred transfer of bidirectional TVS diodes occurs because the applied pulse needs to rise above a certain threshold (the VB value of the upper half of the TVS diode) before conduction can occur from GND to the trace.
In PCB layout, there are some important guidelines that should be followed for proper operation of TVS diodes. These include placement, grounding, and the use of any passive components such as resistors or capacitors on the shield.
1. Placement of TVS diodes
Because ESD can occur near exposed conductors in electronic equipment, it is best to place TVS diodes near areas where these conductors are exposed to the outside environment. A simple layout example with a 2-pin connector is shown below.
Place TVS diodes near exposed conductors where there is a risk of receiving ESD pulses
PCB traces have some parasitic inductance that can cause the TVS diode's clamping voltage to increase above its specified limit. The TVS diode traces should also be short relative to the transceiver traces to minimize impedance and ensure excess energy in the surge is dissipated. This will minimize the parasitic inductance in the path to the TVS diode.
2. Grounding
If possible, it is best to connect the TVS diodes to a different ground grid than the component being protected. This does not mean that the ground plane should be split. Instead, the safest type of connection is to connect the TVS diode to a metal element in the chassis ground (if available), usually via a trace connected to a chassis screw or mounting hole.
If this connection is not available, you can connect to an internal plane. However, in environments where there is a strong ESD risk, the device should be enclosed in a chassis with a safe metal chassis ground and ground connection.
3. Eliminate passive shielding
Some components, such as shielded connectors, will have some additional metal shielding to protect the exposed conductors. The shielding on the connector is not meant for mechanical or thermal protection, it is actually to prevent noise reception and protect against ESD.
If ESD hazards exist, shielded connectors can be used with TVS diodes. The TVS diode is connected to the signal line and the shield on the connector is directly connected to ground.
Connection example of two TVS diodes on data line
In the picture above, a direct connection is placed between the chassis and signal ground. The typical approach is to place this connection in a location in the system that ensures uniform ground potential on all conductors but still controls normal return currents so that they do not pass through the chassis.
The same method applies as long as GND is a low-impedance, low-inductance ground plane. If this is an electrically isolated area of the system, it is best to place the connection closer to the connector body, as shown in the image above.
In some cases you may see someone trying to ground the shield through a snubber circuit or a parallel RC circuit. Both of these defeat the entire purpose of using shielded connectors.
Instead, make a direct connection between the shield and the chassis ground (if available) or ground plane, which creates a very low-impedance path to ground, preventing energy from an ESD event from reaching the protected components.
In some cases where you have trouble controlling the return current (e.g. floating ground), a suitable approach is to place a large capacitor between the shield and ground. This ensures that fast ESD pulses can be shunted and will not be affected by two Any offset between grounds causes high frequency noise to be radiated from the system.
Source: Playing with Embedded
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