PCB Design Guidelines for Using a TVS Diode for Transient Protection

August 1, 2018 Altium Designer

Is your TVS diode design optimized for transient protection

 

Although it was highly educational and undeniably inspiring to witness the early days of the Internet, I don’t miss the slow speeds or endless technical issues. In those days, it took ages to check sports stats or open a simple email, not to mention the risk of thunderstorms damaging the computer. On stormy days, it wasn’t unusual to unplug the power to the computer and disconnect the phone cable connecting the computer to the internal dial modem. Back then, that was the best way to protect the system against surges.

 

For hardware engineers, surge protection is more than unplugging a couple of cables. It involves including transient protection components in the PCB design and hoping that they divert the excessive voltage to the ground before it affects vital components within the PCB. Ensuring that your PCB design is optimized for transient protection can mean the difference between a functional prototype and a fried circuit board.

What is a TVS Diode and How Does it Work?

The Transient Voltage Suppression (TVS) diode is a component that is commonly used for transient protection. It consists of a p-n semiconductor junction that becomes conductive during a transient voltage spike. In normal circumstances, it appears as an open circuit despite a small leakage current.

 

When the voltage on the TVS diode rises across its threshold voltage, the avalanche effect of the semiconductor causes the p-n junction to become a low-impedance path that channels away the excessive current. The response time of the TVS diode is extremely fast, often expressed in picoseconds.

Choosing the Right TVS Diode for Your Design

Of course, not all TVS diodes are created the same. Choosing the wrong one for your design may render the transient protection ineffective from the start. There are a few parameters that you’ll need to understand when choosing a TVS Diode:

  • Clamping Voltage (VC) - The clamping voltage is the voltage limited by the TVS diode in the event of a transient within the limit of the specified peak current.
  • Rated Standoff Voltage (VWM) - This indicates the limit within which the TVS diode will operate normally. Within the Rated Standoff Voltage, the TVS diode has a high impedance with only a small amount of leakage current.  
  • Peak Pulse Power Dissipation (PPP) - The TVS diode needs to be able to safely dissipate the excessive current caused by the transient voltage. This is indicated by the Peak Pulse Power Dissipation.

Besides choosing the right TVS diode, the effectiveness of the protection is determined by the PCB layout itself.

PCB Design Tips for TVS Diode

A TVS diode is connected in parallel to the circuit that it is protecting. The schematic below indicates a typical connection of a TVS diode to a MAX3485 transceiver:

Schematic of typical TVS diode connection

Schematic of typical of TVS Diode connection.

 

In a PCB layout, a few important guidelines must be adhered to for TVS diodes to function efficiently.

 

Avoid connecting both the TVS diode and the transceiver to a similar ground. This may cause the surge to be channeled to the digital ground, thus causing a ground bounce. Instead, the TVS diode should be connected to chassis ground with a low impedance trace. If this is not possible, the inductance of the single ground plane must be minimized to prevent ground bounce.

 

Layout of component placement with appropriate board edge clearance

Place TVS diode near the edge of the PCB.

 

To enable the excessive surge current to be channeled to chassis ground before damaging the transceiver, the placement of the TVS diode is also important. It is a good practice to place TVS diodes as close to the edge of the board as possible. The trace of the TVS diode should also be comparatively short to the trace of the transceiver to minimize the impedance and ensure that the excessive energy of the surge is dissipated efficiently.

 

PCB traces produce parasitic inductance that may cause the clamping voltage of the TVS diode to increase above its specified limit. It is, therefore, important to minimize parasitic inductance by shortening the trace length and opting for a multilayer PCB with separate ground and power planes. Besides that, a smaller form factor of the TVS diode can also lower parasitic inductance.

 

Using the integrated library in Altium Designer, you can easily place a TVS diode without risking parasitic inductance or other problems. With the Polygon Pour feature, you can create the appropriate ground plane to efficiently shunt the excessive current. For more tips on using a TVS diode for transient protection in your PCB design, talk to an expert at Altium.

 

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