If you are a power supply designer, then isolation is often a concern when dealing with high-voltage switching converters. It is very common to use some form of isolation between the high-voltage primary voltage and secondary low voltages. The feed-back control loop frequently traverses an isolation boundary, so pulse transformers or optocouplers are the usual solutions since the loop bandwidth is very low – usually less than 1 MHz.
However, if you are working on communications or factory automation systems, you may have requirements to cross isolation boundaries at much higher speeds – and perhaps with much higher voltage differentials. First, you might ask, why would a communications link between closed systems require isolation? It is very often a problem in the way large networks are created and the environment where they exist.
We have all heard about transient noise, such as electrical fast transient (EFT) bursts causing damage. However, networks can suffer severe damage from ground differentials. What is considered “ground” is completely arbitrary. If a cable travels 1000 feet to another building, then it is very likely that the ground point of these two locations are very different.
Consider the figure below which illustrates how ground currents can shift the voltage between two locations that should have the same potential. RG is the resistance of the “earth ground.” As current flows through the load (R1), there is a voltage drop across RG, resulting in a difference between VG1 and VG2. At first it appears to be minimal. In the case of a power supply, the earth ground is simply a reference point for managing faults.
In large industrial applications, electric furnaces used for smelting and large electric motors can draw extreme amounts of current. If there is an imbalance in these currents, the return current in the earth may be very large, resulting in a large differential voltage. Also, lightning-induced ground currents can shift the differential ground voltage as well.
Since these differential voltages may very well be static, a transient-voltage-suppression (TVS) diode cannot be used. If the differential ground voltage is high enough and there is sufficient current, which there always seems to be, it will simply leave a charred hole in the PCB where it was installed. I’ve seen this first hand. Also, TVS diodes have high capacitance, which may affect line signal integrity for drivers and receivers.
A far better solution is to use isolation in the network. This is commonly done with optocouplers. However, LEDs degrade with time – and more rapidly with higher temperatures found in many industrial applications. A superior solution is to use silicon devices specifically designed for isolating high-speed signals. TI’s capacitively-coupled digital isolators are fabricated using a silicon dioxide (SiO2) insulator that provides thousands of volts of isolation along with speeds in excess of 100 Mbps. TI’s portfolio of digital isolators feature basic or functional isolation up to 4242 VPK per VDE 0884-10. Look for more information on TI’s reinforced family of digital isolators coming soon.
So, when designing industrial networks or simply needing to control high-power loads, remember the benefits of high-speed digital isolator devices. They can provide the bandwidth as well as the isolation, and help prevent your perfectly crafted circuitry from going up in flames! Till next time…
Additional resources:
- System designers can check out TI’s Digital Isolator Design Guide to begin designing with TI's broad portfolio of digital isolators and isolated functions.
- Check out a video on ground noise transient immunity and how it relates to digital isolators from one of our analog experts, John Griffith.