“The high requirements for the reliability of Electronic systems, especially in the industrial environment, continue to bring great challenges to developers. Overvoltage protection is a key design consideration and challenge, because more components are usually needed to protect the system from overvoltage, but these additional components often affect the system, and in the worst case, even produce Error signal. In addition, these components will add additional costs and will further aggravate the space limitation problem. Therefore, when designing protection circuits, traditional solutions often require compromises between system accuracy and protection levels.
Author: Thomas Brand, Field Application Engineer, Analog Devices
The high requirements for the reliability of electronic systems, especially in the industrial environment, continue to bring great challenges to developers. Overvoltage protection is a key design consideration and challenge, because more components are usually needed to protect the system from overvoltage, but these additional components often affect the system, and in the worst case, even produce Error signal. In addition, these components will add additional costs and will further aggravate the space limitation problem. Therefore, when designing protection circuits, traditional solutions often require compromises between system accuracy and protection levels.
Usually, a common simple design method is to use an external protection diode, usually a transient voltage suppressor (TVS) diode, installed between the signal line and the power line or ground line. TVS diodes are quite useful because they can quickly react to transient voltage peaks. Shown on the left side of Figure 1 is this type of external overvoltage protection.
Figure 1. Traditional overvoltage protection design with additional discrete components
If a positive transient voltage pulse overvoltage occurs, current will flow to VDD through D1 to clamp this positive transient voltage pulse. The voltage is therefore limited. The clamping voltage is equal to VDD plus the forward voltage on the diode. If the pulse is negative and less than VSS, then the above function is also applicable, except that it is clamped to VSS by D2. However, if the leakage current caused by overvoltage is not limited, the diode may be damaged. For this reason, a current limiting resistor will be added to the path. Under very harsh environmental conditions, the bidirectional TVS diode at the input is usually used for enhanced protection.
The disadvantages of this type of protection circuit include increased signal rise and fall times and capacitance effects. In addition, when the circuit is in a power-off state, no protection is provided.
Actual devices, such as analog-to-digital converters (ADC), operational amplifiers, etc., usually have built-in protection functions. As shown on the right side of Figure 1, this protection function consists of a switch architecture. It can also be seen from Figure 1 that both sides of the power rail are equipped with input and output protection diodes. This setting has disadvantages. When the floating signal appears in the power-off state (the IC is not powered), the switch may appear to be in an active state (even if it is set to off), and current will flow through the diode and the power rail. This phenomenon will allow current to flow through the signal line, causing the signal line to lose its protection.
Fail-safe switch architecture
One way to solve the above problems is to use a fault-protected switch architecture equipped with a bidirectional ESD unit, as shown in Figure 2. Now, the ESD unit will clamp the instantaneous voltage by constantly comparing the input voltage with the voltage on VDD or VSS, instead of using the input TVS diode. When the voltage is over for a long time, the downstream switch will automatically open. As a result, the input voltage is no longer limited by the protective diode clamped on the power rail, but by the maximum rated voltage of the switch. In addition, higher system robustness and reliability can be achieved without affecting the actual signal and its accuracy. In addition, the leakage current is very low when the switch is turned off, so there is no need to use an additional current-limiting resistor.
Figure 2. Overvoltage protection with integrated bidirectional ESD unit
The four-channel single-pole single-throw (SPST) switch ADG5412F provided by ADI (ADI) uses this input structure. Regardless of the size of the existing power supply, this switch can withstand permanent overvoltages up to ±55 V. The integrated ESD unit on each of these four channels can clamp transient voltages up to 5.5 kV. Under overvoltage conditions, only the affected channel will be opened, and other channels will continue to work normally.
Thanks to this overvoltage protection switch, the circuit can be greatly simplified. Compared with traditional discrete solutions, this solution has obvious advantages in terms of ensuring excellent switching performance of the precision signal chain or in terms of optimizing space utilization. Therefore, the overvoltage protection provided by ADG5412F is particularly suitable for high-precision measurement applications in harsh environments.