Designing Medical Devices for ESD protection and compliance
ESD or Electrostatic Discharge is the transfer of an electrostatic charge between two objects. When two objects of different electric potential come into contact there is a surge of electric current.
An object may become negatively charged by gaining electrons on its surface from another object. The object that transfer electrons become positively charged. In practical medical application a charged human body or object can come in contact with the medical appliance. The extra charge will discharge in the appliance and may or may not cause failure. The designer must ensure that the medical appliance is able to withstand static electricity.
Failures caused by Electrostatic Discharge can happen in a number of different ways. The electrostatic discharge can propagate to the ICs fusing together a junction. It may permanently damage the medical device. In medical applications it can cause the device to malfunction in the middle of the process of data acquisition or transmission causing potentially serious medical hazard condition. The handheld medical devices are more susceptible to human induced ESDs. While permanent failure of the ICs are less likely, though not impossible, the button malfunctioning can induce functional errors. Designers must resolve and test these issues.
Methods of Preventing ESD and ESD induced errors
A number of different strategies exist to prevent and ESD induced error. The designer should implement these to comply with the ESD safety standards as well as to avoid any failure and malfunction.
Shunting ESD Energy
Zener Diodes, Metal Oxide Varistors, transient voltage suppression (TVS) diodes, and regular (CMOS) or bipolar clamp diode can be used to shunt the ESD energy thereby providing protection to the ICs. These are designed to absorb electrostatic discharge (ESD) energy that is introduced from I/O ports and travels through the connector onto the system board. ESD-protection diodes thus provide protection against ESD-induced system malfunction and/or damage to ICs. The designer may add an ESD diode at all the connectors pins to prevent any ESD from entering into the ICs.
These devices work very well for smaller data rates, for example RS232, power supplies, low data rate IOs, buttons and switches.
For signals at higher data rates ( for example USB 2.0, IEEE 1394, DVI, PCI Express), the parasitic impedance of these protection devices can create signal integrity issues.
Figure 1 : Shunting ESD Energy using protection diodes.
PCB Layout for ESD
PCB spark gaps can supplement the Electrostatic Discharge protection. The Spark gap is created between the Signal Pins and the connector etch pad. The etch structure provides a small gap between points. This encourages a breakdown from high voltage across the clearance. The clearance area must be free from solder mask because the solder mask has the tendency to increase the voltage breakdown of the gap. The breakdown voltage of a small spark gap is given by V = (3000pd+1350) where p is the pressure in atmospheres(1) and d is the distance in mm. A 20 mil (0.508 mm) gap will provide a voltage breakdown of 2850V. This will help designs pass the required ESD levels.
Software Key Debouncing
In handheld medical devices using membrane buttons, ESD can momentarily induce pulses that can induce a false button press signal. In certain conditions it is equivalent to creating hazard condition. For example, assume a medical appliance in which patient has to answer questionnaire about if he had taken a medicine. A false button press due to ESD may generate erroneous alarm and incorrect conclusions.
The designer may implement a software debounce technique in addition to the ESD diode protection to overcome this type of failure. The ESD is a very short duration phenomenon ( usually in nano seconds) and therefore a software debounce technique where the it checks for key press after an interval of 20 to 100 milliseconds provides sufficient safegaurd against the ESD induced malfunctions.
Vikas Shukla
Vikas Shukla is currently working as Senior Design Engineer at BL Healthcare. He has degree in Computer Science and Engineering from IT-BHU, Varanasi, India. Mr. Shukla has over 15 years of experience in design of microprocessor-based systems. His expertise includes signal integrity, architecture and design of remote patient monitoring systems. The views expressed are his own.