Specifications

PL 3120/PL 3150/PL 3170 Power Line Smart Transceiver Data Book 191
Design for Electrostatic Discharge (ESD) Compliance
Reliable system design must consider the effects of ESD and steps must be taken to protect sensitive components. Static
discharges occur frequently in low-humidity environments when operators touch electronic equipment. Keyboards,
connectors, and enclosures themselves can provide paths for static discharges to reach ESD sensitive components such
as the PL Smart Transceiver.
ESD requirements for products sold throughout Europe are provided in IEC 61000-4-2 Electromagnetic Compatibility
(EMC), Part 4-2: Testing and measurement techniques – Electrostatic discharge immunity test [4]. Conforming to this
standard in other parts of the world is good practice and highly recommended.
There are two general methods that are used to protect products from ESD. The first is to seal the product in order to
prevent static discharges from reaching sensitive circuits inside the package. The second method is to provide a path for
ESD currents to be shunted around sensitive circuitry on the way back to ground. If the first method is selected care
must be taken to ensure that the creapage path from all points of entry to any internal conductive elements is >1cm. Note
that many LEDs and switches will not satisfy this requirement – unless they are positioned away from the enclosure
boundary and their reach is increased by means of an insulated extender (e.g., a light pipe for an LED).
If the developer chooses the second method of ESD protection (providing a safe path back to ground) particular
attention must be given to device ground paths. ESD currents should not be allowed to pass near to the PL Smart
Transceiver or other sensitive circuits. Note that if the device is floating with respect to earth ground then the ESD
current will return by capacitive coupling to earth via the power supply wires and/or the PCB ground plane.
User accessible circuitry requires explicit diode clamping to shunt ESD currents from that circuitry to earth ground
using a path that will not disturb sensitive circuitry. For example, if a PL Smart Transceiver scans a keypad using I/O
lines, then the I/O lines to that keypad will need to be diode-clamped as shown in Figure 6.2. If a negative ESD hit
discharges into the keypad, the grounded diodes shunt the ESD current to the ground plane. If a positive ESD hit
discharges into the keypad, the V
DD5
diodes shunt the current to the ground plane via a 0.1µF decoupling capacitor that
is placed directly adjacent to the clamp diodes. The keypad connector, diodes, and decoupling capacitor should all be
located so that ESD current does not pass through sensitive circuitry as it exits the PCB.
In addition to the above considerations, the product's package should be designed to minimize the possibility of ESD
hits arcing into the device’s circuit board. If the product's package is plastic, then the PCB should be supported in the
package so that unprotected circuitry on the PCB is not adjacent to any seams in the package. The PCB should not touch
the plastic of an enclosure near a seam, because a static discharge can “creep” along the surface of the plastic through
the seam and arc onto the PCB.