An ESD can strike across isolated electrical systems. This does not relate to the protection devices on the printed circuit board.
International Safety Standards
The international standard IEC 61000-4-2 specifies system level immunity performance. The strike event sample is charged human discharging through a metallic tool to a system in an application environment of end customer’s typical operation.
The qualification requirement can be as high as 8000 volts contact discharge and failure is soft functional type or the hard physical damage type. This is different from the component level of ESD human body model where event example is charged human discharge through the skin to a component IC in a controlled environment of factory production and assembly. The qualification requirement is lower voltage and failure is hard leakage or physical damage type.
ESD events and its robustness are important of study and system design. In the medical devices, a recent revision in the medical standard IEC 60601-1-2 to the fourth edition requires higher ESD immunity level. The change addresses the trend of more medical devices use outside the hospital and controlled environments where medical devices can face more electromagnetic noise.
ESD Immunity Test Setup
Across isolated electrical systems, an optocoupler or isolator must be robust against the ESD. The most ideal performance criteria are to allow no performance degradation during and after testing, while the worst performance criteria are the unrecoverable failure or permanent damage to the device.
In the test setup to simulate ESD immunity test, “AA” size batteries provide power supply and floating ground to an oscillator or crystal that generate signal square pulses to the optocoupler or isolator’s input channel. Then, an ESD gun applies a contact discharge of 8000 volts at the trace of the optocoupler’s LED anode or cathode.
At the other side of the device’s insulation barrier, the output channel is monitored with an oscilloscope. The ESD gun discharge return cable connects to the output side’s power supply reference and earth. In this setup, the ESD zaps across the device’s insulation barrier. The output signal and the device’s current supply consumption are observed for any functional performance degradation (see Figure 1).
Figure 1. ESD immunity test across an optocoupler.
Using Broadcom’s optocoupler ACNT-H61L, an ESD contact discharge of 8000V across the optocoupler shows normal functional operation during and after the ESD strike. There is no performance degradation (see Figure 2).
Figure 2. Broadcom optocoupler ACNT-H61L DUT
On another digital isolator ACML-7410, ESD contact discharge of 8000V across the isolator shows normal functional operation during and after the ESD strike. There is no performance degradation (see Figure 3).
Figure 3. Isolator ACML-7410 DUT
ESD Immunity Test Results
Galvanic isolation products perform differently in the ESD immunity test. Optocoupler and some isolators do not suffer performance degradation and during the test and still able to function normally after the test. Other devices suffer a loss of function or may even damage permanently as the device gets very hot. It is important to choose a robust isolation product during the system design of ESD immunity.
To see these tests in action, check out the video below.
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