Technical Article

Internal Faraday Shield and Small Ci-o Friendly Optocoupler Galvanic Isolation Performance

March 24, 2020 by Lim Shiun Pin

This article highlights Broadcom isolation technologies, namely the optocoupler, magnetic coupler, and capacitive coupler.

High voltage isolation in today’s context involves integrating subsystems with large voltage differences and systems ground potentials. This enables isolation applications ranging from power supply, motor control circuit of servo automation systems and industrial robots, battery management systems, photovoltaic (PV) inverters, electric vehicle (eV) inverters, ultra-fast charging and wireless charging stations to data communication and digital logic interface circuits.

Basically the most important components that provide electrical isolation that allows integration of different subsystems by breaking direct conduction paths are called the isolators or couplers. Integrated circuits (IC) can be combined into the isolators for various electrical functions like driving power electronic devices, high accuracy current and voltage measurements, analog and digital communications and logic interfaces, and isolated power supply conversions.

 

Isolation Technology

There are three main types of isolator technologies, namely, optocoupler, magnetic coupler, and capacitive coupler. Table 1 below shows the key differences between the different isolation techniques, component safety certifications and lifetime reliability failure mechanisms.

 

Internal Faraday Shield and Small Ci-o Friendly Optocoupler Galvanic Isolation Performance
Table 1: Key difference between different isolation techniques, component safety certifications and lifetime reliability failure mechanisms.

 

The optocoupler transmits an electrical signal through the isolation barrier by converting electrical signal to optical signal using an LED and on the other side of the isolation barrier of thickness 0.08mm to 2mm, converts the optical signal back to electrical signal through the photodiodes. In terms of lifetime reliability, the integrity of an optocoupler insulation material can be predicted by partial discharge measurement.

Theoretical dielectric strength values of insulation materials would always apply if optocoupler manufacturers could produce consistently pure insulation barriers. Often, however, high voltage dielectrics contain defects like voids and inclusions of air or other impurities. These voids will have lower breakdown strength than the surrounding dielectric and will discharge or arc when their breakdown strength is reached. The discharge, however, is limited to the length of the void, and after discharging, will slowly recharge with the limited current available through the good dielectric.

The void eventually recharges to the breakdown voltage and discharges again, as the process continues as long as the applied electric field remains high enough. These discharges are considered “partial” because they occur across the void in a limited portion of the length of the dielectric barrier. Partial discharges, which cannot be detected by leakage current measurements, can over time spread in the insulation that eventually leads to complete insulation breakdown. The problem then is to detect the presence of partial discharge during a manufacturing test in order to prevent this phenomenon from degrading devices in the field.

The broadcom optocoupler's high voltage insulation strength is further enhanced with three key design methods. The first is by inserting a clear polyimide called Kapton tape in between the LED and photodiode. The second method is the use of a proprietary, low-cost Faraday shield which decouples the optocoupler input side from the output side. Figure 1 illustrates the isolation construction of Broadcom optocouplers. The third method is by unique package design which is optimized to minimize input-to-output capacitance, Ci-o. The importance of the three design methods will be discussed in detail in this technical paper with an accompanying high voltage surge test as proof.

 

Figure 1: Broadcom optocouplers isolation construction which incorporates Kapton tape and Faraday shield for enhance insulation strength.
Figure 1: Broadcom optocouplers isolation construction which incorporates Kapton tape and Faraday shield for enhance insulation strength.

 

Magnetic coupler uses two coils that are stacked on top of each other with a separating polyimide material of about 0.02mm in between. The application of an AC signal creates a magnetic field, which in turn induces an electric field in the secondary coil. Since the transmission is by magnetic field coupling, the magnetic coupler is also susceptible to nearby magnetic interference.

Figure 2 shows an example of magnetic coupler isolation construction with a single pair of top and bottom coils with polyimide insulation material in between the coils. To double the insulation strength, two sets of magnetic coils are used for one isolation path achieving an insulation thickness of about 0.04mm. The failure mode of the magnetic coupler insulation material is space charge degradation.

 

Figure 2: Magnetic coupler isolation construction with a single pair of top and bottom coils with polyimide insulation material in between.
Figure 2: Magnetic coupler isolation construction with a single pair of top and bottom coils with polyimide insulation material in between.

 

The construction of a capacitive coupler, as the name implies, is quite similar to a ceramic capacitor, whereby silicon dioxide (SiO2) dielectric of a thickness of about 0.015mm is sandwiched in between two metal plates, usually aluminum (Al), in close proximity. The SiO2 crystal is grown on top of the Al plate. Transmission of signal through the capacitive isolation barrier is usually AC electrical signal.

One of the factors that may affect the insulation strength of the capacitive coupler is how well the SiO2 crystal is grown. Defects in the crystal will weaken the insulation material. The lifetime reliability failure mode for the capacitive coupler is a time-dependent dielectric breakdown (TDDB). Similar to a magnetic coupler, to double the insulation strength, two sets of capacitors are used for one isolation path and the insulation thickness is doubled to about 0.03mm. Figure 3 shows a typical double capacitor isolation construction.

 

Figure 3: Capacitive coupler isolation construction with two series caps where SiO2 dielectric is sandwiched in between by two Alu metal layers.
Figure 3: Capacitive coupler isolation construction with two series caps where SiO2 dielectric is sandwiched in between by two Alu metal layers.

 

Optocouplers are certified to components safety certification of IEC 60747-5-5 for reinforced isolation. This international certification recognizes partial discharge as the failure mechanism for insulation material breakdown. As such, the certification only applies to optocouplers. The alternative isolation technologies like the magnetic and capacitive are certified German standard VDE 0884-10/11. Though the insulation material strength is determined by a partial discharge test, this may not be suitable to predict the lifetime reliability of the magnetic (space charge degradation) and capacitive couplers (TDDB).

 

High Voltage Surge Test

A quick bench test setup can be easily assembled to compare the insulation strength of various isolators. Figure 4 below shows the test setup where the high voltage surge is applied using an ESD gun. The voltage profile of the ESD gun has a very fast rise time of about 1ns and a slow fall time of 30ms. This surge profile is different from the IEC 60060-1 standard surge profile of 1.2µs / 50µs, but it is sufficient for the purpose of comparisons of high voltage strength of different isolation technologies.

 

Figure 4: High voltage surge test setup shown on the left and on the right side, the high voltage surge profile.
Figure 4: High voltage surge test setup shown on the left and on the right side, the high voltage surge profile.

 

Three random samples each from two optocoupler manufacturers, Broadcom and Isolator A, one magnetic coupler (Isolator B) and one capacitive coupler (Isolator C) were selected for this high voltage surge test. These isolators are high precision current sensing sigma-delta modulators with an internal clock generator built into an 8-pin stretched surface mount package outline (SSO8).

The isolation withstands voltage, Viso of this type of SSO8 package is rated 5kVrms per minute and with creepage and clearance distance of minimum 8mm. Figure 5 below shows the schematic diagram of the PCB used to hold the device under test (DUT).

 

Figure 5: Schematic diagram of PCB board used of high voltage surge test.
Figure 5: Schematic diagram of PCB board used of high voltage surge test.

 

Both isolated side power supplies were provided from 9V batteries separately and regulated down to 5V through an LDO voltage regulator on each side of the isolation. The test was carried out by applying a high voltage level starting from 14kV between Gnd1-Gnd2 of the sigma-delta modulators.

The output clock and data signal were observed for any anomalies. If the outputs resumed normal functionality after the high voltage surge, the voltage level was increased by 1kV and the test continued up to 25kV test limit. If the output clock and/or data signal latched, the test would be stopped.

 

i) Faraday Shield

The high voltage surge will induce high-density displacement current from Gnd1 to the input circuitry of the isolator and then transmit over to output circuitry and Gnd2 via capacitive structures or parasitic capacitance formed throughout the isolation barrier. Figure 6 below shows the various parasitic capacitance paths formed between the wire bonds of the input circuitry/input leadframe to the Faraday shield of Broadcom optocoupler.

The Faraday shield is grounded to Gnd2 and provides an electric and magnetic shielding to remove the displacement current. In capacitive or magnetic couplers, the Faraday shield is not a viable solution. A Faraday shield would block the electric or magnetic fields used for data transmission in addition to transients.

 

ii) Input-Output Capacitance, Ci-o

In addition to the Faraday shield, Broadcom optocoupler leadframe and package design is optimized for smaller combined input to output capacitance, Ci-o. Table 2 shows the comparisons of Ci-o of various isolators. Displacement current follows the relation of i=c*dv/dt. With a smaller Ci-o, smaller displacement current is induced during the occurrence of a high voltage surge.

 

Sigma-Delta Modulator SSO8 Package Isolation Technology Internal Faraday Shield Typical Ci-o
Broadcom Opto Coupler Yes 0.5 pF
Isolator A Opto Coupler Yes 1.0 pF
Isolator B Magnetic Coupler No 2.2 pF
Isolator C Capacitive Coupler No 1.0 pF
Table 2: Input to output capacitance comparisons between various isolators.

 

Table 3 shows the results of high voltage surge test on the isolators with different technologies. As evident from the test, Broadcom optocouplers are the most robust against high voltage surge whereby no failure is observed for all the units under test up to 25kV test limit. Isolator A (optocoupler) outputs permanently latched starting from 16kV upwards, while Isolator B (magnetic coupler) from 14kV upwards and Isolator C (capacitive coupler) from 15kV upwards respectively. Although Isolator A, B and C started to fail at about the same level, Isolator C recorded the widest range of the high voltage surge levels at which the test units failed.

 

Sigma-Delta Modulator High Voltage Transient across Gnd1-Gnd2 before failure occurs Failure Mode
Broadcom (optocoupler)

DUT 1: No failure up to 25 kV test limit

DUT 2: No failure up to 25 kV test limit

DUT 3: No failure up to 25 kV test limit

No failure observed
Isolator A (optcocoupler)

DUT 1: 16 kV

DUT 2: 18 kV

DUT 3: 17 kV

Output clock and / or data latch permanently low / high
Isolator B (magnetic)

DUT 1: 15 kV

DUT 2: 18 kV

DUT 3: 14 kV

Output clock and data latch permanently low / high
Isolator C (capacitive)

DUT 1: 21 kV

DUT 2: 15 kV

DUT 3: 17 kV

Output clock and data latch permanently low / high
Table 3: Results of high voltage surge test on different isolators.

 

Being one of the advocates for the highly reliable optocouplers galvanic isolation technology, Broadcom’s portfolio covers some of the industrials highly adopted internally clocked sigma-delta modulators for precision shunt-based current and voltage sensing solutions. Table 4 shows Broadcom product offerings of internally clocked sigma-delta modulators housed in the SSO8 package format. ^ Automotive AEC-Q100 qualified and Ta, max.=125°C.

 

Broadcom Part Number Input Linear Range Input Full Scale Range Clock Frequency Typical Signal to Noise Ratio (SNR) Typical Offset Temp. Drift (TCVos)
ACPL-C740 ±200 mV ±320 mV 20 MHz 86 dB

0.3 μV/°C

ACPL-C797 ±200 mV ±320 mV 10 MHz 78 dB 1.0 μV/°C
ACPL-C797T ^ ±200 mV ±320 mV 10 MHz 79 dB -
ACPL-C799 ±50 mV ±80 mV 10 MHz 77 dB 0.3 μV/°C
ACPL-C799T ^ ±50 mV ±80 mV 10 MHz 77 dB 0.1 μV/°C
Table 4: Broadcom internally clocked, optically isolated, CMOS output, high precision sigma-delta modulators housed in SSO8 package format.

 

References

  1. “Optocoupler Designer’s Guide”, av02-4387en_dg_opto_2014-01-03.pdf, Jan 3, 2014
  2. “ACPL-C740 Optically Isolated Sigma-Delta Modulator”, ACPL-C740-DS103 Datasheet, Apr 01, 2019
  3. “Ignore Detection of Partial Discharge Failures NOW--Pay Massive Amounts Later!”, Partial Discharge White Paper, Stephen Chaikin (Harris Tuvey)
  4. “The ISO72x Family of High-Speed Digital Isolators”, Application Report, Kevin Gingerich and Chris Sterzik, Aug 2018

 

This article originally appeared the Bodo's Power Systems magazine.