Cascading insulation barriers to satisfy high creepage requirements

-December 09, 2012

Creating Double Insulation barriers with sequential optocouplers is problematic because the data integrity is poor and there is no compact inexpensive way to provide power to the interface between the barriers. 

With the advent of high performance digital isolators, creating high voltage isolation barriers by layering isolators is now a viable solution.  The recent rapid expansion of new battery and power generation industries has created demand for interfaces that have high working voltages and require reinforced insulation.  For example a solar inverter application could have the following requirements:


Working Voltage             800Vdc

Pollution Degree             2

Overvoltage Category    III

Under IEC62109 for reinforced insulation this would require:

Impulse withstand voltage of 6000Vpeak

Working voltage of 800Vdc

Reinforced Clearance of 8mm

Reinforced Creepage of 16mm

This creepage is not possible with current packaging.  However, if the barrier can be broken into a basic and supplemental barrier, then the requirement for each barrier is:

Impulse withstand voltage of 6000Vpeak

Working voltage of 800Vdc

Basic/Supplemental Clearance of 5.5mm

Basic/Supplemental Creepage of 8mm

The basic/supplemental insulation creepage and clearance and impulse voltage can be met with iCoupler digital isolators in SOIC16W packages.  The following block diagram shows how an isoPower device and a standard high voltage iCoupler digital isolator can be cascaded to provide the required isolation.  Care must be taken to determine the overall performance of the data channels. 

The propagation delay, pulse width distortion and channel matching values will add between the two components.  The maximum data rate will be limited by the slower of the two devices.  isoPower provides the power to run the intermediate interfaces.  At data rates up to 1mB/s the entire barrier will require about 20mA of power at 5V.  At higher data rates, additional power will be required.

If power must be transferred across both barriers to power an isolated load, then two devices can be cascaded as shown below.


This configuration is compact, but the total power efficiency will be low.  The graph below shows the efficiency of the power sections at the load.  If data is being transferred at rates above 1Mbps, then the data transfer will be using part of the available power and the power consumed by each stage will have to be calculated in detail.  The application as shown draws about 40mA from the primary side input to create the entire interface.

This method was described to solve a problem in a solar inverter application.  Under different standards and applications, this method can be applied to achieve different goals depending on the details of the particular system standard.  There are many combinations of isoPower devices, digital isolators and interface devices available to create isolated digital, I2C and USB interfaces with and without power to the end load.

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