Does Active Isolation Pass UL/IEC Testing?

Currently there is no allowance for “transformerless” isolation in UL or IEC. IEC 61558-1:2017 was recently updated to disallow transformerless isolation and now states that "The input and output circuits shall be electrically separated from each other and the construction shall be such that there is no possibility of any connection between these circuits, either directly or indirectly, via other conductive parts, except by deliberate action." That statement makes Active Isolation technology a “what if” technology where researchers are in a quest to see if we can meet the limits of the UL/IEC standards with transformerless technologies. However, UL/IEC would not accept this technology today for providing Galvanic Isolation for safety purposes.” That does not mean Active Isolation could not be used in a system... I will explain this later...

This is the table that the researchers are trying to see if they can beat…

Table 1: Table of allowable leakage current

So, what researchers like me are trying to do is make a transformerless technology that can meet this < 0.5mA touch current specification- as a “what if” scenario.

How can Active Isolation technology be used if it does not meet the current UL/IEC definitions of Galvanic Isolation? There needs to be at least one transformer-based power supply in an isolated system so that the transformer can provide the function of "safety transformer." Safety is not the only use of transformers. Transformers are also often used to provide level-shifting functions. One example is to provide power to high-side MOSFET gate drivers in a power electronics system. Active Isolation technology would find application as an Integrated Circuit (IC) solution for gate drivers ICs. Or, the isolated power could be integrated inside the gate drivers IC.

There are a multitude of applications for Active Isolation. Any system that already contains a non-electronic isolation mechanism- including transformers and air-gap insulating devices like optical, magnetic, etc- could allow the use of Active Isolation as part of the sub-system. For example, in a PV array, the solar modules are no longer required to have (-) MINUS terminal connected to earth ground per recent NEC specifications. As such, the PV module is allowed to float relative to earth ground and only the PV metal chassis must be earth grounded. As a consequence, a multitude of various transformerless inverters topologies have been proposed in research, as shown in table 2 below:

Table 2: Summary of Single Phase Transformerless Inverters

As shown, Common-Ground (CMN-GND) transformerless topologies provide near zero leakage currents. As such, it has been shown that Active Isolation cascaded with any power stage can create a common-ground topology for use in PV inverter systems to meet leakage current specifications, as shown in Fig. 1 Below:

Fig. 1: Active isolated buck-boost converter cascaded with Transphorm inverter Evaluation Board

Fig. 2: Microinverter experimental waveform showing output voltage (Vcd), output

current (Icd), inverter input voltage (Vab), and isolation voltage (Vdb).

Author:

Clint Halsted, Ph.D., CIT, EIT, KD5SBQ