By CHRISTIAN JONGLAS, Applications & Technical support manager,
Considering isolation in switching mode power supply (SMPS) we immediately refer to the power transformer and the isolation it provides between primary and secondary side.
In addition, the feedback loop of an SMPS needs also to be isolated to transmit output voltage information to the primary side switcher. This isolated feedback is necessary to perform SMPS output voltage regulation.
Most of SMPS use opto-coupler because they can transmit a DC current with good linearity, and they are cheap and easy to use.
The opto-coupler seems to be the ideal component for SMPS feedback. However, opto-couplers have a major drawback: their transfer function referred as current transfer gain or current transfer ratio(CTR) drift over time. This drift is due to LED efficiency degradation, and it is increasing with biasing current and temperature. According to conditions of use, it can reach some 5% up 50% per year. This drift can become fatal to SMPS regulation over some 5 to 10 years of continuous operation. In this case, the MTBF at 40°C of such power supply will be somewhere around 30 000 hours. Typical electronic get-around like reducing the CTR range or using compensation, can help reducing this drift, however, the MTBF will still be limited and the advantage of a simple and cheap isolated feedback will be broken.
An alternate solution to definitively remove this weak point of isolated SMPS is to use a magnetic feedback. This solution consists of transmitting the output voltage information to the primary side switcher through a small signal transformer that remains stable over time. The transmitted signal is not a simple DC current, but an AC signal image of output voltage.
High Reliability SMPS designed by Gaïa-Converter are all using magnetic feedback designs:
As example, the 8 watt family MGDD08 operating up to 105°C displays more than 1.100 000 hours MTBF.
About GAIA Converter
Gaia Converter was created in the early 1990’s. Since that time the company has developed one of the most comprehensive ranges of modular power components in the electronics market place. Gaia Converter also brought to the market its now well established concept of Modular Power Architecture simplifying the design of complex power supplies. With more than 3500 standard power component references serving the industrial, transportation, aerospace and military markets, Gaia Converter is recognised as one of the world leaders of “power module” solutions.
For more information: contact: www.gaia-converter.com