Optoisolation for Intelligent Power Modules

Microcontrollers and FPGAs are very sensitive to the electrical noise generated by high-power switching devices such as IGBTs and intelligent power modules (IPMs). Electrically isolating noisy IPM-based power trains from sensitive control circuitry for safety and performance is the ideal application for a class of high-speed optocouplers commonly referred to as IPM drivers. Here is what to look for when specifying an IPM driver -- Jose Espina

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Consequently, it is necessary to first calculate the maximum Δtphl = tphl(max) – tphl(min), and minimum Δtplh = tplh(min) – tplh(max) . Given this, the maximum dead time that can be accounted for by the IPM driver is the difference between maximum Δtphl and minimum Δtplh (max. dead time = maximum Δtphl – minimum Δtplh). In short, most current IPM driver applications, which are limited to switching frequencies of less than 20 kHz, can be satisfied with IPM drivers with data rates of 1 Mbps/1 MBd or 5 Mbps/5 MBd.

Another important consideration when specifying IPM drivers is their ability to provide isolation from hazardous electrical circuits, as well as noise isolation between noisy circuits, such as IPM modules, and sensitive control circuits.

Noise isolation is not as easy to quantify as safety isolation. There are two parameters that can be used to quantify the value of an IPM driver with regards to this most important function. One is the coupling capacitance from the primary side of the IPM driver to the secondary side. The larger the coupling capacitance, the less effective the IPM will be in terms of noise isolation; however, capacitance in most optocouplers is given as <

Arguably, a more meaningful parameter that quantifies the noise isolation capability of a device is common-mode transient immunity (CMTI). The test set-up for this parameter is shown in Fig. 3.

The test methodology is conceptually simple. A high transient voltage potential is placed between the primary and secondary sides, and the amount of the transient that leaks across the isolation barrier measured. An IPM module should have a high CMTI greater than or equal to 10 kV/µs.

There are other parameters of concern to designers when designing opto-isolators, such as operating temperature and isolation voltage values; however, the issues of timing and noise isolation are of paramount importance to IPM drivers in particular. A careful application of opto-isolated IPM drivers can greatly increase reliability and reduce overall costs in new high-power switch-mode designs.


The author is senior product marketing manager, optocouplers, at Vishay Intertechnology. He holds a bachelor’s degree in electrical engineering and a master’s degree in economics from San Jose State University, USA

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