Solid State Circuit Breaker

Posted Nov 29th 2011
solid state circuit breaker switch

Until very recently, few alternatives to electromechanical and magnetic circuit breakers existed. Designers were forced to live with such undesirable characteristics as arcing and switch bounce (with corresponding noise and wear), while accomodating large unwieldly packages in their high power systems.

Solid state technology applied to this traditional device has resulted in circuit breakers free from arcing and switch bounce, that offer correspondingly higher reliability and longer lifetimes as well as faster switching times. A typical solid state circuit breaker will switch in a matter of microseconds, as opposed to milliseconds or even seconds for a mechanical version.

New solid state products currently on the market utilize the many benefits associated with power MOSFETs to deliver a product far superior to earlier silicon versions. Power MOSFETs offer low on resistances (as compared to bipolar transistors), low voltage drops, low EMI, faster switching times and good thermal stability of key parameters.

However, two key advantages that the electromechanical devices have over the solid state versions are simplicity and low cost. For example, a simple commercial circuit breaker relay combination will sell for $4.00 to $6.00 in low volume. The existing solid state circuit breakers will run from several times that amount, and typically include many bells and whistles that the average designer can do without. This cost difference is somewhat less in military versions, as the mechanical devices must also undergo extensive testing.

One reason for the corresponding complexity of the silicon based systems is the power MOSFET drive circuitry required. If N-channel FETs are to be used (N-channel FETs are preferable to P-channel as they have roughly 2.5 times lower RDS (On) and correspondingly lower cost), a charge pump or voltage tripler must be supplied to provide sufficient gate enhancement to turn on the FET. This involves supplying an oscillator as well as the necessary diodes and capacitors, which definitely take board/hybrid package space.

A simple, inexpensive solid state circuit breaker can be made using the MIC5013 power MOSFET predriver with overcurrent sense. This predriver was designed for driving N-channel FETs, and has an on-board charge pump to provide sufficient gate enhancement. This eliminates the issue of providing this enhancement externally; providing a one component solution to what once consumed extensive “real estate”.

As any size FET can be driven by the MIC5013, almost any load can be accomodated. High inrush or inductive loads are driven with equal ease, greatly expanding the realm of possibilities for these circuit breaker topologies.

An internal comparator is used to sense an over-current condition; this feature allows the use of this product as a circuit breaker that can be programmed to trip at a specified current via choice of an external sense resistor. An overcurrent flag provides this information externally, allowing easy digital interface /control of the device. This feature allows its use in more complex, remotely controlled designs such as those currently used in high reliability applications.

Using this highly versatile device, four circuit breaker configurations have been devised; a low parts count, low cost externally resettable version, a minimal parts count remotely resettable version with indicator, a minimal parts count automatically resettable version, and a full blown power controller design with Z8â„¢ microcontroller interface. Typical applications for the first three versions include a variety of commercial, industrial and military applications, such as battery pack circuit breakers/current limiting, electric vehicles, and heavy machinery. The latter design is useful in high end applications such as military avionics or industrial automation. It offers a substantial cost savings over the currently available remotely controllable electromechanical units, as well as most currently available hybrid designs of this complexity.

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