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Reverse polarity protection methods (Part 1)

Posted: 09 Sep 2014 ?? ?Print Version ?Bookmark and Share

Keywords:circuit protection? power supply? Series Diode? Schottky? Diode to Ground?

???Increased window of operation compared to the stand-alone TVS approach
???PTC resistor adds a level of safety in the event the TVS overheats and fails
???Integrated over-voltage protection

???Viability must be evaluated on a case-by-case basis, and care must be taken to match the TVS power dissipation capability with the PTC trip response to assure the PTC trips before the TVS overheats and fails.
???Using a thermistor will introduce series resistance. The thermistor requires series resistance in order to sense current and trip. Therefore, if it is to provide protection, it must have sufficient resistance to activate. Unfortunately, that same resistance generates system power loss.

Method 5: TVS and fuse
This method is ideal for helping protect the end-user from short transients (both positive and negative) and for preventing catastrophic failure in the event of extended transients. Protection issues are similar to a stand-alone TVS, but safety is significantly improved. Unfortunately, it is not a resettable solution in response to sustained faults. The fuse will fail open-circuit permanently once triggered.

On the other hand, permanent open-circuit has advantages in both cost and space requirements. Because the fuse can be designed to open-circuit in a sustained fault, the designer no longer needs to size the diode and its associated heat-sink structures to support sustained over- or under-voltage conditions. The designer can now size the diode based on expected transient events, and what is the required fault condition to open-circuit the fuse. As a result, the diode costs and size can be reduced, saving board space.

Another advantage of this method is that fuse resistance is typically lower than PTC resistance, but it will still introduce some series resistance and power loss.

Figure 5: The fuse and TVS method.

???A very safe approach
???When used in combination with a series fuse, the TVS will still clamp over-voltage transients, as well as under-voltage and negative transients.
???Integrated over-voltage protection

???Not resettable: If over-voltage or negative voltages are sustained, this method is not resettable by design. Once sufficient current flows through the fuse, the fuse will open permanently. Some care must be taken to assure expected transients do not blow the fuse.
???Power loss from series resistance: Based on its operating mode, and that it opens due to I2R heating, the fuse must have series resistance to function. If it is to provide protection, it must have sufficient resistance to activate. That same resistance generates some system power loss, and can heat and thermally cycle the fuse in normal operation.
???Fuse fatigue: Fuses have a well-known mechanism for pulse current fatigue. Pulse currents generate heat inside the fuse (via I2R heating). Repeated pulses can thermally cycle the fuse, ultimately leading to its degradation. Many of the fuse technologies with the lowest resistance are also the most susceptible to fatigue. For this reason, care should be taken when selecting a fuse to understand fatigue and assure field failures do not result from extended normal operation.
???Diode matching: This can be an issue during over-voltage events, when the diode shunts current to ground and pulls enough current to droop the power supply to the clamping voltage of the diode. Although the diode does not need to be designed to sustain this for extended periods, it should be sized such that it will draw sufficient current to open the fuse before the diode fails or overheats. If sized inappropriately, the diode can overheat the board without opening the fuse or the diode can fail open leaving downstream circuits unprotected. Either instance can lead to a downstream thermal event where the fuse does not open and thereby defeats the purpose of using the fuse in the first place.

About the author
Adrian Mikolajczak is with Fairchild Semiconductor.

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