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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?

A short warning must be said before we start. If the power source has reversed polarity, some of the solutions proposed protect the device by shorting the power supply. If the power supply does not have embedded short circuit protection, the power supply, device connectors, and / or the protection circuit can all be damaged due to sustained high short-circuit currents. The table shows that, of the ten methods we review in this article, 6 can result in system damage if the power source is reverse polarized and unprotected. If extended reverse polarity of the power supply is a concern, these 6 solutions (highlighted in red or yellow) should be avoided, or at least very carefully evaluated.

Table: Risk of damage from extended reverse polarity.

Method 1: Series diode
The Series Diode method is a good choice if the design can accept large series voltage drops (1 V) and the operating currents are low (

Figure 1: Series diode method.

Strengths
???Low-cost, simple solution
???Fast blocking, resettable
???Potential for very high breakdown (up to 1000 V+)

Limitations
???The cost benefit is quickly minimised as operating currents go up. At higher currents, the increased power consumption ultimately requires a larger, more expensive IC with a more thermally conductive package and heat-sinking structure.
???The voltage drop and power consumption associated with this method typically rule out implementation in all but a few applications.
???In low-voltage systems (5V), the diode drop may require additional downstream boost circuits, making what is intended to be a low-cost approach actually quite expensive.

Method 2: Series Schottky
The Series Schottky method is similar to the Series Diode method, but with less voltage drop and lower associated power consumption. It is another excellent choice if the design can accept large series voltage drops (0.3-0.6 V) and the operating voltages remain fairly low (

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