Protecting against reverse polarity: Methods examined, Part 1

-August 22, 2014


Editor’s note: Please see this Planet Analog blog, Protecting Against Reverse Polarity: “Which Method is Right for You?” for determining the best method for your particular design.

Steve Taranovich

 

Preventing Damage to the Power Source

 

This article reviews the pros and cons of each method, but we need to begin with a short warning. 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. Table 1 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 1 Risk of Damage from Extended Reverse Polarity

Method

Risk of Damage in Extended Reverse Polarity via Short Circuit Current Protection Method

1. Series Diode

No

2. Series Schottky

No

3. Diode to Ground

Yes

4. TVS to Ground and PTC

Depends

5. TVS to Ground and Fuse

Depends

6. Schottky to Ground

Yes

7. Schottky to Ground and PTC

Depends

8. Series  MOSFET

No

9. Multi-Function, Monolithic ICs

Depends

10. Fairchild Dedicated Reverse-Polarity Protection Devices

No

 

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 (<100 mA). This method typically involves the use of a series PN diode and is typically used in low-current systems where efficiency isn't the main concern, or in systems with power rails that are greater than 5 V, such that diode voltage drop can be tolerated. In applications with low operating current, power consumption of the diode is minimal, so there is less need for heat sinks and the costs are lower. If system efficiency is a critical concern, this solution is not recommended.



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 minimized 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.
   

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