Automotive reverse polarity protection
In this video we will review the reverse battery protection stage of the automotive frond-end power stage. To protect the downstream circuits from negative polarity resulting from accidental reverse connection of battery terminals different methods are used.
The simplest method is to use a Schottky diode. When the reverse voltage is applied the diode becomes open circuit and protects other components. Because of the diode drop, there is power loss associated with diode conduction (Current x VF) during normal operation. Therefore this method is limited to low current designs.
Another common method is to use a PFET along with some discrete components. For correct battery connection the PFET turns on and provides a low resistance path for the current. For reverse voltages the PFET turns off thereby protecting downstream circuits. This method is more efficiency than a diode. However PFETs have worse characteristics than NFETs and need external circuit such as zener clamps, and resistor.
Smart diodes combine the superior performance of NFETs and the simplicity of a diode.
The second slide compares the solution size of the three solutions using Schottky diode, PFET, and smart diode repectively for a 6A/10V solution. As seen here the Schottky diode solution area is the biggest, followed by the PFET. The Smart diode based solution results in about 50% smaller size than the diode based solution. This 3rd slide shows the thermal performance of the three solutions. For the same load current of 6A, the Smart Diode solution runs much cooler than the larger Schottky and the PFET solution.
The transient performance of a PFET based solution and a smart diode + N-channel FET solution are also compared (slide 4). The PFET turn-off is slower in response to the reverse voltage. As a result of which there is negative current flow which causes output voltage to dip. The smart diode responds faster to turn off the n-channel FET and as a result it prevents dip in output voltage.