Another use for load switches: reverse battery protection


Electronic devices are certainly ubiquitous today, with the average individual coming into contact with three to five handheld or mobile devices daily. All of these devices, however, need a portable power source or battery to keep the device running. But what happens when users insert batteries in reverse or connect wires with an unintentional negative voltage? In some cases, this could damage the Microcontroller (MCU) or other sensitive devices. In other cases, the designer has considered these possibilities and implemented a protection circuit into the system. In this post, I’ll cover one example of reverse battery protection that you can implement very easily, and with minimal addition to your circuit.

Using a FET to protect the circuit

The simplest way to protect the circuitry is to use a diode in series with the battery. During normal operation the diode biases and allows current through, but when the battery is in reverse current cannot flow. However, using diodes has disadvantages such as needing a voltage drop of ~0.6V.

Current FETs on the market feature very low on-resistance, causing a lower drop across the FET when operating. You can apply a P-channel FET (PFET) to this circuit, as shown in Figure 1.


Figure 1: A P-type metal-oxide-semiconductor (PMOS) FET in a battery power path

How does it work?

The high-current PFET connects to the power path to disconnect the circuit when the battery polarity is incorrect, as shown in Figure 2. Connecting the gate of the PFET to the negative terminal of the supply ensures a negative Vgs voltage for the correct battery polarity (Figure 2a), turning on the FET and enabling the supply to connect to the load. With the battery polarity reversed, as shown in Figure 2b, the Vgs voltage is positive, ensuring that the battery disconnects from the load.

 

Figure 2: Correct battery polarity (a); and reverse battery polarity (b)

One of the key considerations when using a PFET for reverse battery protection is the direction of the body diode. The body diode of the PFET should point in the direction of the load. If the body diode isn’t directed toward the load, the body diode could conduct even though the FET is off, thereby completing the reverse-polarity conduction path.

Figure 3a displays a battery tester without the correct orientation, with the TPS27081A providing protection to the board circuitry. After placing the battery into the socket incorrectly, Figure 3b shows the battery tester with the correct polarity. The battery tester reads the voltage of the battery indicating that the circuitry is not damaged and is working appropriately and with no change to reverse polarity.

Figure 3: AA battery tester with incorrect polarity (a); and correct polarity (b)

 

Example schematic and layout

Figure 4 is an example of a schematic when using the TPS27081A load switch. The pinout for this device is designed for a load-switching application, so when using the device for reverse battery protection, VOUT is connected to the supply (battery) and VIN is connected to the load. The R1/C1 pin must be connected to the negative terminal of the battery. You can leave the R2 and on/off pins floating, but I recommend connecting them to the negative terminal of the battery to improve thermal performance.

 

Figure 4: Circuit schematic

Figure 5 is an example layout for using the TPS27081A as reverse battery protection. Use polygon pours on all pins to help with thermal dissipation when the load requires a large supply current. I recommend vias to the board ground for better thermal performance.

 

Figure 5: Board connection layout

Using this method in your next battery-based design will protect your system well in case of incorrectly installed batteries or misconnected wires.

Additional resources

  • That TPS27081A is a nice versatile chip, not much to it really just a swiss army knife PFET/NFET switch.  I've used it in its original intended use case (separate power domain for external peripherals that are not ULP-designed, with a TXB0104 or similar fencing the I/O pins) but it's fun to see it applied to a simple example like this!

  • Thanks Eric! You got it! The idea is to promote simple solutions like this, and we also have the WEBENCH tool for this device linked above, so feel free to take a look for your next design.

  • A harder problem is when you have a multi-cell battery back and a user inserts one correctly and one backwards.  Solved that recently for Li-Ion cells.