Protect your system from reverse current

At some point while working with electronics, you have inevitably smelled the unmistakable scent of burning silicon. That’s what reverse current can do to your system. Reverse current is an event in which current travels in the opposite direction it should be moving through a system due to a high reverse bias voltage; from output to input. Fortunately, there are a handful of ways to protect your system from reverse current. This is the first blog of a series about reverse current protection, and will give a high-level overview of the solutions that exist.

 

Causes

The most common cause of reverse current, reverse bias voltage, is having a higher voltage on your output than on your input, inducing current to travel through your system in the opposite direction from what you intended. This can be seen below in Figure 1.

Figure 1: Reverse current

This can happen because suddenly V_IN becomes zero due to a power loss, leaving a higher voltage on the systems output than the input. Or perhaps power MUXing has accidentally caused a reverse voltage event.

 

How do you prevent it?

Reverse current can potentially damage both internal circuitry and power supplies such as batteries. In fact, even cables can be damaged, and connectors degraded. This is why things burn up, because the large currents lead to an exponential rise in power dissipation.

Protection necessitates keeping reverse current flow very low. This means limiting reverse voltage. There are three common ways to protect from reverse current: designing a system using diodes, FETs, or load switches.

 

Diodes

Between diodes and FETs, diodes cost less and are simpler to integrate. They are great for high-voltage, low-current applications. However, if you’ve ever used a diode, you’re surely familiar with the forward voltage drop that it causes, which shortens battery life and limits V_CC by (generally) around 0.6-0.8 V. This voltage drop can lower the efficiency of the power circuitry, and increase the total power dissipation in the system.

For these reasons, a Schottky diode is a popular alternative; they have lower forward voltage drops. But they’re also more expensive and have higher reverse current leakage, which could cause problems for a system. Figure 2 shows a diode used in a system to block reverse current.

Figure 2: Diode reverse current protection

FETs

FETs are useful due to their low forward voltage drop and high current-handling capabilities, which are helpful when you must maintain lower power dissipation. For an N-type metal-oxide-semiconductor (NMOS) FET set in the ground path, you orient the body diode in the direction of normal current flow. That way, if someone installs the battery incorrectly, the gate voltage is low, which prevents the FET from turning on. When the battery is installed correctly, however, the gate voltage is high and its channel shorts out to ground.

You can also put FETs back to back in order to block current in both directions when the FETs are turned off. Compared to the diode solution, there is a lower voltage drop from the power supply to the load, but this implementation takes up a larger amount of board real estate. Figure 3 shows an example of back to back FETs used for blocking reverse current.

Figure 3: Reverse current blocking with dual FETs

Load switches

TI load switches are integrated electronic relays used to turn power rails on and off. Most basic load switches consist of four pins: input voltage, output voltage, enable, and ground, and include a multitude of features, including reverse current protection, in a small, integrated package. Figure 4 shows a load switch used to block reverse current.

Figure 4: Load switch reverse current protection 

For example, Texas Instruments’ TPS22963 3A load switch integrates reverse current protection and more, all in a 1.4mm x .9mm package. Figure 5 shows typical placement for the TPS22963 in a system.

Figure 5: TPS22963 load switch

Reverse current can be caused for a few reasons, whether it be a sudden loss of V_IN or an accident in power MUXing. This can result in system damage or even power-supply damage. TI load switches can be a size-efficient, cost-effective solution for managing reverse current.

Additional resources:

Learn more about TI’s load switch solutions with reverse current protection.

Read the blog: What is a load switch?