There are some well known-techniques for handling source voltage reversal. The most obvious is a diode from the source to the load, but it has the downside of extra power dissipation due to the diode forward voltage. As elegant as it is, a diode will not work in portable or backup applications since the battery must sink current when charging and source current when not.

Another approach is to use one of the MOSFET circuits shown in Figure 1.

Figure 1
Conventional load side reverse protection

For load-side circuits, this approach is superior to the diode since the source (battery) voltage enhances the MOSFET, yielding less voltage drop and effectively higher conductance. The NMOS version of the circuit is preferable over the PMOS version due to the higher conductivity, lower cost, and better availability of discrete NMOS transistors. In both circuits, the MOSFET conducts when the battery voltage is positive and disconnects when the battery voltage is reversed. The physical “drain” of the MOSFET becomes the electrical source since it is the higher potential in the PMOS version and the lower potential in the NMOS version. Since MOSFETs are electrically symmetrical in the triode region, they will conduct current in both directions equally well. With this approach, the transistor must have a maximum VGS and VDS rating greater than the battery voltage.

Unfortunately, this approach is only valid for load side circuits and will not work with a circuit that can charge the battery. The battery charger will produce power, re-enabling the MOSFET and reestablishing the connection to a reversed battery. An example using the NMOS version is shown in Figure 2 where the battery is shown in the fault state.

Figure 2 Load side protection circuit with a battery charger

When the battery is connected, and the battery charger is inactive, the load and battery charger are safely decoupled from the reversed battery. However, if the charger becomes active, for instance if the input power connector is attached, then the charger produces a voltage from the gate to the source of the NMOS, enhancing it, resulting in conduction. This can be visualized better in Figure 3.

Figure 3 Conventional reverse battery protection fails for battery charger circuits

The load and charger are isolated from the reverse voltage but the protection MOSFET now suffers exceedingly high-power dissipation. In this scenario, the battery charger becomes a battery discharger. The circuit will come to equilibrium when the battery charger produces enough gate support for the MOSFET to sink the current delivered by the charger. For instance, if the VTH of a strong MOSFET is around 2V, and the charger can deliver current at 2V, then the battery charger output voltage will settle at 2V with the drain of the MOSFET at 2V plus the battery voltage. The power dissipation in the MOSFET is ICHARGE • (VTH + VBAT), heating the MOSFET until it flows off the printed circuit board. The PMOS version of this circuit suffers the same fate.

[Continue reading on EDN US: N-channel MOSFET design]

Steven Martin is a battery charger design manager at Analog Devices.

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