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. In this scenario, the battery charger becomes a battery discharger. The load and charger are isolated from the reverse voltage but the protection MOSFET now suffers exceedingly high power dissipation. Conventional Reverse Battery Protection Fails for Battery Charger Circuits This can be visualized better in Figure 3. 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. When the battery is connected, and the battery charger is inactive, the load and battery charger are safely decoupled from the reversed battery. Load Side Protection Circuit with a Battery Charger An example using the NMOS version is shown in Figure 2 where the battery is shown in the fault state. The battery charger will produce power, reenabling the MOSFET and reestablishing the connection to a reversed battery. Unfortunately, this approach is only valid for load side circuits and will not work with a circuit that can charge the battery. With this approach, the transistor must have a maximum V GS and V DS rating greater than the battery voltage. Since MOSFETs are electrically symmetrical in the triode region, they will conduct current in both directions equally well. 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. In both circuits, the MOSFET conducts when the battery voltage is positive and disconnects when the battery voltage is reversed. 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. Conventional Load Side Reverse Protectionįor 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. 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.Īnother approach is to use one of the MOSFET circuits shown in Figure 1. 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. There are some well known techniques for handling source voltage reversal. Reverse Voltage Protection for Battery Chargers
0 Comments
Leave a Reply. |