Motor uses simple reverse-battery protection
Edited by Bill Travis
Dongjie Cheng, Allegro Microsystems, Warminster, PA -- EDN, March 18, 2004
This Design Idea presents a low-side, reverse-battery-protection technique for a dc-motor system. The system in Figure 1 incorporates two protection options. The common practice of using a diode for reverse-battery protection does not work with dc motors' inductive loads. In Figure 1, an Allegro (www.allegromicro.com) A3940, a low-cost power MOSFET controller, drives a dc motor. An H-bridge for driving the motor comprises the n-channel MOSFETs Q0 to Q3. To change the motor's direction from forward (current flowing from Phase A to Phase B) to reverse, the direction of current in the motor winding must reverse. This reversal means that Q0 and Q3 switch from on to off, and Q1 and Q2 switch from off to on. For an inductive load, the induced voltage, E, produced by a change in current is E=L(di/dt), where L is the inductance, and di/dt is the rate of change of the current. The induced voltage opposes any change in current. Therefore, after the switches attempt a current reversal, current continues to flow forward (forced by the induced voltage) from ground back to the power supply through Q2 and Q1. The current gradually decays to zero and then reverses direction. If a reverse-battery-protection diode is present in the current path, the decaying current is blocked and a large voltage can develop across the protection diode. Thus, the normal current recirculation meets interference. Further, the protection diode may break down, and potentially destructive voltages can appear on the FETs and the IC.
Figure 1 shows how to implement reverse-battery protection using the n-channel MOSFET, Q6, at the ground (low side) of a power supply. You can use either Option A (solid line) or Option B (broken line) to complete the circuit protection. If you use Option A, remove Q5. If you use Option B, you should cut open the bold trace marked "Option A." In both options, Q6 is connected such that its source connects to the H bridge, its drain to the power-supply ground, and its gate to the VREG13 output (a regulated 13.5V). At power-up, the body diode of Q6, D1, is forward-biased and provides the dc current path that allows the IC to power up. As the VREG13 regulator powers up, it turns on Q6, which provides a lower resistance path to ground than does the body diode, D1. Thus, Q6 connects the IC's ground and the power supply's ground. In normal operation, the motor current in the H bridge can flow from the power supply to ground or from ground back to the power supply through Q6. In the case of a reverse-battery condition, Q6 stays off, because the VREG13 voltage is not available and the Q6 body diode, D1, is reverse-biased, preventing any reverse- current flow.
We devised Option B because of a concern that switching noise may appear at the IC's ground if Q6's on-resistance is not low enough. By opening the connection labeled Option A, you isolate the IC ground from the potentially noisier connection at the source of Q6. Q5 is configured and operates in the same fashion as Q6. Q5 can have higher on-resistance than Q6, and, in this configuration, you may relax the on-resistance requirements for Q6. Experiments have demonstrated that both options work equally well if you carefully choose Q6's on-resistance.
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This reverse battery protection technique can be found in Patent #6,043,965
Lawrence Hazelton - 2004-3-5 06:34:00 PDT
