United States Patent 4581565

An improved H-bridge power amplifier for controllably driving a dc motor is presented. The motor is connected in the center of the H-bridge in conventional manner. The H-bridge includes a current sense resistor placed between the power supply and a common tie point of the upper transistors of the bridge. Free wheeling diodes are connected from the motor directly to the power supply. This arrangement allows current to flow through the current sense resistor only when one of the upper transistors is on. The power amplifier is operated by selecting an appropriate pair of upper and lower transistors on opposite sides of the bridge so as to cause the current to flow through the motor in a desired direction. Both the upper and lower transistors of the selected pair are held ON until the current as sensed in the current sense resistor equals or exceeds a specified value, typically a desired current level. When this occurs, the upper transistor is turned OFF for a fixed off-time period, during which time the lower transistor is maintained ON. The motor current thereby decays through a path comprising the ON lower transistor, one of the free wheeling diodes, and the motor, which decay path significantly reduces the ripple associated with the motor current. At the end of the fixed off-time, the upper transistor is turned ON, and the process repeats itself. If braking is desired to slow the motor down, a plug inhibit mode is enabled wherein the lower transistor is turned off and the upper transistor is alternately turned ON and OFF.