Circuit efficiently drives inductive loads

Carlisle Dolland, Honeywell Engines and Systems, Torrance, CA -- EDN, 5/25/2000

In the driver circuit in Figure 1, the system controller provides the V_COMMAND signal. V_COMMAND equals the desired load current multiplied by R₈. When the controller applies this voltage to R₁, the output of IC₁ goes high, applying voltage to the gates of Q₁ and Q₂. These transistors turn on, allowing load current to flow to ground through Q₁ and R₈. The current in the load ramps up, and a voltage proportional to the load current, sensed by R₈, feeds back to the inverting input of the comparator IC₁. When this voltage exceeds the voltage at the noninverting input, the output of IC₁ goes to ground. Q₁ and Q₂ then switch off. The load current now circulates around the loop comprising D₁ and L₁. During this time, the slope of the load current becomes negative because of the dissipation in D₁ and the load resistance. The duration of this phase of the circuit's operation is a function of the hysteresis (set by R₁, R₂, and R₄) and the decay of the voltage across C₂ (essentially a function of R₉). C₂ and R₉ determine the ripple current in the load. The circuit cannot use a power MOSFET for Q₂, because of the intrinsic drain-to-source diode. You must use a device without the intrinsic diode, such as a 3N71. (DI #2535)