Negative transformer inverter circuit design

　　A simpler approach for any high-side overcurrent protection circuit is to reference the entire circuit to the high-side rail. Such circuits consume almost no power, and a small three-terminal linear inverter regulator can easily power them. However, this approach requires an unusual configuration that uses a negative transformer with its ground pin connected to the high-side rail and the input connected to system ground. There are no other connections to system ground. All of the "ground" points of the overcurrent protection circuit are connected to the output pins of the transformer.

　　The figure above depicts a two-phase stepper motor fast-acting self-resetting high-side circuit breaker, with 24V power going to the motor and 12V power going to the circuit breaker, which is referenced to 24V. The circuit breaker sees the 24V motor rail as 12V referenced to its local ground, which is provided by the output of the transformer. Like all negative linear transformers, this circuit requires a 6.8μF tantalum capacitor.

　　R10 and R12 are 0.33Ω 1W resistors that provide current sensing for both phases. High-side power flows through the sense resistors and P-channel MOSFETs to the high-side input of an H-bridge (not shown), which drives one motor winding. Current in either phase can cause the sense voltage to rise to 0.5V, thereby tripping the circuit breaker. The circuit responds by turning off both MOSFETs. It then waits 20ms and turns them on again, automatically clearing the momentary short circuit.