Low-electromagnetic interference (“EMI”) switching systems, or switched circuits, are generally known in the art. These systems employ the use of electronic switches, such as transistors, to rapidly connect and disconnect a load, a power source, a signal, or other electrical circuitry within the system. Often, these systems utilize multiple switches, and often instances exist when one or more switches are to be engaged at a same desired time that one or more other switches are to be disengaged, or visa versa. To cost-effectively control EMI emissions of switching systems, the engaging and disengaging of switches is overlapped using pre-calculated timing in an effort to rid the system of fly-back voltage and shoot-through current without the need for additional external filtering components.
The switch overlap can be realized and controlled by dividing each switch into multiple independently-controlled switches in parallel with varying impedances (essentially creating a composite switch). When these parallel switches are operated sequentially, the impedance transition of the composite switch is slowed. Applying this technique to multiple switches and overlapping transitions can effectually eliminate both fly-back voltage and shoot-through current. Additionally, this decrease in high-frequency energy may help result in lower EMI.
Although effective for a wide range of output power levels, this technique's performance can be less than optimal when the output power falls outside of the effective power range of the pre-calculated timing values. Particularly, if output power is too low, the overlap time may be too long, resulting in excessive shoot-through current. This excessive shoot-through current may dominate the quiescent current of the system as a whole in low power applications where often it is desired to keep quiescent current to a minimum. Conversely, in high output power applications, the overlap may be too short, resulting in fly-back voltage, and thus defeating the desired low-EMI effect of the circuit.