Problem statement
Switch-mode systems such as power converters usually generate significant electromagnetic interference (EMI) in the form of high-frequency current at their input (e.g., the grid) and/or output (e.g., processor supply voltages), which must be filtered to meet product specifications and/or regulations. This is typically accomplished through the use of passive inductors, capacitors, and resistors, adding substantially to the weight and size of the system. Active EMI filters are sometimes designed that attempt to cancel the interference current through broadband feedback; this approach is often power-hungry and complex. Finally, interleaving is often applied at high power but carries an additional cost burden by demanding more high-voltage components.
Solution
This invention permits cancellation of a great majority of the undesired interference current by use of a circuit that is switched synchronously with the main switch-mode system without the need for feedback, sensing, or independent actuation. The circuit is connected to the main bus through a DC blocking capacitor which allows the circuit to operate at much lower voltage than the main system—40 times lower or more in a power factor correction system, for example—which keeps the cost of semiconductor devices and the size of passive components low. The invention may be coupled with additional EMI filtering schemes, for example a very small passive EMI filter to fully eliminate the highest-frequency components of the interference signal.
Features
The invention consists of a circuit that is coupled to the main bus through a DC blocking capacitor. E.g., the circuit may resemble a boost converter to filter the input current of a boost PFC. The switching action of the circuit ideally mirrors the behavior of the boost PFC such that the input ripple is exactly canceled.
Benefits
The circuit requires no high-voltage components other than the DC blocking capacitor and is only rated for the ripple current to be filtered. For a boost PFC operated in continuous conduction mode, the invented circuit could have much lower voltage and current ratings, and therefore very small physical size and cost. Cost is further minimized by using the same command signals as the main system and avoiding the need for additional integrated circuits; much of the invented circuit could be integrated with a PFC controller, for example.
In a typical application such as a grid-connected power factor correction converter, the invention may be supplied by a voltage less than 40 times that of the output voltage of the PFC, resulting in a subcircuit with 40 times lower voltage rating on switches and 40 times less magnetic energy storage. One would expect sizes on the order of 2-3% of total power converter volume; compare with passive EMI filters which sometimes make up 20-40% of the total power converter volume.
Markets
Power factor correction converters for anything connected to the grid; DC-DC converters in electric vehicles; high-voltage pulsed power systems, including medical oncology and imaging systems; point-of-load (e.g. microprocessor) power converters
Development stage
Proof of concept through simulation; lab prototype forthcoming
