The need for a small, economical solution for switching power supplies and for battery chargers has increased. These applications generally require both voltage and current limiting. The supplies must be very efficient and produce a low amount of EMI radiated noise. ON Semiconductor's MC33364 is a critical conduction-control IC that can be configured in a resonant turn-off mode to reduce the amount of EMI generated during the turn-off of the power MOSFET. The part is a secondary-side controller that contains the band gap reference to provide a tight-tolerance voltage reference. An additional part, the MC33341, also allows for current limiting thus providing an almost square-loop of voltage and current needed for lithium-ion battery charging. Combining these two devices produces a much more cost-effective solution than other approaches offer.

The Classic Approach

In prior years, a very cost-efficient design approach was demanded for battery chargers and power supplies. The unit had to be current limited on the secondary side and provide the proper voltage output. The supply often did not support universal input. One circuit approach is a classical blocking oscillator (see Fig. 1.) This circuit has its roots back to vacuum tube days, and was updated by Bob Haver in 1984 and presented at various switching power supply seminars [1], [2]. This is a variable-frequency unit that operates in what is known today as the critical-conduction mode.

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