Offline ac/dc PFC flyback regulator eschews optocoupler; simplifies, improves design

Bill Schweber, EE Times -- EDN, January 5, 2012

Going from ac-line to dc constant-current/constant-voltage power is a challenge because you must balance the need for sufficient regulation, efficiency, and PFC with the need for low parts count and cost. The LTC3798 from Linear Technology Corp manages the ac/dc transition using a single-stage flyback topology along with isolation. It achieves this task without an optocoupler, or optoisolator, because it senses the output voltage through the primary-side feedback signal. Target applications include high-end LED drivers; 12, 24, and 48V bus converters; and avionic supplies.

Active PFC yields a correction factor greater than 0.97, and the IC typically operates over a 90 to 277V-ac span, supporting output power as high as 100W. Associated external components primarily determine the input- and output-voltage limits, so you can also configure the device for applications requiring input voltages of 400V ac or more. These applications include industrial systems, electric and hybrid vehicles, and medical equipment. The device includes a 1.9A gate driver for controlling an external MOSFET.

Efficiency is typically better than 86%, and output-voltage and -current regulation are typically within ±5%. Using the LTC3798, a designer can develop a circuit that both meets the IEC-61000-3 harmonic-emissions specification and complies with Energy Star requirements of less than 0.5W no-load power.

Feedback does not require the use of a transformer; instead, it occurs through an extra winding off the power transformer as part of the CRM (critical-conduction) topology. With nonoptically isolated feedback, the minimum-load requirements are lower than those in optically isolated designs. In a 24V, 1A isolated design, for example, approximately 2 mA goes to the output clamp, and input power is approximately 100 mW.

The LTC3798 has an operating-temperature range of −40 to+125°C and sells for $2.25 (1000). Another version operates at −40 to +150°C. The IC comes in a thermally enhanced, 16-lead MSOP.

Linear Technology Corp