Dear Dennis... analogZONE's Dennis Feucht answers your design queries in his new Circuit Design Clinic! January 2007

analogZONE presents an interactive analog design service to readers! Send us your design questions (with relevant data; schematics in JPEG or GIF, please) for some free engineering advice from analogZONE's circuit consultant, Dennis Feucht, on how you might solve a design problem or improve circuit performance. Submissions may be edited for clarity or brevity, and submitters and their email addresses will remain anonymous (unless otherwise indicated). Please send your questions to Dennis here.

Power Converter Power Supply Design Problems

Q: I am designing a cost-sensitive off-line power converter with a PFC input stage that also needs to dissipate low standing power -- that is, the power used by the PFC itself must be kept minimal. I need to power the control circuitry on the primary side of the converter, and if I use a resistive divider down from the unregulated supply, it will dissipate too much power. Any recommendations on a lower-power circuit?

A: This problem can be solved by using two different schemes to power the control circuitry. The first derives power in the manner you describe above, through a dc connection to the unregulated supply. However, once start-up occurs, a separate supply provides power much more efficiently. The start-up supply then must somehow be turned off without causing a discontinuity in power to the control circuits.

Read the entire response here (131 kb Adobe Acrobat PDF file)