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Subject: Re: DC/DC Converter

Date: 12/18/02 at 7:38 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: DC/DC Converter posted by Francis LHeureux on 12/17/02 at 11:54 AM:

Hello Francis,

A buck converter is probably the topology you'll want to use, assuming you don't need galvanic isolation between input and output.

I expect you will need to use an external switching MOSFET given that input voltage range. The 85V input will be much too high for switchmode regulator I.C.s with a built-in switching transistor.

There's a lot of controller I.C.s around that will drive an external MOSFET - try Linear Technology, Maxim, Texas Instruments, National Semiconductor, Micrel . . . Some websites organize the range of controller I.C.s according to the topology you intend to implement (buck, boost, flyback, etc); that helps to narrow down the choice. The datasheets and application notes nearly always have examples of circuits, so you might well find just what you need there - or at least hints on what MOSFET and other components to use with the controller I.C.

Some I.C.s offer extra features such as a logic-level shutdown input, soft-start, or current-limit setting using an external resistor. But one characteristic that you will need to look at is the operating frequency. Controllers offering a higher switching frequency will allow you to choose smaller inductors and capacitors, allowing the regulator to be made physically smaller - but the downside is, the efficiency tends to get lower because the switching losses increase with frequency.

Hope these hints are useful.

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