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23/02/2009
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Automotive applications benefit from multiphase boosters.
 High power step down dc/dc converters have long benefited from multiphase operation. But the advantages of multiphase operation, such as reduced input and output ripple currents, lower output noise and lower component stresses, can also be realised in step up applications.
Until recently, most high power step up converters have used non optimised solutions due to the lack of an available multiphase boost controller. A common solution has been to use the top side drivers of a two phase synchronous step down controller configured to drive two low side power mosfets 180° out of phase. Another solution has been to use two or more single phase step up controllers and an external clock circuit to achieve the required channel to channel phase relationship. However, these solutions suffer from significant drawbacks.
In the automotive environment, the next generation of low emissions diesel fuel injection systems requires a 2A supply at a voltage between 70 and 110V, delivered from a 12V battery that can vary from 9V to 28V. This input to output voltage conversion requires a boost converter with a duty cycle of more than 92% and constant frequency operation.
Meanwhile, high power car audio amplifiers often need a main supply ranging between 25 and 35V, with the ability to supply up to 1000W. This makes multiphase operation essential. By splitting the power stage into multiple paralleled phases, thermal stress is reduced on the power components, as are output voltage ripple and noise. This allows the use of smaller output capacitors and improves system efficiency.
As power densities continue to rise, multiphase boost designs become a necessary option to keep input currents manageable, whilst increasing efficiency and power density. With mandates on automotive energy savings more common, a multiphase converter topology may be the only way to achieve these design objectives. A two phase or higher converter built around Linear Technology’s LTC3862, can demonstrate the benefits of this approach.
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Author
Bruce Haug and Tick Houk
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