Design Tools

Commonly Used Switching Topologies

Buck Synchronous Buck

V IN V IN I SWITCH I SWITCH 1I I OUT LOAD V OUT V OUT I SWITCH 2 Function: Step-down (V
OUT < V
IN ) Function: Step-down (V
OUT < V
IN ) When to use: Typically when V
IN is 3x to 5x V
OUT and I
OUT is> 0.5A and < 5A When to use: When high efficiency is required with high-output current (> 5A) or low duty cycles (V
IN > 5 x V
OUT and/or I
OUT < 0.5A) Characteristics: Easy to design and good efficiency for the above-mentioned typical V Characteristics: A second switch replaces the diode in the basicbuck topology, reducing losses in the conditions mentioned above
IN /V
OUT /I
OUT conditions Devices to use: All buck integrated regulators and controllers Devices to use: Any “synchronous rectification” buck integratedregulator or controller Comments: Can do multi-output or isolation through coupling a second inductor to the one shown in the basic circuit

Boost

I LOAD 1V V

Flyback

V IN IN I OUT OUT 1 V OUT I SWITCH OUT 2 I SWITCH I LOAD 2V Function: Step-up, step-down, invert or buck-boost Function: Step-up (V
OUT > V
IN ) When to use: Typically when multi-output or isolation is required, when step-up beyond 8 x V When to use: Typically used when transformerless, regulated output voltages larger than input voltages at output currents beyond 100 mA - 200 mA are required Characteristics: Best for low-power conversion (up to 10W or 20W)and output voltages less than or equal to 7x the input voltage
IN is required or when the max. voltage or current of the switch needs to be extended in order to take advantageof the turns-ratio conversion from the transformer Devices to use: All boost/flyback regulators and controllers Comments: Output current V Characteristics: Ideal for medium-power conversion (5W to 100W)
IN /V
OUT x 0.7 x switch current Devices to use: All boost/flyback regulators and controllers

SEPIC

Cuk

V LOAD IN V IN I LOAD V OUT V OUT I SWITCH I SWITCH SEPIC = Single Ended Primary Inductor Converter Function: Inverting (V
OUT is negative; V
IN is positive) Function: Buck-boost (V
IN max. > V
OUT > V
IN min.) When to use: When a regulated, negative, low-ripple voltage is needed from a positive supply When to use: Characteristics: Excellent option when buck-boost operation is needed and no transformer is desired Lower input ripple than flyback, no snubber circuitry required Characteristics: Continuous current at input and output translates into a very low-ripple/very low-noise design Devices to use: Any boost/flyback regulator or controller Comments: Also useful for replacing boost circuits when true shutdown is required Devices to use: Any boost/flyback regulator – this is easiest with parts that have a negative FB input pin Comments: OK for isolation when a 1:1 transformer is added

46