disadvantages of a conventional buck-boost converter

[biswaIITH]

i need sb's advice regarding the disadvantages of a conventional buck-boost converter . I read some where that it's negative polarity output and floating drives for switch are its main disadvantages.

 

[BradtheRad]

Although it inverts the supply polarity, this is not a problem if you only need it to provide one supply.

And the buck-boost has advantages too.

It's ideal if you have a positive power supply and you wish to obtain a negative supply from it, so you can have bipolar supplies.

A buck-boost gives you a smooth continuous range of available output voltage, ranging from stepping up to stepping down.

 

[biswaIITH]

ohk..will it(i.e. the inverted polarity) be a problem when used as a PFC(power factor corrected converter) in LED lighting applications.

 

[BradtheRad]

It is common to use a control IC which is made to regulate a switched-coil converter.

I suppose you will use one which works from a positive supply. Obtain an IC which specifically states it can create a buck-boost converter. Then it should be able to read feedback from the negative output voltage, one way or another. (Or else it is common to use an optocoupler.)

As for its ability to drive the switching device... If you want to make the design easy, use one transistor/mosfet. And make the other switching device a diode.

Sorry I lack the experience to say what problems might arise.

 

[mtwieg]

Even if you want the negative output voltage, the simple non-isolated buckboost still has some unique disadvantages. First, if you want to use an N channel FET, its source will be at the switching node, and must swing above and below ground. One trick to deal with this is to reference your whole control IC to the switching node (though then you need to have a more specialized output voltage sense configuration). Or you can use a P channel FET, though this isn't a great solution either.

Also the buckboost has discontinuous currents on both the output and input, which is bad for PFC applications. For that reason alone you may want to consider a SEPIC, or even a Cuk topology.