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[SOLVED] Buck with FET in the low side

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CataM

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Hello everyone,

The attached buck converter uses the MOSFET in the low side, however the drawback is the floating output. This makes it easy to drive the MOSFET and even can be used as buck converter for input voltage as high as rectified mains because no high voltage capacitor is needed for the bootstrap.

1) Why isn't this topology as popular as the usual one ? Only because of the floating output ? Is there any issue I do not see ?
2) I do not see any problem in changing the output filter inductor at the cathode of the diode, am I right ?

Thank you for your time !

Buck with low side switch.png
 

Hi,

I think there are some issues with your idea:
* Cin needs to be high voltage
* no bootstrap capacitor needs to be high voltage
* this circuit shows a typical buck converter circuit. The only difference is the reversed polarity. It has the same advantages and disadvantages than a positive buck converter.

Your assumption 2) is wrong.

Klaus
 
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    CataM

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Yes, I made a mistake in the original post regarding the bootstrap capactior that it needs to be high voltage.

Anyway, this circuit does not need a high side driver, it can go along with a low side driver for the FET, that is an advantage.

Probably I have explained wrongly assumption 2). I meant to say moving the output inductor at the cathode of the diode but still in the same mesh as in the circuit in the original post, NOT in series with the diode. In other words, the output inductor would have been connected between the cathode of the diode and the Vout(+) (positive plate of the cap).

If Synchronous Buck, then it has no advantage whatsoever.
 
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I've seen his circuit mostly in LED drive circuits, since in those applications it does not matter that the output negative rail is not common with the input's negative rail.

Feedback may be achieved with common optoisolator techniques.
 
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    CataM

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Are you asking about this topology?

Buck with low side switch2.png

It's really bad related to generated common mode noise.

As already stated, the low side buck switch is an option for floating (not ground referred) loads. Need to check your application requirements if it's suitable.
 
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