Viper06 Buck mode converter, and AN4260

[GGibb.1]

Hello, I need to be able to generate a small DC voltage from an AC voltage.  

The App Note 4260 looked absolutely perfect - exactly what I needed - 12V 150mA buck converter.  Simple. no flyback transformers, etc.  But I was a bit confused as to the GND connection on the IC.  It looks like it is connecting to the V-Out instead, with a diode to gnd.

So i went into the edesign suite and used the exact same parameters.  the design it shows before you hit "Start design" also shows the diode to gnd on the GND pin, but it also shows another ground symbol.  Can you clarify the two different grounds?

The other thing I noticed - it shows a bridge rectifier on the AC input before you tap "start design", but the app note and design you see after you tap "start design" just show the simple single diode rectifier. Can you clarify if I need to add a bridge rectifier on the front? Do I also need the other diode then?

[Peter BENSCH]

Please do not be confused by the somewhat confusingly drawn GND triangles. They are actually intended to show the flyback regulator, which is "misused" as a classic buck regulator, as a buck. The GND with the small triangles is an internal connection only, NOT to be connected to GND with the three dashes!

The regulated output voltage can then be tapped between VOUT and the GND with the dashes, which is also connected to the negative pole of the input capacitors, for example.

The bridge rectifier is inserted by default because it enables the smallest input ripple current. Of course, the rectifier can then be changed to half-wave rectification with a single rectifier by clicking on it, but using this significantly increases the above-mentioned ripple current and also deteriorates other parameters of the design.

Does it answer your questions?

Regards
/Peter

[GGibb.1]

So none of the triangle grounds are connected to my PCB ground?

[Peter BENSCH]

Well, that's a question of definition: what else is your PCB GND connected to?

If I assume that your PCB GND is connected to the GND with the three dashes and therefore to one pole of the mains voltage in the case of half-wave rectification, then the GND with the triangles must NOT be connected to your PCB GND under ANY circumstances.

You can also remove the triangles and connect their nodes with a wire, then the schematics might be a little easier to understand.

Please keep in mind that the converter is a non-isolating one, i.e. with galvanic connection to the input voltage, usually mains.

Good luck!

Regards
/Peter