Tapped Buck Converter

[eem2am]

tapped buck converter

Hello, i cannot understand why the inductor in the "tapped" buck converter needs to be tapped

...or see page 6 of the following App Note:-

http://www.powerint.com/sites/default/files/PDFFiles/der186.pdf

it seems to me that the inductor between terminals 1 and 4 is not needed?

do readers know why it is there?


thanks in advance

Added after 4 minutes:

also, when the switch is OFF, the current in the inductor between 5 and 6 will be the same as the current in the inductor between 7 and 8 because these have the same number of turns.

....but that would mean there was 350mA flowing in the R1 and R2 which would cause a massive voltage of some 20000 volts across these resistors.

How can this circuit possibly work as this voltage would destroy it?

 

[VVV]

tapped inductor buck converter

Tapped inductor buck solves the problem of low duty-cycle at high input voltages. As you probably know, the duty-cycle of a regular buck is given by the output voltage divided by the input voltage (approximately, the exact formula is a little more complicated). As you can imagine, for a buck that works up to 265VAC (375VDC), with an output voltage of say, 6VDC for 3 LEDs in series, the duty-cycle would become 6/375=0.016. At 100kHz, the switch would be on for only 160ns. That is not practical.

The calculation of the duty-cycle for the buck is governed by Faraday's law. That requires that the volt*seconds applied to the inductor during the on time be equal to the volt*seconds during the off time. In a regular buck the voltage across the inductor is clamped by the catch diode to the output voltage (actually output voltage plus the catch diode drop; this is one of the corrections to the duty-cycle formula: DC≈(Vout+Vdiode)/(Vin+Vdiode) ).

With the tapped buck the voltage is clamped to the output voltage only across the section of the inductor that is connected to the catch diode. The voltage across the other section of the inductor will be simply the output voltage multiplied by the turns ratio of the two sections. The inductor behaves like a transformer now.
All this means the voltage across the inductor during the transistor off time is no longer limited to the output voltage plus one diode drop, but it is much larger, it is the output voltage plus diode drop times 1 plus the turns ratio

Voff=(Vout+Vdiode)*(1+N)

With this, the on time can be much longer and it can become practical. That is why you have the tapped inductor buck.

Regarding your second question, no the voltage is not calculated that way. As I said above, the inductor behaves like a transformer and it behaves like a flyback transformer. So the voltage across the feedback winding will be the output voltage plus a diode drop times the turns ratio between the output section and the feedback section. The current in the feedback winding will then be that voltage divided by the value of the resistors.

The main thing to remember is that it works much like a flyback and the output section of the winding is the one that clamps the voltage and the other windings just follow that with their respective turns ratios.

One thing I think is missing in that schematic is a clamp for the transistor. As in any transformer, there will be leakage between the sections of the inductor, causing leakage spikes. These can destroy the switcher. So if you are going to build it, add a snubber of some sort to the drain of the switcher.

 

[eem2am]

tapped buck

thankyou, you explained much better than the App Note for sure!

regarding the clamp...do you think this will suffice:-

-----------------
Also, regarding the clamping of the turn off spike...

page 8, 3RD paragraph down of....

http://www.powerint.com/sites/default/files/PDFFiles/der186.pdf

says...

"The leakage energy in the input section of T1 (pin 4 to pin 1) causes a voltage spike at
turn off. This spike is limited by the intra-winding capacitance of T1. This parasitic
capacitance is sufficient to keep the voltage spike from exceeding the BVDSS (700 V) of
the MOSFET internal to U1."

...In light of this, do you believe i can forget about clamping and just leave it to the intra-winding capacitance?

 

[VVV]

tapped buck converter over regular buck converter

Well, the appnote says that, and it may be correct for teh inductor they built. But can you guarantee that your inductor will behave the same way. Usually interwinding capacitance is something you want to be as low as possible. So making a better transfomer damage your IC?

I would recommend you add the snubber and if it turns out it is not necessary, you leave it unpopulated. But if it's required, you have it on the PCB.

 

[pianomania]

you can refer to the book : switch-mode power supplies, SPICE simulations and Practical designs. P.204 . author is basso.

there are detail descriptions about tapped buck converter.