TOP switch as a Buck converter

Hi


This part of the forum is usually for PIExpert questions. There is a section dedicated for PSU support.


From a schematic concept point of view, it is relatively simple to adapt a TOPSwitch to work as a buck converter (see attached image, made for an high voltage application, but you can adapt it to your needs). You could use 132kHz switching frequency to reduce inductance size, but because of the relatively high output current your conduction losses can be relatively high.


In order to estimate losses and calculate the inductor value, you can use the general formulas in the appendix a of the application note 37

Thank you for the schematic.
Please explain me how to select R3 & VR1 for the 12V Output.
I have connect 1k and 10v zener for 12v output but its not working.

Hi.


Please keep in mind that the control pin is at 5.9V approximately. So if you want 12V, a likely candidate a 5.6V zener in order to get 12V on the output. You should probably also reduce (for the first tests at least) the resistor to something around 100Ohm.


Do not try the circuit at no-load (the voltage will simply keep rising). But do start by trying with a low load (e.g. a 5k resistor). Also keep in mind that the inductor should not saturate at the peak drain current of the IC. You can adjust the maximum peak current by change the resistor on the X pin.


I hope this helps

Sir,
still I'm not capable to get output.
sending you my schematic pls comment on it
my output requirement is 12V,1A if it can't work on 1A, 250ma is also OK

First I want to operate TOP switch as a buck.

Hi,


Firstly I recommend that you put a capacitor on the input side, just so that the drain current has a small loop.


Secondly, if you want 12V you really should drop your zener to a 5.6V low test current (BZX55, or equivalent).


Having said all this, I would still expect you to see something, even if just auto restart. Can you measure the voltage between the control pin and the source pin? You should have around 6V there.


What resistor did you select for the X pin? A too large resistor may also limit the current too much (check figure 55b of the datasheet).


I also believe that you will need a bigger inductor than 33uH for your spec, but even with this value you should see some switching. Try and measure the current on the drain pin. If it's in auto-restart the IC may stop switching periodically for around 1.5 seconds, but you should still be able to see something

Sir,

As per your suggestion I made few changes to my circuit as attached here.

but still not working.

I'm measuring 13.6V at output but its not stable...It is not working as buck...
Output is not constant as I change the input....Output is increasing and deceasing as changing input.....

Control pin voltage wrt source pin is 0.25V WHILE control pin voltage wrt GND is 13v.....

I need constant 12V OUTPUT with input range of 30-48VDC.

Don't know why it is not working????
As i BELIVE Vout=Vc+Vz

please reply

Hi


I made some tests and successfully managed to have the IC running without issues. As I mentioned in previous messages you should have ultrasfast diodes for the freewheeling and for the feedback (I used UF4004).


The concept I sent you (and what I tried in the lab) switched at 66kHz (your board is configured for 132KhZ, and that theoretically should be ok, but I didn't try it).


I also used a 330uH inductor, and managed to go to 1A of load (no thermal testing though). you do need a minimum load, but if you check my concept (with 5.6V zener in series with 100Ohm), it should work well as long as you pay some attention to the layout. The 100nF cap right next to the C pin with dedicated trace to source; the IC, inductor, freewheeling diode, and output cap should be all close to each other...

could you please send me circuit details including each and every value.

This time i would like to test same as you tested....I will also keep every component same including top-switch also....

TOP switch TOP271KG as a Buck converter

Dear Sirs,

I am desgining non-isolated step-down converter(buck) using TOP271KG with this spec:
- wide DC voltage input range from 30 to 210 VDC
- and a fixed output 12V and about 1A.

I used these papers:
- Stefan Baurler "Buck Converters Using TOPSwitch Family"
- an-37

I tried the following circuit (see attached 172001.gif)

But the output voltage is not stabilized - it changes when the input voltage changes.
See the table here:
http://94.127.69.99/top271/top271kg.html

Please tell me what can be the reason?

Please tell me is there is the spice model for TOP271 ?

--------------------
Thank you for support
Serg

Hi.

R1 is a bit low, I suggest you increase it to 100Ohm.

D2 has a test current of 45mA. This is way too high. You need to use a low test current zener (like a BZX79 or equivalent)

You need to change your components on the C pin (100nF + RC combination). See the schematic I placed on my first post in this thread.

Your 100nF cap between D2 and D3 should be perhaps a bit bigger, at least for your first tests (I tried with 10uF on my basic proto board, but probably something like 1uF should also be enough).

You need a much bigger low ESR capacitor on the output. I tried with 680uF/25V LXZ series, but you really need to see your worst case inductor current ripple and have the capacitor rated for that ripple.

You can take a look at the attached picture to get the idea of how the layout should look like (more or less at least). You can see that the source of the IC is very close to the freewheeling diode and to the inductor. also the diode and the capacitor for the feedback circuit are very close to the inductor (the zener and resistor are on the other side of the board). I used a 100nF ceramic cap right next to the Control pin of the IC (above that you can see the RC circuit with 47uF and 6.8Ohm also connected between the C and S pins).

For my simple test I used a TOP257PN switching at 66kHz and a 330uH inductor from Coilcraft (RFB1010 series). I didn't check Safe Operating Area, nor saturation of the inductor, but these are critical tests that you'll have to do for your project.

I did not have the issue you are reporting. The TOP-JX does have lower Control pin current that the TOP-HX (that I used for my simple test). You may need to increase the resistor R1 to a value above 100Ohm.

I hope this helps.