Buck converter mains voltage

Question

I'm working in a buck converter to step down 127VDC to 5VDC/1A. I know that boost converters have limitations, thus it's not recommended to step up 5VDC to 127VDC. But I couldn't find anything about buck converter limitations. After Reading this document from TI I figured out some values for inductors, capacitors and so on. But my doubts still remain: Is it okay to step down that amount of volts? Will I find any trouble with my common buck converter circuit?

Obs.: The load will be constant and it is not sensitive to voltage oscillation around the 5V tension.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Yeah, I know. But I really would like to learn more about buck converters and it's possible applications on rectified mains voltage. So, no _easy way_ this time :3 . — salgado, Oct 01 '17 at 00:18
High ratio can be achieved but is not common due to the limitations several people have mentioned. I designed a 12-200 VDC in / 12 DCout buck converter for use in exercise equipment that used an alternator as the user load. It worked but efficiency was low near Vout max. Still FAR better than a linear regulator. — Russell McMahon, Oct 01 '17 at 11:35

Kevin White · Accepted Answer · 2017-10-01T17:12:08.587

2

One problem with a large voltage ratio buck converter is that the duty-cycle becomes very small - the ratio of the voltages - and if you maintain a reasonable frequency of operation the ON time gets very short.

Also the way you have it here with an N-channel FET the PWM signal is required to be over 130V in amplitude - this will be difficult to create, especially as it is above the maximum Vgs of the switching FET.

A buck converter is non-isolating so the output is galvanically connected to the AC input - potentially a safety issue.

Also - why do you specify 127V as the input? If it is really AC line power at 120V when you rectify it you will get ~165V.

The normal solution to all these problems for low to medium power is a flyback converter using a transformer. These can be very simple, although it is usually simpler to purchase a ready made unit.

From SMPS schematic

Design Guide for Flyback Converter

edited Oct 01 '17 at 17:12

answered Oct 01 '17 at 00:22

Kevin White

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Yeah, you are right man, I forgot to edit that voltage values (in fact, AC lines are 127V here). According to that TI paper that I mentioned, the duty cicyle will be 3.3% and frequency 100kHz. Is it possible to work? Considering the modifications that you proposed for the MOSFET. I will consider the flyback design, I really liked the idea. Thanks for the answer! – salgado Oct 01 '17 at 00:27
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Another drawback of high voltage ratio buck (and boost) converters is the need for both switching devices (MOSFET and diode in this case) to withstand both the full input voltage *and* the peak inductor current. In a transformer -based DC/DC converter (like a flyback) this isn't the case. – jms Oct 01 '17 at 00:30
@Kevin White, not sure about the winding dots but considering the two diodes and the output inductor \$L_2\$ it looks like you've inserted the picture of a forward converter : ) Most of the small non-isolated converters found in the white-goods market are buck converters (up to 276 V rms). A tapped version can help for the duty ratio but the flyback converter is the easiest way for low-power isolated stuff. – Verbal Kint Oct 01 '17 at 07:51
@VerbalKint - good catch. My error - Sorry about that, more haste less speed. – Kevin White Oct 01 '17 at 17:09
@Kevin White, no problem. I can see you have inserted the pix of a nice ringing-choke converter. I remember seeing these in cell-phone chargers back in 1997. They don't like to be unloaded (at least this simple version) hence the fairly chubby bleeder in the output. – Verbal Kint Oct 01 '17 at 18:22
@VerbalKint - I was looking for something simple enough to see the operation. Agreed being self-oscillating it has its limitations. – Kevin White Oct 01 '17 at 21:37

Buck converter mains voltage

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