Hi,
I have an identical/similar problem: SMPS (on breadboard) that regulates 5V well with 1 mA load, when I connect the 50 Ohm resistor Vout falls to 2.5V. I've tried lots of things but not eliminating problem.
Klaus already told me, several times that breadboards are wrong for prototyping SMPS, he and another member also pointed out that may not be driving switch MOSFET hard/high (or low) enough, also op amp/comparator may not be appropriate part for switching speed. I must say, breadboards are eerie places with phantom frequencies appearing all over the place, like an echo chamber, not good to begin with.
I'm stuck with my circuit, I'm guessing these may be several possible problems with woeful Iout/full load problem and VOut droop:
a) What EDA members advised me about.
b) Error in calculations and therefore inductors wrong value, or being +-20% could influence "best laid plans."
c) Incorrect error amplifier compensation.
d) Wrong frequency.
e) Sawtooth and error amplifier signals wrong ratio.
f) Insufficient capacitance, I doubt, as can go from calculated 22uF Cout to OTT 492uF Cout or Cc and nothing improved.
g) MOSFET with RDSon that is far from ideal at ~1.4Ohms.
I'm wondering if I haven't undercalculated switch gate drive, which could count as h).
I noticed that "slow" op amp gives calculated 5Vout, much faster op amp gives an unwanted 7Vout. No idea why, maybe guess b) affects that.
I'm convinced that incorrect or lack of compensation contributes to this a lot, but no doubt I'm wrong, 'though compensation is a must-have for regulated outputs.
Some SMPS need a minimum load, don't they?
I'm clueless, but if you need to compensate that IC, I'd review the values, as it sounds like the IC loses regulation, which may be reason for needing to reset (unplug, and/or let it cool down). Can you get 1A out of that chip? Does it have user-friendly thermal limit calculations in the datasheet, that relate frequency, Vin/Vout and Iout, etc.?
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Hi again,
Reading this datasheet,
https://file.yizimg.com/332467/2009072110594237.pdf
I calculated the inductors for, respectively: 5Vin/5Vout, 0.1A and 1A (50Ohms and 5 Ohms), and got 16uH and 795nH.
I desisted from the compensation calculations as, as per usual with compensation documents and app notes and books, too many terms are included that have no definitions, or necessary equations (and/or are comprehensible to less experienced users) or even cryptic hints as to where to locate that information, nor how to calculate them, as it seems to be expected that every reader will understand these terms (the same as poorly written software manuals that skip important steps that your average computer user who won't have IT qualifications will never know they need to do..., but the manual writer expects everyone to know...) sorry.
Also, even when I got 7V out on my SEPIC/SMPS, Vout still dropped to half the output voltage, seems to be a pattern: attach a "small" load, 100mA, or 50mA and the regulated Vout falls by half. ??