Voltage drop in SMPS output (ACT4060)

Hi
I designed device which is supplied by 5V provided by ACT4060 (SMPS regulator).
Regulator produces stable 5V but only when there is no load connected. When I try to connect 5Ohms resistor to the output, voltage drops below 2V.
I use 22uH inductor and 22uF ceramic resistor. When I tried to replace inductor to 10uH It was a little bit better but the problem still existed.
Please Help.

BR
Grzegorz

Hi,

There are a lot of 22uH inductors. But usually in the datasheet you find additional information how to choose the right inductor.
Did you follow the rules?

Btw: We are talking about a well designed PCB layout? Breadboard gives no meaningful results.

Klaus

Unfortunately in the datasheet I can't find any additional information regarding to inductor choose (except inductance value).

Of course everything is soldered on PCB.

BR
Grzegorz

Hi,

Why do you say this? This is wasting our time.

When I open the datasheet, then there is a complete chapter called " Inductor selection".
And as expected they show the saturation current requirement.

Klaus

Sorry, You are right. Shame on me.
I will replace the inductor and let know the results.

Grzegorz

I replaced inductor to 22uH with Isat = 7A but It didn't help.
Additionally I figured out that when I connect load (5 Ohm resistor) SMPS chip warming up very fast and it persist even if I then disconnect load (as well as voltage drop).
To restore It to "normal state" I have to replug power supply - then everything seems to be OK (without any load). Any ideas?

Hi,

* PCB layout.
* wrong wiring
* wrong parts (capacitors)

--> show us your PCB layout, your schematic with all the informations of the used devices, a photo of the circuit.

Klaus

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.?

- - - Updated - - -

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. ??

d123 said:

5V well with 1 mA load, when I connect the 50 Ohm resistor Vout falls to 2.5V.
...
regulated Vout falls by half. ??

Click to expand...

A typical cause is oscillations which travel the range between supply rails. Hence their average voltage is V/2.

Or perhaps the frequency is too fast for the device to handle, so that the waveform hovers near V/2, and never reaches its proper high or low extreme travel.

Often these are parasitic oscillations. They may be difficult to prevent.

Hi,

Some datasheets are more detailed than others.

There is a problem when writing a datasheet. It should be small that the reader can go through it within minutes and easily find the informations....and it should contain all informations about device selection, theory of operation, explaining the function, PCB layout and all background information for a stable circuit.
Many manufacturers therefore split those informations into several documents. The datasheet itself, application notes, design notes, other informations and often a design kit which include a PCB.

A switching regulator is far more complex for the designer to understand than a 7805 linear regulator.
* with a 7805 you only need one (two) uncritical standard capacitors. It is table with a wide range of capacitace value.
* but with a switching regulator the designer needs to calculate the values. The inductance for example: you need to take car about inductance value, saturation current, gapped or non gapped, DC current, switching frequency and ferrite material (don't use a EMI ferrite for a storage inductor). Often even for an experienced designer it is hard to find all the necessary informations in online shops. Often the only solution is to go through the inductor datasheets and the find the informations in tables, charts, text...

This isn't a task for some minutes..

I know, the recommendation for a "good PCB layout" sounds so simple, but I know it isn't. So much details to take care of.
A switching regulator needs knowledge about
* precision analog signal routing
* HF signal routing
* high current signal routing
* thermal considerations
And so on

With the years of experience you will become faster in desgning such a circuit.

We all needed to go through it.
So don't give up. Take your time. Read documents. Try to go step by step. Try to make it work..then try to modify and improve it.

Klaus

Hi my friends.
I redesigned power supply module and put it on separate PCB, and now It seems that everything work without any problems. Output voltage is very stable and SMPS doesn't warm up even on load.
Unfortunately I'm not sure what is the reason of my problem - probably messy PCB layout. Is It possibly that layout of PCB has such strong influence on SMPS working?
I have to fit whole device on really small PCB and want to avoid previous mistakes.

BR
Grzegorz

Hi,

Why do you think I mentioned "PCB layout" in three of my posts within this thread?

Yes, the PCB layout IS that important. A wrong PCB layout can have these effects.

Klaus