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From: Spirit on 23 Feb 2006 11:33
I built variable switching power supply using schematic from this address:

http://delabs.netfirms.com/cirdir/power/del20020.pdf

I don't know if this schematic has any bug, but my switcher which is built
using this schematic not works good :(
I checked, and rechecked all connections several times, but everything is
like on this schematic diagram.
Voltage and current regulations are very bad. I conect stabilised 13V to
input, and from another stabilised power supply I connect 12V for SG3524 and
LF358.
For testing I use mosfet BUZ11, and various ferrite inductors, but results
are bad.

On output I cant get more than 6V, and output 470uF elco sounds like
"bzzzzzzzzzzzzzzz"

Did anybady built any working switcher with this schematic?

Maybe is problem in inductor. Is it critical?
I tryed with few toroid and ETD44 EI core with 4 to 40 turns of wire, with
no luck :-(
Which corre and numbers of turns is suggested for this type of application?


Spirit


From: Tom Bruhns on 23 Feb 2006 13:27
Without seeing your construction it is difficult to say for sure. The
inductor is a good place to start. It must not saturate, up to perhaps
10 amps. Can you measure the waveforms with a scope? Can you measure
inductance? Did you notice the note on the schematic by the inductor
that says "air gap"? (An air gap will greatly reduce the tendency of
an inductor wound on a ferrite core to saturate, though you need more
turns to get the same inductance, compared with an ungapped core.)

Another item: be sure the current shunt (200mV(a)5A = 40 milliohms) is
very low inductance, and the leads to it are vanishingly short. It
must be able to dissipate 1 watt.

Suggest you get someone local to you who has experience with switching
power supplies to have a look at it. It may well be obvious on visual
inspection, and certainly should be using a scope to look at waveforms.
In a suppy like this, layout and construction practice in some areas
are critical.

Cheers,
Tom

From: Tim Williams on 23 Feb 2006 13:32
"Spirit" <spirit810(a)hotmail.com> wrote in message
news:dtko3u$r57$1(a)ss405.t-com.hr...
> http://delabs.netfirms.com/cirdir/power/del20020.pdf
<snip>
> Maybe is problem in inductor. Is it critical?
> I tryed with few toroid and ETD44 EI core with 4 to 40 turns of wire,
> with no luck :-(

Well, it's certainly a hell of a lot more critical than "4 to 40". That's a
range of A HUNDREDFOLD for inductance! Not even the broad side of a barn.

> Which corre and numbers of turns is suggested for this type of
> application?

It sounds like you didn't read the schematic. It states "30uH 5A" for the
inductor...

You can measure inductance by moving R15 in series with the inductor and
watching voltage across the resistor. When terminal voltage (of the
inductor) is known (20V by the looks of it), you can measure the slope of
the increase in current and calculate inductance by dI/dt = V/L.

In addition, you can evaluate the maximum current capacity by noting at what
current level inductance decreases, due to core saturation (current starts
rising steeply).

Average (actually RMS) current capacity corresponds to the wire gauge.
You'll need reasonably heavy (circa 16AWG or better) wire to handle 5A at
that frequency.

Once you have the inductor in place and you are certain it isn't the
problem, you can try evaluating the feedback circuit: voltage and current
regulation.

For now, I would suggest you disconnect the error amplifiers and use a
potentiometer to control duty cycle (route pin 1 to pin 9, remove R3 and
disconnect pins 4 and 5).

Use a large resistor (say, 15 ohms 25W) on the output to provide a load.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Rene Tschaggelar on 23 Feb 2006 15:14
Spirit wrote:

> I built variable switching power supply using schematic from this address:
>
> http://delabs.netfirms.com/cirdir/power/del20020.pdf
>

Unfortunately a switcher is one of these things
where a schematic is only half the story. The
other half is the layout / 3D build and the
waveforms. You should have a few waveforms to
compare to as well.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
From: Fritz Schlunder on 25 Feb 2006 16:37

"Spirit" <spirit810(a)hotmail.com> wrote in message
news:dtko3u$r57$1(a)ss405.t-com.hr...
> I built variable switching power supply using schematic from this address:
>
> http://delabs.netfirms.com/cirdir/power/del20020.pdf
>
> I don't know if this schematic has any bug, but my switcher which is built
> using this schematic not works good :(
> I checked, and rechecked all connections several times, but everything is
> like on this schematic diagram.
> Voltage and current regulations are very bad. I conect stabilised 13V to
> input, and from another stabilised power supply I connect 12V for SG3524
and
> LF358.
> For testing I use mosfet BUZ11, and various ferrite inductors, but results
> are bad.
>
> On output I cant get more than 6V, and output 470uF elco sounds like
> "bzzzzzzzzzzzzzzz"
>
> Did anybady built any working switcher with this schematic?
>
> Maybe is problem in inductor. Is it critical?
> I tryed with few toroid and ETD44 EI core with 4 to 40 turns of wire, with
> no luck :-(
> Which corre and numbers of turns is suggested for this type of
application?
>
>
> Spirit



If I'm not mistaken the schematic seems to be using a very funky weird means
of driving the MOSFET gate. In this case, funky weird is decidedly not a
desirable aspect. The SG3524 has a pair of uncommitted output transistors
inside, but they are designed to be used to drive power bipolar transistors.
They aren't designed to be used to make a totem pole driver for driving
MOSFETs directly like shown in the schematic...

Due to this bizarre gate drive connection scheme, it appears to me the
circuit cannot produce more than roughly half duty cycle. This would
explain why you can't get more than around 6V output with 13V in. In an
ideal buck converter operating in continuous conduction mode the output
voltage is Vin*duty cycle = Vout. Since the gate drive connection scheme
prevents large duty cycles, the maximum output voltage with any kind of
decent current capability is limited to roughly half the input voltage.

To fix this, the SG3524 output transistors should be "wired ORed" (connect
collectors together, emitters together, collectors to +12V, emitters to pull
down resistor) and the output should drive a MOSFET gate driver IC such as
the TC4427 by Microchip.

Given the nature of this conceptual error in the schematic, I wouldn't be
surprised if there are other hidden problems that I have not noticed in it
as well.


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