SG3524 switcher problem

[Spirit]

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

 

[Ross Herbert]

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

Looks like a damned awful design to me....

I thought the main reason for using switch mode techniques for PSU's
was to eliminate the need for heavy iron-cored transformers. Yet this
design still uses a normal 15V/5A mains transformer and bridge
rectifier to supply the rated output voltage of 15V at 4A....

The SG3524 appears to be used only as an ancillary device to control
the duty cycle of the -VE return rail for voltage control and to
switch the supply off for over-current conditions.

Surely, if you are going to use a standard full wave bridge rectifier
and mains transformer to supply full current and voltage, then far
simpler well tried analog techniques can be used for voltage and
current control, without using a SG3524.

 

[Fritz Schlunder]

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.