Help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (I am sorry to keep

[Scott Wiper]

I am stumped here.

I have bought some books on switcher supplys. I am beginning to understand
the three types

A: flyback type switchers.
B: Step up buck switchers.
C: Stepdown buck switchers.

I am trying to do this with transistors for this project.
http:www.travel-net.com/~swiper/switch.gif
I am trying to understand the function of the stepdown buck switcher. I am
useing a ne555 and a 4013 as the timing or switch. The primary DC is comming
a 100W 24VAC stepdown transeformer. I have done it like this. Vout~vz+Vce.

My first question is the output inductor.

Is this inductor used to store energy into the output capasitor?

Is the way I have this setup (Link above)... Will this make the caps burst?

Will this also make the 2N6284 Darlingtons heat up to a point that you can
boil water on them?

I am sorry to keep hounding on this issue but I have built it. I am affraid
what will happen on pwer up.


My last question is I have a LM2574HVT buck switcher that can be adjusted. I
want to use a 12.6V/1.5AH battery to power my portable CD player. For
stupidly long periods of time through the small power socket on the back.

Will there be noise in the audio even if I use the 330uH indctor and the
330uF cap as stated in the aplication notes on the data sheet?

--
My cat Tigger says every morning...
"Before my morning coffee... I might as well be a dog!"
To contact follow the link below.
http://www.travel-net.com/~swiper/email.htm

 

[Jan Panteltje]

**** Post for FREE via your newsreader at post.usenet.com ****

Looking at your circuit as it is now, I see you are using 144kHz?
Or is it 144.0 (as in 144Hz),
The diode D9 must be a fast recovery type if 144kHz.
For 144kHz 10mH is rather big...
So 144Hz then... not very good, you need a large L.

You also are using 15V zener drop in the driver.
You have somehow combined 2 very different circuits in one...

In a real switchmode the transistor is really only on or off.
The on / off duty cycle will control the output voltage.
The coil stores energy when thr transistor switch is on, when it is off
it releases that energy via the diode D9 into C2.
The amplitude of the ripple current in L together with the value of C sets
the output ripple.
Q = C.U = i.t
So for a current of 1A in a 1F capacitor you get 1V rize after 1 second.
For normal operaton the current variation in L will perhaps be about
40% of the output current.

Do not switch this thing on.
Start with the basic circuit, (L, C diode, switch) and try to get an
oscilloscope, or if not available, model it in LTspice.
Make the pulse (on time) somehow variable, and start with a short on time.
Measure the current in L with a small series resistor and a scope.
And see what happens at different frequencies and values of L.
Limit the current for your experiments in some way (some resistor, or low
value fast fuse, whatever).
Start simple!
JP

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[colin]

hmm, i spent a while loooking at that circuit and found quite a few simple
things wrong with it before i even go to the complicated bits, but i didnt
realy recognise any of the necesary components of a switch mode suply.

i sugest you try make things a lot easier for yourself and use one of the
many PWM PSU controller chips available. many are realy cheap now and
directly drive a mosfet with only a few other resistors/capcitors, in fact
some even have built in transistor such as the 78s40, l297 etc .. itl end
up a lot easier and better and probably cheaper too in the long run. also
many of them come with extensive application notes.

but il try and clear a few things up if i can.

with a step down converter like this u have a switch on the hi side of the
supply, this switches fully on and provides voltage to a inductor, curent
builds up through this inductor storing enrgy and curent also flows through
the load as well.

when the transistor turns off the curent in the inductor continues to flow
due to the stored energy and now flows thru the diode conected to ground.

the switching transistor needs to be switched hard on for maximum eficiency
but with this sort of circuit in order to acheive maximum eficiency u often
need a boost bias supply, this enables the switching transistor to be driven
hard on by a voltage higher than the suply its switching. this is often
acheived by using the output from the switch output to drive a charge pump,
ie a capacitor-diode pump.

usualy u have a clock wich produces a ramp (or sawtooth waveform) and a
voltage feedback or error amplifier. u then have a comparator wich compares
the error voltage to the ramp and this produces a puls width wich is
proportianal to the error voltage so that as the voltage falls slightly the
switch is turned on more.

an alternative is to use the curent in the switching transistor as the ramp
itself, this in fact not only makes the feedback loop easier to stabilise
but also doubles up as a curent limiter too.

the oscilation frequency shld be above 20khz or youl end up listning to
screach eminating from the inductor.
but in some situations tho it is posible that the switch only turns on for
every other pulse or fewer thereby efectivly reducing the frequency.

however u shld easily be able to filter it enough so that the voltage ripple
does not efect the operation of the device you are powering.

what u apear to be doing is to drive the switch from a much reduced suply
with a fixed pulse width, thereby making the whole thing probably as
ineficnet as a simple linear voltage regulator and with exceedingly poor
regulation. - although maybe i have misunderstood the purpose of the circuit
somewhat.



"Scott Wiper" <nobody@devnull.spamcop.met> wrote in message
news:89uzc.5459$vO1.45638@nnrp1.uunet.ca...

I am stumped here.

I have bought some books on switcher supplys. I am beginning to understand
the three types

A: flyback type switchers.
B: Step up buck switchers.
C: Stepdown buck switchers.

I am trying to do this with transistors for this project.
http:www.travel-net.com/~swiper/switch.gif
I am trying to understand the function of the stepdown buck switcher. I am
useing a ne555 and a 4013 as the timing or switch. The primary DC is
comming
a 100W 24VAC stepdown transeformer. I have done it like this. Vout~vz+Vce.

My first question is the output inductor.

Is this inductor used to store energy into the output capasitor?

Is the way I have this setup (Link above)... Will this make the caps
burst?

Will this also make the 2N6284 Darlingtons heat up to a point that you can
boil water on them?

I am sorry to keep hounding on this issue but I have built it. I am
affraid
what will happen on pwer up.


My last question is I have a LM2574HVT buck switcher that can be adjusted.
I
want to use a 12.6V/1.5AH battery to power my portable CD player. For
stupidly long periods of time through the small power socket on the back.

Will there be noise in the audio even if I use the 330uH indctor and the
330uF cap as stated in the aplication notes on the data sheet?

--
My cat Tigger says every morning...
"Before my morning coffee... I might as well be a dog!"
To contact follow the link below.
http://www.travel-net.com/~swiper/email.htm