buck converter output

fifahaywire · Mar 5, 2012

Hi..i'm working with a buck converter and my output with a 150Ω load is as follow
View attachment 61839
with a duty cycle of about 11%
View attachment 61840

is this the correct output?

thanks.

crutschow · Mar 6, 2012

You tell us. We have no idea what the circuit looks like, what it input and output voltage and current levels are supposed to be, or what signals the oscilloscope photos you have posted represent. So how can we tell you if it is correct?

fifahaywire · Mar 6, 2012

crutschow said:
You tell us. We have no idea what the circuit looks like, what it input and output voltage and current levels are supposed to be, or what signals the oscilloscope photos you have posted represent. So how can we tell you if it is correct?

i'm sorry for the lack of information. here's the circuit..
View attachment 61850

i hope u can help me

ronsimpson · Mar 6, 2012

The output cap 1uF is very small. It is hard to know what you ripple voltage is. You might be using 1:1 or 10:1 or 100:1 probes. With a 1uF cap you ripple will be high. If you look at any supply you will see 100uF or more.

Looks like a good start!

fifahaywire · Mar 6, 2012

ronsimpson said:
The output cap 1uF is very small. It is hard to know what you ripple voltage is. You might be using 1:1 or 10:1 or 100:1 probes. With a 1uF cap you ripple will be high. If you look at any supply you will see 100uF or more.

Looks like a good start!

yeah, it is small. but i'm avoiding the use of electrolytic caps. but i did try it out anyway and the ripple became less
i have polyester 22uF though. so expensive :S
the probe is a 1:1 i think. the one in the blue channel is crazy so i didn't use it. lol.

ronsimpson · Mar 6, 2012

I use SMT ceramic 10uF capacitors. Parallel 4 capacitors.

Gayan Soyza · Mar 6, 2012

ronsimpson said:
I use SMT ceramic 10uF capacitors. Parallel 4 capacitors.

Very nice way to add capacitors instead of a giant reservoir.

crutschow · Mar 6, 2012

You still haven't told us where in the circuit you measured the signals you posted.

WTP Pepper · Mar 6, 2012

What is drawn in the top part of the circuit is the classic buck convertor. However I believe you are deriving the feedback signal from the wrong point. Try moving it to the other side of the coil on the load side so it can monitor the smoothed DC and not the chopped signal which I imagine will throw the feedback circuit all over the place.

fifahaywire · Mar 6, 2012

crutschow said:
You still haven't told us where in the circuit you measured the signals you posted.

View attachment 61839 this is at the R load.
View attachment 61840 this is at the output of the optocoupler HCPL-3140 before entering the gate of IGBT.

WTP Pepper said:
What is drawn in the top part of the circuit is the classic buck convertor. However I believe you are deriving the feedback signal from the wrong point. Try moving it to the other side of the coil on the load side so it can monitor the smoothed DC and not the chopped signal which I imagine will throw the feedback circuit all over the place.

what if i add another inductor (about 500mH) to the load side instead of moving the 1mH?

ronsimpson said:
I use SMT ceramic 10uF capacitors. Parallel 4 capacitors.

hey, thanks! i will be sure to try paralleling 2 or more caps

buck converter output

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