Hello
I am trying to design 220VAC to 28VDC Buck Boost controller. First I try to lower the 220VAC to level around 28VAC, then full bridge rectifying to around 28VDC.
Here is the circuit. ( ) Only the first part.
And Result ( )
But my actual load is 3.14ohm. When I change load to 3.14 result become like that () Is that expected output or something wrong ?
Thanks...

 

If the transformer secondary voltage is 28V RMS then the rectified voltage with no load will be around 38V.
With a 3.14 ohm load the output current will be around 10A and then 1000uF is nowhere near big enough to give low ripple.

 

I think, the waveform that you are seeing with the 3.14 Ohm Load is called RIPPLE.
9 Amps = 28 Volts / 3.14 Ohms

Is that amount of ripple OK for you?
You may need to design your Power Supply to deliver more than 9 amps (peak) to prevent Voltage Sag ( ripple ) during the zero crossings.

Instead of using one larger capacitor, try adding several "Low ESR" Capacitors in parallel.

 

Note that the transformer RMS current rating must be derated when feeding a diode-capacitor filter combination.
Typically that's at least 60%, so for a 9A output, the transformer should be rated for 9A/ 0.60 = 15Arms.

An approximate formula for the ripple voltage at the rectifier output is ΔV = 0.7 * I /(C * F), so you select a capacitor value to get the desired maximum ripple voltage ΔV.

 

An approximate formula for the ripple voltage at the rectifier output is ΔV = 0.7 * I /(C * F), so you select a capacitor value to get the desired maximum ripple voltage ΔV.

Is that 'F' the frequency of the mains or the ripple frequency (twice the mins frequency)?

 

Is that 'F' the frequency of the mains or the ripple frequency (twice the mins frequency)?

It's the ripple frequency, which is the main's frequency for a half-wave rectifier, and twice the main's frequency for a full-wave rectifier.