Hii....

I have built isolated 24v smps using UC3845 pwm controller ic works fine but getting spikes at output side around 1.2v-1.8v at the same frequency on which my mosfet is switching ie. 45khz, if i fed this to the motor driver ic gets damage.

what could be possible the reason? bad pcb design?

 

What filtering do you have on it? A circuit and photo of your build will help.
And it may be your scope probe is picking the noise up directly. What do you see on the scope with the probe tip connected to the ground clip?
PCB layout for switch mode power supplies can be pretty important too.

 

That looks about normal for a flyback converter.

 

It could be that if you lay your scope probe near the supply you will see the same noise. (not connected)
Connect scope probe to it's ground and you probably will see the same noise.

 

The basic switching supply mode does create transient voltages. And currents. That is why designing a switcher supply is actually a complex project. It is also why switcher supplies have earned the reputation of being noisy, which is because , they are noisy. So some filtering is required, and in addition, adjustment to reduce the noise is required. Once the switcher works, the noise must be reduced. That is by no means simple.

 

@dendad

thanks for the help ,using ground clip it shows very lil spike which is acceptable i guess.