Please help - Spikes in SMPS output

Thread Starter


Joined Mar 8, 2007
I am designing a 25A 12V Variable SMPS, offline.
Output voltage is ok, but the ripple is about 35 mV peak to peak triangular, with spikes of about 300 mV on either side, in sync with MOSFET Switching.
How can I arrest these 300 mV sharp spikes?

Details of ckt are :
Half bridge topology, IRF450P MOSFETs, 3825 PWM Controller driving a ferrite core trans 1:1:1.
Transformer & Inductor with 2 separate planar cores.
Primary transformer snubber of 51 Ohms & 1000 pF.
MBR3060 diodes (2 devices) for output, with 10kPF & 10 Ohms snubber across each diode.

I have changed the freq fm 40 kHz to 100 kHz, tinkered around with doubling / halving both the IRF450 / MBR3060 Snubber caps & resistors, with no change in the spikes.

Can someone help me?


Joined Apr 19, 2007
Have you got chokes in series with your outputs after the diodes and capacitor filtering ? Daniel.


Joined Jan 28, 2005
Can you post a schematic of your circuit so that we can better help you diagnose you problem?


Thread Starter


Joined Mar 8, 2007
Thank you very much lighingman & hgmjr. I have not looked at the forum for sometime, and, I am really glad for the help.

I am using a choke & Capacitor filter, but the choke value realised was only 4 uH instead of the 20 uH initially planned, but on recalculation appeared to be ok.
I am attaching the schematics for the Power supply. Transformer T2 is a planar core transformer with primary of 7 turns, and, secondary 2T, 1T, 1T, 2T. The middle 2 Nos. of 1 Turn are for power in centre tap configuration. The outer 2 Nos. of 2 Turns alongwith D10 are for future testing synchronous rectifier scheme using MOSFETs as rectifiers instead of MBR3060 rectifier, and, not used. The secondary turns are in PCB tracks.

The 3825 PWM controller circuit powered by a benchtop power supply of +15V. I have not drawn this ckt as I have just assembled it on a breadboard with the datasheet. I will try to draw the ckt and post it soon.

Thanks again for offering me help.