TOP255PN

Posted by: dayatin on 08/27/2012

with ref to attached schematic, output starts oscillating at 50% load, dont no why? need help, even though heatsink is installed.

Comments

The first thing I think of when I hear "I have an oscillation" is that something isn't setup correctly with your control loop.

Have you verified your power supply control loop and asessed your gain/phase margins at different input voltages and load conditions?

Also, I can't see your schemetic very well. Please create a PDF and attach that directly. Also if you have you PI Expert design files, attach those. If not, please give me the input and output specs of your design as well as the documentation on your transformer construction.

-The Traveler

5V out put dips to zero suddenly when i apply 60% load, it works fine upto 50% load, above which output dips, i have attached pdf file of my schematic, and transformer design files. I dont know what is the problem.

Primary Inductance

1194.523985

μH

Core Type		EF25
Core effective cross sectional area	Ae	0.525	cm²
Core effective path length	Le	5.75	cm
Core ungapped effective inductance	Al	1850	nH/turn²
Bobbin Physical Winding Width	BW	15.1	mm

safety margin	M	3	mm
effective bobbin width	Bwe	18.2	mm
outside diameter of the primary wire	OD	0.259518519	mm
insulation thickness	INS	0.0486016	mm
Bare conductor diameter	DIA	0.210916918	mm
American wire guage	AWG	31.69938734
effective cross sectional area in circular mils	CM	68.60321203	Cmils
	cma	211.9673499	Cmils/A
current capacity for magnet wire	CMA	212.5926727	Cmils/A
maximum flux density in the core	Bm	2451.147174	Gauss
AC Flux Density for Core Loss Curves (0.5 X Peak to Peak)	Bac	1225.573587	Gauss
core gap length	Lg	0.2360653	mm

You feedback circuit components seem generally correctly.

I did notice that your bulk caps on the 5V output seem a bit low. You have 660uF of caps...I would try doubling that.

Also, with the turns ratio you have, primary inductance, etc, you might be in saturation territory for that transformer. I would double check your design and verify drain current and voltage waveforms during operation.

-The Traveler