LNK306Gn HELP

Posted by: gaetan.osvald on 04/21/2009

HEllo,

I have design a simple test PCB for test the LNK306GN device in buck topology with direct feedback.
I want to use this device in a high temperature application Tambient +105°C.

The Voltage input is DC from +70V to 200V
Vout : +15V 200mA

I have use PIXls software to made the calculation.(see attached file for the schematics and layout)

I put 120V dc input with 4,3Kohms resistor load for start my test.
I got 10,8V output (Vfdk = 1,5V).
When I put 120 Ohms resitor load I got 0,3V output (Vfdk = 0,8V).

Why this chip don't made the regulation?

Files

EV016-EVALPSU-A01_Sch.pdf (20.78 KB)

EV016-EVALPSU-A01_layout.pdf (21.18 KB)

Comments

The inductor value recommended for 15V, 225mA output is 1200uH.why dont you try that value.?

I have try with 1000µH but I got the same behaviour.
I will try with 1200µH and change the chip.

Thanks.

What is the value of the free wheeling diode? whether the forward voltage drop of the feedback diode & free wheeling diode are same?

The freewheeling diode is a USB260 from vishay

VF= 1V
trr= 30 ns
IF = 2A
Vrrm = 600V

why dont you try with the same diode for both free wheeling and feedback? is the resistor voltage divider is maintaining 1.65V in the feedback pin?

I have try with the same diode and no change.
The feedback pin stay between 1.0V and 1.1V

Can you adjust the resistor value to make it 1.65V?

Yes I can but I don't understand why I don't get the good value with the 15K resistor.

Am seeing the soft start capacitor is reversed

when you are connecting full load, whether 5.8V is there in BP pin?
why dont you connect 0.1uF in BP pin to source?

try reducing or increase the value of feedback capacitor

OHHHHHHHH I made a mistake with the feedback capacitor, it was a 100pF capacitor on place of 100nF....

I will try this tomorrow.

Thanks.

Is your supply now working and preforming as desired?

Yes Now I get the good output 15v in all load the conditions.

Thanks you very much for this help.
@+

What inductor value are you using now?

Now I use a 1000µH inductance.

I've made the same circuit as you for a little less current (120mA) and sometimes the LNK just dies or explodes! I've checked the voltages andthe BP pin is receiving 5.8V but up to 12V of overshoot. This is above the 9V of the absolute maximum ratings list. Can this be the reason?? We are about to enter production of 100k annually and this issue is about to become a job stopper.

Can anyone help me???