transformer for lnk362

Hi;
Did you tried full wave rectification with 4 diodes?

Hi.


It looks like you are using a very slow diode on the output (500ns). That will be a problem as you can see here (be sure to click "show answer" for a detailed explanation).


You may also require a clamp to limit the peak Vds (Drain to source voltage) of the Mosfet. There are designs that can be clampless, but a careful transformer design is required.


I cannot give you a final answer regarding the transformer because the datasheet only specifies the turns ratio and not the actual number of turns (important for flux density). I believe Wuerth Elektronik should be able to give the answer since they know their transformer in detail, but I suspect that the issue is the clamp and output diode.


I suggest that you take a look at the application note for the LNK-XT for some design tips regarding transformer, diodes, capacitors, clamp circuit, etc.

a

Can you send the layout of the pcb?
I checked the leakage and it seemed it is a bit high. This may cause a damage on internal Mosfet
Try adding a P6KE200 and UF4007 clamp; and use a Vf=0,5V schottky (SB240) for output rectifier.

Hi
i used development board.
i will try to use UF4007 with resistor +capacitor, becuse
i dont have the P6KE200.

also i will replace the output diode to 1n5819.

look in the attached file please.
if you can to help me to found the valu of the resistor and capacitor.

Thanks

Hi;
using a resistor and capacitor may not clamp the voltage at the required level.Try this:connect 5 zener diodes (30V) in series to get 150V zener instead of P6KE150 to clamp Vds.

Determining the clamp resistor and capacitor is a struggling issue,I am also working to determine for a 12W SMPS. But if you do not need a very clean,Clamp diodes UF4007 and P6KE180/200 will work for you

i ordered the parts from DIGIKEY
when i will have the diode i will tell you the result.

many thanks for your support.

Eyal

Hi Eyal;
You told that you were using development board.I guess you had changed some components of the board,am I right?

Also I want to ask that could you get an output with the WE transformer?or LNK362 burned or something else?

Did input resistors,8,2 and 100 ohm were burned?

Did any load was connected to output before power on?If yes how much Amps did you tried to source?

Regards

Hi
i dont change parts is the same part numbers like in the circuit.
i only build a diode bridge 4x1n4007.
after i replaced yhe ic's lnk362 + pc817:
in the firat time when i power on the output is 3.78V
if i add load of resistor 100R the output is the same.
but when i power off and on again after 10sec the 100R(R25)
burned, and the lnk362 + pc817 finished.
i tryed to change the feedback resistor R27 to 330R + trimmer 100R
and no effect on the output voltage (only ~50mv).

when i will gat the parts from digi key i will try again.
i did few change in the circuit please look on it

Hi
Schematic seems ok but if you change the secondary side diode to Vf=0.5-0,7 V diode (1N5819 as you told) it would be better.

Also increase C11 to 2,2nF 1kV capacitor,but this value can change as a result of EMI

you can also change R24 to 100R 0.5W and R25 to 1mH bobbin

Add a second capacitor to the output to GND (330uF -470uF is enough)

Are you sure that any short circuit was not happened when you power on the circuit again?

Also can you measure the burnt LNK362 Rds resistance?

Here my theory:Due to leakage,a high voltage spike that is near to Vds breakdown voltage has occured on internal mosfet.When you increased the load,this voltage sipke also increased and damaged the mosfet but somehow it did not burned the IC on first time .But when you powered the circuit with load,this voltage spike increased too much due to start up and burned the IC.

Hi

i got the parts , and now its work .
but the fixing voltage no so good,
without loading the voltage 5.05v ,with load of 200ma the voltage drop to 4.5-4.8v,
some time when i power on the voltage 4.9v.

look in the attached file for the value of the resistors R26,R27,D12.

many thanks for your support.

eyal

Hi ;
If you want to get ~5V output; try changing D12 with 4.7V zener diode
You can try adding an LC filter to output.see attachment.

Change 330uF capacitance (C12) with 1000uF electrolytic (10V will work for you)

hi, ı use p3 schematic but I use D11 and D13 UF4007 and I use 3,3uH for R25 and output inductor. Unfortunately,i cannot look 5V.When i measure output without load ı look 15V but connect load voltage drop and it is changing like saw (0 to 5 V).

Hi.

Please be aware the transformer (assuming Wuerth 749118105) on schematic P3 is incorrectly placed. You need to connect pin 1 to VDC+, pin 3 to drain, pin 4 to anode of output diode, and pin 6 to the output GND.

UF4007 for D11 and D13 should be ok.

Hi, Thanks your interest,

I change connection of secondary and ı see demand output,