Help with audible noise with INN2904K

Posted by: daniel_bezerra on 07/07/2017

We're developing a tiny 12V@1A - 80-280Vac for a client, and we decided to use the INN2904K and most of the hardware is based on the already functional Reference design RDR-531.

Unfortunately as is the first time I'm using this chip (and don't have any avaliable model to simulate it, I need to gess many and many stuff with it ***) an audible and persistent problem came.

So, after many days trying to figure out, i'm asking for some help with this layout (single and even double sided board)/schematic, in order to supress this terrible audible noise - BTW many of the components are the same of the RDR-531.

Attached: The schematic; the single side board; the double side version.

Thanks for any help.

Luiz Daniel.

Files

Composite Drawing_v01_doubleLayer_PB073A.pdf (24.81 KB)

Composite Drawing_v01_singleLayer_PB073A.pdf (23.48 KB)

Schematic Prints_v01.pdf (22.49 KB)

Comments

Hi Daniel,

Just want to confirm, is your transformer varnished?

PI-AP

No, we didn't varnished it, just dropped the 5V secondary (from the Application Report) and gapped it with the same RM8 core and 380uH primary inductance.

Hi Daniel,

Please varnish the transformer as that reduces the audible noise significantly.

PI-AP

Now I remember that I dropped the shield winding...

I'm going to rebuild another transformer with some changes on the primary winding (the end of the primary winding will be the +Vcc in the hope it will shield the high dV/dt on MOSFET's Drain), also including the finished varnished transformer, and the shield winding.

Hope it will work.

Varnishing the transformer helped much! The noise is gone!
Now I have a start-up issue... If the power supply is connected under load (12R@12V) to the grid (250Vca) it tries to start with bursts of ~640us pulses and doesnt start at all. But if I lower the grid voltage (90Vca) it starts normally under load. Any tips?

Could you try lowering the R8,R9,R12,R14 resistor values from 2.4 Mohm to say 2 Mohm each and see if that helps?

Unfortunately is still erratic on the high voltage > 200Vca startup. Maybe inserting a tiny capacitor (like 1000pF or 330pF) from V to GND may help (i think is a high frequency noise going into V-pin and it could shutdown the start-up procedure)?

Tested with a tiny capacitor from V to GND: 330pF and didnt worked, the start-up still erratic, and only if the voltage is < 130Vca it starts without any problems (under or without load). If the voltage is higher than 130Vca, even without load it still starts erratically.

Can you please try disabling the V pin ?

Also is there an overshoot in the output voltage during the burst attempts?

Can you check how much is the bias winding capacitor voltage? At high line, if it might be injecting more than needed current then also it would not startup.

You can also try replacing the SRFET with just a schottky diode to see if it helps starting up at high line.

PI-AP