LNK3296G-TL with different output frequency.

Posted by: victorfg on 11/27/2020

Hello.

Recently I acquired, via Digikey, some LNK3296G-TL to build a 15V~5W power supply. I'm using a typical buck topology and I follow the application note and datasheet to determine my resistors/capacitors values.

The converter is regulating the output at 15VDC as designed. However, the operating frequency is between 22kHz and 16kHz. I know that the variation is because of Jitter but I don't understand the low frequency. It was supposed to be around 66 kHz. I tried to vary the feedback and external resistors but without success.

My input source is set to 100 VDC and the inductor has 1.2mH and I am using a 68-ohm resistor at the output leading to a 3W consumption.

Could you help me solve this problem?

I send the schematic, the pcb, and the waveforms attached. In blue the inductor's current, in green the Voltage Drain to Source (VDS), and in red the output voltage.

Files

Attachment	Size
schematic.JPG	43.93 KB
pcb.JPG	91.22 KB
20201127_120202.jpg	2.42 MB
20201127_120043.jpg	2.47 MB
20201127_120123.jpg	2.54 MB

Comments

Hi victorfg,

Thank you for providing the design files.

What you are observing is normal behavior for LNK-TN2 family of ICs. LNK-TN2 actually has ON/OFF feedback control, instead of traditional fixed frequency PWM control. The 66kHz in the datasheet is the maximum frequency for this device. You can read more about feedback and ON/OFF control in the device datasheet (Functional Description, page 3).

You can do a quick calculation of output power using the scope measurements and P=0.5*L*I^2*Fsw=0.5*1.2mH*(640mA^2)*16kHz > 3W, so this operating frequency is realistic.

I also noticed that you can still reduce the inductance which can improve your efficiency. For that, I recommend you use the PI Expert online tool which can provide you with optimal component sizing for your design.

https://piexpertonline.power.com/

Hi Jango,

Thanks for the help.

My biggest problem was the audible noise that I thought was the inductor frequency oscillation coming from the ON / OFF control.

I tested it with a higher inductance value to take it out of the discontinuous conduction mode (approx. 2mH) but the average frequency decreased, keeping it in the same conduction mode.

Finally, I reduced the inductor to 500uH, and I got variations with higher frequency and the audible noise stopped. I believe that the inductor was close to saturation in my fast design.