LNK3207

Posted by: Kelvin Cheng on 09/03/2024

Dear Sir/Madam,

LNK3207(High-Side Buck – Direct Feedback, 5V output) will be used in my new product development and now we are considering the light-load power.

I tested that even I did not connect any load at 5V output sides, the input is still dissipated ~0.09W. May I know how i can reduce the input power / improve no-load & light load efficiency?

Many thanks

Kelvin

Comments

Hello Kelvin,

Good day and thank you for choosing Power Integrations.

For a 5 V output, one way to significantly reduce the no-load input power of the design is to implement an external bias resistor (Rex) that is highlighted in the attached image to externally supply the LinkSwitch-TN2 device. Please refer to page 16, "5 V Output With RBIAS" and page 12, step 10, of the LinkSwitch-TN2 design guide for more detailed information.

A good starting value for a 5 V output is 1.5 kΩ.
To verify if the LinkSwitch-TN2 device is being supplied externally at no-load condition, the BPP voltage across capacitor CBP should be equal around 5.2 V to 5.3 V. If the voltage across the BPP capacitor (CBP) is only 5 V at no-load condition, then you need to decrease further the value of the external bias resistor until the BPP voltage is equal to 5.2 V.

It would be really great as well if you could share your schematic diagram with us, so we can review and give feedback on what may be improved further to reduce the no-load input power consumption of the design.

I hope that the information above helps. Thank you.

Regards,

Tommy

Hi Tommy,

Thanks for your quick response! I have already add the external bias resistor in my circuitry.

Please find my circuitry in attachment

Many Thanks,

Kelvin

Attachment	Size
Screenshot 2024-09-05 092831.png (47.01 KB)	47.01 KB

Hi Kelvin,

Thank you for sharing the schematic.

Can you check the voltage across bias capacitor C1 at no-load condition? The voltage reading should be around 5.2 V to confirm if it is externally supplied. If the voltage reading is 5 V only at no-load condition, please reduce further the resistance value of R1 to around 1.5 kΩ and check again the bias capacitor voltage will become 5.2 V.

Please refer to the following points below for some additional suggestions to reduce the no-load input power consumption.

Replace the fusible resistor (R14) at the input section with a fuse.
Remove both L2 and L4 since the current design has already implemented a PI filter (consists of C5, C6, and L3). Please do check the EMI performance of the design and ensure that it is still passing after removing L2 and L4.
Choose an inductor with low DCR value for both L1 and L3.
Increase and tweak further the value of pre-load resistor R5. According to the LinkSwithc-TN2 design guide, a good starting point for the pre-load resistor should be selected to give a minimum output load of 3 mA at no-load condition. However, the value of pre-load resistor (R5) should be optimized further such that the no-load input power is within the acceptable requirement and the output voltage at no-load condition is within the tolerance requirement.

I hope the above suggestions helps with your no-load input power. Looking forward to the votlage measurement across the bias capacitor C1.

Regards,

Tommy