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LinkSwitch-HP Family Power losses increase at high temperatures

Posted by: Umaraani Subramanian on

Dear Sir,

We are using LNK6769E for 33W output SMPS. Our device is an adapter with IP65 enclosure.
At ambient temperatures it works well.If we place our device in temperature chamber and test,over temperature shutdown happens.
Losses is 8W. We designed heatsink for this losses. But at higher ambient temperatures losses increase to 16 W. Then the heat sink size is not enough.So,over temperature shutdown happens.

Please provide Rdson curve for various temperatures and the MOSFET dynamic parameters,capacitance,gate charge values to calculate switching losses.
Please assist us in solving this issue.

Thanks,
Umaraani Subramanian.

Comments

Submitted by PI-Mallora on 08/13/2020

Hi Umaraani Subramanian,

We only have data at junction temp 25°C (1.3Ω typ, 1.7Ω max) and 100°C (2Ω typ, 2.6Ω max) in page 16 of the specs. Did you set your current limit at 100% setting? Reducing the current limit setting might help if you can still deliver the full power. You can check the chip consumption including gate drive losses by putting a resistor and measuring measuring the current with voltage drop across. With that you can monitor how it is changing with load and ambient temperature.

Submitted by Sakthivishnu on 08/18/2020

Hi Mallora,

Thanks for your response. In continuation with #1 from Umaraani, our designed output is only 33W and the current limit is programmed to 40%(minimum programmable as per datasheet). Even then we could see an increase in losses when the temperature increases more than 70C. Looses at 100C - 12W, which will be the operating temperature of LNK at 50C ambient.

1. Is there a way to program current limit below 40%?
2. What the reason for the increase in losses(not linear) when the temperature increases?
3. Is LNK6769E is designed to work on the current limit setting?

Thanks.

Submitted by PI-Lupin the Third on 08/18/2020

Hello Sakthivishnu,

Good Morning.

Thank you for reaching out to us through this page. We would definitely help you in your inquiry. However, if may I request, please create a "new thread" for this. If you think your inquiry is somewhat related to this post then you may include this link.

Looking forward on your new post.

Thanks and Regards,

PI-Lupin the Third

Submitted by Sakthivishnu on 08/19/2020

Hi Lupin,

I find difficulties in creating a new post so I am writing here. I and Umaraani are from the same organization and Umaraani could not reply to the existing post too.

Please do respond to the above queries in the same thread. We need to understand the reason for higher power dissipation at the earliest. It will help us a lot.

Thanks,
Sakthivishnu

Submitted by PI-Mallora on 08/26/2020

Hi Sakthivishnu,

If you are already on the minimum current limit, it might be the switching loss and gate drive loss that is causing thermal shutdown. Do you still have margin on your transformer to operate in higher current limit/lower switching frequency. Try 60% or higher and check if you can see improvement. Just watch out for your transformer saturation.
The switching loss might be the bigger cause of increase in package temperature then RDSon increase with I^2R dissipation plus increase in ambient caused it to reach thermal shutdown. The current setting is giving you the freedom to explore between switching loss and conduction loss. Thanks.