Design help needed for thermal issue in LNK306D based design.

Posted by: jkaramchandani… on 03/12/2014

Hello PI support,

We are working on LNK306D based design with 12VDC, 360mA output and 95VAC to 300VAC input. We are using design components and values as per application notes AN-37 and attached spreadsheet. We are using opto based design as per attached schematic.

During prototype testing (attached PCB layout in pdf) we found LNK306D goes in thermal shutdown at 300VAC input at 350mA load current after few minutes. At lower loads we don't have the shutdown issues. Pertaining to this issue we have following queries,

1. Is there any equation to calculate power losses in LNK306D IC to know amount of copper pouring needed? Datasheet already specifies thermal impedances.

2. There is one parameter PLOSS_LNK inside spreadsheet, can we get near exact results using this parameter and thermal impedances specified in datasheet?

3. In datasheet on page-9, thermal impedance is specified with some copper clad area. Pl advice if this copper pouring has to be on top/bottom layer or internal planes will do?

Awaiting your valuable support,

Thanks,

jagdish

Files

Layout of LCSLC SMPS_V.1.pdf (19.35 KB)

LinkSwitch-TN Buck Design-360mA.pixls (12.64 KB)

Schematic Prints_14.pdf (44.99 KB)

Comments

yes you can use the equation in the pogram "PLOSS_LNK "

I will find out the eauation today later. But yes you can use PLOSS_LNK results

Hi Worf,

Thanks for your response. Meanwhile I shall appreciate if you can reply for point-3 in my original comment regarding location of copper pouring area in top/bottom or internal planes.

Awaiting your support.

Jagdish Karamchandani

Technical Architect (R&D)

Cimcon Software India Pvt. Ltd.

+91-079-26578639

Fax: +91-079-26578659

The copper clad was on the bottom side

Hi Worf,

OK got, as per your response we have tried to pour ~350mm2 in our PCB for SMD package. We are considering ambient temp of 60'C and output of 12V at 350mA. PLOSS_LNK in spreadsheet is 0.56W. So IC temperature in worst case with thermal impedance of 100'C/W (from datasheet) comes to near 60'C+[0.56X100 ]=116'C.

We have spread copper clad on top as well as bottom and have connected them with vias. Pl find attached .PCBDOC in zip format. Pl refer SMPS section in that.

Do you think we still need to increase copper are or 350mm2 is sufficient as per our application?

Awaiting your response.

Thanks,

jagdish

Jagdish Karamchandani

Technical Architect (R&D)

Cimcon Software India Pvt. Ltd.

+91-079-26578639 Fax: +91-079-26578659

This is what I would do if I were you. May not be the answer you expec but is is the most practical answer.

Build a prototype. Measure the temperatures. Then you will find out if you need more copper. This may be not the most proffesional answer but is the most acurate. Thermal models will be acurate if you have all the variables in the model. But very often something is missins. You still need to build a prototype anyway.

Hi Worf,

We always build prototype and test the SMPS but this is something we are taking your inputs based on your experience and expertise. It will help us to narrow down our prototype runs. Since there is no exact thermal equation, I had to rely on PLOSS_LNK, a person from PI can only know how much accurate it is and hence we took your help.

Jagdish Karamchandani

Technical Architect (R&D)

Cimcon Software India Pvt. Ltd.

+91-079-26578639 Fax: +91-079-26578659

It is not an easy task to get a precise results for thermal issues when you need to calculate the thermal impedance from heat sink to ambient. If I try to do it I will need to justify the time with my boss. You can try by using the emissivity of the copper plate.

In the past I have been used the below link to calculate the area of copper plate with fair results

http://www.daycounter.com/Calculators/Heat-Sink-Temperature-Calculator.phtml

Keep in mind that this is not a PI document and that if you decided to use it is under your own risk

Hi Worf,

Thanks for your response, we shall make the prototype of PCB and make measurements. Will take your support if needed later.

Jagdish

sure

Hi Worf,

Pl note that we are working on the prototype of PCB in which we faced thermal issues. We have certainly seen improvement in thermal performance with DIP package and increasing cooper clad area but we are still evaluating the design for exact thermal improvements.

1. Meanwhile I noticed one issue during testing that output of our circuit goes in restart mode across 150VAC to 207VAC input range at full load of ~350mA. I don't know why is such behaviour?

2. We have designed circuit of 12VDC at 360mA as per PI software. Below 150VAC and above 207VAC output is fine even with 350mA load.

3. If I reduce load to 320mA instead of 350mA then output works fine even in 150VAC to 207VAC range.

4. Contrary if I increase inductance from 1500uH to 2700uH then also output works fine even with 350mA load across 150VAC to 207VAC range. PI software design gave 1500uH value, 2700uH is random value just kept to check performance. We need to tune to find exactly how much increase is needed.

Can you pl help us out of this trouble as to why such kind of behaviour is seen? I believe if circuit works in worst case of 90VAC input at 350mA then it should also work even across 150VAC to 207VAC.

Awaiting your help.

Jagdish Karamchandani

I suggest you hook the current probe to the DRAIN node. Then see your the behavior of the switching and the peak of the current

As you change the voltage see how the bevhavior changes. see what happes when the power supply goes into autorestart. pay attention how often the controller skip cycles.