LM2576: How to design a heat sink?

Ras Sharif50

Part Number: LM2576
Other Parts Discussed in Thread: LMR14030, LMR16030

Dear All,

Please consider an adjustable version of LM2576 supplied with a 24V or 15V ( if possible ) at input and the output= 13.2V @ 3A.

The ambient temperature range: 25-60 degC

If input voltage= 24V, the dissipation power= 2.5W and for 15V is 3.9W.

Please suppose the maximum of junction temperature= 125 degC.

For example: 60 degC

24V: the junction-to-ambient thermal resistance= 26 degC/W.

15V: the junction-to-ambient thermal resistance= 16.7 degC/W.

Should I to place a copper area on the PCB to sink the heat?

How can I calculate a heat sink embedded in the PCB?

Best Regrads, Ras

over 5 years ago

0 Andy Chen over 5 years ago

TI__Mastermind 27825 points

Hi Ras,

Can you take a look at our new generation converter like LMR14030 and LMR16030 which offer you much more compact solution size?

0 Ras Sharif50 over 5 years ago in reply to Andy Chen

Genius 3455 points

Hi Andy,

Thank you for the reply.

No. I should to use LM2576.

Regards, Ras

0 Ras Sharif50 over 5 years ago in reply to Ras Sharif50

Genius 3455 points

In the figure 17 of the datasheet, maximum of the dissipation is 2W at 60 degC ( ambient temp. ) and the lowest accessible of 32 degC/W heatsink made with 1.6 square inch is suitable ( and more plane gives no extra sink capability ). This thermal resistance calculated with a 4-Layer PCB.

If I use a two layer PCB and connect double sides with narrow vias, does it meet my request to achieve for example 26 degC/W?

Regards, Ras

0 Daniel Li14 over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27180 points

Hi Ras:

pls refere below apps notes for thermal consideration (TO-263 and TO-PMOD pacakge)

http://www.ti.com/lit/an/snva424a/snva424a.pdf

for 2layer , the apps notes introduce in the figure 4 of this apps notes.

below is the heatsink serano for refer.

thanks

Daniel Li

0 Ras Sharif50 over 5 years ago in reply to Daniel Li14

Genius 3455 points

Hi Daniel,

Thank you for the app note. It helped me to understand better the thermal effect of pcb on smd switchers.

I need an approximate formula to calculate the area required for my application.

Can you please give me a rough equation to calculate the area?

Or if not possible, can you tell me how much area is required for my application ( 26 degC/W )?

Regards, Ras

0 Ras Sharif50 over 5 years ago in reply to Ras Sharif50

Genius 3455 points

From figure 4 in snva424a.pdf, a 8.5x5.4 mm bellow of the LM2576 and 25.4x25.4 mm on bottom layer yields 28.3 degC/W. ( 1.07 square inchs )

On the other hand, figure 17 in datasheet illustrates 32 degC/W for 1.6 square inchs. ( 4-Layer )

These values do not have proportion?

Regards, Ras

0 Andy Chen over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27825 points

Hi Ras,

Per our estimation on 2 layer 1oz PCB, it require 2x2 inches cooper area on top and bottom layer for ThetaJA round 23C/W, it also require 12 vias with 12mil diameter on thermal pad.

0 Ras Sharif50 over 5 years ago in reply to Andy Chen

Genius 3455 points

Hi Andy,

Please consider three values:

Figure 17 in the datasheet says 1.6 square inchs corresponds to 32 degC/W.

Figure 4 in the snva424a.pdf says 8.5x5.4 mm+25.4x25.4 mm ( 1.07 square inches ) corresponds to 23.8 degC/W.

You estimates 4 square inches for 23 degC/W.

I have tow question:

1) Which of the above value is true?

2) How do you estimate?

Regards, Ras

0 Andy Chen over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27825 points

Hi Ras,

Pls let us know the full P/N of LM2576 you used.

LM2576 data sheet Fig10 is for Efficiency vs Vin, can you double check or download latest data sheet on TI website?

0 Ras Sharif50 over 5 years ago in reply to Andy Chen

Genius 3455 points

Hi Andy,

Thank you for the reply.

Full product number: LM2576S-Adj

Excuse me for wrong Fig. number. The correct is Fig. 17 in page 10.

Regards, Ras

0 Andy Chen over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27825 points

Hi Ras,

LM2576 data sheet Figure 18 says 1.6 square inchs corresponds to 32 degC/W. I can verify the result with below conditions:

Power Dissipation: 3.5W

Ta=25C

Theta_JC=2C/W

2 layer PCB, size 3x3 inch, 1oz copper

Copper heat sink on top and bottom: 1.3x1.3 inch

Vias on thermal pad: 3x5

Calculated Theta_JA is 33.4C/W which is close to 32C/W.

I estimate 23C/W based on below:

Power Dissipation: 2.5W

Ta=25C

Theta_JC=2C/W

2 layer PCB, size 2x2 inch, 1oz copper

Copper heat sink on top and bottom: 2x2 inch

Vias on thermal pad: 3x4

Figure 4 in the snva424a.pdf says 8.5x5.4 mm+25.4x25.4 mm ( 1.07 square inches ) corresponds to 23.8 degC/W, I don't find out more detail conditions as above to verify it.

0 Ras Sharif50 over 5 years ago in reply to Andy Chen

Genius 3455 points

Hi Andy,

Thank you for the explanation.

I need a formula or a simple simulation tool which using it I could calculate the values for various conditions.

Regards, Ras

0 Andy Chen over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27825 points

Hi Ras,

Pls send me email at andy.chen@ti.com, I will send you the calculator.

0 Ras Sharif50 over 5 years ago in reply to Andy Chen

Genius 3455 points

Hi Andy,

Thank you for your honest cooperation.

I sent you the email.

Regards, Ras

0 Ras Sharif50 over 5 years ago in reply to Ras Sharif50

Genius 3455 points

Hi Andy,

I am still waiting for your email.

Regards, Ras

0 Andy Chen over 5 years ago in reply to Ras Sharif50

TI__Mastermind 27825 points

Hi Ras,

Do you have other email box?

I already sent email to you but it bounced as below:

Delivery is delayed to these recipients or groups:

r.sharifi@alumni.iut.ac.ir

Subject: RE: [EXTERNAL] Ras email address replying to TI E2E Power management forum: LM2576: How to design a heat sink?

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