LM2678 Output overvoltage problems

Mauro Gatti

Prodigy 40 points

Other Parts Discussed in Thread: LM2678

Good evening.

I designed a supply-stadio with LM2678-5. I used the follow configuration:

Vinmin = 10,

Vinmax = 40v

Vout = 5v

Iout = 5A

I used WEBENCH tool to calculate the components.

I can have different load so I can have output current variable from 1.0A to 4.5A.

When the load requires low current (1.5A .. 1.8A), When Vin>30v, I have a spike overvoltage (aboout 7v for 100ms) at the moment when I turn on the device.

This overvoltage, often, breaks the LM2678 and they sometimes breaks the load also.

I tried to change the inductor value, the input and output capacitor but I have not found solutions.

I tried to insert the delay between the Vin voltage and ON/OFF signal, but I don’t found solutions.

Do you have some suggestions for me?

Thank you.

Mauro Gatti

over 10 years ago

0 Alan Martin over 10 years ago

TI__Expert 4365 points

Hello Mauro,

The input capacitance value for the circuit is too small and the lead inductance on the input power leads results in input side intability or oscillation that cause the IC input voltage to swing above maximum rating. Place a 50V rated aluminum electrolytic capacitor in parallel with the two 10.0uF ceramic caps that are given by the Webench application. Choose a capacitance value of 150uF to 270uF and that should resolve the issue.

Alan Martin

0 Mauro Gatti over 10 years ago in reply to Alan Martin

Prodigy 40 points

Hello Martin and thanks for your answer.

However, to night I try to insert threee 1nF capacitor in parallel to catch diode. This has solved the problem.

0 Mauro Gatti over 10 years ago in reply to Mauro Gatti

Prodigy 40 points

I try to use a 220microFarad input capacitor (without m,y capacitor in parallel to diode) but the problem don't solved.

0 Alan Martin over 10 years ago in reply to Mauro Gatti

TI__Expert 4365 points

What is the part number of the catch diode you are using?

Alan

0 Mauro Gatti over 10 years ago in reply to Alan Martin

Prodigy 40 points

The diode is a SB540 in DO-204 package.

0 Hoa Nguyen4 over 7 years ago in reply to Alan Martin

Prodigy 70 points

Hi Alan,
I was using the webbench to build the DC-DC converter by using the LM2678T-5V. What i have for the C-in is 470uF Al cap coupling with 0.1 and 0.01uF ceramic cap. On the output side, i am using the 14.8uH-5A-27mOhm, with a Cout of 330uF tantalum cap coupling with 0.1 and 0.01uF ceramic caps.
The problem i have is the Vout is in the range of 8.2-8.4V. I have tried to insert three .01uF in parallel with the diode, the output remains in the range of 8.2-8.4V.
Can you give some suggestion?
Thank you for your help.

0 Frank De Stasi over 7 years ago in reply to Hoa Nguyen4

TI__Guru 57555 points

What is your load? and input voltage? This device may need a few mA of load under certain conditions.

0 Hoa Nguyen4 over 7 years ago in reply to Frank De Stasi

Prodigy 70 points

Thank You Frank for your help. The unit does required mA load to output correctly.

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LM2678 Output overvoltage problems