LM5045: The value of SS cap and Rd resister

user1257299

Prodigy 90 points

Part Number: LM5045

Hi Expert.

Ｉoperating the LM5045 open loop, with no feedback.

I have 2 questions.

Q1.I have heard that the recommended range of the SS cap is 0.1uF to 1uF .

I change SS cap is 1uF.But inrush current is large,(inrush current is peak 10A)

I want to suppress. Could I set a higher value? (ex. 2.2uF)

Q2. I operate LM5045 open loop, with no feedback. I think secoundary dead time is 0sec.

Could I set Rd resister is 0Ω？

over 5 years ago

0 Peter Meaney over 5 years ago

TI__Mastermind 31580 points

1. can you check if the inrush current is caused by the application on the input voltage source to the input capacitance or if it caused by the ramping of the output voltage charging the output capacitors. Please post the waveforms so we can review them.

2.Section 7.3.13 on page 18 of the datasheet gives the formulae for calculating RD1 and RD2 to set the delay times between the primary signal gate drives and the SR gate drives. I would advise setting the deadtime at a 200ns as a starting value and then reducing this value depending on the delays in the SR signals crossing the isolation barrier and delays in the gate drive circuits.

Regards

Peter

0 user1257299 over 5 years ago in reply to Peter Meaney

Prodigy 90 points

Hi Peter

2234.WAVE.pdf

I answer 1st question. Please cheak the wave form.

Regards

0 Peter Meaney over 5 years ago in reply to user1257299

TI__Mastermind 31580 points

I think the waveform looks normal, what inrush current specification do you have to meet ?

The magnitude of the inrush current will also depend on how much output capacitance is fitted to the design. What is the input current value when the unit is operating at full load and minimum input voltage?

Thanks

Peter

0 user1257299 over 5 years ago in reply to Peter Meaney

Prodigy 90 points

Hi Peter

The output cap of this circuit is 700uF. I think there are many.

But I would like to reduce inrush current about 5A.

I think SS cap increase. (2.2uF is the limit)

How do you think?

Regards

0 Peter Meaney over 5 years ago in reply to user1257299

TI__Mastermind 31580 points

I think 2.2uF should be ok.

Where do you see 2.2uF as the max value for the SS pin capacitor in the datasheet?

Thanks

Peter

0 Peter Meaney over 5 years ago in reply to Peter Meaney

TI__Mastermind 31580 points

It has been over a week since my last response. I hope you have made progress on this issue and I am going to close the post.

Please open a new post if you have more issues.

Regards

Peter

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LM5045: The value of SS cap and Rd resister