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Inductance oversized

user3757022
Prodigy 40 points
Other Parts Discussed in Thread: LM2576, LM2696, LM2596

Hello, this is my first time because I really need help.

I'm trying to build a voltage regulator for tattoo machines. I'm almost cloning a commercial power suply and the regulator uses the LM2696 or LM2576 circuit. (I don't know wich component they are using because the reference is removed with a sandpaper.

I have identified the circuit but when I tried to design the circuit choosing the value of the aditional components, I realized the commercial power suply is oversized. The specifications are: Vi=24v Vomax=20V Imax 3A, with those specs I calculated a Inductance about  22uH, but the commercial suply inductor is about 90uH.

Also the capacitors are oversized: 470uF in the commercial and less for the theoretical value but this is less important, in my opinion.

I would like to understand why 90uH (real measure is about 87uH and nearest commercial value is 92uH, Würth elektronik 7447013 , but I can't get it in RS neither mouser etc..). I'm trying the circuit with other inductances and I don't see significant differences between 30uH and 150uH, but just connecting the tattoo machine, I don't have a oscylloscope. I want to know if I can put a 50uH or 150uH inductance and the possibles issues.The ircuit is the page 9 (second circuit) and it doesn't have capacitor in the Feedback.

I just want to know if it is the same with 50uH or 100uH (not any value, just more than the theoretical calculated in the table)

This is the schema of the commercial power supply

Thank you very much.

  • 0
    TI__Genius 11085 points
    We are out of office on a holiday today. We will get back to you tomorrow.
    Best regards, Yang
  • 0
    TI__Genius 15810 points

    Hello,
    You mentioned you do not know which device was used in the schematic and there lies the confusion. The LM2596 and LM2576 have different switching frequencies which will change the value of the required inductance. The LM2576 has a switching frequency of 52kHz and the LM2596 has a switching frequency 3 times higher than that of LM2576 (150kHz). For your requirements of 24Vin to 20Vout at 3A and assuming ideal condition of no voltage drops and a 30% ripple current through the inductor, then you would need 71.2uH if you were using LM2576 and 24.69uH if you were using LM2596. 

    You can change the value of inductance and that will change the ripple current amplitude flowing through the inductor. The advised range of ripple current amplitude is between 20% and 40% of the max load current (which in your case is 3A). This ensures that your ripple current is large enough for the internal current sense circuitry to perform accurately and it is small enough to not cause a very large voltage ripple at the output. The fact that you see a 90uH inductor being used suggests that the device being used is the LM2576.

    I hope this clears the confusion.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 40 points

    Yes, thank you, I have to recalculate it all because my results were different that yours in the case of the Lm2576.

    There is something else: usually the machine works at 1,5A maximum and reachs 3,4A only if you force the tattoo machine, but that's not normal because that is not the way of work. I mean, for tattooing it demands less than 2A. This aspect can influence the selection of the inductor?

    Thanks again.

  • 0 in reply to user3757022
    TI__Genius 15810 points

    Hello,

    With changing load current, your ripple current will change. But that will not affect the operation of the device. Designing the inductor for max load is a better option out. At lower load currents the ripple will be a little higher.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 40 points
    Ok, Thank you very much. I have the last question, I think..

    I can build the circuit with the LM2596 with smaller components due to his high frequency, but... Is it possible that it would gett hotter than the LM2576? It won't have heatsink and I'm worried about that.

    Is the efficiency the same in both regulators?

    Thank you again
  • 0 in reply to user3757022
    TI__Genius 15810 points
    Hello! The efficiency depends on numerous internal device parameters. You are right in assuming that since the switching frequency is higher, some losses will be higher. What package are you using? If you use the TO263 then you should be fine because the power pad of the IC can be soldered to the copper plane on the board. If you are planning to use the TO220 package then you absolutely need some external heat sink. If the heatsink is not used then the device will not be able to drive the rated load current.

    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Prodigy 40 points
    I use TO-220 but the point ins that the normal use of the tattoo machine is demanding 1,6A maximum, and it is not constant because the footswitch is connected to the ON/OFF, so the regulator is resting a lot. and the commercial power supply doesn't have any heatsink. My question is wich one is the best option for my application? I'm scared if I choose the LM2596 with about 30uH and it couldn't work properly because of the frequency is higher. So It is better to use the LM2576 because the losses are less.

    Thanks a lot again.