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LM317HV: 24VAC RMS to 24VDC converter

Danilo A.
Mastermind 26500 points
Part Number: LM317HV
Other Parts Discussed in Thread: LM317

Hi Team,

This inquiry of our customer is related to the other thread below

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1133060/lm2575-n-24vac-rms-to-24vdc-converter/4205970#4205970

According to our customer,

I am about to run some tests with the LM317HV. I talked to the client and it seems that the system will run for about 2-5 minutes and draw 150mA and then be at rest for the remaining of the time. Maybe for 1-2h before it is run again for maybe 30 seconds or so. The system is also in a controlled environment with cool air. 
Nominally I will step down 35V to 24V or a drop of 11V at 150mA.  The IC would heat up but should not be that significant with 1.6W. 
Do you think if this might still be a good solution. I want to do a Switcher but all solutions I am looking for is out of stock. Or the other support components such as inductors are out of stock. 
Do you think if this would work? I will also put a zener to clamp at the input to make sure that I do not go past 60V input limit. 
Regards,
Danilo
  • 0
    TI__Mastermind 44705 points

    Hi Danilo,

    I don't see an issue with this. The junction temperature difference throughout the cycle is not enough for me to be concerned with thermal cycling issues since the package for this device has very good thermals, and the current is low so I don't have concerns with electromigration failures. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Mastermind 26500 points

    Hi Nick,

    Thank you for your response. Our customer has follow up inquiries below.

    I have a question for general knowledge. 
    1.  On the datasheet page 5 it says that the maximum input is 40V but on page 7 it says that it is actually the Vout to Vin difference that should be less than 40V. What is the theoretical limit of this? 
    2. When doing thermal calculation, I see a value for junction to air and then junction to board. How do I normally calculate if I want both a heat sink and a thermal pad+plane to dissipate heat?
    3. Could I still make it with the LM317? The HV version is not in stock right now. 
    Regards,
    Danilo
  • 0 in reply to Danilo A.
    TI__Mastermind 44705 points

    Hi Danilo,

    1. I'm not seeing what they're saying. I see on page 5 of the LM317 datasheet the input-to-output differential voltage spec but page 7 does not have anything like they are saying. Page 5 of the LM317HV datasheet does not have the same spec. In response to their question, though, I'm not sure what they are asking. Are they asking, for example, can the device support VIN = 540V, VOUT = 500V etc.? If so, I think the limitation is not in the LDO; the limitation would be in the DC voltage rating for the feedback resistors. There should be no real theoretical limit as long as the input-to-output differential voltage rating is met.
    2. I'll have to ask about this. 
    3. VIN - VOUT = 11V is fine for LM317 as well, and the LM317 in the KCS package has pretty much the same thermals as the LM317HV in the NDE package, so no issue there either. LM317 should work just as well as LM317HV here.

    Regards,

    Nick