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LM2575-N: 24VAC RMS to 24VDC converter

Danilo A.
Mastermind 26720 points
Part Number: LM2575-N
Other Parts Discussed in Thread: LM2575, LM2574, TPS560430, TPS54231, TPS54233

Hi Team,

Our customer is looking for a 24VAC RMS to 24VDC converter and is considering the LM2575-N. I understand that the peak voltage of a 24VAC RMS is equal to 24VDC / 0.7071 = 33.94V. Can we use the LM2575-N to convert a 24VAC RMS to 24VDC with a bridge rectifier in series with the regulator?

Regards,

Danilo

  • 0
    TI__Guru* 75450 points

    Hi Danilo,

    The main concern of applicability is the duty cycle.  The LM2575's max duty is 93% according to the datasheet, which requires the rectifier's output voltage to be (24V +1V)/0.93=26.9V, in order to maintain the output regulations.  Therefore, for the 24VRMS input,  the filter (C or L-C) after the bridge rectifier should be large enough to produce a voltage >26.9V in order to use the regulator. 

    Hope this clarifies,

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Mastermind 26720 points

    Hi Youhao,

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

    I get it. I will use a half wave rectifier which is a demand in the application. 24VAC*1.414= 33.9VDC and minus the drop on the rectifiers and Capacitor bank I can get an easy 20% tolerance for fluctuations. 
    I was also looking into the LM2574. Would that one also work? I tired a design with the LM2575 but the output inductor is so big and costs is quite sensitive for this project. I was hoping to drop the capacity to 500mA which is still double of what the application needs but should cost less. 
    Regards,
    Danilo
  • 0 in reply to Danilo A.
    TI__Guru* 75450 points

    Hi Danilo,

    I think the main cause is the switching frequency, which is 52kHz.  These are legacy devices and TI does release newer part, like the TPS560430 which is also mentioned in the latest LM2574 datasheet.  The TPS560430 operated at 1.1MHz and it should help reduce the solution size.  Could you take a look of it?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Mastermind 26720 points

    Hi Youhao,

    Thank you for your suggestion. The TPS560430 and other similar devices are currently out of stocks. The TPS54231 and TPS54233 are the buck converters with higher switching frequency (570kHz and 300kHz respectively) that we have stocks. Are these devices suitable for this application?

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Guru* 75450 points

    Danilo,

    These two parts belong to BSR-MV team.  I am directing this thread to their attention, and they will be happy to support you.

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Mastermind 30780 points

    Hello Danilo, 

    These devices are up to 28 V Input Voltage Range. 

    TPS54233 with fixed 300 kHz Switching Frequency. 

    TPS54231 has fixed 570-kHz Switching Frequency.

    please provide more details on you requirement, Ruby Li will confirm if this a good fit.  

    Thanks! 

    Tahar

  • 0 in reply to Danilo A.
    TI__Expert 7430 points

    Hi Danilo

    Would you please tell me some details about the application? Such as, input voltage, output voltage, load current, switching frequency or so on.

    Thanks.

    BR

    Ruby

  • 0 in reply to Ruby Li
    TI__Mastermind 26720 points

    Hi Ruby,

    The specifications of the project are as follows.

    Input voltage = 24VAC RMS, 50Hz
    Output voltage = 24VDC
    Load current = 150mA
    Switching frequency = no preferred frequency 

    Here are some information about the customer's application.

    "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."

    Can you please suggest a device with stocks that would fit their application? 

    Regards,

    Danilo

  • +1 in reply to Danilo A.
    TI__Expert 7430 points

    Hi Danilo

    The application is a AC to DC conversion, but our device is DC to DC. This may be other product line.

    Hope this helps.

    BR

    Ruby