• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Subscribers 63 subscribers
  • Views 445 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

need 11 amps at 24 volts from input as low as 4 Volt

Rob vreden
Intellectual 310 points
Other Parts Discussed in Thread: LM3481, LM2587, LM2588

Hello,

I need to make 24 Volt form an input Voltage starting from 4 Volt and max input Voltage is 35 Volt. I have been testing with the LM3481 in SEPIC mode but this was not working very well. Very, very hot FET; could not solve the issue. Have spent a lot of time but no luck. Also posted this issue at this forum.I tried different type of FETs and changed the coils with models with higher current.

What dit work was the LM2587. This simple switcher picks up smooth at 4 Volt and works its way up to 24 Volt. But the output current is nog enough. I need an 11 amps at 24 Volts.

The variable Voltage input ( 4-35 Volt) is capable of delivering the 11 amps current  from 14 Volts input. Below 14 Volts the currrent decreases rapidly.

So any ideas how to solve this or are there components available to make such an wide input step up at this high current rate?

regards,

rob van vreden

  • 0
    TI__Mastermind 19891 points

    Hello Rob, 

    At 24V output and 11A of current, your input source must be capable of supplying around 70A of current at 4V. Is your source capable of this current?


    Regards, 

    Denislav

  • 0 in reply to Denislav Petkov
    Intellectual 310 points

    Hello Denislav,

    no, but it can deliver about 2 amps at 4 Volt. this increases quite fast to 4-5 amps at 14 Volt. So from 14 Volts and up it is no problem to get the amps from the source.

    I have seen that it is possible to increase the power of the LM2587 by paralleling them. TI has an paper on that issue. I will test that. 

    But maybe there is an better, more ecomical way to generate this power using an other design. 2x lm2587 is quite expensive for this project and it takes quite some pcb space. If you have suggestions?

    regards,

    rob

  • 0 in reply to Rob vreden
    TI__Mastermind 19891 points

    Hello Rob,

    Let's discuss the source first. The output power you are targeting is 264W (24V x 11A). Assuming 85% conversion efficiency as a starting point, you would need 310W of available input power. At 14V input, this translates to 22A of source current.

    Assuming that the source can provide that kind of power, you will not be able to have a single switch solution. You would need some sort of parallel combinations with enough heatsinking. Whatever solution you pick, it will take quite a bit of board space to drive this much power.  

    You could try to parallel LM2588 boost converters (same as LM2587 but has frequency synchronization), here is a diagram (http://wwwd.national.com/national/PowerMB.nsf/(approved-attachments)/213EFF179DB67284882568CD006874CE/$File/Parallel+Regulators+to+Boost+Current.pdf). However, you would need more than 2 of them to get the output current. Using an external clock to drive the sync pin and offset the switching phases would be helpful. Also good board layout will be very critical in this case for proper operation. 

    Out of curiosity, what is your application that needs 24V and 11A? Is it some sort of a motor? Also, if you don't mind sharing, what are you using for your source?

    Cheers, 

    Denislav

  • 0 in reply to Denislav Petkov
    Intellectual 310 points

    Hello Denislav,

    Thanks for the reply.

    I will try 2x lm2588. I had a look at them before but could not figure out how I could benefit from the synchronisation pin. I will study it once again.

    Dissipation is not a problem, but I want to keep loss as low as possible. 

    It is indeed some sort of motor. 

    Thanks for your help sofar.

    kind regards,

    rob

  • 0 in reply to Rob vreden
    Intellectual 310 points

    Hello Denislav,

    If the set Voltage is met; say 24 Volts, and the input Voltage rises above that, say 26 Volt. what would happen? will the schottky and coil conduct the current( a kind of bypass) ?

    and if this is the case, will the lm2587 stop or shut completely? or will it once in a while, start with very short pulses? 

    rob

  • 0 in reply to Rob vreden
    TI__Mastermind 19891 points

    Hi Rob, 

    In that case the LM2587 will not switch. The schottky and the inductor will provide the current path from VIN to VOUT. Once the input voltage falls under 24V (+ diode and inductor drop), the feedback will tell the boost to switching again in order to maintain the output at the set voltage.

    Regards, 
    Denislav 


     

  • 0 in reply to Denislav Petkov
    Intellectual 310 points

    hello Denislav,

    ok, thanks for the info. I hoped so, that the switcher would stop completely.

    in case he would still swithc once in a while I was afraid of RFI. very short pulses. 

    I have completed the layout of the pcb and will have it made. I read on this same forum that the slave IC should carr a 0,01 Ohm resistor in series with the diode to balance better. but I have not seen it in the schematic.

    regards,

    rob