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LM25037EVAL: Hacking this to test for a wide Vin and large boost output.

Keith Cress
Prodigy 140 points
Part Number: LM25037EVAL
Other Parts Discussed in Thread: LM25037-Q1, , LM5022

Hello,

I'd like to use the LM25037-Q1 in a vehicle CDI power supply design:

6V < Vinput < 32V

===============================================

Vout = 280V

0.1A < Iout < 0.2A

It needs to have a fairly small footprint so I'm expecting f to be a ways north of 100kHz.

===============================================

Do you see any reason this won't work?

Will the wide input voltage cause problems?

Do you think one could successfully hack the LM25037EVAL board to prove this out? 

  • 0
    TI__Mastermind 31580 points
    Hi Keith

    The LM25037-Q1 device will operate fine over this input voltage range. The Power stage will have to change its duty cycle to cope with the input voltage variation. So at 6V it will be a Dmax 100% duty cycle, then at 36V it will be a 25% cycle. The efficiency will drop at higher Vin due to switching loss and higher RMS currents due to the smaller duty cycle. This will result in higher thermal dissipation which may be tolerable if the operation at the extremes of Vin is only transient in nature.

    Feel free to hack the EVM but you will need a new transformer, you can try wurth-midcom, Renco or GCI depending on your location.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 140 points

    Peter;  That helps my understanding.

    Vehicles have different nominal supplies so 32V would not be transient. 

    What do you think of a tapped winding on the transformer with an input voltage monitor that automatically directs the FET drives to an alternate set of FETs connected to the taps.  Essentially, changing gears on the transformer for a high input voltage. 

    I understand the transformer change is needed. Thanks for the recommendations for that.

    Thanks so much for your response.

  • 0 in reply to Keith Cress
    TI__Mastermind 31580 points
    Hi Keith,

    The challenge with a tapped transformer is there will be step change in the operating point of the power stage and that transient will travel dierctly to the output where the output filter cap will take up some of the energy but the control loop will have to catch up. You will see an output voltage transient during the step. You will also need to add hysteresis on the step threshold to prevent chattering. Its the kind of approach that quickly grows arms and legs....

    An alternative is to do a two stage approach, boost the input to 40V and run the dc/dc isolation and close to Dmax all the time for maximum efficiency.

    The 4:1 input voltage range is not untypical and is seen in telecoms systems dc/dc converters, in general their efficiency is lower that design that have to cope with more moderate input voltage ranges, like 2:1. So It can be done but at a cost of efficiency.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    Prodigy 140 points
    Arms and legs indeed! Exactly my concern and your two stage gambit is what I was going to ask about next. I was just looking at an LM5022 42V output design in the Webench thinking 42V might be more likely than 40V but the same thing essentially. I think I'll give the two-stage a try.
    Thanks again.
  • 0 in reply to Keith Cress
    TI__Mastermind 31580 points
    Hi Keith,

    Its nice to work with someone who knows what they are doing, if you are ok for now please press the resolved button. If you have more questions on the LM5022 just start a new post.

    Thanks

    Peter