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LM5175: Operates in diode emulation mode (DEM) - Battery charger application

Vi Tran
Prodigy 70 points
Part Number: LM5175
Other Parts Discussed in Thread: LM5176

Hi,

We have been developed battery charger using LM5175. We are currently encountering a problem with a high side MOSFET (boost leg) getting very hot; particularly, when we have a battery connect on the output of charger before it was powered up. 

I find a below post on the TI forum addressing exactly what we have been dealing with at the moment.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/651682/webench-tools-lm5175-lm5175-problems-with-lead-battery-and-supercapacitor-charging

 

According to this post, there are 2 solutions to solve this issue:

1/ Add extra Schottky diode in parallel with high side MOSFET (boost leg).

2/ Or use LM5176 IC.

We cannot sourcing LM5176 else where at the moment so we cannot really use this option. In regard to option #1, we are still not quite satisfy with a result as the Schottky diode still getting quite hot at full load (20A). The IC operates in diode emulation mode (DEM).

To tackle this issue, we have been playing around with a compensation RC circuit. And we find out that reducing cross-over freq prevents diode from getting hot and IC from operating in diode emulation mode (DEM).

However, it flags another concern that will it cause any "side effects"?, e.g. instability, unit faulty in long-term, etc.

Please see the bode plot below when using a compensation RC circuit as TI's suggestion and our experimental compensation RC circuit.

Last but not least, do you have any guideline to select compensator poles and zero?

What is the relationship between those parameter and cross-over freq?

Any advice would be  appreciated.


Thanks,

VT

  • +1
    TI__Guru 65170 points

    Hello VT,

    Please check section 9.2.2.14 in the datasheet if it answers your questions to the relationship between poles, zeros and crossover frequency.

    We do have a lot material on compensation available, if it is really helpful in this case, I do not know. Here 2 examples:

    https://www.ti.com/lit/an/slua101/slua101.pdf

    https://www.ti.com/lit/an/slva352a/slva352a.pdf

    In general when reducing the crossover frequency, the converter gets more stable with the drawback that it needs longer to react on transients.

    So make the crossover frequency as low as possible and as big as necessary is a general saying in power supplies.

    If your system is working fine with a low crossover frequency and you do not have problems with a slow reaction (I do not expect that this is an issue in a battery charger), reducing the crossover frequency is fine.

    Nevertheless, I would always recommend to check the system at minimum and maximum input voltage, load current, temperature, ... and with worst case transients on the input in your case as well with the behavior of a faulty battery.

    Best regards,
    Brigitte