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Инструменты WEBENCH® / LM5118:Problem with the charger on the chip LM5118

Sergey Shusharin
Intellectual 260 points
Part Number: LM5118

Tool/software: Инструменты дизайна WEBENCH®

Good afternoon. I make an uninterruptible power supply for a system installed in railway transport. I decided to use the LM5118 chip as a charge controller. An ideal diode controller LM74700 is used as an output diode, ACS712-05 is used as a current sensor. The charge voltage is controlled by the IRLML6346 transistor, which shunts the 1KOM resistor. Management is via PWM from the microcontroller. The main problem is the large current consumption, or rather, the feeling is that for some short period the LM5118 controller which triggers the protection of the DC-DC converter TEP 75-7215WI. For example, I charge a battery with a current of 500 milliamps and a voltage of 28.8 volts. Moreover, the input voltage at Lm5118 is 24 volts. DC-DC converter goes into protection. Or it starts to work in some kind of resonance. I tried to exclude the DC-DC converter from the circuit and connected the LM5118 directly to the laboratory power supply. The power supply starts to ring, the current consumption is also high. I tried to put a choke, a choke and a capacitor between the DC-DC converter and the charge controller. Tried to manipulate the working frequency and ramp of the LM5118. Always one result. The circuit makes a loud ringing. And she has a peak large current consumption. I am attaching to the question the input voltage waveform on the LM5118, the design of the printed circuit board and the printed circuit board itself, perhaps I was mistaken with the wiring. I made the circuit diagram and face values ​​of the radio components according to the proposed TI designer on the site. Which way should I dig, is it a calculation error, or an error in the PCB layout? I would also like to say that such miracles happen only with the battery. The load in the form of a lamp or resistor works correctly, there is no ringing or increased current consumption LM5118. With a resistive load, the circuit works perfectly !!! Where does such self-excitation come from when working on a battery?

  • 0 in reply to Sergey Shusharin
    Intellectual 260 points

    The message above is a schematic diagram. In this message I am attaching another photo of a piece of the printed circuit board and the printed circuit board itself in the PCAD 2006 format ИБП_РА_3(1.6мм шелкография).pcb

  • 0 in reply to Sergey Shusharin
    TI__Mastermind 21720 points

    Hi Sergey,

    Sorry to hear the you are having issues with your application.

    To double check the component values I suggest using the LM5118 quick-start calculator. Can you provide me the application specifications?

    VIN range:

    VOUT

    Max output current:

    When the input voltage is ringing is the output voltage stable?

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Intellectual 260 points

    Good afternoon. Input parameters of the circuit - direct current 24 volts from a DC-DC converter. The output voltage is adjustable from 28.8 (cyclic charge mode of the battery) to 27.4 (buffer charge mode) The maximum output current is not more than one ampere. I was able to defeat the problem. It turned out to be my carelessness. I made a mistake with the calculations and soldered a current-limiting resistor at 0.025 ohms, after replacing it with 0.075 ohms, the circuit began to work perfectly. the ringing disappeared, the current consumption decreased. The unfortunate DC-DC Tracopower TEP 75-7215WI finally became kind and stopped restarting. All this time, the chip was in critical on-off mode, as evidenced by the saw on the SS leg. Accordingly, the circuit was not in the current limiting mode, as intended, but in the continuous emergency on-off mode. I still have to adjust the feedback circuit, but now it’s clear that the circuit is working. One thing I can’t understand is why the inductance is so hot. She is at 8 amperes, SRP1265 inductance is 30uH. Very hot, with a charging current of 1 ampere and an output voltage of 28.8 volts. There is a current reserve. Is it really necessary to take an inductance with an even higher current? And another question, I’m completely confused, how knowing the input and output voltage, the current is to choose the inductance correctly? I tried 30 uH, 47, 100, 120. And the circuit behaves the same. How to calculate it correctly?
                             In general, I would like to thank the developers of texas instruments for this chip. She is very cool, and she has no analogues in her segment in the world. BUCK -BOOST, input voltage up to 75 volts, small wiring, current limitation. I have not seen anything like this among competitors. Our company has repeatedly used your chip and I hope that it will not be discontinued for many years.

  • 0 in reply to Sergey Shusharin
    TI__Mastermind 21720 points

    Hi Sergey,

    I am happy to hear that you like the IC and that you got the circuit switching. 

    Is is possible to probe the inductor current? How hot is the inductor getting? As saturation current is much higher than the calculated inductor current I am surprised to hear the inductor is getting hot.

    Thanks,

    Garrett

  • 0 in reply to Garrett Roecker
    Intellectual 260 points

    Unfortunately, I don’t have time to measure the inductor current. A prototype was sent yesterday. I’ll just put a high current inductance for the future. Now I am using inductance  47uH  6A  (SRP1265A-470M)

    Yes, and the diodes warm decently, although it seems that the current is not large (1 ampere). But, unfortunately, the product was taken from under my nose, not allowing it to be adjusted to the end.

  • 0 in reply to Sergey Shusharin
    TI__Mastermind 21720 points

    Sergey,

    Understood. If you have any questions in the future please reach out.

    Thanks,

    Garrett