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LM5118: Design for Solar Panel and Lead-Acid Battery

Part Number: LM5118

Hello,

Currently I am trying to use the LM5118 with a solar panel at the input to charge a 12V lead acid battery as a load.

The solar panel is rated for 50W and when tested outdoors has a Voc = 22V and Isc = 3A.

The specifications used for the device are as follows:

Vin = 8 to 25V

Vout = 13V

After simulating using the unencrypted spice files it seems that the inductor current far exceeds the limit available from the solar panel in Buck-Boost mode. At Vin >= 18V the device operates in Buck Mode and the current from the panel is not limited. However it is unlikely that the panels voltage will ever exceed 18V in direct sunglight. I was curious if there is a way to reduce the current spike when Buck-Boost mode becomes active at around the 69% duty cycle mark or if this IC is just not suitable for this design and a simple Buck IC would be more appropriate. I should also mention that I have a second panel which could be put in parallel effectively giving me around 4.5 to 5A average.

Load of 18 ohms was chosen to stay in CCM and keep current through inductor down.

  • Hi Colin,

    Just let you know that we are reviewing this and will get back to you in two days.

    Thanks,
    Youhao Xi, Applications Engineering
  • Ho Colin,

    Thank you for consider the LM5118. I am confused by our question. What exactly is the issue? For the green trace, I believe it is the input current from V1, right? Can you align it with the Vout waveform?

    Thanks,
    Youhao
  • The device is functioning properly. I can only supply a maximum of 5A from the solar panels in parallel. The trace is the input current which corresponds to the amount of current in the inductor. This current spikes as soon as the device is operating in buck-boost mode which I understand is normal when reffering to the datasheet component calculations. However, this current can far exceed my current limitation from the source even with a small input range of 8-25V and and Vout approximately 13V. My question is, is there a way to reduce the amount of current through the inductor when Q2's gate starts to switch activating buck-boost mode? I know it will always be greater than buck mode current but Id like to find a way to reduce the difference so I won't be current limited. If there is no way then I guess I will only run the device in buck mode.

  • Hi Collin,

    One possibility is to add an average input current limit. This can be done with and current sense amplifier and voltage reference (error amplifier) to pull the COMP pin low when the input current is at 5A. This might be the best solutions for limiting the input current for all operating conditions.

    Thanks,

    Garrett
  • Hi Garrett,

    Thank you for the advice I will test it out! Another quick question. I am currently testing the device at 25 Vin and 12.7 Vout with a 5 ohm load. Simulation in spice and using WEBENCH show that the device is running in CCM however upon testing the current through the load is only 400mA (should be seeing just over 1A)  no matter what changes are made (different inductor size, changes to comp network, changes to Rsense).

    The UVLO pin is always at 5.1V and EN is tied straight to the 25V. The comp pin however is reading 2.73V up to 4V and the soft start capacitor has never once reached 1.23V. Instead of seeing the 12.7V out im lucky to see 5V with a feedback pin voltage of 350mV and CSS voltage of 500mV. I don't know if the chip is not functioning properly or if this might be human error.

    I understand that CSS has to reach 1.23V for the internal amplifier to start using 1.23 as a reference voltage however mine will never even reach 1V.

    The ramp signal seems to be the only thing functioning properly with a nice clean waveform on the scope. Im using two IRF540's as switching devices and a 100nF bootstrap capacitor where the HS pin is reading just over 3V and the gate of the buck switch is approximately 6V. Any advice on common solutions or if this may just be a bad chip is greatly appreciated.

  • Hi Colin,

    Please send us some key waveforms like the switch node voltages, current sense signals, COMP pin voltage, etc.

    Best Regards,
    Youhao
  • Hi Colin,

    Any update? Can I assume the issue is resolved? We are required to close the thread if no activity for a few days. You can always reopen it by a new post.

    Thanks,
    Youhao