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I am designing a 2s Li-ion battery charger using a buck converter from a 10W (18V, 0.55A) solar panel. So I understand that the duty cycle controls the ratio of the input power vs output power and therefore Io = Is/DC and Vo = Vs.DC, but when I increase my duty cycle both the voltage and current increase to a point where all 0.55A are being drawn and then the voltage becomes limited causing a massive voltage drop. I am using an arduino uno to generate a 20kHz switching frequency and the circuit diagram can be found below.I am using an 8 ohm power resistor in place of the battery to ensure I dont damage the battery. Any help/advice would be greatly appreciated.

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Edited: Updated circuit, NMOS MOSFET changed to PMOS MOSFET for M2

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  • \$\begingroup\$ Do realize that playing with this in LTSpice is OK for educational purposes. However, for "real world" applications I advise you to use a proper IC designed for charging Li-Ion cells. One reason for this is that there are no safety measures in your design, if the Arduino crashes and keeps the MOSFET on, the batteries will overcharge and get damaged and/or smoke and/or catch fire. On ebay there are plenty buck converters and charging circuits for sale which will provide you with a much more robust and reliable solution. \$\endgroup\$ – Bimpelrekkie Jun 3 at 7:05
  • \$\begingroup\$ I am aware, will that cause this problem? \$\endgroup\$ – Zee96 Jun 3 at 7:07
  • \$\begingroup\$ I agree with you @Bimpelrekkie, I would love to use an IC module but the project specs say I have to design one. And I agree, LTSpice is so annoying. \$\endgroup\$ – Zee96 Jun 3 at 7:09
  • \$\begingroup\$ @HarrySvensson I realised tat I was connecting source in and drain in the incorrect way in SPICE, I agree, a PMOS is a better fit. \$\endgroup\$ – Zee96 Jun 3 at 7:17
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    \$\begingroup\$ @HarrySvensson Ive spent the day debugging and have narrowed down the issue to the rectifier side of the circuit. I used a signal generator and mimicked the signal and placed the input at the inductor, I then varied the duty cycle and measure output voltage and current delivered to the load and even then, as I increase my duty cycle both output current and output voltage increase. \$\endgroup\$ – Zee96 Jun 3 at 16:52

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