I'm currently testing a buck converter I designed but I was running into some issues with it.

Vin = Solar Panel (30W, 17V at Vmax, 1.76A,Imax)

Vout = 12V battery

Inductor = 150uH

(Switching frequency= 62.5kHz, Iout = 2.5A, Current Ripple = 20%*Iout

When I test the MOSFETs by themselves they are switching nicely as expected, but the moment I connect the filtering components (inductor, capacitor, output load) my MOSFETs start to heat up terribly and my voltage at the output is in the order of milliVolts (should be 12V since load will be a battery). I went back to recheck my calculations for the inductor and cap value and it was all good. I then decided to measure the current at the input and that was only about 30mA and my output current was only about 2mA. I'm thinking that since the current I'm getting from my dc supply is lower than what I would be expecting from the solar panel, my inductor (which was sized for a 2.5A output) causes my buck to go into DCM and maybe what I need to do is use a different source than can supply more current? Does this sound right, or are there any other possible factors I'm not aware of? Thanks for the help.

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    \$\begingroup\$ Show us your circuit. Also, how exactly are you planning to get 2.5A out of a 5W panel? \$\endgroup\$ – Dave Tweed Nov 14 '15 at 23:27
  • \$\begingroup\$ I apologize, I took down the specs from the wrong solar panel by accident. I have fixed the error now \$\endgroup\$ – L_t130 Nov 15 '15 at 1:21

Let's do some calculations (and use round numbers to make things a little more obvious:

18 V in and 12 V out ==> D% = 67 %. At 62 kHz, this is TON = 0.67 * 1/62k = 11 us; TOFF = 5 us. DeltaI = (18-12)*11u/150u = 0.44 A. This is your ripple current.

Given your peak input power of 5 W, 0.28 A, your inductor current would reverse in CCM. Your system would be somewhat more efficient in DCM.

What controller are you using ? You can probably just replace the lower FET with a diode (or just disconnect its gate and connect it to the S instead).

  • \$\begingroup\$ In orthodox speak reversal of inductor current wastes power. \$\endgroup\$ – Autistic Nov 28 '15 at 5:03

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