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Link to circuit schematic simulation

The circuit is from the above link. There is some problem with the control loop. The output voltage doesnt stay constant within the input range of 12-24v for which it is designed. Even for any simple boost/buck circuit I am having the same problem with the control loop.


Boost regulators are notorious for needing a minimum load to keep them in regulation. Think about it a little. At the lowest duty cycle possible the inductor is charged for a very short time and this gathers in energy. Then that energy is released via D2 into the output circuit. You do this maybe 50,000 times per second and that means you are transferring real power to the output but, there's too light a load and the output just keeps rising. The higher your input voltage the more energy per cycle transfers so, when you raise your input supply, the output rises so more.

The 1 kohm (R5) you have may not be enough. Try putting 100 ohm on the output and see what happens.

  • \$\begingroup\$ Hi Andy, Thanks for the reply. I tried adding a 100 ohm load but it is still the same. When the input voltage is 12v the output is 30v and for 20v input the output is 40v. \$\endgroup\$ – Mounika Reddy Vanga Aug 4 '16 at 17:23
  • \$\begingroup\$ What do you think the transistor ON voltage should be? \$\endgroup\$ – Mounika Reddy Vanga Aug 4 '16 at 17:32
  • \$\begingroup\$ The transistor should turn on to a low milli ohmrange but i dont think that is related. What is the minimum duty cycle you can get and what operating frequency are you running at. Also, what is the intended output voltage. Is anything getting overly hot? \$\endgroup\$ – Andy aka Aug 4 '16 at 19:49
  • \$\begingroup\$ I am doing the simulation part, the intended output i s 48V and operating frequency is 80KHz \$\endgroup\$ – Mounika Reddy Vanga Aug 4 '16 at 20:06

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