I'm using following circuit for a boost converter using MC34063A IC. enter image description here

The circuit is designed for 24 V, 250 mA output. The simulation works fine but the actual circuit on a PCB behaves weirdly. In case of no load I get the very high output of 43 V which doesn't change even if I change the value of potentiometer (R45). This is, I think, the no load behavior of a boost converter, we need a minimum load, so this makes sense. So if I put a load of 1 kohm, the output drops to 25-29 volts, over the complete range of potentiometer(R45). Now if use a load of ~150 ohms (160 mA at 24 V), the output drops to ~16.8 V and doesn't change by changing the value of R45. What could be the issue here?

Similarly, I use following circuit for a buck converter, designed for a 3.3 V, 100 mA output. With 7 V input the converter works fine, but with 5 V input the voltage drops to 2.9 V, and doesn't go to 3.3 V at any value of potentiometer. enter image description here

Edit 1: Following is the layout for boost converter. I had to remove the rest of the pcb's components and tracks to make it more readable. enter image description here

  • \$\begingroup\$ Did you follow design advice on the choice of inductor? You should also show the PCB layout for a switcher as minimizing loop area is critical. \$\endgroup\$
    – user133493
    Jun 22, 2018 at 7:41
  • \$\begingroup\$ I didn't build my own inductor. I bought some inductors online, and there is no detailed datasheet for it, only specs are 220uH and 2A. the pcb has a lot of other circuitry as well, so it will be very difficult for someone to figure out the tracks for boost converter, I'll try to trim the layout down and post it here, \$\endgroup\$
    – Salman
    Jun 22, 2018 at 7:51

1 Answer 1


What could be the issue here ?

Look at figure 9 in the data sheet. There is a table indicating the line regulation is 30 mV from Vin = 8.0 V to 16 V, IO = 175 mA. Your Vin is 7.8 volts AND you are using a 220 ohm resistor in position R2 instead of the recommended 180 ohm. I would also suggest that your inductor is probably not good enough for the job.

but with 5v input the voltage drops to 2.9v

Your expectations of it being a low dro-out buck regulator may be unfounded. Look at figure 11 - it is a 5 volt regulator and it specifies Vout from input voltages as low as 15 volts only.

It also appears that you haven't closely followed the PCB layout guidelines given on page 9.

  • \$\begingroup\$ I borrowed an inductor of same value from a friend(it has drum core) and replaced the inductor on pcb with this one, the results got much better, only the voltage regulation is still very poor, so I think the inductor is the main problem, I'm going to design an inductor myself and use it, then I will post about the results. \$\endgroup\$
    – Salman
    Jun 25, 2018 at 10:26

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