2 buck converters from same source, voltage drop

Thread Starter


Joined Jan 21, 2021
Hello there ,

Previously, I opened a topic about Mcp16311 buck converter. But now I think the problem is different.

In the simulation scheme I set up with MC34063 integrated, I use 2 converters that convert 24V-12V and 12V-5V together.

In this simulation, both circuits work fine when they are separate from each other. I can get the voltage and current outputs I want from both.

However, the problem is that when I use 2 of them together as I will use in the final circuit, that is, when I take the 12v converter from the output of the 24v converter, the output of the 12v-5v converter is only up to 4v.

I could not find the reason. I would be very happy if those who have information help.

Simulation scheme:

Previous thread : https://forum.allaboutcircuits.com/...converter-output-problem.175978/#post-1591584


Joined Aug 21, 2008
As Papabravo mentioned, D2 should be on the end of L1 that connects to pin 2 of the MC34063 where it limits the negative excursion of the pulse at that point, AND it should be a high speed rectifier diode like a 1N5819 and It is not a good idea to put Zeners on the outputs of power supplies.

There is the same error on the 12V power supply, and that should also be corrected. Without the proper rectifier you might be exceeding the maximum voltage rating of the chip, so it might appear to be working but it may also be slowly killing the MC34063.

Also, there is no reason you can't make the 5 volts from 24 volts as crutchow suggested - that would be more efficient because it takes a lower average input current and you would only be converting the power once.


Joined Aug 21, 2008
Wait - Is there are reason for the Zener diodes?

It will be best to remove the zeners on the outputs of the power supplies. You are probably wasting power and asking for trouble because of the tolerance of the power supplies and the Zeners.


Joined Jun 5, 2013
The chip regulates the output. It probably was not able to do that in your original, incorrect circuit. With the circuit fixed, it should have a steady, regulated output.