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I am having an issue with the circuits that I am working on in Multisim. The first one is supposed to convert 24VDC into 27VDC/200mA. And it is working just fine. When I substitute VDC source with 220VAC and a transformer the result is almost the same.27VDC/200mA

The second circuit that gives 12VDC/50mA output also works with this configuration. But when I attach it to the same 220VAC source it gives rather strange results. During the first few milliseconds of simulation Iin shows value of ~-2A. 12V/50mA My questions are as follows:

  1. How do I fix the negative Iin current in beginning in the fourth scheme?
  2. Is there a simpler way to connect MC34063 to the transformer?
  3. How can I connect both step-up and step-down converters to the same 24VDC source?
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  • \$\begingroup\$ Are you sure that those 2A are going backwards from buck convertor to input linear regulator? There should be no difference between 3rd and 4th circuit. \$\endgroup\$
    – Chupacabras
    Aug 17, 2016 at 6:27
  • \$\begingroup\$ @Chupacabras , yes, here is a screenshot captured at ~0.5ms imgur.com/a/jIKS8 \$\endgroup\$
    – Lee Brown
    Aug 17, 2016 at 7:09

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  1. I am not a Multisim user, but the IC=??V specifications on the capacitors probably stand for "Initial Condition". Since the input filter capacitor starts with 30V, current flows backward from that capacitor to charge the 470uF capacitor. Another hint of that is Uin gets to be more than 24V. So, don't specify invalid initial conditions.

  2. & 3. You can connect the two converters directly to the transformer rectified output.

But, if the transformer has 30VAC output, you can easily get a rectified voltage that does not dipped below 30V. Why linear-regulate to 24V and boost it back to 27V? Regulate the rectified voltage directly to 27V.

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  • \$\begingroup\$ Thank you! It's the first time when I use SPICE, I'll keep information about IC in mind. Now it works just as expected. I will try regulating it as you suggested. \$\endgroup\$
    – Lee Brown
    Aug 17, 2016 at 8:26
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Is there a simpler way to connect MC34063 to the transformer?

MC34063 can handle input voltage up to 40V. So you can use 28V (or lower) transformer instead of 30V you use. And you can skip linear regulator if you want to simplify it.

How can I connect both step-up and step-down converters to the same 24VDC source?

There's no need to special care. Just connect Vin of those converters to 24VDC source.

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  • \$\begingroup\$ The problem with simultaneous connection is that it gives unusual current values during first few milliseconds. imgur.com/a/ez3N7 \$\endgroup\$
    – Lee Brown
    Aug 17, 2016 at 7:12
  • \$\begingroup\$ But you are comparing circuit 1 and 4. Compare circuit 3 and 4. \$\endgroup\$
    – Chupacabras
    Aug 17, 2016 at 7:36
  • \$\begingroup\$ I am not sure that I understand -- I am trying to connect both of my circuits to the same power source, but when I do it, it gives me unacceptably high currents Iin and Ipk at 27V circuit and negative current Iin at 12V circuit. Eventually it gives correct values, but I am worried that these initial spikes might cause problems. \$\endgroup\$
    – Lee Brown
    Aug 17, 2016 at 7:50
  • \$\begingroup\$ Short spikes at the beginning is normal. It is called "inrush current". You have capacitors there, they are empty at the beginning, so when you turn power on, capacitors need to be filled with charge and that is why there is current spike in the beginning. \$\endgroup\$
    – Chupacabras
    Aug 17, 2016 at 7:53
  • \$\begingroup\$ I have understood your question that there is a problem only with circuit 4. That is why I suggested to compare circuit 3 and 4, because inrush current should be the same. \$\endgroup\$
    – Chupacabras
    Aug 17, 2016 at 7:56

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