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In the circuit I am working on there is a requirement to create 30V DC source for a very light load (couple of mA max). I am aware that there a specialized ICs that can do that but I would like to achieve that with the parts that I already have.

What I came up with is this: enter image description here The idea is that 555 IC configured in astable mode drives a voltage multipler (configured as tripler on the schematic), a fraction of output voltage is compared with known source (1/3rd of 15V) via an op amp and the amplified error is fed into NPN so that when base voltage is ~0 control and reset pins of 555 IC are triggered.

Simulation in LTSpice does produce a desired output: enter image description here And a closer loot at 555out: enter image description here enter image description here

The 15V and -15v are generated at the same time with two MC34063 hence they are not stable at the startup: enter image description here enter image description here

What I was wondering is:

  1. Is there any reason this circuit would fail in the real life?
  2. Are there any possible improvements that might simplify/improve the schematic?
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    \$\begingroup\$ I've not looked over the circuit much. But I do notice that perhaps an LM239 (or LM139 or LM339), if you have one, already includes a nice open-collector output and might be able to replace all that 'error amplifier' and 'feedback' stuff. Apparently, you have +/-15 V that are also starting up. It would be very easy to use another LM239 (there's four available in the IC) to monitor the 15 V rail and enable the circuit, if you wanted that feature. \$\endgroup\$
    – jonk
    Jun 25, 2022 at 22:40
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    \$\begingroup\$ @jonk , thank you. Simulation with LM339 works like a charm. I have an equivalent chip 1401SA1 at my disposal. \$\endgroup\$
    – Ivan Demyachenko
    Jun 26, 2022 at 7:54

2 Answers 2

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C14 is useless, you only need the other 4 caps for this tripler and Cout can also be 0.1UF if you raise the frequency from 10 kHz.

The voltage ratio will follow the transformer impedance ratio rules of Load/source ^2 so with 30V and 10k load the unregulated voltage will drop from 45V-diode drops by the amplified source impedance of 9x plus diode impedance. Effectively you need a source impedance of < 100 ohms or or so to drop 43V to 30V with load of 3 mA. (Guestimate)

I don't see any reliability issues but definitely see cost/simplification opportunities.

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Using the TL431 for the error feedback control may simplify the design.

555 boost

Start

Output response.

Step zoom

No load starting.

No load

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