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I am trying to design a step-up converter with the following specifications:

  • Vin: 3.0 - 3.7 V
  • Vout: 30 V
  • Iout(max): 50 mA

With such a high boost ratio, my calculated duty cycle would be ~92% which is near the maximum for most controller ICs. Is there a topology that uses a transformer to achieve such a high boost ratio?

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    \$\begingroup\$ Any transformer based. I would recommend a flyback or possibly push-pull. \$\endgroup\$
    – winny
    Jun 7, 2022 at 20:38
  • \$\begingroup\$ would there be any difficulty with using such a low Vin? \$\endgroup\$ Jun 7, 2022 at 20:39
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    \$\begingroup\$ Given to low current requirement would daisy chaining boost converters be more practical? \$\endgroup\$
    – Bryan
    Jun 7, 2022 at 20:49
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    \$\begingroup\$ Something as this could help MAX15032 500kHz, 36V Output, 600mW PWM Step-Up DC-DC Converter \$\endgroup\$
    – Antonio51
    Jun 7, 2022 at 20:52
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    \$\begingroup\$ @RGBEngineer Webench has a check box if you need output isolation, try checking that. Also, you could use a tapped inductor if you don't need isolation (which isn't really different from a flyback transformer with one side of the windings tied together.) \$\endgroup\$
    – John D
    Jun 7, 2022 at 20:54

2 Answers 2

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With such a high boost ratio, my calculated duty cycle would be ~92% which is near the maximum for most controller ICs

It's ~90% duty when operating in continuous conduction mode (CCM) but, if you lower the value of the inductor, you could operate in DCM (discontinuous conduction mode). If your controller permits DCM, it will be at a lower duty cycle. Here's a sample of what I mean from my basic website with a 33 μH inductor in CCM operating at 100 kHz and 90% duty: -

enter image description here

If you reduce the inductor to 22 μH you get an 80% duty cycle and DCM operation: -

enter image description here

The calculator is for ideal components so the percentage duty will be a tad higher. If you wanted to run at closer to 60% duty, you could choose an inductor of about 12 μH.

Is there a topology that uses a transformer to achieve such a high boost ratio?

Consider the above modifications first before taking the step to go to a flyback controller (more complex and requires careful selection of the MOSFET and/or leakage flyback snubbing).

If you decide that you need a CCM flyback circuit (1:4 step-up transformer) you can use this basic "ideal" calculator (also from my website): -

enter image description here

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This probably isnt the answer you're looking for:

I threw this together because I had a silly idea after suggesting daisy chained boost converters: what if you were super cheap and didn't want to pay for two controllers? Anyways I napkin'd this and then did the math, which is neat (neglecting diodes, assuming cont curr, etc):

\$V_{stage1}=\frac{V_i}{(1-D)}\$ and \$V_{stage2}=\frac{V_{stage1}}{(1-D)}\$ so if we rearrange for final \$V_o\$ from \$V_i\$ we get:

\$V_{stage2}=\frac{V_i}{(1-D)^2}\$ which means you could get a 10x boost ratio with a 68% duty cycle - cool!

schematic

simulate this circuit – Schematic created using CircuitLab

One immediate improvement I see is if you managed to use a synchronous boost converter for the first stage you would lose a lot less voltage. You wont be able to pirate the synchronous rectifier signal for the second stage, but it won't mind the diode drop as much as the low voltage stage.

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  • \$\begingroup\$ or perhaps a boost converter chip with multiple outputs? \$\endgroup\$ Jun 8, 2022 at 3:58
  • \$\begingroup\$ @RGBEngineer yeah that’s the smarter, less entertaining way to do it. Looks like a few dual boosts would support this application \$\endgroup\$
    – Bryan
    Jun 8, 2022 at 4:09

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