I am trying to make a boost converter. I'm stepping up 5V to 12V.

I have all the required components, including a power MOSFET(IRLZ44N), a Schottky diode (SR260) and an astable multivibrator that produces a 71.4kHz square wave for my MOSFET.

I just don't have an inductor, but I have a small ferrite core transformer in my parts bin. I got the transformer from an old SMPS that uses one power transistor to step down the AC.

If I use the secondary winding (winding with low resistance) as an inductor:

  1. Will it work?
  2. Will there be a high voltage at the primary that is unconnected?
  3. The boost converter has a feedback. Will this damage the transformer?
  4. Will my MOSFET and diode get damaged?

Transformer 1

Transformer 2

  • \$\begingroup\$ We don't know the specs for your transformer, or for the rest of your boost converter, so at this point, no-one can really say whether any of your components will be damaged. Do you know the current rating of the converter it came out of? Do you know the maximum current that your boost converter will draw from its input power source? \$\endgroup\$ Mar 13, 2021 at 22:15
  • \$\begingroup\$ The transformer is from a Cheap tiny little SMPS. It's a li-Ion battery charger that plugs into a wall socket actually. It uses the TC3582DA IC. \$\endgroup\$ Mar 13, 2021 at 22:21
  • \$\begingroup\$ The maximum output current is 500mA, So the maximum input current should be about 1.5A, that gives me an 80% Efficiency, which is okay. \$\endgroup\$ Mar 13, 2021 at 22:24
  • 2
    \$\begingroup\$ @MiracleTECH You need the specs (for all), else if the inductor needs to be (say) 100 uH, but is, instead, 10 uH, the current will be 10x times larger. You can't just throw in some parts that resemble some known schematic and hope to work, properly or not. \$\endgroup\$ Mar 13, 2021 at 22:25
  • 1
    \$\begingroup\$ You should post a schematic of the proposed circuit in order to get good answers as to whether it will work or not. \$\endgroup\$ Mar 14, 2021 at 3:14

1 Answer 1

  1. Will it work?

It will work as an inductor. However the amount of energy it can store will be a tiny fraction of that a proper inductor of the same physical size can store, because the core is ungapped, built for high permeability rather than high energy storage. You'll need to measure its value to see if it's suitable for your design. Measure the value at a range of currents, you may be surprised at how low the saturation current is. At the saturation current and above, the inductance collapses to a very tiny fraction of its low current value, which could cause a damaging current to flow in your driver.

  1. Will there be a high voltage at the primary that is unconnected?

Yes, the voltage will be proportional to the number of turns on the winding. Having an open circuit high turns winding on the same core will reduce the SRF dramatically from what a single winding would give you. This may or may not cause you trouble at 71 kHz. Having only half the winding volume carrying your inductor current means higher losses than for a proper inductor.

  1. The boost converter has a feedback. Will this damage the transformer?

Without a schematic, nobody will be able to tell you. I'm not sure what 'a feedback' is, or how you have it connected.

  1. Will my MOSFET and diode get damaged?

Maybe. The on time of the 71.4 kHz signal could take the inductor current up beyond its saturation level, and so to very high indeed, which could damage your MOSFET, if your power supply has the current capability. When working with an unknown inductor, you should take baby steps, so measure the current while starting with a current-limited supply and using short pulses. You really need an oscilloscope for this. Note that using short pulses and observing the current with an oscilloscope is the same as 'measuring the inductance at a variety of currents'.

  • \$\begingroup\$ By feedback, I mean a Circuit that adjusts the duty cycle of the converter depends on the output load. \$\endgroup\$ Mar 14, 2021 at 13:46
  • \$\begingroup\$ @MiracleTECH Yes, I know what 'a feedback' sort of sounds like, but if you've bothered to mention it, then it deserves a schematic. Normally, providing feedback to control the output of a converter doesn't affect that converter's safety at all, but you never know for sure until you see the details. \$\endgroup\$
    – Neil_UK
    Mar 14, 2021 at 14:26
  • \$\begingroup\$ Okay. I've not yet drawn a schematic yet, because I'm still curious about the transformer. But if I disconnected the primary winding, would it just act as an inductor? Since there is no primary winding? \$\endgroup\$ Mar 14, 2021 at 14:28
  • \$\begingroup\$ @MiracleTECH Have you read my first paragraph? \$\endgroup\$
    – Neil_UK
    Mar 14, 2021 at 16:18
  • \$\begingroup\$ I have read it. Thanks For clarifying me. \$\endgroup\$ Mar 14, 2021 at 16:24

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