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I was experimenting with a low watt boost converter,12V to 100V,50mA using the venerable MC34063A and an external MOSFET switch..I calculated the inductor value to be about 100uH.So I made two inductors using a drum core and toroid ring,both of which are 100uH.The drum core works well,but under load the toroid drops to 60V or so. Is the toroid saturating? If so ,should i use a larger toroid to overcome this problem? TIA.

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    \$\begingroup\$ Show schematic. Show core calculation LI=NAB. \$\endgroup\$ – winny Mar 23 '18 at 18:30
  • \$\begingroup\$ Thanks for the reply,winny.The schematic is exactly the same as : threeneurons.wordpress.com/nixie-power-supply/hv-supply-kit. I did not do any calculations for the core,just used some inductor cores lying around and wound wire to get the required inductance. \$\endgroup\$ – EmbSysDev Mar 23 '18 at 19:18
  • \$\begingroup\$ There’s your problem! Now calculate the peak flux density and peak current ripple of your converter. \$\endgroup\$ – winny Mar 24 '18 at 6:27
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A 5:1 ratio is already on top of what is possible with a simple boost converter. The turn-off time of your switch has to be extremely short for higher ratios, otherwise you won't get a high boost. A flyback topology is often simpler to implement.

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  • \$\begingroup\$ Thanks Janka.Yes,am aware of that. My application needs less current ,so as is,my requirement is fulfilled.Was just looking for some empirical suggestions,on sizing toroids. \$\endgroup\$ – EmbSysDev Mar 23 '18 at 19:17
  • \$\begingroup\$ What material is the toroid core? \$\endgroup\$ – Janka Mar 23 '18 at 19:21
  • \$\begingroup\$ Its ferrite , about 16 mm OD,and height about 6mm.Putting 2 turns(2 passes of wire through the center) gives about 15uH. \$\endgroup\$ – EmbSysDev Mar 23 '18 at 19:32
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Just commenting on Janka's idea that a 5:1 ratio is the practical limit to boost converter design. These kinds of limits only apply for continuos mode boost converters. In discontinuous mode, there are no such limits at all, I've built converters of this type with a 50:1 upconverting ratio, while maintaining reasonably high efficiencies. Since control circuits are needed anyways to better stabilize the output of a continuous mode boost converter, it makes little sense to adhere to the continuous mode which has the disadvantage of needing a minimum load at all times. You yourself may be well aware of these considerations.

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  • \$\begingroup\$ Welcome to EE.SE. Your post should normally be a comment, as it doesn't address the question directly. If you instead formulate a new question, which can be answered by your post, it will be much more transparent for other readers. Nevertheless this is a lot more work and has to be written well, too. \$\endgroup\$ – Ariser 19 hours ago

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