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Q&A How to define Vsat for MOSFET as a switch?

I was looking at this design: It uses MC34063 as switching controller. I wanted to adapt this circuit, and I was busy trying to pick R1 value. The datasheet has following formulas: The issue I hav...

2 answers  ·  posted 3y ago by Kranulis‭  ·  last activity 3y ago by Andy aka‭

Question boost-converter MOSFET
#2: Post edited by user avatar Kranulis‭ · 2020-09-24T19:43:08Z (over 3 years ago)
  • I was looking at this [design](https://threeneurons.wordpress.com/nixie-power-supply/):
  • ![Image alt text](https://electrical.codidact.com/uploads/Zp78CAHwK89eqGAFHZSzXMzr)
  • It uses [MC34063](https://datasheet.lcsc.com/szlcsc/1809191921_HTC-Korea-TAEJIN-Tech-MC34063BD_C126079.pdf) as switching controller. I wanted to adapt this circuit, and I was busy trying to pick R1 value. The datasheet has following formulas:![Image alt text](https://electrical.codidact.com/uploads/JLGD1skM5JfjoYreaJVtihZv)
  • The issue I have is with calculating ton/toff, since Vsat is defined as saturation voltage of switch. The switch is [IRF470](http://pdf.datasheetcatalog.com/datasheet/fairchild/IRF740.pdf), and since it is a MOSFET, it does not have a defined saturation voltage, but Rds. Therefore, the saturation voltage is dependent on drain-source current. This is where I get confused, and uncertain regarding how to approach this. How do I find Ipeak that is required for selecting R1 using MOSFET as a switch?
  • I was looking at this [design](https://threeneurons.wordpress.com/nixie-power-supply/):
  • ![Image alt text](https://electrical.codidact.com/uploads/Zp78CAHwK89eqGAFHZSzXMzr)
  • It uses [MC34063](https://datasheet.lcsc.com/szlcsc/1809191921_HTC-Korea-TAEJIN-Tech-MC34063BD_C126079.pdf) as switching controller. I wanted to adapt this circuit, and I was busy trying to pick R1 value. The datasheet has following formulas:![Image alt text](https://electrical.codidact.com/uploads/JLGD1skM5JfjoYreaJVtihZv)
  • The issue I have is with calculating ton/toff, since Vsat is defined as saturation voltage of switch. The switch is [IRF740](http://pdf.datasheetcatalog.com/datasheet/fairchild/IRF740.pdf), and since it is a MOSFET, it does not have a defined saturation voltage, but Rds. Therefore, the saturation voltage is dependent on drain-source current. This is where I get confused, and uncertain regarding how to approach this. How do I find Ipeak that is required for selecting R1 using MOSFET as a switch? It seems like datasheet was written with the assumption that an NPN transistor would be used as a switch.
boost-converter MOSFET
boost-converter MOSFET
#1: Initial revision by user avatar Kranulis‭ · 2020-09-24T19:41:32Z (over 3 years ago) 
How to define Vsat for MOSFET as a switch?
I was looking at this [design](https://threeneurons.wordpress.com/nixie-power-supply/):
![Image alt text](https://electrical.codidact.com/uploads/Zp78CAHwK89eqGAFHZSzXMzr)

It uses [MC34063](https://datasheet.lcsc.com/szlcsc/1809191921_HTC-Korea-TAEJIN-Tech-MC34063BD_C126079.pdf) as switching controller. I wanted to adapt this circuit, and I was busy trying to pick R1 value. The datasheet has following formulas:![Image alt text](https://electrical.codidact.com/uploads/JLGD1skM5JfjoYreaJVtihZv)
The issue I have is with calculating ton/toff, since Vsat is defined as saturation voltage of switch. The switch is [IRF470](http://pdf.datasheetcatalog.com/datasheet/fairchild/IRF740.pdf), and since it is a MOSFET, it does not have a defined saturation voltage, but Rds. Therefore, the saturation voltage is dependent on drain-source current. This is where I get confused, and uncertain regarding how to approach this. How do I find Ipeak that is required for selecting R1 using MOSFET as a switch?
boost-converter MOSFET