Communities

Writing
Writing
Codidact Meta
Codidact Meta
The Great Outdoors
The Great Outdoors
Photography & Video
Photography & Video
Scientific Speculation
Scientific Speculation
Cooking
Cooking
Electrical Engineering
Electrical Engineering
Judaism
Judaism
Languages & Linguistics
Languages & Linguistics
Software Development
Software Development
Mathematics
Mathematics
Christianity
Christianity
Code Golf
Code Golf
Music
Music
Physics
Physics
Linux Systems
Linux Systems
Power Users
Power Users
Tabletop RPGs
Tabletop RPGs
Community Proposals
Community Proposals
tag:snake search within a tag
answers:0 unanswered questions
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
votes:4 posts with 4+ votes
created:<1w created < 1 week ago
post_type:xxxx type of post
Search help
Notifications
Mark all as read See all your notifications »
Q&A

Post History

60%
+1 −0
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 4y ago by Kranulis‭  ·  last activity 4y ago by Andy aka‭

#2: Post edited by user avatar Kranulis‭ · 2020-09-24T19:43:08Z (about 4 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.
#1: Initial revision by user avatar Kranulis‭ · 2020-09-24T19:41:32Z (about 4 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?