EEVblog #110 - Let's Design a DC to DC Switchmode Converter

• EEVblog #110 - Let's Design a DC to DC Switchmode Converter
  Posted by JonnyJacksonX on 26 Mar, 2016 15:42
• Hello.
  
  I'm new to electronics and I need to make a step down circuit.
  I want to design a bike led lamp feed by 3 18650 li ion battery, so 11.1V Vin.
  Then I need to step down the voltage with a buck converter, as a linear converter seems to be really inefficient.
  I find the #110 video from eevblog about the venerable MC34063.
  For calculating the values of the in and out parts, like inductors, resistors, capacitor... I need use the frequency as a variable.
  But, here is the issue, what is the frequency to use with a battery? Aren't the voltage a constant? The frequency is 0 (I know it's not)?
  
  Thanks for answering my noob question. And sorry for the bad english.
   
  
  
  
• #1 Reply
  Posted by Simon on 26 Mar, 2016 16:05
• You have to pick a frequency you want to use, that is the frequency the chip will oscillate at. This comes with the knwledge that the higher the frequency the less efficient it may be but the smaller the parts. Just go for a ballpark 40KHz to get you going.
• #2 Reply
  Posted by JonnyJacksonX on 26 Mar, 2016 16:44
• well.
  
  I didn't understanded that the frequency was about the IC, not the source of power. So I will use 40 kHz.
  
  So thank you Simon for your rapid answer.
  
  
• #3 Reply
  Posted by ghulands on 03 Apr, 2016 00:41
• Quote from: JonnyJacksonX on 26 Mar, 2016 16:44

  well.
  
  I didn't understanded that the frequency was about the IC, not the source of power. So I will use 40 kHz.
  
  So thank you Simon for your rapid answer.
  

  
  The 3 18650's are a DC source so they don't oscillate - frequency of 0. I found afrotech's video on converters helpful in understanding their operation.