Inductor selection for MC34063

[indecided]

Hi..

i've got a ciggeratte lighter powered 5v switching regulator (to emulate the charging facility of a USB port in the car) and I wanted to make a duplicate of it.

Having figured out the heart was a switching reg MC34063, I soon discovered that it was a generic reference design the manufacturer had used.

It takes in about 12v - 14.4v (theoretically) and puts out 5v, 850mA (says on the label).

Now, i've gotten all the values of the parts save one - the inductor.

I know there's a method of calculating the inductance value but the problem is I can't actually figure out how.. the datasheet askes for Vsat (voltage saturation?) and it's not something i've done before, so I'm rather clueless.

Can someone recommend a method/solution/value for this? The enclosed circut is how everything is pretty much set up, under the "medium power" design in the PDF.

https://www.electro-tech-online.com/custompdfs/2005/04/AN920-DPDF.pdf

Also, it would be nice to explain the particular significance of the inductor in this circut (being in Malaysia, you either have to buy 10 pieces at an extremely inflated price off RS, or.. scrounge around miserably in the dotting electronic shops around) as per reason it would be very pleasant to scrap it.

Lastly, there was something unusual - the manufacturer had put a bit of heatshrink around one of the capacitors - the 47uF one going from +ve to ground. There are no moving parts/naked wires, any idea of its purpose?


Thanks!

 

[Russlk]

Vsat is the saturation voltage of the transistor. It is given as 1.3 volts max in the data sheet. I calculate that 600 microhenries will work.

 

[Nigel Goodwin]

Lastly, there was something unusual - the manufacturer had put a bit of heatshrink around one of the capacitors - the 47uF one going from +ve to ground. There are no moving parts/naked wires, any idea of its purpose?

Although it's not much help!, in the UK the Digital Satellite TV service is called BSkyB, and uses specific digital satellite receivers. One of the manufacturers was Grundig (who went bankrupt in 2003, and no longer exist - although the name lives on, placed on cheap rubbish, sold by a large UK retail chain), a common fault on these receivers is capacitor failure in the PSU (high ESR).

Anyway, the reason for this story is that a number of them (on some models) have heatshrink sleeving around one or more capacitors - and the capacitor kits you can buy to repair them include heatshrink to fit on the new ones.

I've always wanted to know the reason as well!.

 

[Exo]

maybe to keep the pieces together when they explode :?:
:wink:

 

[fredsm]

600 uH looks a bit much for a 12v/12v converter.
Without doing any calculations, it looks like 100 uH
would be more like it.
<als>

 

[Russlk]

100 uH is used in one of the application circuits, a high current, 5v supply.

 

[indecided]

oh dear, looks like i didn't correct one of the values in the schematic

it should be 5v/850ma and not 12v/850ma.

sorry..

 

[Russlk]

But you said originally that it was 5 v, 850 mA, so that is what I used to calculate 600 uH.