My project requires 30V DC and I need to get the bulky transformer and AC out of the enclosure. The plan is to use an external off the shelf mains 12V power supply and which will then allow it to work off a 12v battery if required.

Fortunately the current required is very low, less than 50mA, so I want to shoot for something not greater than 100mA.

If possible I would like to use the MC34063A as it is readily available here. Any inputs and suggestions are welcome.
Here is my idea at the moment:

 

You have it nailed
Did u spice it

 

You have it nailed
Did u spice it

No, cannot simulate it. I have never done any simulation before. I have not even built it on breadboard as I will need to first order the parts and wanted to make sure I am close.
Thanks for the feedback.

 

Don't worry ..
give it time and bid daddy will spice it up for u..

 

Don't worry ..
give it time and bid daddy will spice it up for u..

Thanks!!!
Do you know of the current rating for the L1 coil is determined by the output current. I can only locate small ones that look like small resistors at 220uH here. http://shop.rabtron.co.za/catalog/inductors-c-360.html?osCsid=814b567d36b1d78de7675de4beb0539f

I have just opened a 12V cellphone charger and it has a nice torroidal coil about 10 mm diameter without the copper with one full layer of wire and uses the same IC. I have no idea if its even close.

 

current rating is not easy to measure, as they are switched.
I do not use spice.
Sgt is familiar with it.

You have to wait till he sees this, I think he is a bit bzy.

By the L1 should be strong enuf in switched mode cases. U can salvage an inductor from ATX supply

 

The load current is not equal to the inductor current.
Notice the switch is in between the two.

Anyway, the simplest relationship is the following...

You have power out = power in * efficiency.
For DC quantities:
Pin = Vin * Iinductor
Pout = Vout * ILoad

Vout * Iload = Vin * Iinductor * Eff

If Iload < 100 mA then:

Iinductor < Vout/Vin * Iload / Eff

(say efficiency is 70%)

Iinductor < 30/12*100mA / 0.7= 360 mA

Keep in mind that's DC value, not peak.