iWatt Power Supply Schematic

(From www.ecnmag.com)

The link post above shows the schematic diagram of iWatt 1810 power supply. I want to know that why did they use the combination of L1 & FB1 after the bridge rectifier.What is the purpose of connecting such a components with the C5 & C2? Does they form some sort of filters?

I guess maybe the designer has put these resistors (R2 & R5) as a damper for possible resonant oscillations in the rectifier output pi filter that C5, L1 and C2 form. Is it true or somthing I missing?


2 Answers 2


L1, Bead, C5, C2: You are guessing right, these components act as a filter. The transients caused by U1's switching need to be kept away from the mains input. While you can read the diagram from left to right when thinking about the direction of the power flowing from the input via the rectifier to the transformer, the switching noise has its source at the node labeled "C" (U1.1, U1.2). One way for the noise to leave the device is from right to left, via the rectifier towards the input, and the filter is right in this path.

R2, R5: These are called start-up resistors. A small current through R2 and R5 charges C3 and C6, and likely, an internal Z-Diode from U1.4 to U1.5 holds the voltage across C3 and C6 at 10...15 V. Once C3 and C6 are charged, U1 is able to start and from there, D6 provides the auxiliary power to C3 and C6 and for U1. R2 and R5 do not act as a filter (as long as you don't get too theoretical and call R2+R5 and C3+C6 a low pass RC filter, which would of course be true but not very practical). Actually, there even is a resistor to dampen the filter, but this is the one they've put parallel to L1 (R10).

If you want to understand the circuit, it'll help a great deal to redraw it with the major internal components and building blocks of U1, and not sticking to the IC's pinout when drawing the schematic.


I too am guessing, and guess similarly to you.
The circuit shown is a common one when used in balanced form but much less common when used unbalanced as you show here.

enter image description here

They appear to have cut corners by using an unbalanced "Pi filter" and the added the bead to try to achieve emission standards.

A less corner cutting circuit is seen here http://media.digikey.com/photos/rdl/rdr_159_schematic.jpg
In your circuit they use a 1 mH inductor and 4.4k parallel resistor. Th inductor impedance equals resistor impedance at very roughly the 1MHz range, limiting the filter Q at very high frequencies.

enter image description here

Design page for that smps here

Digikey smps design resources - 5 pages !!!.

Diagrams can be viewed with right click then 'open image in new tab'.

Following are all realted but varoiably 'on target' for main question. By showing what people do in a range of supplies you gt a feel for how useful a given cct is liable to be.

Here is a switch integrations version with unbalanced filter, no damping and no bead! - note that sole inductor is in the "ground" leg. Interesting.

And here Linkswitc use balanced filter with damping on one side only

Viper don't need no steenking filter

Classic line side filtering - one stage

... and two stages

NO filtering - but has series string VCC fed with divided cap to get voltage rating for startup with winding fed takeover.


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