DocID025611 Rev 1

11/20

AN4414

Description of SMPS

20

Case 1

Assuming Lp = 400 µH

Equation 28

as

VR = 10 V,

so

Equation 29

Np is two times of Ns.

Using a lesser diameter wire (DCR will obviously be high, but select in order to have

it < 10 E in any case to avoid copper losses), a large number of turns can be accommodated

in the available bobbin. Here we can consider the EE19 bobbin. Common practice to fix this

type transformer specification is to wound primary turns as a selected AWG wire on the

bobbin till the required inductance (Lp) is not achieved. Freeze Np at desired Lp. Select

secondary turns, Ns = Np / n and auxiliary turns to maintain a minimum VCC bias voltage to

the device.

With Lp = 400 µH and Pout = 0.5 W in Table 3 are the test results:

Table 3. Test results

We can see that keeping the primary inductance as 400 µH, the drain current reaches to its

internal limit threshold level, which is not desirable as the device activates the hiccup mode

of operation at this current level, so we need to increase the primary inductance in

reasonable limits to avoid higher drain current.

Vmains (VAC) Ipkp (mA)

Turn-on time (ns)

Comments

80

175

700

-

120

210

Ton > Tonmin

-

180

290

Ton ~ Tonmin = 400

-

230

350

Ton ~ Tonmin = 400

-

265 and above

~ 400

Ton ~ Tonmin = 400

Device current reaches to Idlim and enters into

hiccup mode

n

VR

Vout VF

+

---------------------------

2

==

(ignoring VF)