Description of SMPS

AN4414

12/20

DocID025611 Rev 1

Case 2

Let's take Lp = 680 µH and increasing the reflected voltage slightly: VR = 18 V.

Equation 30

So

recalling the equation:

Equation 31

In our case

V = voltage at flyback input,

L = primary magnetizing inductance, Lp

and dt = turn-on time

With 680 µH as a primary inductance, let's analyze the flyback stage at different DC input

conditions. At Vdcmin ~ 100 VDC (which is corresponding to 80 VAC), calculating the peak

current through the device in a discontinuous conduction mode.

Equation 32

This gives:

Equation 33

Dmax = 15.2%

Equation 34

Ton = Dmax x Ts = 1.33 µs

Equation 35

Looking into waveforms in Figure 2 and Figure 3., we have the same practical results. The

waveforms are captured at different input voltage conditions.

As long as the turn-on time of the flyback switch is greater than its minimum turn-on time

(internally fixed, 400 ns in the VIPER17), the device peak current is maintained as per the

duty decided by the feedback loop. As input voltage increases and duty reduces in order

“Tonmin” is achieved, there is no further decrease in turn-on time with a further increase in

input voltage and the device peak current will increase as input voltage increases and it can

be calculated using Equation 35 at fixed Tonmin = 400 ns. Since the MOSFET clamp circuit

ensures the maximum DC input to flyback as per the Zener biasing and maintains maximum

n

VR

Vout VF

+

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

3.2 considering VF

5V

=



==

VL

di

dt

-----



=

VR

Dmax

1Dmax

–



------------------------------ Vdcmin



18V

==

Ipkp

Vdcmin Ton



Lp

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

196mA

==