AN2753 Datasheet, PDF (10/31 Pages) STMicroelectronics – 6 W single-output VIPer17 demonstration board

Testing the board
AN2753
Figure 11 shows the drain current and the voltage on the feedback pin in a time interval of
about 10 ms. The system is working with a constant load but the voltage on the feedback
pin is a triangular wave shape as well as the peak drain current. These changes are the
result of the frequency jittering.
In a fixed frequency flyback converter, operating in discontinuous conduction mode, the
output power is proportional to the switching frequency according to the following formula:
Equation 1
P_OUT
=
1_--
2
⋅
L_P
⋅
I^2PK
⋅
f_SW
⋅
η
where L_P is the transformer primary inductance, I_PK is the drain peak current and η is the
converter's efficiency. The VIPer17 internal oscillator gives a switching frequency modulated
by a triangular waveform of 250 Hz (typ.). The power demand of the load is constant, but,
due to the variable switching frequency, the power delivered is not constant if I_PK is constant.
The control loop reacts to the unsteady switching frequency, modulating the feedback pin
voltage and then, the drain peak current.
Figure 11. Frequency jittering (115 V_{IN_AC}, full load)
CH2: VFB 200 mV/Div (Light blue)
CH4: I_DRAIN 50 mA/Div (Green)
Time: 1 ms/Div
10/31
Doc ID 14654 Rev 2