Datasheet

td Lp COSS= p · ·
Freq 2 Lp COSS= pg g
LM5023
OUT
QR
CS
VCC
GND
90-264 VAC
High
Voltage
Start-up
VSD
Enhancement
Mode FET
CVCC
R
START-UP
LM5023
www.ti.com SNVS961D –APRIL 2013–REVISED JANUARY 2014
Figure 9. Start-Up With an Enhancement Mode FET
Quasi Resonant Operation
A Quasi-Resonant controlled Flyback converter operates by storing energy in the transformers primary during the
MOSFETs on-time. During the on-time (T
ON
) VIN is applied across the primary of the transformer. The primary
current starts out at zero and ramps towards a peak value (I
PEAK
). When the peak primary current reaches the
feedback compensation error voltage the PWM comparator resets the output drive, turning off the MOSFET. Due
to the phasing of the transformer, the output diode is reversed biased during the MOSFET on-time.
During the MOSFETs off time the output diode is forward biased and the stored energy in the transformer
primary inductor is transferred to the output. The voltage seen on the secondary inductor is V
OUT
plus the output
diodes forward voltage drop, V
F
. The current in the output inductor linearly decreases from I
PEAK
• Ns/Np to zero,
refer to Figure 11.
When the current in the secondary reaches zero, the transformer is demagnetized, and there is an open circuit
on the secondary, and with the primary MOSFET also turned off, there is an open on the primary. A resonant
circuit is formed between the transformers primary inductance and the MOSFET output capacitance. The
resonant frequency is calculated by:
During the resonant period the Drain voltage of the MOSFET will ring down towards ground, refer to Figure 10.
When the Drain voltage is at its minimum the Flyback MOSFET is turned back on. The point where the voltage is
at its minimum is calculated by:
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