4
Fig. 3 Basic configuration
Fig. 5 Characteristics
Fig. 4 LC series resonant circuit
VL
e
4. Outline of the Current Resonant Type
Supply
Fig. 3 shows the block diagram for the current resonant
switch mode power supply. The primary side is an LC
series circuit. It consists of the primary winding of the
transformer and a resonant capacitor in series. Two
power MOS FET’s in a push-pull configuration drive
the primary side of the transformer.
The switching action on the primary side of the trans-
former produces the main B+ on the secondary side.
5. Fundamental Theory of LC Series
Resonant Circuit
The LC series resonant switch mode power supply is a
frequency regulated power supply operating above reso-
nance. When the the load increases on the secondary
side of the transformer, the frequency decreases (oper-
ates closer to resonance) and the current increases. Con-
versely, when the load decreases, the frequency increases
and the current decreases.
VL (v)
e
Resonant point
Frequency
1
f=
2
p
LC
Increased Load
Decreased Load
Normal
Operating
Frequency
The main B+ is regulated by negative feedback. The
main B+ is fed into an error amplifier and outputted to a
photo coupler. The output of the photo coupler is ap-
plied to the primary side of the power supply to control
the switching speed.
DRIVE
Oscillator
PHOTO
COUPLER
B+
ERROR
AMP
