Application Information: continued
be used in synchronized operation, the inductor value
should be recalculated to work with the slope compensa-
tion ramp reduced to 67% of the normal value.
Powering the CS5124/6 from a Transformer Winding
There are numerous ways to power the CS5124/6 from a
transformer winding to enable the converter to be operated
at high efficiency over a wide input range. Two ways are
shown in the application circuits.
The CS5124 application circuit (main application diagram)
is a flyback converter that uses a second flyback winding
to power V
CC
. R4 improves V
CC
regulation with load
changes by snubbing the turn off spike. Once the turn off
spike has subsided the voltage of this winding is voltage
proportional to the voltage on the main flyback winding.
This voltage is regulated because the main winding is
clamped by the regulated output voltage.
In the CS5126 application circuit (below) an extra winding
is added to the forward inductor to power V
CC
. This wind-
ing is phased to conduct during the off time of the forward
converter and performs the same function as the flyback
winding above.
A flyback winding from a forward transformer can also be
used to power V
CC
. Ideally the transformer volt-second
product of a forward converter would be constant over the
range of line voltages and load currents; and the trans-
former inductance could be chosen to store the required
level of energy during each cycle to power V
CC
. Even
though the flyback energy is not directly regulated it
would remain constant. Unfortunately in a real converter
there are many non-ideal effects that degrade regulation.
Transformer inductance varies, converter frequency varies,
energy stored in primary leakage inductance varies with
output current, stray transformer capacitances and various
parasitics all effect the level of energy available for V
CC
. If
too little energy is provided to V
CC
, the bootstrapping cir-
cuit must provide power and efficiency will be reduced. If
too much energy is provided V
CC
rises and may damage
the controller. If this approach is taken the circuit must be
carefully designed and component values must be con-
trolled for good regulation.
CS5124/6
Additional Application Diagram
36-75V
IN
L1
10µH
R2
200k
R1
39k
D2
C1
1.5µF,
100V
R6
17.4k
D3
11V
C5
1µF,
25V
MMBD6100L
C6
390pF
Q1
F2T493
CTX15-14526
T1
CTX15-14527
5V
OUT
C3
0.2µF, 100V
C2
1.5µF,
100V
T2
Q2
IRF634
R4
0.2Ω
1/4W
MBRB2060CT
C12
.01µF
R7
2k
R3
30.1k
C7
47µF
C8
47µF
V
CC
UVLO
ENABLE
R9
SYNC
10k
C4
1000pF
R10
10k
C11
0.1µF
SYNC
SS
Gnd
GATE
IS
U2
C9
.01µF
V
FB
C10
1000pF
TPS5908
R8
10.0k
ISOLATED
RTN
CS5126
48VRTN
48V to 5V, 5A forward converter using the CS5126
9
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
