NCL30081
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17
During the on−time of the MOSFET, the bulk voltage
V
bulk
is applied to the magnetizing and leakage inductors L
p
and L
leak
and the current ramps up.
When the MOSFET is turned−off, the inductor current
first charges C
lump
. The output diode is off until the voltage
across L
p
reverses and reaches:
N
sp
ǒ
V
out
) V
f
Ǔ
(eq. 1)
The output diode current increase is limited by the leakage
inductor. As a consequence, the secondary peak current is
reduced:
I
D,pk
t
I
L,pk
N
sp
(eq. 2)
The diode current reaches its peak when the leakage inductor
is reset. Thus, in order to accurately regulate the output
current, we need to take into account the leakage inductor
current. This is accomplished by sensing the clamping
network current. Practically, a node of the clamp capacitor
is connected to R
sense
instead of the bulk voltage V
bulk
.
Then, by reading the voltage on the CS pin, we have an
image of the primary current (red curve in Figure 43).
When the diode conducts, the secondary current decreases
linearly from I
D,pk
to zero. When the diode current has
turned off, the drain voltage begins to oscillate because of
the resonating network formed by the inductors (L
p
+L
leak
)
and the lump capacitor. This voltage is reflected on the
auxiliary winding wired in flyback mode. Thus, by looking
at the auxiliary winding voltage, we can detect the end of the
conduction time of secondary diode. The constant current
control block picks up the leakage inductor current, the end
of conduction of the output rectifier and controls the drain
current to maintain the output current constant.
We have:
I
out
+
V
REF
2N
sp
R
sense
(eq. 3)
The output current value is set by choosing the sense
resistor:
R
sense
+
V
ref
2N
sp
I
out
(eq. 4)
From Equation 3, the first key point is that the output
current is independent of the inductor value. Moreover, the
leakage inductance does not influence the output current
value as the reset time is taken into account by the controller.
time
time
Figure 43. Flyback Currents and Auxiliary Winding Voltage in DCM
V
aux
(t)
t
on
t
demag
t
1
t
2
I
sec
(t)
I
pri
(t)
N
sp
I
D,pk
I
L,pk
