SI9140CQ VISHAY [Vishay Siliconix], SI9140CQ Datasheet - Page 15

• Manufacturer Part Number: SI9140CQ
• Description: SMP Controller For High Performance Process Power Supplies
• Manufacturer: VISHAY [Vishay Siliconix]

INDUCTOR
The size and value of the inductor are critical in meeting
overall circuit dimensional requirements and in assuring
proper transient voltage regulation. The size of the core is
determined by the output power, the material of the core, and
the operating frequency. To handle higher output power, the
core must be larger. Luckily, a higher switching frequency will
lower the inductance value, decreasing the core size.
However, a higher switching frequency can also mean greater
core loss.
In applications where the dc flux density is high and the ac
flux density swing is only 100 to 200 gauss, the core loss will
be negligible compared to the wire loss. Kool Mu is the best
material to use at 500 kHz to deliver 30 W in the minimum
volume.
0 A
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I
P
P
I
P
I
I
Current
PEAK
RMS SW
RMS RECT
PP
I
IN
Dissipation
Dissipation
=
=
=
------------------------------------ -
L
I
=
V
------------------ -
PEAK
O
Ferrite has a lower core cost and loss at this
f
P
-----------------------------------------------------
=
OSC
V
IN
I
=
PEAK
in switch
in synchronous rectification
O
I
+
O
–
2
I
PEAK
0.5
I
V
I
PEAK
IN
V
O
2
V
+
=
2
O
I
+
PP
I
RMS SW
I
PP
2
I
+
2
I
+
PEAK
I
2
PEAK
R
SW
I
PP
=
I
+
PP
I
Q
--------------------------------------------- -
RMS RECT
----------------- -
3
SW
V
V
----------------------
3
O
V
IN
IN
V
–
2
V
IN
V
2
IN
O
f
R
OSC
I
RECT
PP
+
I
---------------------------------------------------- -
time
+
PP
I
R
I
R
Q
Q
V
V
I
f
RMSSW
RMSRECT
O
OSC
Q
--------------------------------------------------- -
C
IN
O
SW
RECT
SW
RECT
RECT
V
I
O
O
frequency, but the core size is fairly large. If the power supply
is designed on the motherboard and space is not a critical
issue, ferrite is a better choice.
The higher switching frequency reduces the core size by
decreasing the amount of energy that must be stored between
switching periods. It also accelerates the transient response
to the load by decreasing the inductance value.
inductance is calculated with following equation:
maximum output current.
I = desired output current ripple.
2
L
V
2
C
IN
=
=
=
=
=
=
=
=
=
=
=
=
=
f
OSC
-------------------------------------- -
V
f
Switch rms current
Switch on resistance
Synchronous rectifier rms current
Synchronous rectifier on resistance
Total gate charge of switch
Total gate charge of synchronous rectifier
Input voltage
Output voltage
Output current
Switching frequency
efficiency
Crossover time
IN
OSC
V
O
I
2
f
OSC
Vishay Siliconix
S-58034—Rev. G, 15-Mar-99
Typically
I = 25% of
Si9140
The
15