SI9140 Datasheet, PDF (15/15 Pages) Vishay Siliconix – SMP Controller For High Performance Process Power Supplies

Si9140
Vishay Siliconix
P_Dissipation
in switch
=
I_RM
S
2
SW
×
R_SW
+
-^Q----^S----^W-----^×-----^V-----^I-^N-----^×-----^f--^O---^S----^C-
2
+
I_--P----P----×_-----V_----O-----×_-----τ_---C-----×_----f_{--O----S---C--}
2
I_{RMS SW} =
(I_PEAK2
+
I_PP2
+
I_PEAK
×
I_PP)
×
------^V----^O--------
3 × V_IN
P_Dissipation
in
synchronous
rectification
=
I_RMS
2
RECT
×
R_RECT
+
-^Q----^R----^E---^C----^T----^×-----^V----^I--^N-----^×----^f--^O----^S---^C--
2
I_{RMS RECT} =
(
I_P
EA
2
K
+
I_PP2
+
I_PEAK
×
I_PP)
×
V_{-----I-N-----–-----}V_----O--
3 × V_IN
I_PP = I_PEAK + ∆I
∆I
=
--------------^V----^O----²--------------
L × f_OSC × V_IN
I_PEAK
=
P_{-----I-N-----–-----}(_--0_---._--5_-----×_-----V_----O-----×_-----∆_---I_--)_-
V_O
P_IN
=
V_-----O-----×_----I_--O--
η
I_RMSSW = Switch rms current
R_SW
= Switch on resistance
I_RMSRECT = Synchronous rectifier rms current
R_RECT
= Synchronous rectifier on resistance
Q_SW
= Total gate charge of switch
Q_RECT = Total gate charge of synchronous rectifier
V_IN
= Input voltage
V_O
= Output voltage
I_O
= Output current
f_OSC
= Switching frequency
η
= efficiency
τ_C
= Crossover time
Current
I_O
I_PEAK
0A
I_PP
time
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. Ferrite has a lower core cost and loss at this
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. The
inductance is calculated with following equation:
L = ---------------^V----^O----²---------------
V_IN × ∆I × f_OSC
∆I = desired output current ripple. Typically ∆I = 25% of
maximum output current.
FaxBack 408-970-5600, request 70026
www.siliconix.com
S-58034—Rev. G, 15-Mar-99
15