SC4809A Просмотр технического описания (PDF) - Semtech Corporation
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SC4809A Datasheet PDF : 16 Pages
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SC4809A/B/C
POWER MANAGEMENT
Application Information (Cont.)
Control-to-Output transfer function for the flyback power The DC transfer function of a CCM flyback converter is:
stage operating in CCM is given by:
dVO
dD
=
VI
(1− D)2
•
NS
NP
•
1+
S
ωz1
•
1−
S
ωz2
1+
S
ωO •
Q
+
S2
ωO 2
where:
ωz1 =
1
RC •
C
ωz2
≈
(1− D)2 • R
D • LSEC
ωO ≈
1− D
LSEC • C
Q ≈ (1− D) x R
L SEC
C
Control-to Output transfer function for the flyback power
stage operating in DCM is given by:
VO + VD
VIN(min) − VRds(on)
=
1
N
•
Dmax
1− Dmax
where VO = output voltage,
VD = forward voltage drop across rectifier D1,
N = turns ratio, equal to NP/NS,
D = duty cycle.
Transformer Design
The transformer in a flyback converter is actually a coupled
inductor with multiple windings. Transformers provide
coupling and isolation whereas inductors provide energy
storage. The energy stored in the air gap of the inductor
is equal to:
( ) E = LP
•
IPEAK
2
2
dVO
dD
= VI
•
NS
NP
•
R • TS
2 • LSEC
•
1
1
+S
ωP
where:
2
ωP = R • C
Peak current mode control requires simpler
compensation, has pulse-by-pulse current limiting, and
has better load current regulation. Primary and secondary
RMS currents can be up to two times higher for
discontinuous mode than for CCM. Discontinuous
conduction mode would require using a transistor with a
higher current rating. Because the output ripple current
is less than it would be continuous mode were used, the
output capacitors are smaller. Continuous conduction
mode (CCM) was therefore chosen.
where E is in Joules, LP is the primary inductance in
Henries, and IPEAK is the primary peak current in Amperes.
When the switch is on, D1 is reverse biased due to the
dot configuration of the transformer. No current flows in
the secondary windings and the current in the primary
winding ramps up at a rate of:
∆IL
∆t
=
VIN(min) − VRds(on)
LP
The output capacitor, COUT, supplies all of the load current
at this time. Because the converter is operating in the
continuous conduction mode, ∆IL is the change in the
inductor current which appears as a positive slope ramp
on a step. The step is present because there is still
current left in the secondary windings when the primary
turns on. When the switch turns off, current flows through
the secondary winding and D1 as a negative ramp on a
step, replenishing COUT and supplying current directly to
the load.
2005 Semtech Corp.
9
www.semtech.com
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