LT1103_15 Datasheet, PDF (25/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
APPLICATIONS INFORMATION
where N is the transformer turns ratio of secondary to
primary and DC is the duty cycle. This formula can be
rewritten in terms of duty cycle as:
( ) ^{DC =}
V_OUT
V_OUT + N • V_IN
It is important to define the full range of input voltage, the
range of output loading conditions and the regulation
requirements for a design. Duty cycle should be calculated
for both minimum and maximum input voltage.
In many applications, N can vary over a wide range without
degrading performance. If maximum output power is
desired, N can be optimized:
( ) ^{N(OPT) =}
V_OUT + Vf
V_M – V_IN(MAX) – V_SNUB
where
Vf = Forward voltage of the output diode
V_M = Maximum switch voltage
V_SNUB = Snubber clamp level – primary flyback
voltage.
In the isolated flyback mode, the LT1103/LT1105 sense
and regulate the transformer primary voltage V_PRI during
“switch off” time. The secondary output voltage will be
regulated if V_PRI is regulated. V_PRI is related to V_OUT by:
( ) _{VPRI =}
V_OUT + Vf
N
This allows duty cycle for an isolated flyback converter to
be rewritten as:
( ) DC = Duty Cycle = VPRI
V_PRI + V_IN
An important transformer parameter to be determined is
the primary inductance L_PRI. The value of this inductance
is a trade-off between core size, regulation requirements,
leakage inductance effects and magnetizing current ∆I.
Magnetizing current is the difference between the primary
current at the start of “switch on” time and the current at
the end of “switch on” time. If maximum output power is
needed, a reasonable starting value is found by assigning
∆I a value of 20% of the peak switch current (2A for the
LT1103 and set by the external FET rating used with the
LT1105). With this design approach, L_PRI is defined as:
L_PRI
=
V_IN
(∆I)(f)^{}1 +
V_IN
V_PRI


If maximum output power is not required, then ∆I can be
increased which results in lower primary inductance and
smaller magnetics. Maximum output power with an isolated
flyback converter is defined by the primary flyback voltage
and the peak allowed switch current and is limited to:
( ^{( )} ) ^{( ) POUT(MAX) =}
V_PRI
V_PRI + V_IN

^VIN

_IP
–
∆I
2 
–
Ip ²R^E

where
R = Total “switch” on resistance
I_P = Maximum switch current
E = Overall efficiency ≈ 75%
Peak primary current is used to determine core size for the
transformer and is found from:
( )(( ))(( ₎ ) _{IPRI =}
V_OUT I_OUT
E V_PRI
V_PRI + V_IN
V_IN
+ ^∆I
2
A second consideration on primary inductance is the
transition point from continuous mode to discontinuous
mode. At light loads, the flyback pulse across the primary
will drop to zero before the end of “switch off” time. The
load current at which this starts to occur can be calculated
from:
( ⁽ ) ( ⁾ )( )( ) ^{IOUT(TRANSITION) =}
V_PRI • V_IN ²
V_PRI + V_IN ² 2V_OUT
f
L_PRI
The forward converter as shown below is another
transformer-based topology that converts one voltage to
either a higher or a lower voltage.
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