Datasheet

LT3511
13
3511fc
at maximum load and minimum input voltage. The switch
current is highest at this point along with the energy stored
in the leakage inductance. A 0.5W Zener will satisfy most
applications when the highest V
ZENER
is chosen. Choosing
a low value for V
ZENER
will cause excessive power loss as
shown in the following equations:
DZ Power Loss =
1
2
•L
C
•I
PK(VIN(MIN))
2
•f
SW
•
1+
N
PS
•V
OUT
+ V
F
()
V
ZENER
–N
PS
•V
OUT
+ V
F
()
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
L
C
= Leakage Inductance
I
PK(VIN(MIN))
=
V
OUT
•I
OUT
•2
η •V
IN(MIN)
•D
VIN(MIN)
f
SW
=
1
t
ON
+ t
OFF
=
1
L
PRI
•I
PK(VIN(MIN))
V
IN(MIN)
+
L
PRI
•I
PK(VIN(MIN))
N
PS
•V
OUT
+ V
F
()
Tables 2 and 3 show some recommended diodes and
Zener diodes.
Table 2. Recommended Zener Diodes
PART
V
ZENER
(V)
POWER
(W) CASE VENDOR
MMSZ5266BT1G 68 0.5 SOD-123 On Semi
MMSZ5270BT1G 91 0.5 SOD-123
CMHZ5266B 68 0.5 SOD-123 Central
Semiconductor
CMHZ5267B 75 0.5 SOD-123
BZX84J-68 68 0.5 SOD323F NXP
BZX100A 100 0.5 SOD323F
Table 3. Recommended Diodes
PART I (A)
V
REVERSE
(V) CASE VENDOR
BAV21W 0.625 200 SOD-123 Diodes Inc.
BAV20W 0.625 150 SOD-123
Leakage Inductance Blanking
When the power switch turns off, the flyback pulse
appears. However, a finite time passes before the trans-
former primary-side voltage waveform approximately
represents the output voltage. Rise time on the SW
node and transformer leakage inductance cause the
delay. The leakage inductance also causes a very fast
voltage spike on the primary side of the transformer.
The amplitude of the leakage spike is largest when
power switch current is highest. Introduction of an
internal fixed delay between switch turn-off and the
start of sampling provides immunity to the phenomena
discussed above. The LT3511 sets internal blanking to
150ns. In certain cases leakage inductance spikes last
longer than the internal blanking, but will not signifi-
cantly affect output regulation.
Secondary Leakage Inductance
In addition to primary leakage inductance, secondary
leakage inductance exhibits an important effect on ap-
plication design. Secondary leakage inductance forms
an inductive divider on the transformer secondary. The
inductive divider effectively reduces the size of the
primary-referred flyback pulse. The smaller flyback
pulse results in a higher regulated output voltage. The
inductive divider effect of secondary leakage inductance
is load independent. R
FB
/R
REF
ratio adjustments can ac-
commodate this effect to the extent secondary leakage
inductance is a constant percentage of mutual inductance
(over manufacturing variations).
Winding Resistance Effects
Resistance in either the primary or secondary will re-
duce overall efficiency (P
OUT
/P
IN
). Good output voltage
regulation will be maintained independent of winding
resistance due to the boundary mode operation of the
LT3511.
Bifilar Winding
A bifilar, or similar winding technique, is a good way to
minimize troublesome leakage inductances. However, re-
member that this will also increase primary-to-secondary
capacitance and limit the primary-to-secondary breakdown
voltage, so bifilar winding is not always practical. The
Linear Technology applications group is available and
extremely qualified to assist in the selection and/or design
of the transformer.
APPLICATIONS INFORMATION