Power integrations LinkSwitch-4 LNK4*15D Series Application Note page 11

AN-69
Step 7 – Bias / Feedback Winding Design Parameters
The bias / feedback winding performs two functions, as its name
suggests. Firstly, it provides power to the controller after the start-up
period and secondly, it provides the feedback signal to monitor output
voltage and bulk capacitor voltage.
During the start-up period, the controller is powered by the charge
held on the V capacitor C . Before the voltage on C
CC VCC
V , 4.5 V, the output voltage from the bias winding must exceed
VCC(SLEEP)
V +V . The voltage from the Bias winding during start-up is
VCC(SLEEP) DB
related to the voltage on the secondary winding, which in turn, is
related to the voltage on the output capacitor, which is charging from
0 V to V . Hence more turns on the bias winding results in easier
OUT
starting with high output capacitance or constant current loads, but
results in a higher no-load power. Conversely, a lower number of
turns results in a reduced output capacitance or constant current load
start-up capability, but lower no-load power. A good starting point is
to aim for a no-load V voltage of between 8 V and 9 V, 7 V is the
CC
minimum. The no-load level of V
the secondary and bias turns, whereas the V
is increased by energy scavenged from leakage inductance and is not
practical to calculate. Check the V
checking that at the maximum AC input voltage and at maximum load
or start-up, V does not exceed 16.5 V.
CC
V
^
CC No Load
N =
BIAS V
The bias supply diode should be a silicon junction device, a Schottky
has too much reverse leakage and may prevent start-up. The spread-
sheet has 0.7 V entered as the default forward volt drop of the bias
supply diode, suitable for a silicon junction diode. An alternative
value may be entered into [B106] if required.
The spreadsheet defaults to calculating the required number of bias
wind turns to give at least 7 V with an integer number of turns. The
number of turns calculated is given in [E105] and the resulting V
level, at no-load, is given in [E109]. As previously discussed, between
8 V and 9 V is recommended, so enter '8' into [B107]. The spread-
sheet will recalculate the number of turns to achieve a level of at
least 8 V. The revised number of Bias wind turns are given in [E105]
and the actual V level, at no-load, is given in [E109].
CC
www.power.com
falls to
VCC
is determined by the turns ratio of
CC
level with load applied
CC
level on the completed design,
CC
+ V
h
- DBIAS
N
#
+ V S
OUT DOUT
For applications that have challenging start-up conditions, CC load
and/or high output capacitance, a higher value of target V
entered into [B107], 9 V for example. Note however, that the Bias
winding power, given in [E108], increases with Bias voltage. Check
the no-load power estimation in [E174] is within specification. Check
the V level on the completed design, confirm that at the maximum
CC
AC input voltage and at maximum load or start-up, V
exceed 16.5 V.
Bias Supply Diode Selection
In most circumstances a 1N4148 is adequate. In situations where
more leakage energy scavenging is required, to assist start-up for
example, a higher current ultrafast silicon diode may be used.
The maximum reverse voltage across the Bias diode is calculated in
[E129], add margin to this value before selecting a suitable part.
Suggested parts are given in Table 7.
V
^
DBIAS MAX
Where V = 16.5 V worst case.
CC(MAX)
Type T (ns)
RR
1N4148 4
1N4933 50
SF11G 35
UF4001 50
BYV27-50 25
UG1A 25
ES1A 35
STTH1R02 15
Table 7. Suggested Bias Supply Diodes.
CC
Application Note
CC
N
A
= 2 V + V
#
h ^
N
ACMAX CC MAX
P
V (V) V (V) @
RRM FD
75 1.0
50 1.0
50 1.0
50 1.0
50 1.0
50 1.0
50 1.0
200 1.0
may be
CC
does not
h
I (A)
F
0.15
1.0
2.0
2.0
2.0
3.0
3.0
3.0
11
Rev. B 10/17