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| 型號(hào)： | LC5511D |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | POWER FACTOR CONTROLLER, 18 kHz SWITCHING FREQ-MAX, DIP7 |
| 封裝： | DIP-8/7 |
| 文件頁(yè)數(shù)： | 13/40頁(yè) |
| 文件大?。?/td> | 888K |
| 代理商： | LC5511D |
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20
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
28106.02
Initially, target 20 V in a transformer design, and then optimize its
winding turns in a way that VCC voltage stays within that speci-
fied range over the conceivable input voltage range and output
load conditions.
Figure 20 shows the VCC voltage behavior at the startup phase.
Immediately after the controller circuit starts operation, the aux-
iliary winding voltage, VD has not yet reached its design target
value, which is determined by the transformer auxiliary winding
turns. Therefore, as shown figure 20, VCC voltage starts decreas-
ing after the startup circuit turns off at VCC(BIAS)2 = 16.6 V. After a
while, if the VCC voltage reaches the Startup Current Threshold
Biasing Voltage-1, VCC(BIAS)1 = 11.0 V, the bias assisting func-
tion is activated in order to avoid further voltage drop and VCC
voltage becomes nearly constant. Thanks to this function, the C4
value can be small, which results in shortening the startup period
and improving the response time of the VCC pin overvoltage
protection.
Figure 21 shows the positive dependency of VCC voltage on
output current. This is caused by the surge voltage, which occurs
on the D/ST pin at the turn-off edge of the incorporated power
MOSFET. The surge voltage is coupled to the auxiliary wind-
ing and it charges-up C4 more than the design target. In order to
avoid this, insert R1 in series with D5 as shown in figure 22, and
choose a value for it between several ohms to several tenths of
ohms.
In addition, the transformer winding structure has influence on
VCC fluctuation and the two items below are examples of worsen-
ing it:
Poor coupling between the primary and secondary windings
(this causes high surge voltage and is seen in a design with low
output voltage and high output current).
Poor coupling between the secondary winding and the auxiliary
winding D (this increases the effect of the surge voltage on the
auxiliary winding voltage).
Against those items, the two items below are commonly used as
techniques for improvement:
Separate the auxiliary winding D from the primary windings
P1 and P2 (figure 23(A)); P1 and P2 are two separated primary
windings.
Place the auxiliary winding D within the secondary winding
S1 in order to improve the coupling of those windings (figure
23(B)); S1 is the secondary output winding.
IOUT
VCC
With R1
Without R1
D
Vcc
D5
C4
2(4)
1(2)
LC55xxD
(LC55xxF)
S/GND
R1
Added
Figure 21. VCC versus IOUT with and without resistor R1
Figure 22. VCC pin peripheral circuit with R1
Figure 20. VCC at startup period
Time (t)
= 15.1 V
= 11.0 V
VCC
VCC(OFF)
VCC(ON)
= 16.6 V
VCC(BIAS)2
VCC(BIAS)1
= 9.4 V
Operation start
Startup
successful
Startup circuit off
Bias assisting
Startup failure
Figure 23. Transformer winding structures: (A) auxiliary winding apart from
the primary windings, and (B) auxiliary winding within secondary winding
P2
D
P1 S1
S1
P1 S1 D
P2
S1
P1, P2: Primary Winding
S1: Secondary Winding
D: Auxiliary winding
Core
Bobbin
P1, P2: Primary Winding
S1: Secondary Winding
D: Auxiliary winding
Core
Bobbin
(A)
(B)
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