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參數(shù)資料
| 型號： | LC5521D |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 2.5 A POWER FACTOR CONTROLLER, 18 kHz SWITCHING FREQ-MAX, DIP7 |
| 封裝： | DIP-8/7 |
| 文件頁數(shù)： | 29/40頁 |
| 文件大?。?/td> | 888K |
| 代理商： | LC5521D |
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35
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
28106.02
Here is the example of design specification and calculation:
Given:
AC input voltage: 85 to 265 VAC
Output power: 40 W
Transformer primary winding: 40 T
Transformer auxiliary winding: 6 T
IDP at the minimum AC input voltage: 3.0 A
I'DP at the maximum AC input voltage where the output power
of the maximum AC input voltage becomes the same as that
limited by OCP at the minimum AC input voltage: 1.9 A
ROCP = 0.2 Ω
R3 = 220 Ω
DX1 forward voltage: 0.8 V
OCP input compensation starting voltage: 120 VAC
1. Calculate Efw1 at 120 VAC input:
=
Efw1
NP
ND
==
40
6
× VIN(AC)× √2
× 120 √2
25.4 (V)
(16)
Thus, select 27 V as the Zener value for DZX1.
2. The compensation current, I, is calculated using equation 10:
=
I
0.2 (Ω)
220 (Ω)
= 1 (mA)
(3.0 (A) – 1.9 (A)) ×
3. RX1 can be calculated using equation 14:
=
RX1
40 (T)
1 (mA)
28.4 (kΩ)
6 (T) × 265 (VAC)√2 – (27 (V) + 0.8 (V))
Thus, select RX1 = 27 kΩ out of the E12 resistor standard value.
Finally, ensure that these values work to achieve the target output
power through experiments, and adjust them if necessary.
Thermal Shutdown Protection
Thermal Shutdown protection is activated at TJ(TSD) = 135°C
(min). When it activates, the IC enters a latched mode.
Maximum On-Time Limiting Function
The maximum on-time, set at tON(MAX) = 40 μs (figure 50), limits
lower side switching frequency, and it minimizes audible noise
from the transformer at low AC input or during transient periods
such as at switching AC mains on or off.
Ensure that the actual on-time at the minimum AC input and the
maximum load condition does not reach tON(MAX) = 40 μs. If that
does happen, redesign the transformer, such as by reducing the
primary inductance or reducing the duty cycle by lowering the
turns ratio of NP / NS.
Design Considerations
Peripheral Components
Choose appropriate components that are matched to actual use
conditions.
Consider design margins for ratings of ripple current, voltage,
and temperature in selecting the output capacitor. A low-
impedance capacitor, designed to be tolerant against high ripple
current, designed for a SMPS, is recommended.
Consider design margins for temperature rise, resulting from
copper losses and core losses, in designing or selecting a
transformer.
Switching current contains a high frequency component that
causes the skin effect; therefore, consider a current density of
3 to 4 A/mm2 and select a wire gauge based on rms current. In
the event further temperature measurement is necessary and it is
necessary to increase surface area of the wire, try the following
measures:
Increase the quantity of parallel wires
Use litz wire
Increase the diameter of the wires
Choose a low equivalent series inductance and high surge
tolerant type for the current sensing resistor. If a high
inductance type is used, because of the high frequency current
running through it, it may cause malfunctioning.
Transformer Design
The transformer design is the same as for an RCC (ringing choke
converter, or self-oscillation flyback converter) transformer
design. However, a quasi-resonant operation includes a certain
delay to turn-on, so duty cycle must be compensated.
I
D
V
DS
time
Maximum
On-Time
Figure 50. Maximum On-Time
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