LYT2002-2005
Applications Example
R1
3.9 kΩ
1/8 W
R3
T1
EE19
22 V - 48 V,
180 mA
220 kΩ
1
2
10
9
L1
3.9 mH
C3
TP3
C6
D3
US1G
470 pF
1 kV
R10
1.2 MΩ
100 µF
63 V
RTN
TP4
R6
2 Ω
R4
560 Ω
R5
560 Ω
C7
1 nF
500 V
1/8 W
BR1
B10S-G
1000 V
5
4
F1
2 A
D1
S1ML
L
TP1
C1
C2
12 µF
400 V
12 µF
90 - 265
VAC
400 V
R7
D2
BAV21W-7-F
60.4 kΩ
1%
N
LYTSwitch-2
U1
LYT2004E
1/8 W
TP2
D
S
FB
BP
R2
R8
6.98 kΩ
1%
3.9 kΩ
R9
C4
C5
1/8 W
12 kΩ
1 µF
50 V
1 µF
1/8 W
1/8 W
50 V
L2
3.9 mH
PI-7280-051414
Figure 4. Energy Efficient 8.6 W LED Power Supply (>86 % Average Efficiency, <30 mW No-load Input Power).
LYT2004 Primary
Circuit Description
The LYTSwitch-2 family (U1) incorporates the power switching device,
oscillator, CC/CV control engine, start-up, and protection functions.
The integrated 725 V power MOSFET provides a large drain voltage
margin in universal input AC applications, increasing reliability and
also reducing the output diode voltage stress by permitting the use of
higher transformer turns ratios. The device is completely self-
powered from the BYPASS pin and decoupling capacitor C4.
This circuit shown in Figure 4 is configured as a primary-side
regulated flyback power supply utilizing the LYT2004E from the
LYTSwitch-2 family of ICs. This type of LED driver design is typical
for an external ballast application where safety isolation is required
while power factor correction is not. The output can drive an LED
load from 48 V to 22 V with a constant output current of 180 mA ±5%
across input range of 90 VAC to 265 VAC and ambient temperature
range of 0 ºC to 60 ºC. It has an average efficiency of >86% and
<30 mW no-load input power measured at nominal input voltages
(i.e. 115 VAC and 230 VAC). This design easily meets the most
stringent current energy efficiency requirements.
The optional bias supply formed by D2 and C5 and R6 provides
operating current to U1 via resistor R9. This reduces the no-load
consumption from 200 mW to less than 30 mW. The bias supply also
increases light load efficiency.
The rectified and filtered input voltage is applied to one side of the
primary winding of T1. The other side of the transformer’s primary
winding is driven by the integrated power MOSFET in U1. The
leakage inductance drain voltage spike is limited by an RCD-R clamp
consisting of D1, R3, R4, R5, and C3.
Input Filter
AC input power is rectified by bridge diode BR1. The rectified DC is
filtered by the bulk storage capacitors C1 and C2. Inductor L1, L2,
C1 and C2 form a pi (π) filter, which attenuates conducted differen-
tial-mode EMI. Resistors R1 and R2 placed across the inductors
damp the Q to improve frequency noise filtering without reducing low
frequency noise attenuation. A small value Y capacitor (C7) across
the transformer was used to reducer common-mode noise currents.
The fuse F1 provides protection against catastrophic failure. This
can be replaced by a fusible resistor for cost reduction but should be
suitably rated (and typically a wire wound type) to withstand the
instantaneous dissipation experienced during input capacitor charging
when first connected to the AC line.
Output Rectification
The output from the transformer is rectified by D3, a 1 A, 400 V
ultrafast recovery type diode (for higher efficiency), and filtered by
C6. In this application C6 was sized to meet a (typical) ripple
requirement of less than 10% without the need for an additional LC
post filter.
4
Rev. B 09/15
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