DN86
Reduced component count and compact reference
design for MR16 replacement lamps using multiple
1W LEDs
Silvestro Russo, October 2007
Introduction
MR16 lamps are one variety of Multifaceted Reflector (MR) lamps that usually employ a halogen
filament capsule as the light source. They are used in many retail and consumer lighting
applications where their size, configurability, spot-lighting capability and aesthetics provide utility
and creativity. Low efficiency, heat generation and halogen capsule handling issues are among
the disadvantages of the technology. They typically operate from 12V DC or 12V AC, using
conventional electromagnetic transformers.
LEDs offer a more energy efficient and no radiated heat solution to replace some halogen lamp
applications.
This reference design is intended to fit into the base connector space of an MR16 style LED lamp.
The design has been optimized for part count and thermal performance. The design can be used
with up to 3 1W LEDs in the Lens section. This can be arranged to suit the luminary designer's
requirements.
Figure 1
Data sheet
MR16 application with ZXLD1350
It is recommended that this design note is used with the data sheet for the ZXLD1350 see
http://www.zetex.com/3.0/pdf/ZXLD1350.pdf
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
1
www.zetex.com
DN86
Description
The system diagram of the MR16 lamp solution with ZXLD1350 and ZXSBMR16PT8 is shown in
Figure 2, and Table 1 provides the bill of materials.
Figure 2
System diagram of ZXLD1350 MR16 Lamp Solution
The
ZXLD1350
is designed for LED current drive applications of up to 350mA. The monolithic
NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 400mA, which
with the hysteretic mode of operation (the inductor current waveform will ramp +/-15% about the
nominal current set point) provides sufficient margin. The main features of the ZXLD1350 are:
•
•
•
•
•
Up to 380mA output current
Wide input voltage range: 7V to 30V
Internal 30V 400mA NDMOS switch
High efficiency (>90% possible)
Up to 1MHz switching frequency
The
ZXSBMR16PT8
is a new space saving and thermally efficient device specifically designed for
the critical requirements of MR16 applications. The device encompasses a full bridge and a
freewheeling diode realized using extremely low leakage 1A, 40V Schottky diodes to allow a
nominal 12V AC input operations. The Schottky bridge together with the embedded freewheeling
diode enhance the system efficiency compared to the standard silicon diodes in a compact
format. The reference design has solder tag pins to bypass the bridge rectifier should the final
lamp design be used for purely DC operation.
As the ZXLD1350 has a hysteretic switching topology, the switching frequency is dependent on
several factors - input voltage, target current and number of LEDs. An Excel based calculator is
available for system initial evaluation and component choice.
See http://www.zetex.com/3.0/otherdocs/zxld1350calc.xls
System efficiency and LED current have been measured keeping the ADJ pin floating and the
current in the device at its rated value. The input impedance of the ADJ pin is high (200K) and is
susceptible to leakage currents from other sources. Anything that sinks current from this pin will
reduce the output current. In order to avoid any kind of electromagnetic coupling a guard track
around this pin is used.
www.zetex.com
2
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
DN86
Quantity
1
2
1
1
1
1
1
Part reference
R1
C1, C2
C3
C4
L1
U1
U2
Value
0.33
150µF/20V
0.1µF/25V
1µF/25V
100µH
ZXLD1350
ZXSBMR16PT8
Description
Resistor, 1%, 0805
Type D SMD Tantalum Cap
SMD 0805 X7R
SMD 1210 X7R
MSS6132-104
LED driver IC
Schottky bridge rectifier
and freewheeling diode
Source
Various
Kemet
NIC
Componenents
NIC
Componenents
Coilcraft
ZETEX
ZETEX
Table 1
Bill of Material
Referring to circuit schematic in Figure 2; the jumper connection could be used utilizing a zero
ohm resistor, in order to enable the pure DC operations.
Care has to be taken in this case, since the system is not reverse polarity protected.
In Figure 3 the circuit layout is shown, highlighting its space saving features and compactness.
Both bottom layer and top layer are shown to display effective devices arrangement.
TOP COPPER AND SILKSCREEN
BOTTOM COPPER AND SILK SCREEN
Figure 3 Circuit layout
The main layout design suggestions are:
•
•
•
•
All thin devices on one side
Employ a star connection for ground tracks
Use a ground ring protecting ADJ pin
Check that:
•
•
•
Tracks connecting R1 to ZXLD1350 are as short as possible (being sense tracks)
The filter capacitor C3 is connected as close as possible to the V
in
pin
The freewheeling current path is as short as possible to ensure system precision and efficiency
3
www.zetex.com
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
DN86
Circuit board views
Top Layer
Bottom Layer
MR16 Pins
- Anode
LED connections
+ Cathode
Figure 4 Circuit board views
Choice of Inductor and switching circuit layout
A 100 µH screened inductor was chosen to set the nominal frequency around 250kHz. A screened
inductor is chosen to minimize radiated EMI. The layout with any switching regulator is crucial to
minimize radiated EMI. This reference design keeps the critical track lengths to a minimum.
Ground areas have been maximized around critical areas.
Circuit performances
Circuit performances have been evaluated taking into account two main parameters, the system
efficiency and the current precision.
The reference current is set to a nominal 300mA but can be adjusted to any value up to 350mA
by changing the sense resistor R
sense
according to the formula:
I
ref
= 0.1/R1
For
R1 = 0.33
[A]
I
ref
= 300mA
In Table 2 the data related to the system supplied with a DC voltage ranges from 12V to 15V. For
these tests the Schottky bridge was included. The most important parameters are the system
efficiency and the error between the rated LED current (300mA) and the actual LED current. In the
DC case the frequency ranges between 150kHz and 300kHz, depending on the input voltage.
Whatever the input voltage, the efficiency is higher than 87% and the error lower than 2%.
Vin [V]
12.000
13.000
14.000
15.000
Iin[A]
0.275
0.252
0.232
0.220
Vout[V]
9.80
9.78
9.76
9.75
Iout[A]
0.296
0.294
0.294
0.294
Efficiency
87,9%
87.7%
87.6%
87.4%
Current
Accuracy
1.3%
2.0%
2.0%
2.0%
Table 2 DC input voltage
Table 3 shows the data related to the system supplied with an AC electromagnetic transformer.
Using a SMD tantalum capacitor will save space and avoid using a larger aluminum electrolytic
capacitor. This will improve the reliability of the system and stabilize performance during its
lifetime. There is a trade off between physical size, reliability, cost and average LED current.
Typical output voltages from a nominal 12V AC transformer can be ±10%. With 3 LEDs the voltage
across these will be around 10V. If the input capacitor value is lower then 200µF, the AC input
waveform is distorted (as can be seen in figure 8). When the rectified AC is not sufficiently
www.zetex.com
4
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
DN86
smoothed the ripple may drop below the combined LED forward voltage which stops the
switching regulator and so reduces the average current in the LEDs. This will also reduce the
average lumens output.
C1 [µF]
100
150
200
300
Vin [V]
12.70
12.60
12.53
12.50
Iin[A]
0.303
0.394
0.432
0.386
Vout[V]
9.28
9.50
9.55
9.70
Iout[A]
0.225
0.271
0.293
0.295
Efficiency
54%
52%
52%
60%
Current
Accuracy
25%
10%
2%
2%
Table 3 AC input voltage
Figures 5 to 7 show the input voltage ripple and LX voltage varying the input capacitance value
C
in
= C1 + C2 + C3. The higher the input capacitance the higher to output current precision and
the average lumens outputs. The case with C
in
=300µF has the best performance both as efficiency
and current precision. Reducing the input capacitance the output current precision will decrease
up to 25% with system efficiency always above 50%.
Figure 5 C
in
=300µF
Figure 6 C
in
=200µF
Issue 1 - October 2007
© Zetex Semiconductors plc 2007
5
www.zetex.com
京公网安备 11010802033920号
EEWORLD
