DN92

 A Product Line of
Diodes Incorporated
DN92
ZXLD1360 and Super Barrier Rectifier ideally suited
for 700mA LED current in MR16 application
Introduction
MR16 solutions with LEDs are intended to replace halogen lamps with a more efficient solution. Often
this is associated with problems in thermal management, both from the LED and the driver.
Since the LED mainly dissipates the heat through conduction rather than irradiation, the replacement
system has to be carefully designed in terms of both shape and material, and in consideration of the LED
derating curve. Also, the driving circuit has to manage increasing amounts of current, while the space is
constrained by the limits of the base connector in an MR16 style LED lamp (GU5.3 standard).
MR16 systems typically operate from 12V AC or 12V DC, using conventional electromagnetic or
electronic transformers, and the efficiency as well as the LED current is strongly affected by the power
source used.
This design can be used for a system employing a single 3W LED in the lens section, and this can be
arranged to suit the luminary designer's requirements.
.
Figure 1- Combined strengths of ZXLD1360 and SBR2A40P1
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Description
The system diagram of the MR16 lamp solution with ZXLD1360 and SBR2A40P1 is shown in Figure 2,
while Table 2 provides the bill of materials for this reference design.
SBR 2A 40 P1
D1
D2
U1
D3
VIN
S B R 2A4 0P1
IS EN SE
R1
S B R 2A4 0P1
C4
C3
AD J
C 1//C 2
S B R2A4 0 P1
LX
Vsupply
GN D
Z XL D1360
S B R 2A4 0 P1
D5
D4
L1
Figure 2 - MR16 circuit diagram
The ZXLD1360 is a hysteretic converter designed for LED current drive applications of up to 1A. The
monolithic NMOSFET is sized appropriately to provide a cost-effective die size and is rated to 1A current.
[1]
The main features of the ZXLD1360 are:
•
Up to 1A output current
•
Wide input voltage range: 7V to 30V
•
Internal 30V 1A switch
•
High efficiency (>90% possible)
•
Up to 1MHz switching frequency
As the ZXLD1360 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 to
support initial system evaluation and component selection and is available for download from
www.diodes.com.
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 (200kΩ) 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.
The SBR2A40P1 belongs to the patented Super Barrier Rectifier (SBR) family, and has proven to be very
stable at high temperature. It can be used both in the input bridge rectifier section and in the freewheeling
section. [2]
The new SBR technology offers to combine the high performance of Schottky diodes while possessing
the high reliability characteristics of p–n diodes. In particular the device combines low forward voltage,
high switching speed and low leakage at high temperature. In an MR16 application the SBR technology
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displays reduced a temperature coefficient when compared to Schottky parts, as is highlighted in table
1. This feature is fundamental since the diodes work predominantly in blocking phase in a MR16
application.
Table 1
Part No.
SBR2A40P1
ZLLS2000
Typical Ir [mA]
Ir @ 30V, 25˚C
0.013
0.025
Ir @ 30V, 85˚C
0.24
1.7
Ir @ 30V, 150˚C
5.5
80
Also, the SBR for MR16 can be housed in a proprietary package, the PowerDI™123, that combines both
space saving and high power density due to its excellent thermal performances, making the solution
reliable at higher temperature. [2]
Bill of Material
Table 2 presents the BOM for the reference design.
Table 2
Quantity
1
5
1
1
1
1
1
1
Part reference
R1
D1, D2, D3, D4, D5
C1
C2
C3
C4
L1
IC1
Value
0.15
SBR2A40P1
150μF/20V
0.1μF/25V
1μF/25V
Description
Resistor, 1%, 0805
SBR 40V, 2A - PowerDITM123
Not Fitted
Type D SMD Tantalum Cap
SMD 0805 X7R
SMD 1206 X7R
Source
Various
Diodes Inc.
KEMET
NIC Components
NIC Components
33μH
ZXLD1360
LPS6235-333MLB
Zetex LED driver IC – SOT23-5
Coilcraft
Diodes Inc.
Figure 3 and 4 show the circuit layout, highlighting its space saving features and compactness. In
particular, both bottom layer and top layer are shown to display effective devices arrangement.
Figure 3 - PCB layout – Top Layer
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Figure 4 - PCB Layout – Bottom Layer
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P1 and P2 on the top layer are the pads for the supply voltage connection, while K and A pads on the
bottom layer connect the board to the LED cathode and anode respectively.
Figure 5 - Demo Board - Top Layer
Figure 6 - Demo Board – Bottom Layer
Choice of Inductor and Switching Circuit Layout
Inductor selection influences the performance of the system in terms of switching frequency and EMI
emission. This section presents the critical choices and their implications.
A 33μH screened inductor was chosen to set the nominal frequency around 210kHz.
Radiated EMI is minimized using a screened inductor, keeping the critical track lengths to a minimum.
Ground areas have been maximized around critical areas, to allow a more efficient heat transfer.
Circuit Performance
Circuit performance has been evaluated taking into account two main parameters; the system efficiency
and the LED current precision.
The reference current is set to a nominal 660mA but can be adjusted to any value up to 1A by changing
the sense resistor R1 according to the formula:
Iref = 0.1/R1 [A]
Iref = 660mA, for R1 = 0.15Ω,
In Table 3, the data shows the system supplied with a DC voltage ranging from 10V to 15V. The most
important parameters are the system efficiency and the error between the rated LED current (660mA)
and the actual LED current. In the DC case, the frequency ranges between 270kHz and 300kHz,
depending on the input voltage.
Whatever the input voltage, the efficiency is higher than 77% and the LED current error lower than 2%.
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Table 3- DC input data
Vsupply
[V]
10V
11V
12V
13V
14V
15V
Iin
[A]
0.247A
0.227A
0.210A
0.195A
0.182A
0.171A
Pin
[W]
2.47W
2.50W
2.52W
2.54W
2.55W
2.57W
Vout
[V]
2.97V
2.97V
2.99V
2.96V
2.96V
2.96V
Iout
[A]
0.647A
0.650A
0.655A
0.660A
0.665A
0.669A
Pout
[W]
1.92W
1.93W
1.96W
1.95W
1.97W
1.98W
Efficienc
y
[%]
77.9%
77.3%
77.7%
77.1%
77.3%
77.2%
Error
[%]
2%
2%
1%
0%
1%
1%
Figure 7 highlights the main waveform in the circuit during AC operations when C2=150uF and Vsupply
is 12VAC (waveform CH1 is the device VIN voltage and CH2 is its LX voltage).
Note that in the single LED application, the VIN voltage can have a bigger ripple compared to a 3 LEDs
application. This is because in a standard buck configuration the only constraint is VIN > VLEDs, with
VIN greater than the driver minimum voltage. The possibility of having a larger voltage ripple allows
efficiency and accuracy to be maintained using a smaller input capacitor when compared to a 3 LEDs
MR16 solution. [3]
In Table 4 the data related to the system supplied with a 12V AC voltage. System efficiency is better than
50% and the error between the rated LED current (660mA) and the actual LED current is usually lower
than 1%.
Table 4 - DC Input data
C1
[uF]
150
Vin
[V]
12
Iin
[A]
0.328
Pin
[W]
3.936
Vout
[V]
2.97
Iout
[A]
0.667
Pout
[W]
1.981
Efficiency
[%]
50.3%
Error
[%]
1%
Furthermore, in the AC case the frequency ranges between 240kHz and 300kHz, performing a natural
frequency dithering that simplifies the eventual EMC filter design.
Figure 7 - Main waveforms
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Thermal Considerations
An important consideration in MR16 systems is the difference between the ambient temperature and the
operating temperature of the driving circuit. The driving circuit temperature is determined by; the size of
the MR16 connector, the proximity of this circuit with the LED thermal dissipation area, and the material
used to manufacture the lamp. The temperature range seen by the driving circuit within the connector of
an MR16 lamp is usually between 75˚C and 90˚C, even with a max ambient temperature of 40˚C. This
condition makes the environment very hard for both the freewheeling diode and the bridge rectifier
diodes, but is managed by SBR technology’s key characteristics of high temperature stability and higher
thermal runaway capability compared to a Schottky diode.
Table 5 shows the maximum operating temperature for MR16 replacement systems based on SBR and
Schottky diodes.
Table 5 - System maximum operating temperature
System tested
MR16 with SBR2A40P1 SBR
MR16 with ZLLS2000 Schottky
diodes
Input
Voltage
12AC
12AC
Output current
660mA
656mA
Maximum system
temperature
105˚C
80˚C
It is clear that the SBR2A40 outperforms the conventional Schottky diode in this application, being itself
immune to the run away problem typical of Schottky diodes.
Conclusion
In conclusion SBR technology displays high temperature stability and higher thermal runaway capability
compared to a low leakage Schottky diode. This characteristic, in conjunction with the excellent features
of the PowerDI™123 package and the ZXLD1360 led driver, allow easy design of MR16 systems coping
with the most demanding environmental conditions.
This reference design, in summary, provides all the elements to design a 700mA single LED MR16
system, capable of managing the most demanding specifications.
References
[1] ZXLD1360 datasheet
www.diodes.com
[2] SBR2A40P1 datasheet
www.diodes.com
[3] DN86 MR16 reference design
www.diodes.com
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