DN89

A Product Line of
Diodes Incorporated
DN89
MR16 EMC compliant reference design
Adrian Wong, Systems Engineer
Introduction
As a supplement to DN86, this design note aims at depicting the essential EMC tests for a standalone MR16 lamp with up to three power LEDs in series at 350mA. The LED driver is simply
implemented by a switching buck regulator employing the ZXLD1350 hysteretic current controller
with the ZXSBMR16PT8 dedicated and new full bridge/freewheeling combo extremely low
leakage schottky diodes. Throughout the compliant verification, the driver was operated from a
12Vac source by means of a typical linear step down transformer directly connected to AC mains.
For the demonstration purpose, the application circuit diagram along with the part list is
presented as a ready-to-use reference design tool.
Circuit description
In MR16 lamp applications, the hysteretic converter is more preferable than the fixed frequency
PWM converter because of the reduced component count and its lower switching frequency that
minimizes the radiated emissions. Furthermore, an EMI filter along with frequency dithering is
adequate for conducted emission attenuation as well as better immunity.
The basic circuit with the EMC modifications is shown in Figure 1.
EMI
R ec eiv er
R1
L2 - L3 - L4
C1// C2
C3
ISENSE
C4
LED2
ADJ
C5
LX
C6
C5
GND
Vin
IC1
VIN
LED1
IC2
LED3
L1
S w itc hing C onv erter
(N ois e S ourc e)
E M I F ilter
Figure 1 - System diagram of ZXLD1350 MR16 lamp solution with an EMI filter
The LED driver is simply implemented by a switching buck regulator employing the ZXLD1350
integrated controller with the ZXSBMR16PT8 dedicated and full bridge/freewheeling combination
extremely low leakage schottky diodes. The controller has an internal switch that greatly reduces
both the PCB size and the component count. Two input energy storage SMD tantalum capacitors;
C1+C2, of total value 300uF have been optimized as the minimum capacitance in terms of
efficiency and LED current accuracy.
The following components were needed for EMC considerations. Three 0805 type SMD commonmode chokes in series, L2, L3 and L4, were verified to be the minimum requirement to pass the
conducted EMI test. A screened inductor L1 is selected to minimize the radiated EMI from the
switching operation.
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The variable switching frequency of the system can be derived as below with reference to the
ZXLD1350 internal block diagram as shown in the datasheet.
1
f SW =
L ΔI
VIN − VLED − I AVG (R S + rL + R LX )
+
VLED
LΔI
+ 2TPD
+ VD + I AVG (R S + rL )
where:
L is the coil inductance (H)
rL is the coil resistance (Ω)
IAVG is the required LED current (A)
ΔI is the coil peak-peak ripple current (A) {Internally set to 0.3 x IAVG}
VIN is the supply voltage (V)
VLED is the total LED forward voltage (V)
RLX is the switch resistance (Ω)
VD is the diode forward voltage at the required load current (V)
TPD is the internal comparator propagation delay
In this example, we have
L=L1=100µH±10%
rL=0.82Ω
IAVG=303mA
ΔI=90.91mA
VIN=14.5V±2.5V
VLED=8.7V~12.3V (typical 10.8V)
RLX=1.5Ω (max. 2Ω)
VD=420mV (max. 485mV) at IF=303mA
RS=R1=0.33Ω
TPD=200ns
As can be calculated, the inherent variation of the switching frequency in relation to the input
variation of 14.5V±2.5V is an advantage. It was found to be from around 24 kHz at minimum
voltage to 420 kHz in the peak voltage in a worse case analysis. In this way, the ZXLD1350 offers
a ±90% deviation of its nominal switching frequency of 230 kHz thereby facilitating the frequency
dithering. The effect is shown in Figure 2.
20
18
1 2 V A C AinC pinput
u t V /oVlt a g e
16
14
12
10
8
6
4
2
0
5
10
15
20
25
30
Tim e / m S ec s
35
40
45
50
5 m S ec s / div
f= 2 4 kH z
f= 4 2 0 kH z
Figure 2 - Switching frequency variation with 12Vac input voltage
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In addition, an EMI filter is composed of two X-capacitors, C5 and C6, and three common-mode
chokes in series, L2, L3 and L4. The 3dB cutoff frequency fC is estimated as below.
Half inductance of an individual common-mode choke = L1/2 =
Z
67
=
= 0.1066μ H
2π f 2π × 100 × 10 6
Total inductance of all common-mode chokes = L SUM = L 2 + L 3 + L 4 = 3 × 2 × L 1/2 = 0.6396μ H
∴ 3dB cutoff frequency fC =
1
2π L SUMC 5
=
1
2π 0.6396 × 10 −6 × 1× 10 −6
= 200kHz
Hence, the filter can provide a supplementary attenuation of -40dB/decade from 200 kHz. This is
just below the nominal switching frequency. In other words, it is effective to suppress the higher
harmonics.
PCB layout considerations
There are many critical EMC considerations in the PCB layout as below:
•
A star ground connection is employed to avoid the common impedance effect
•
A ground ring is used to protect the ADJ pin against any kind of electromagnetic coupling
•
The sense tracks connecting R1 to ZXLD1350 are as short as possible
•
The decoupling capacitor C3 is placed as close as possible to the Vin pin
•
The freewheeling current path is as short as possible to ensure system precision and
efficiency
•
Power and ground tracks have been maximized around critical areas on both sides to create
the intrinsic capacitors for high frequency filtering
Top copper and silkscreen
Bottom copper and silkscreen
Figure 3 - Circuit layout
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Top layer
Bottom layer
Figure 4 - Circuit board views
Quantity
1
2
1
1
1
1
1
3
1
1
Part reference
R1
C1,C2
C3
C4
C5
C6
L1
L2,L3,L4
U1
U2
Value
0.33Ω
150µF/20V
0.1µF/25V
1µF/50V
1µF/50V
0.1µF/50V
100µH
[email protected]
ZXLD1350
ZXSBMR16PT8
Description
Resistor, 1%, 0805
Type D SMD Tantalum Cap
SMD 0805 X7R
SMD 1210 X7R
SMD 1206 X7R
SMD 0805 X7R
MSS6132-104
744231061
LED driver IC
Schottky Bridge rectifier and
freewheeling diode
Source
Various
AVX
NIC Comp AVX
NIC Comp AVX
NIC Comp AVX
NIC Comp AVX
Coilcraft
Würth Elektronik
Diodes Inc
Diodes Inc
Table 1 - Bill of materials
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EMC test results for the reference design
Radiated electromagnetic disturbances
The test was performed in accordance with the general lighting standard EN 55015: 2006. The test
setup block diagram together with results is shown as below. The radiated emissions from the
switching converter can be minimized due to the proper PCB layout, the usage of a screened
inductor and the shielding effect of the sealed housing.
Computer
Pre-amplifier
EUT
Turn-table
Receiver
Ground Plane
Figure 5 - Block diagram of test setup for radiated electromagnetic disturbances
Figure 6 - X direction radiated emission
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Figure 7 - Y direction radiated emission
Figure 8 - Z direction radiated emission
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Electrostatic discharge
The test was performed in accordance with IEC 61000-4-2 Level B. The EUT was operating
normally without shutdown when subjected to ±4kV contact discharge onto the surface of the
metallic coated housing while ±8kV air discharge near the insulated input connector. Obviously,
the control circuit was not interfered by the ESD with the aid of the housing.
Conducted EMI (optional, customer-driven)
In general, the test is not required under such low input voltage condition. However, it was still
performed in accordance with EN 55015 with the special LISN relocation at the 12Vac input
terminals to represent a more stringent assessment. The scans with data analysis are shown
where the emissions can be suppressed below the limit line by the addition of the input EMI filter
with the above mentioned radiated emissions considerations.
Att 10 dB
dBµV
120
RBW
200 Hz
MT
100 ms
PREAMP OFF
100 kHz
1 MHz
10 MHz
EN55015Q
110
1 PK
MAXH
100
90
TDF
80
70
PRN
60
EN55015A
50
6DB
40
30
20
10
0
9 kHz
30 MHz
Figure 9 - Live conducted EMI scan
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RBW
200 Hz
MT
20 ms
PREAMP OFF
Att 10 dB
dBµV
100 kHz
120
1 MHz
Marker 1 [T1 ]
49.37 dBµV
486.000000000 kHz
10 MHz
EN55015Q
110
1 PK
MAXH
100
90
80
TDF
70
PRN
60
EN55015A
50
1
6DB
40
30
20
10
0
9 kHz
30 MHz
Figure 10 - Neutral conducted EMI scan
Live
Frequency
(MHz)
0.238
0.290
0.354
0.486
QP (dBµV)
Neutral
AV (dBµV)
Reading
Limit
Reading
Limit
57.6
54.4
55.6
46.5
62.2
60.5
58.9
56.2
36.1
27.5
29.7
26.6
52.2
50.5
48.9
46.2
Frequency
(MHz)
0.230
0.266
0.310
0.378
0.466
QP (dBµV)
AV (dBµV)
Reading
Limit
Reading
Limit
55.6
54.8
55.2
33.9
42.9
62.4
61.2
60.0
58.3
56.6
30.1
27.6
28.1
20.9
20.5
52.4
51.2
50.0
48.3
46.6
Table 2 - Conducted EMI data record and analysis
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Conclusion
A comprehensive EMC compliant reference design is suggested using ZXLD1350,
ZXSBMR16PT8, and a few associated passive components for a 3x1W LED MR16 lamp. It features
on board EMI filter and low part count. The design exhibits a performance that is excellent in
electrical and EMC performance. It can be readily incorporated with a traditional 50/60Hz linear
transformer to form a specific EMC compliant lighting system. However, if an electronic
transformer is intended to be used, the system EMC compliance level is unknown at this moment.
In which case, the EMI filter may need to be re-designed for optimization. The circuit passed a precompliance test to the general lighting standard EN 55015: 2006. As with any design, EMC
performance is dependent on many factors and users must verify the suitability of their own
designs. The purpose of this design note is to show that with careful design and taking advantage
of the variable frequency nature of the hysteretic converter an EMC compliant MR16 lamp can be
achieved.
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Definitions
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The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
assumed by Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property
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1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
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cause the failure of the life support device or to affect its safety or effectiveness.
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The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
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Product status key:
“Preview”
Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
“Last time buy (LTB)”
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Datasheet status key:
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This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
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