Post on 31-Mar-2015
LED Driver Load -- 63110A
Introduction
Working on Better Solutions
Working on Better Solutions
Agenda
1. LED & LED Driver Introduction
2. Disadvantage of General E-load for Testing LED Driver
3. Chroma 63110A LED Driver Load Introduction
4. Conclusions
Working on Better Solutions
LED Driver E-load
LED Driver E-Load
LED Driver
LED
1. UUT : LED driver2. LED driver powers the LEDs3. E-load simulates the LEDs
∥
Working on Better Solutions
The Category of LED
LED is classified by POWER and CURRENT
1. Traditional LED: For indicator light. Current: 10mA – 80mA Voltage: 0.7V Power 0.06W≦
2. Power LED: High brightness for lighting. Current: 350mA, 500mA, 700mA Voltage: 3.2V Power: 1W, 1.5W, 3W
Working on Better Solutions
Power/High brightness LED Application
Lighting: Use Power LED to be lamps and lanterns.
1. Outdoor lighting: Table lamps, Wall lamps, Ceiling lamps, Integrated
lighting.
2. Outdoor lighting: Garden light, Street/Road lamps.
Traditional lighting like halogen bulb/tube will be replaced by LED.
LCD backlight: Monitor, Notebook, TV.
1. Because of the low monitor cost and low notebook power(3-4W), their test
requirements are limited.
2. TV backlight: LED’s features include “lightweight, power saving (to meet the
requirement of Energy Star), RGB high chromaticity.” But it is more complex,
with high quality requirements, and therefore has greater testing requirements.
Working on Better Solutions
LED Driver Introduction
LED driver are required to power LEDs.Power LED driver: Classify by power
1. 1W-5W: a. Driven by IC directly, less components, integrated circuitry. b. Less test requirements.
2. ≥ 5W: a. AC or DC input, like general power supplies
b. Most LED drivers are designed as Constant Current, the
voltage will increase with LED numbers.
c. Probably independent product, more components and greater
testing requirements.
Working on Better Solutions
• LCD Backlight: TV– Including power supply outputs (4 channels) and multiple LED Drivers– Voltage< 500V (Commonly), Current < 100 mA
• Indoor/Outdoor Lighting: Table lamps, Wall lamps, Ceiling lamps, Integrated lighting, Garden light, Street/Road lamps. (>5W)– Voltage < 500V, Current < 1A – If the LED Driver is CV+CC type, its current may be 1A≧ Even up to 20A.
63110A Target Market
Working on Better Solutions
LED Characteristic1. Nonlinear V-I curve.
2. Forward Voltage (VF).
3. Small Current (several hundreds mA).
4. Brightness decided by POWER.
5. Small equivalent capacitance.
≠�Rb
RL
VFVf
I
V
LED curve
Vo
Io Rd
LED Introduction
Working on Better Solutions
• LED driver Type• Voltage source
• Current source
• Voltage Source LED driverDisadvantage : Due to the different LEDs having different forward
voltages(VF), the current flowing through the LEDs will vary, causing a difference in the brightness of the LEDs.
• Current Source LED driver• The brightness of the LED is determined by power, if Vf is fixed then the
brightness can be controlled by the current ( P= I ×V).
• Hence most LED driver designs are of the Constant Current type.
LED Driver Introduction
Working on Better Solutions
LED Driver Introduction
Power LED driver: CV+CC Type of LED driver1. The output is constant voltage under no load.2. When the loading current reaches a certain point, the output voltage drops
and the LED driver operates in constant current mode.3. Similar to DC Source design, and different from constant current LED
drivers.4. Has larger output current specification (>2A), and requires current
balance control circuit or device.
Can be tested using general E-Load by CV mode,but can not simulate “Turn on” status.
Use multi-channels of LED load simulator paralleling to get higher current.
Balance device
Working on Better Solutions
• 6% V error => 60% power error!!
• 6% I error => 6% power error!
• From the LED V-I characteristics a slight change in the voltage will cause a big change in the current.
• Hence most LED drivers are of constant current design.
Why LED Drivers are Designed as Constant Current?
Working on Better Solutions
LED Driver Structure
Dimming duty control
Current feedback
Power LED driver: CC Type of LED driver
Control output current, output voltage depends on LEDs, unlike SMPS with constant output voltage
OVP
LED in series
Working on Better Solutions
LED Driver Specification
Test Equipment
AC Source
Power Meter
LED Load Simulator
AC Source +Power Meter
Working on Better Solutions
• Temperature/aging of the LEDs may cause different test results
• Different series number of LEDs required for testing different voltage ranges of the LED drivers, not convenient
• Different types of LED have different VF 、 Rd values, users will need to prepare many different LEDs for verifying the LED drivers functions.
• More often than not if one LED of Light Bar is damaged during testing it may not be obvious.
• A LED Load Simulator is suggested to be a standard load for LED driver test.
Why LEDs are Not Suggested to Use for LED Driver Testing
Working on Better Solutions
Problems with General E-Loads (1)
I
V
LED curve
Vo
Io Rd
CR mode (R=Vo/Io) CV mode
Only able to test LED stable characteristics
1. Unable to test and verify LED driver startup characteristics and simulate different LED characteristics. 2. Unable to simulate the ripple current.
Stable operating point
Working on Better Solutions
Problems with General E-Loads (2)
Internal impedance of E-load may cause LED driver OCP or OVP.
General E-load
Resistance Load General E-load CR Mode
63110A
63110A
V/I overshoot
Overshoot then fail
Working on Better Solutions
Problems with General E-Loads (3)
General E-load response is too slow, it can not support to test PWM mode of dimming for LED driver.
63110A E-load has increased bandwidth to meet the test.
63110A LED modeLED load
Working on Better Solutions
Problems with General E-Loads (4)
General E-Load may encounter unstable voltage and current measurements.
First, check the UUT output is stable or not. Users can use Average
method for stable measurements
Max: 64 times
6310A series set the average times
Working on Better Solutions
LED Driver Load
Chroma designed an E-Load especially for LED Driver
Model 63110A: dual channel LED Driver Load Voltage Range: 0 ~ 60V / 0 ~ 500V Current Range: 0 ~ 0.6A / 0 ~ 2A Mode: LED mode, for simulating LED characteristics also included are CC, CR, CV mode (CC mode does not support dynamic loading)
Working on Better Solutions
Purpose of LED Driver Load
Purpose of 63110A: Simulate LED characteristics, and be used for verifying LED Driver.
1. Check the turn on condition for different LED. OCP/OVP is not allowed.
2. Check the inrush current when turn on. If over spec, it will damage LED.
3. Check if all spec is OK within the operating voltage range (LEDs in series).
4. Check the accuracy of output current when it stays on stable condition
5. Check the ripple current when it stays on stable condition
6. Check the operation is OK when PWM dimming. OCP/OVP is not allowed.
7. Check the current balance for PWM dimming of multi-channel driver
Working on Better Solutions
63110A designed to simulate LED characteristics, the following parameters are used to determine the loading characteristics
1. Vo: LED driver operating voltage
2. Io: LED driver operating current
3. Rd Coefficient: Operating resistor coefficient
4. Rr: High frequency resistor
Setting Parameters
LED mode
Turn ON “LOAD” before turn on Driver
Working on Better Solutions
1. Vo: operating voltage
2. Io: operating current
Press MODE key to choose LEDH (High V range) or LEDL (Low V range), then press ENTER to edit Vo and Io.
Vo & Io Setting
Working on Better Solutions
Rd or Coefficient: Operating resistor
coefficient
Press CONF. key, then press key to
choose Rd or Rd Coeff
Rd Coeff.: 1. Default.=0.1 2. Set range: 0.001 ~ 1.
Rd Coefficient Setting
V
I
Vo
Io
Vf
Working on Better Solutions
Definition:Vo: LED operating voltageIo: LED operating currentVF: LED Forward voltageRd: Operating resistor
What is Rd Coefficient?
VF Vo
Io Rd
V
I
The relationship of the four variables above can be described by the following equation
doFo RIVV
oF VaV 1aEg. ,
o
o
o
oddoodooo I
VCoeff
I
VaRRIVaRIVaV .)1()1(∴
Rd is therefore calculated as the ratio of Vo and Io multiplied by a coefficient (Coeff<1)
Working on Better Solutions
Method 1: Example: The V-I curve of a LED is shown in the diagram below. For a light
bar with 10 LEDs, the LED driver will need to output 350mA
How to get a suitable Rd Coefficient?
How is Rd calculated for a LED with operating current of 350mA, and a operating voltage of 3.44V?
From the V-I curve on the right, Rd can be calculated from the slope of the line
Rd of the one LED
7.13.04.0
35.352.3dR
Therefore the Rd Coeff. is:
173.0.7.135.0
44.3.. CoeffCoeff
I
VCoeffR
o
od
Working on Better Solutions
The Advantage of Setting the Rd Coefficient
Why set Rd Coefficient instead of the operating resistor Rd?
1. LED drivers are usually specified with an output voltage range (eg. 3~36V), this indicates that the LED driver can drive different number of LEDs in series.
2. When testing LED driver, the voltage range must be tested.
3. Different output voltage means different numbers of LEDs in series, hence it has different Rd values.
4. It is therefore inconvenient to change the value of Rd every time the output
voltage changes.
Working on Better Solutions
The Advantage of Setting the Rd Coefficient
Why set Rd Coefficient instead of the operating resistor Rd?
5. Because Rd is proportional to the output voltage, therefore by setting the
Rd Coefficient, 63110A will calculate the corresponding Rd value.
V
I
Vo1 Vo2 Vo3 Vo4
IoRd1 Rd2 Rd3 Rd4
Working on Better Solutions
7.13.04.0
35.352.3dR
287.0.3035.0
5.36. CoeffCoeffRd
33.04.0
5.38.3dR
173.0.1735.0
4.34. CoeffCoeffRd
Verifying Different LEDs
Different types of LED need to be considered when testing LED drivers.
A example: LEDs with 350mA current will have different Rd and Rd coefficient values. By setting the different Rd coefficients 63110A can simulate different types of LED for LED driver testing.
Working on Better Solutions
How to get a suitable Rd Coefficient?
Method 2:LED driver designers sometime do
not know the character of LED they
used. So, they can not calculate Rd
Coefficient directly.
From the following two formula, V
I
Vo
Io
Vf1 Vf2 Vf3
Rd1/ Rd2/ Rd3
doFo RIVV
o
od I
VCoeffR . o
fo
V
VVCoeff
.We can get a new formula:
That means Rd Coeff can be calculated from Vf.
Working on Better Solutions
How to get a suitable Rd Coefficient?
When output voltage is over Vf, the current stat to flow. Users can get Vf
value from the power-on waveform of LED driver.
From the formula , we can get Coeff = (42-37)/42 = 0.119o
fo
V
VVCoeff
.
Vo = 42V
Vf = 37V
V
I
Example:
Working on Better Solutions
How to get a suitable Rd Coefficient?
Set parameters on 63110A, Vo=42V, Io=750mA, and set Rd Coeff = 0.119. The test result is very similar to the real LED load. Users can fine-tune Rd Coeff to approach and get better waveform.
If users set a wrong value of Rd Coeff (=0.5), they will get a different result.
V
I
V
I
Working on Better Solutions
1. 63110A will calculate and simulate the LED characteristics from the
Vo, Io, Rd coefficient settings, as shown in the diagram below, Vo and
Io are not the real loading values.
2. Io is determined by the LED driver, if the Io varies from the setting
value then Vo will also vary. This is different concept from CC and
CV mode of normal E-load.
doFo RIVV
VF Vo
Io Rd
V
I
Vo & Io Setting and Measured Vo & Io d
Working on Better Solutions
Vo & Io Setting and Measured Vo & Io d
1. For example, if Io is set to 100mA, and the real LED driver output Io’=110mA, then Vo will not be the setting value. So, the real Vo ’ will be higher than Vo.
2. User set the character of LED (by Vo/Io/Rd Coeff), then change the
parameter Vo for verifying current spec (±5%) in the voltage range.
V
I
Vo Vo’
Io’Io
Working on Better Solutions
Ripple Current of LED Driver
1. Under stable operation(Vo / Io), Iripple is caused by the LED driver Vripple and the
LED operating resistor Rd, Vripple / Rd = Iripple.
2. There are two kind of voltage ripple: 100/120Hz from main (50/60Hz), and
the switching frequency which can be as high as 100kHz.
I
V
LED curve
Vo
Io Rd
100Hz
100KHz
Working on Better Solutions
1. 63110A is an active load, the internal control circuits have a limited bandwidth. The Rd setting is able to simulate for 100/120Hz current ripple, but it is unable for the high frequency components (>100kHz).
2. Rr can be set similar value of Rd for simulating high frequency resistance. Then, it is recommended to use an oscilloscope to observe the ripple current while using LEDs as loading, and using 63110A adjust Rr to achieve the ripple current waveform as the LED loading.
Set high frequency resistor – Simulate Ripple Current
63110A as load
Working on Better Solutions
Rr: High frequency resistor
Press CONF. key, then press key to choose Rr, then press ENTER to setup.Rr: 1. Default. 2.Set. Rr range: 5Ω ~ 250Ω.
Rr is used to adjust the ripple current by adjusting the internal resistance of the
63110A. it is recommended that the Rr function is switched off to prevent the LED
driver from triggering OCP when turn on the power.
Only when testing ripple current should the Rr function be switched ON
Set high frequency resistor Rr – Simulate Ripple Current
Working on Better Solutions
Most general E-Load draw full load to simulate a short circuit test.
LED drivers have small output current, the 63110A is designed with internal relays for short circuit testing.
LED driver - Short Circuit Testing
Working on Better Solutions
1. The LEDs may have been packaged in parallel for some high power application. It needs LED Load Simulator connecting in parallel to provide high current or power.
2. The setting for Io of each channel of 63110A also need to be separated. For example, use two channels of 63110A to simulate a 2.8A of LED module. Each channel need to set Io=1.4A. But Rd Coeff still keep the same value.
3. When parallel, users may need to set 63110A a lower bandwidth if the loading current is unstable. There are 5 range of bandwidth to select in Config.
Parallel for higher Current
2.8A
Working on Better Solutions
1. Users can set the different value of parameters for each channel of 63110A. Therefore, it performs multi-section of LED I-V curve.
2. It can be used for simulating different type of paralleled LED. It also can simulate the nonlinear part of Vf.
Parallel for Different Type LEDs
V
I
V
I
V
I
Working on Better Solutions
• Most E-Load manufacturers have similar designs and are all unable to
simulate the I-V curve of the LED, as well as the internal capacitances
causing abnormal OCP, OVP. Unable to simulate LEDs, general E-Loads
are not suitable for testing LED drivers.
• 63110A was specifically designed to simulate LED characteristics and
therefore suitable for testing the functions of LED drivers.
• Can 63110A not only test stable conditions, it can also test turn on and
PWM dimming characteristics of the LED driver. The Rd value can be
adjusted according to the LED V-I curve making it ideal for LED driver
testing.
Conclusions
Thank you very much
Working on Better Solutions
Creation and InnovationCreation and Innovation
For more information contact ValueTronics 800-552-8258
Please visit our website www.valuetronics.com