LED Driver Load A Introduction 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 driver 2. LED driver powers the LEDs 3. 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: LEDs 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 A Target Market

Working on Better Solutions LED Characteristic 1.Nonlinear V-I curve. 2.Forward Voltage (V F ). 3.Small Current (several hundreds mA). 4.Brightness decided by POWER. 5.Small equivalent capacitance. Vf I V LED curve Vo Io Rd LED Introduction

Working on Better Solutions LED driver Type Voltage source Current source Voltage Source LED driver Disadvantage Due to the different LEDs having different forward voltages(V F ), 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 driver 1. 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 V F R d 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 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 A E-load has increased bandwidth to meet the test A LED mode LED 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. V o : LED driver operating voltage 2. I o : LED driver operating current 3. R d Coefficient: Operating resistor coefficient 4. R r : High frequency resistor Setting Parameters LED mode Turn ON LOAD before turn on Driver

Working on Better Solutions 1. V o : operating voltage 2. I o : operating current Press MODE key to choose LEDH (High V range) or LEDL (Low V range), then press ENTER to edit V o and I o. V o & I o Setting

Working on Better Solutions R d or Coefficient: Operating resistor coefficient Press CONF. key, then press key to choose R d or Rd Coeff R d Coeff.: 1. Default.= Set range: ~ 1. R d Coefficient Setting V I VoVo IoIo Vf

Working on Better Solutions Definition: V o : LED operating voltage I o : LED operating current V F : LED Forward voltage R d : Operating resistor What is R d Coefficient The relationship of the four variables above can be described by the following equation Eg. 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 R d 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, R d can be calculated from the slope of the line R d of the one LED Therefore the R d Coeff. is:

Working on Better Solutions The Advantage of Setting the R d Coefficient Why set R d Coefficient instead of the operating resistor R d ? 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 R d values. 4. It is therefore inconvenient to change the value of R d every time the output voltage changes.

Working on Better Solutions The Advantage of Setting the R d Coefficient Why set R d Coefficient instead of the operating resistor R d ? 5. Because R d is proportional to the output voltage, therefore by setting the R d Coefficient, 63110A will calculate the corresponding R d value. V I Vo1 Vo2 Vo3 Vo4 IoIo Rd1 Rd2 Rd3 Rd4

Working on Better Solutions Verifying Different LEDs Different types of LED need to be considered when testing LED drivers. A example: LEDs with 350mA current will have different R d and R d coefficient values. By setting the different R d coefficients 63110A can simulate different types of LED for LED driver testing.

Working on Better Solutions How to get a suitable R d 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 VoVo IoIo Vf1 Vf2 Vf3 Rd1/ Rd2/ Rd3 We can get a new formula: That means Rd Coeff can be calculated from Vf.

Working on Better Solutions How to get a suitable R d 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 = Vo = 42V Vf = 37V VIVI Example:

Working on Better Solutions How to get a suitable R d Coefficient Set parameters on 63110A, Vo=42V, Io=750mA, and set Rd Coeff = 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. VIVI VIVI

Working on Better Solutions A will calculate and simulate the LED characteristics from the V o, I o, R d coefficient settings, as shown in the diagram below, V o and I o are not the real loading values. 2. I o is determined by the LED driver, if the I o varies from the setting value then V o will also vary. This is different concept from CC and CV mode of normal E-load. V o & I o Setting and Measured V o & I o d

Working on Better Solutions V o & I o Setting and Measured V o & I o 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 IoIoIoIo

Working on Better Solutions Ripple Current of LED Driver 1. Under stable operation(V o / I o ), I ripple is caused by the LED driver V ripple and the LED operating resistor R d, V ripple / R d = I ripple. 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 A 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 R r : High frequency resistor Press CONF. key, then press key to choose R r, then press ENTER to setup. R r : 1. Default. 2.Set. R r range: 5Ω ~ 250Ω. R r 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 A 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

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