LED Driver Test Description

1 LED Characteristic 1.1 Fig. 2 shows Fig.1 LED’s equivalent circuit, there is a forward resister series a forward

voltage Vd, the exponential V-I characteristic curve as Fig.3 When voltage across of LED larger than forward voltage Vd, then LED current Io is (Vo-Vd)/Rd.

1.2 When LED driver is constant-current type, the voltage across of LED is Vd+(I*Rd)=Vo,

Actually the Vd have a negative temperature coefficient (NTC -2mV//- -4mV/) with respect to voltage, e.g. the LED forward voltage Vd decreases as the LED gets warmer, causing the Vo voltage decreases as temperature goes up.

1.3 Right figure shows LED driver constant current output

Io has a current ripple, The voltage across of LED also got a Io*Rd voltage ripple(Normally is a high frequency triangle waveform)

1.4 Several LED cascading connection : Several LED cascading connection can get more

output brightness, Vd and Rd also will increase as series. Fig. 4. Shows 3 LED series, Fig. 5. and Fig. 6. are the LED’s equivalent circuit, Fig. 7. shows exponential V-I characteristic curve

Vo

Io

1.5 In parallel connection stacks of LED : Several LED stacks in parallel connection is also

can get more output brightness, Vd also will increase as series, Rd is according to the cascade and parallel. Fig. 8. is 2 stacks LED to parallel, Fig. 9. to Fig.11. are the LED’s equivalent circuit, Fig. 12. shows exponential V-I characteristic curve.

2 Real LED lamp load and electronic DC Load：

Below will explain what is the different in the test between real LED lamp load and electronic DC load, and what is the different of CC, CR LED mode of electronic DC load. 2.1 Real LED lamp load (3W/3.85V/700mA series 10 LED )

2.2 Electronic DC load： As the actual connection to the LED driver of the LED will be by brand, size, cascading, in parallel and then various different load conditions, if each test is required should be get the expensive cost of testing, the use of electronic load to simulate various combinations of LED to test can achieve fast and low cost.

2.2.1 CR mode；setting R= Vo/Io, 38.5/0.7 = 55Ω

2.2.2 CV mode；setting CV mode, V=Vo

2.3 LED mode；LED mode electronic load is simulating LED’s equivalent circuit Fig. 2. , Combine the CR and CV mode, The 3340G series LED mode need to setting Vd and Rd, The output current will be as section 2.1 real LED lamp output current.

3340G LED mode electronic load voltage Vo and current Io waveform, which when the output voltage reaches 25.8V, Io current began to increase, the same equivalent circuit with LED

3 How to setting Vd, Rd and Vo parameters for LED mode electronic load a. If the LED lamp’s brand, parts no and specifications is available then follow the LED data

to setting the parameters. b. If the LED lamp’s data information is not available, Normally, you can use the LED driver’s

specification to setting the parameters, Vo is the LED driver’s output voltage specification, Vd can predict 70~90 percent of Vo (initial setting to 80%), Rd = (Vo-Vd)/Io, Io is the LED driver’s output current specification.

3.1 Brand example I : LUXEON Rebel(Philips) specifications table

3.1.1 As specifications sheet you can find out Vo (it is equivalent to Vf, Typical Forward

Voltage), Normally in the specification Vf (Forward Voltage) from LED manufacture is the LED’s output voltage Vo.

3.1.2 By the characteristic curve, as shown above the blue line drawing along the LED

characteristic curve to find out forward bias voltage point Vd, Vd = 3.05V 3.2 Brand example II : CREE, model XLamp-L

3.2.1 How to setting 3340G series LED mode parameter Vo, Vd, Rd and No

First press mode key to LED mode as shown abovePress the "PRES" key to set the number of LED in series or series-parallel (initial number is 1, in general, LED Driver output specifications have been listed Vo directly to the final value, so you can generally set to 1.)

Press "PRES" key to set Vo.CREE LED according to specifications set Vo = 25.000V

Press "PRES" key to set Vd.According to CREE LED curve, set Vd = 22.500V

Press "PRES" key to set Rd. According to (Vo-Vd) / Io, set Rd = 16.666Ω

Press the "LOAD" key, the 3340G series electronic load Is ready to sink current from DUT (LED driver).DUT connected to the load terminals, and then turn on the Input source of DUT power supply

LED Mode No (LED Quantity) VO VD RD

Preset ONPress Mode key to

Preset OFF

3.3 Vo & Io setting and test result

3.3.1 3340G series LED mode according with user’s setting Vo, Vd and Rd to simulating the LED characteristic curve as Fig. 3. LED characteristic curve.

3.3.2 As Io is LED driver output, If LED driver actual output current not equal to specification output current that will be effect the output voltage Vo, EX. Io setting 150mA, actual LED driver output current 140mA, So the actual output voltage also will lower than the Vo specification, That can not use the ordinary electronic load’s CV or CC mode’s concept to check the Vo and Io, If use the actual output voltage test value to check the result that is not in accordance with Vo specification, in fact, this is the LED Driver’s output current has not correctly.

4 LED driver’s short test 4.1 Constant current output LED driver can not use the ordinary

electronic load to do short test. 4.2 3340G series LED mode load built-in a 12Vdc power supply for

short relay control the external relay to do short test.

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3341G

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5 LED driver’s dimming test

LED Driver dimming device can be TRIAC dimming or PWM dimming, the general E-Load response is too slow, can not be do LED Driver dimming test, 3340G Series LED mode Load increase the bandwidth (100KHz or more to) Improve the speed of response. 5.1 TRIAC dimming is the use of TRIAC dimmer (currently on the market has been in use for

many years incandescent dimmers, to use the TRIAC to adjust the voltage phase, to adjust the brightness of incandescent bulbs), LED Driver input connected to the dimmer, you can control the voltage phase changes of TRIAC dimmer, to adjust the LED Driver output current as well as the brightness of LED lights.

5.2 PWM dimming mode dimming test it, must provide a control signal to the LED Driver dimming control input dimming for LED Driver test, 3340G Series LED mode Load provides a DIM Ctrl O / P voltage signal (adjustable voltage levels 0 ~ 10V, Duty Cycle 0.01 ~ 0.99 (1 ~ 99%), Frequency DC, 10Hz ~ 1KHz) as a module to achieve the function of the system.

6 How to adjudge LED driver test is PASS or FAIL LED driver’s equivalent circuit as right figure, The LED driver’s tolerance as following 6.1 LED driver’s current source Io. 6.2 Different brand and specification have different Vd, Rd. 6.3 Different quantity LED cascading. EX. 3 LED cascade,

5 LED cascade will change with it. 6.4 When temperature changes the Vd will decrease with

increasing temperature -2 mV / ~ - 4mV / , Rd also temperature coefficient, so the above test, the factors to be consider. Therefore, To test LED driver, set a different Rd, Vd have different Vo, (Vo = Io* Rd + Vd), not as a general constant voltage output Power Supply, with the CC test, the output voltage / current have general specifications (5%). Do not like the usual constant current output Charger, with the CV test, the output voltage / current with the general specifications), LED driver output Vo will be with Vd, Rd the set and have similar Corresponding changes.

6.5 So test items in general have Vo, Io, short circuit voltage and PWM dimming function.

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