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. 

1.3 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.

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. 

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 3340F series LED mode need to setting Vd and Rd, The output current will be as section 2.1 real LED lamp output current. 

3 How to setting Vd, Rd and Vo parameters for LED mode electronic load

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 

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

• Application Note

• Non-Linear V-I curve
• Forward Bias (Vd) and Dynamic Resister (Rd)

Io=Vo-Vd/Rd

• Power Rating

• LED mode load for LED Power Driver test.
• CC, CR, CV, CP, LED and Dynamic mode.
• Simulate LED Forward Bias Voltage (Vd) and Resistance (Rd).
• Fast Response for PWM dimming test.
• Built-in dimming control signal for PWM dimming test.
• Short circuit test by external relay (built-in short relay driver circuit). 
• 5 digital V / A / W Meter.
• Protections against V, I, W, and ℃.
• Can be configured in 3302F〔single channel mainframe〕、 3305F〔two channels mainframe〕or 3300F〔four channels mainframe〕and the mainframe with 150 sets Store / Recall memory.
• Optional Interface：GPIB、RS232、USB、LAN.

• Only need 0.8V/0.3V to draw rated current
• Programmable slew rate, up to 1A/uS
• Precision Voltage/Current/Power measurement 
• OCP, OPP, Short circuit simulation
• Full Protection : OV, OC, OP and OT protect 
• Auto Sequence operating
• GPIB, RS-232, USB, LAN interfaces

• LED Driver test with different Load

• LED Driver test with different Load

• LED Driver test with different Load

• LED Driver test with different Load

• LED Driver test with different Load

－LED mode – Vd+Rd

• LED Data information
• Real LED test result
• Prodigit 3340D Led load

• LED Parameters : Vd (Forward voltage), Io 

LED Driver Specifications

Drawing a slop line

get Vd

How to get Rd ; (Vo– Vd) /Io, (3.38V-3.05V)/0.7 = 0.4714Ω

Check LED driver with real LED lamp

• Real LED testing and record the voltage & current waveform
• LED Driver Specification：30W / 0.7A
• LED 3W(3.85V, 700mA) Series 10 PCs
• Test Result : Vo≒38.5V, Io≒700mA, Vp-p=2.4V, Ip-p=0.130mA(CH1 10V/DIV, CH2 250mA/DIV

Find out Vd from LED Driver turn on character curve

• you can use the LED driver’s specification to setting the parameters, 

LED mode testing procedure

• 3340F LED mode setting : No =1, Vo=38.5V ； Vd=25.8V ；Rd=18.007Ω
• LED Driver Specification：30W / 0.7A
• Test Result : Vo≒38.5V, Io≒700mA, Vp-p=2.8V, Ip-p=0.120mA(CH1 10V/DIV, CH2 250mA/DIV

Current Ripple

• Iripple is produced by the LED driver's Vripple of LED'scaused by the equivalent impedance Rd, Vripple / Rd = Iripple.
• Adjust Rd to modify current ripple
• Bandwidth set to Hi

• DIM : Io (Level) control by analog voltage control

• DIM : PWM Io by analog voltage control

• PWM Freq. DC~1KHz, Duty 0.01~0.99(1~99%)

Short Test

• Constant current output LED driver can not use the ordinary electronic load to do short test. Because the LED Load input impedance.
• 3340F series LED mode load built-in a 12Vdc power supply for short relay control the external relay to do short test. 

Short Relay

• Direct Plug in Short Fixture
• No Relay Power Required
• Two 250V relay cascade  to get 500V
• Easy Relay Replacement

How to choose proper Model of 3340F series 
 

• Linear Start up model or PWM start up mode 

• Linear ramp up type LED driver – SanSung STOOPA5804 58Vdc / 450mA LED Lamp 3W x 14

• 3340F LOAD set 48.4V / 36V / 27.55 ohm