Designing Offline HB LED Current Sources with Primary Side Control Using E-series Fairchild Power Switch (FPS)

Transcription

1 Designing Offline HB LED Current Sources with Primary Side Control Using E-series Fairchild Power Switch (FPS) Carl Walding Global Power Resource Center, Hoffman Estates, IL

2 Overview LED s Characteristics of current sources FPS Flyback converters constant voltage output CV Flyback converters constant current output CC New PSR flyback converter circuit CC Test results of test board Protection Circuitry of PSR LED Driver 2

3 Something about LED s Why LED s are used more and more? -High luminosity -Less power consumption -Easy to mount What are LED s used for? -Consumer Electronics: Lightning for inside and outside -Traffic lights and street lightning -Automotive Why do LED s need constant current? -PN-diodes in forward direction -To keep brightness constant -Temperature rise would lead to an increase of current till the device will be destroyed 3

4 Constant current, constant power and foldback characteristic of current sources Output Voltage [normalied] 1,2 1 0,8 0,6 0,4 0, ,2 0,4 0,6 0,8 1 1,2 Output Current [normalised] Output Voltage [normalised] 1,2 1 0,8 0,6 0,4 0, ,2 0,4 0,6 0,8 1 1,2 Output Current [normalised] Constant current Constant Power Foldback Output Voltage [normalised] 1,2 1 0,8 0,6 0,4 0, ,2 0,4 0,6 0,8 1 1,2 Output Curent [normalised] 4

5 E-series FPS Fairchild Power Switch Internal SenseFET Burst mode operation Precise switching frequency Pulse-by-pulse current limiting Current mode device Adjustable current limit AOCP OVP OLP TSD Auto restart mode UVLO Low operating current (max 3mA) Built-in Soft Start 5

6 FPS - adjustible current limit circuit PWM comparator compares signal from V fb and peak current through SenseFET Current through SenseFET can be adjusted by Rx Rx form a parallel resistor to internal voltage divider The peak current limit can be adjusted through the equation below. I I limit default desired 2. kω = Xk Ω limit 8 where, XkΩ = R R X X 2.8kΩ + 2.8kΩ 6

7 Typical Use of Peak Current Limit Pin Ε = Ε = L i 2 P P HI L i 2 P P LO In a typical voltage regulated power supply, the function of the Peak Current Limit pin is to reduce the current limit trip point. This will reduce the available energy in the event of a fault condition. 7

8 Secondary side regulated flyback circuit CV Optocoupler for isolation Few external components Switch integrated in the FPS FOD2741 combines optocoupler, E/A-amplifier and voltage reference Output voltage very accurate due to voltage reference 8

9 Primary side regulated flyback circuit CV No expensive optocoupler for Isolation Output and Vcc winding have to be coupled very well Output voltage less accurate due to tolerance of ZENER-diode in comparison to voltage reference element For constant current application output voltage doesn t need to be high accurate 9

10 Secondary side regulated flyback, CC, low power For low output currents due to power dissipation in R201 R201 has to drive Q201 (V BE approx 0.7V) Need an optocoupler for isolation 10

11 Secondary side regulated flyback, CC, high power For large output currents Need an optocoupler for isolation Need an auxiliary bias winding for op amps Expensive due to use of op amps 11

12 Primary side regulated flyback, CC, low and high power Isolated Inexpensive Good performance Block 1: Constant Power Block 2: Constant Current R103 prevents FPS before shutdown R106 limits current through D106 R105: Load regulation R108: Line regulation R102: Adjusts current limit of FPS 12

13 How Does the PSR System Work? Recall in a flyback converter operating in discontinuous mode conduction, the power delivered to the output can be expressed as: P O = 1 2 L P i 2 P f SW = V O I O = n V F I O The primary inductance, L P, is constant. The switching frequency, f SW, is constant. If the primary current, i P, can be clamped, then the output power will be constant. Since the output voltage is clamped to n x V F, then the output current will be constant! 13

14 How Does the PSR System Work continued i PCLAMP The primary current is clamped via the Peak Current Limit pin. Setting the resistor to a value that clamps the current to a particular amplitude also clamps the deliverable power. 14

15 Achieving Line Regulation V V HI negdc in N N Vcc P The Vcc winding waveform is an ac coupled replica of the drain-source waveform. The negative portion of the waveform is proportional to the bulk dc voltage which in turn is proportional to the ac line voltage. V V LO negdc in V > V N N Vcc P negdchi negdc LO The negative portion of the waveform is rectified and filtered to create a dc voltage that is proportional to the ac line voltage. This negative dc voltage will modulate the I peak limit pin voltage as the ac line voltage changes thereby achieving line regulation. 15

16 Primary side regulated flyback, CC, low and high power Block 1 -For low output current in constant voltage CV -Load increase leads to maximum duty cycle and maximum peak current => constant power CP Block 2 -Constant current appears at boarder between CV and CP -then an additional current flows from I PK -pin through R105 -which reduces current limit of FPS further -which results in a positive feedback signal => constant current CC -R108 compensates the output current against input line variations 16

17 Possible Output Characteristics 35,0 30,0 Output Voltage [V] 25,0 20,0 15,0 Foldback curve: with lower R105 value Curve with Block1 alone: constant power Constant current curve: with higher R105 value 10, Output Current [ma] 17

18 European Test board - Specification Demo Board Specification Minimum Input Voltage Maximum Input Voltage Frequency Output Voltage and Current 185 V RMS 265 V RMS 50 Hz 12 V 22 V / 700 ma constant current for driving 4 to 7 OSRAM LED s with 3V forward voltage 18

19 Test Board Application Circuit T1 EF20 12V / 700mA R k 0.6W C nF 1000V R D201 ES2D + C uF 35V 2 1 CONN201 B2P-VH CONN101 B2P3-VH Vrms D101 MB8S ~ ~ C uF 400V + LF1 2 x 47mH, 0.25A C uF 400V + 8 Drain 7 D102 RS1K Drain 6 Drain Vcc D103 16V D105 MMBD1503A + C105 10uF 50V W D107 MMBD1503A C107 33nF C108 1uF 50V R W 4 6 C nF 250V 5 IC101 FSDH321L VStr GND 1 Ipk 4 Vcc VFb R102 open 2 3 R K C104 68nF Q101 BC847B R104 D106 18V R106 10k 0.6W D104 FDLL K 0.6W R105 39k 0.6W R K 0.6W R102 is open due to current limit of FPS 19

20 Test Board Selected Performance Results Regulation versus load (voltage) -Output current is normalised on 100% load -Maximum tolerance is 6% -Input voltage was at nominal 230V RMS Regulation [% of nominal] Output Voltage [V] Efficiency -Output at 100% load -Efficiency greater than 84% -Input voltage was varied in its specified range Efficiency [%] Input Voltage [Vrms] 20

21 Test Board Selected Performance Results Constant current characteristic -R102, R105 and R108 values as in test board circuit -Output current at 700mA -Input voltage was at nominal 230V RMS Output Voltage [V] 35,0 30,0 25,0 20,0 15,0 Foldback characteristic -R102 : 2k -R105 : 15k -R108 was not mounted due to constant input voltage -Output current 200mA -Input voltage was at nominal 230V RMS Output Voltage [V] 10,0 35,0 30,0 25,0 20,0 15, Output C urrent [ma] 10, Output Cuurent [ma] 21

22 Test Board Selected Performance Results Constant power characteristic -R102 : 1k5 (Block1) -R105, R108 was not mounted (Block 2) -Output power at 5W 15,0 -Input voltage was at nominal 230V 10,0 RMS Output Voltage [V] 35,0 30,0 25,0 20, Output Current [ma] 22

23 Test board - EMI Test board at its maximum load and nominal Input voltage Easy to meet requirements of EN55011/22 Class B EMI limits 23

24 Protection Circuitry What happens if: LED Opens without LED (load) current the output voltages, including the Vcc (aux) voltage will tend to increase. When the Vcc voltage reaches the OVP threshold, typically 19 V, the system will shutdown. Output Short when the output shorts, all energy stored in the flyback transformer will transfer to the short circuit. The Vcc (aux) supply will also drop in value since there will not be enough energy to maintain the Vcc (aux) voltage. When the Vcc (aux) voltage drops to approximately 8 V (UVLO lockout) switching will terminate. The IC will then go through its startup sequence. When the Vcc voltage reaches 12 V it will start switching again. If the short is removed the system will startup again, otherwise the process will repeat. 24

25 Protection Circuitry Soft Start Another important circuit that increases reliability is the soft start. This circuit allows the duty cycle to increase gradually such that the output capacitors are also charged gradually. This helps to keep the output diodes and MOSFET within their ratings. It also helps to prevent transformer saturation. The typical soft start time is approximately 10 ms. 25

26 Three LED-350 ma PSR Board ( Vac) 26

27 Protection Circuitry Open LED Output (LED) Opens: Drain-Source and Vcc Voltage During Open Load 27

28 Protection Circuitry Shorted Output Shorted Output: Drain-Source and Vcc Voltage During Output Short 28

29 Protection Circuitry Soft Start Drain-Source and Output Voltage During Startup 29

30 Conclusion Different current source characteristics are possible with this application circuit This application circuit can be used for low and high output currents Need of just a few external components Inexpensive circuit due to primary side regulation Inexpensive due to use of Fairchild Power Switch FPS Good output performance Easy to meet EMI requirements 30

31 Thanks for your attention Thank you! 31