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
www.fairchildsemi.com
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
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
Constant current, constant power and foldback
characteristic of current sources
1,2
Output Voltage [normalised]
1
0,8
0,6
0,4
0,2
0
0
0,2
0,4
0,6
0,8
1
1
0,8
0,6
0,4
0,2
0
1,2
0
Output Current [normalised]
0,2
0,4
0,6
0,8
1
1,2
1
1,2
Output Current [normalised]
1,2
•Constant current
•Constant Power
•Foldback
Output Voltage [normalised]
Output Voltage [normalied]
1,2
1
0,8
0,6
0,4
0,2
0
0
0,2
0,4
0,6
0,8
Output Curent [normalised]
4
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
FPS - adjustible current limit circuit
• PWM comparator compares signal
from Vfb 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 lim it default
I lim itdesired
2.8kΩ
=
XkΩ
where,
RX ⋅ 2.8kΩ
XkΩ =
RX + 2.8kΩ
6
Typical Use of Peak Current Limit Pin
Ε=
1
LP iP2HI
2
Ε=
1
LP iP2LO
2
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
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
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
Secondary side regulated flyback, CC, low power
•For low output currents due to power dissipation in R201
•R201 has to drive Q201 (VBE approx 0.7V)
•Need an optocoupler for isolation
10
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
Primary side regulated flyback, CC, low and high
power
•Isolated
•Inexpensive
•Good performance
•Block 1: Constant Power
•Block 2: Constant Current
•R105: Load regulation
•R108: Line regulation
•R102: Adjusts current limit
of FPS
•R103 prevents FPS before shutdown
•R106 limits current through D106
12
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:
1
PO = ⋅ LP ⋅ iP2 ⋅ f SW = VO ⋅ I O = n ⋅ VF ⋅ I O
2
• The primary inductance, LP, is constant.
• The switching frequency, fSW, is constant.
If the primary current, iP, can be clamped, then the output power will be
constant.
Since the output voltage is clamped to n x VF, then the output current will be
constant!
13
How Does the PSR System Work – continued
iPCLAMP
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
Achieving Line Regulation
• The Vcc winding waveform is an ac
coupled replica of the drain-source
waveform.
VnegdcHI ≅ −Vin
NVcc
NP
VnegdcLO ≅ −Vin
NVcc
NP
VnegdcHI > VnegdcLO
• The “negative” portion of the waveform
is proportional to the bulk dc voltage
which in turn is proportional to the ac
line voltage.
• 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
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 IPK-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
Possible Output Characteristics
35,0
Output Voltage [V]
30,0
25,0
Curve with Block1 alone:
constant power
Foldback curve: with
lower R105 value
20,0
Constant current curve:
with higher R105 value
15,0
10,0
0
100
200
300
Output Current [mA]
17
400
500
European Test board - Specification
•Demo Board Specification
Minimum Input Voltage
185 VRMS
Maximum Input Voltage
265 VRMS
Frequency
50 Hz
Output Voltage and Current
12 V – 22 V / 700 mA constant current for
driving 4 to 7 OSRAM LED’s with 3V
forward voltage
18
Test Board – Application Circuit
T1
EF20
R101
100k
0.6W
C103
2.2nF
1000V
R109
100
0.6W
D102
RS1K
D101
MB8S
9
3
8
5
7
4
6
C202
D107
MMBD1503A
2.2nF
250V
+
Drain
2 x 47mH, 0.25A
5
VStr
1
IC101
FSDH321L
GND
185 - 265 Vrms
2
Vcc
6
+
10
C102
4.7uF
400V
D103
16V
Drain
-
LF1
7
~
3 C101
4.7uF
400V
4
Drain
1
+
8
2
~
Vcc
Ipk
2
1
D105
MMBD1503A
VFb
+
C105
10uF
50V
C108
1uF
50V
C107
33nF
R107
47
0.125W
2
3
D106
18V
4
CONN101
B2P3-VH
12V / 700mA
1
Q101
BC847B
R102
open
R103
820K
R106
10k
0.6W
R104
C104
68nF
47K
0.6W
D104
FDLL4148
•R102 is open due to current limit of FPS
19
R105
39k
0.6W
R108
160K
0.6W
D201
ES2D
CONN201
B2P-VH
+
C201
330uF
35V
2
1
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
230VRMS
Efficiency
-Output at 100% load
-Efficiency greater than 84%
-Input voltage was varied in its
specified range
105.0
100.0
95.0
90.0
12
14
16
18
20
22
Output Voltage [V]
90.0
88.0
Efficiency [%]
•
Regulation [% of nominal]
110.0
86.0
84.0
82.0
80.0
180
190
200
210
220
230
Input Voltage [Vrm s]
20
240
250
260
270
Test Board – Selected Performance Results
•
Foldback characteristic
-R102 : 2k
-R105 : 15k
-R108 was not mounted due to
constant input voltage
-Output current 200mA
-Input voltage was at nominal
230VRMS
3 5 ,0
3 0 ,0
Output Voltage [V]
Constant current characteristic
-R102, R105 and R108 values as in
test board circuit
-Output current at 700mA
-Input voltage was at nominal
230VRMS
2 5 ,0
2 0 ,0
1 5 ,0
1 0 ,0
0
100
200
300
400
500
600
700
800
O u tp u t C u rre n t [m A ]
35,0
30,0
Output Voltage [V]
•
25,0
20,0
15,0
10,0
0
50
100
150
Output Cuurent [mA]
21
200
250
Test Board – Selected Performance Results
Constant power characteristic
-R102 : 1k5 (Block1)
-R105, R108 was not mounted
(Block 2)
-Output power at 5W
-Input voltage was at nominal
230VRMS
35,0
30,0
Output Voltage [V]
•
25,0
20,0
15,0
10,0
0
100
200
300
Output Current [mA]
22
400
500
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
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
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
Three LED-350 mA PSR Board (90 – 270 Vac)
26
Protection Circuitry – Open LED
• Output (LED) Opens:
Drain-Source and Vcc Voltage During Open Load
27
Protection Circuitry – Shorted Output
• Shorted Output:
Drain-Source and Vcc Voltage During Output Short
28
Protection Circuitry – Soft Start
Drain-Source and Output Voltage During Startup
29
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
Thanks for your attention
Thank you!
31