NCP1294: High Voltage LED Driver (24 V input to 110 V output @ 100 mA)

DN06062/D
Design Note – DN06062/D
Reference Design
for LCD TV LED Backlighting
Device
Application
NCP1294
LCD TV Back
Lighting
Input Voltage
24 VDC
Output
Power
Topology
25 W
Flyback
I/O
Isolation
NONE
Specifications
DC Input
22V – 26V
190V to 230V
110 mA regulated
Output Voltage
Output Current
Circuit Description
This circuit is proposed for driving a
series string of white LEDs with a forward
voltage up to 230V for use in LCD TV
Backlighting.
The Flyback topology was selected
due to the high transfer ratio from Vin to
Vout. Having a transformer with a turns ratio
allows operation at more reasonable duty
cycles thus improving efficiency and
transient performance. The need for fast
transient response during dimming makes
discontinuous mode the best choice for
operation.
The dimming input is a 500 HZ to
1KHz logic level pulse train that sets the
overall intensity of the LED string by turning
the LEDs off and on at a specific duty cycle.
LED color degradation is avoided by having
fixed LED current during the dim signal high
and zero LED current during the dim signal
low.
Figure 1 shows a schematic of the
reference board.
August 2009, Rev 0
Key Features
y 500 HZ to 1.5 KHz Duty Cycle Dimming
y Discontinuous Mode Flyback
y Wide output operation voltage
y Regulated LED current
y Open LED Protection
y Output Short Circuit Protection
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DN06062/D
Figure 1 – LED Backlight Schematic
August 2009, Rev.1
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DN06062/D
Basic Power Topology
The principle of the Discontinuous Flyback converter is fairly simple (see Figure 2):
While the power MOSFET is in the ON state, the input voltage source is directly applied to the primary
of the coupled inductor. Current ramps linearly in the primary since no current can flow in the secondary
due to the blocking diode on the secondary. Under these conditions the transformer is really functioning
as a coupled inductor. This results in accumulated energy in the core. During this time all of the current
needed to supply the load is provided by the output capacitor since the blocking diode is reverse biased.
The voltage stress on the output Diode during this time due to transformer action is:
Vin
Ns
+ Vout
Np
When the MOSFET turns OFF, the voltage on the secondary is forced to flip and conduction to the load
commences. Since the output voltage is fixed with the output capacitance, current ramps down linearly
flowing both to the load and the output capacitor. The voltage stress on the MOSFET Drain during this
time due to transformer action is:
Vin + Vout
Np
Ns
Once the current reaches zero, the MOSFET Drain voltage falls toward Vin. Since the magnetizing
inductance in series with the MOSFET parasitic Drain-to-Source capacitance forms a tuned circuit a
resonance voltage waveform is typically created. That resonance is:
1
2π LpriCds
It is advantageous to select the capacitance, inductance and operating frequency to coincide with the
lowest portion of this resonance just prior to start of a new switching cycle (See figure 3). This will
minimize turn ON switching loss in the system.
Vin
`
RLoad
Figure 2 – Flyback Operation
August 2009, Rev.1
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DN06062/D
Figure 3 – MOSFET Drain Voltage
The initial spike seen on the Drain is a function of leakage inductance in series with the Drain-toSource capacitance of the MOSFET. The resonance is:
1
2π LleakCds
C16 and R11 are installed in series across the transformer primary winding to snub this turn OFF
waveform. Similar snubbers are installed across the output rectifiers to reduce local leakage inductance
based ringing.
Input Regulator Regulation
The Circuit in Figure 4 is used to create a regulated 7Volts for the NCP1294 to operate. The input
regulator is a simple Emitter Follower with a Zener diode to set the base voltage. A three winding
transformer could be used to back bias this supply, but care must be taken to understand the output
variation on the winding voltage since the main output id current regulated.
Figure 4 – Input Regulator
Current Loop
R11 is used to sense the inductor current and is fed to the FB pin of the NCP1294. When not in
dimming mode, M2 and M5-A FETs are in the ON state allowing basic closed loop operation. Closing
the voltage loop around R1 makes the output current 1.26V/11.5Ohms = 109.5mA. C9 is the
integrating capacitor that stabilizes the main current loop.
August 2009, Rev.1
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DN06062/D
Figure 5 – Current Sense
Input Voltage Feed-Forward Ramp
R23 and C3 form an RC network that sets the ramp voltage for the internal PWM loop. Deriving the
voltage from Vin is beneficial to line rejection since perturbations on the input are directly offset in the
modulator stage.
Soft Start
Flyback power converters must be soft started to avoid saturation level currents from developing in the
primary winding. This is achieved by feeding the soft start voltage into the Iset pin of the NCP1294.
This allows for a variable current limit trip that increases as the Soft Start Capacitor charges at start up.
PWM Dimming Circuit
The PWM dimming circuit is comprised of a number of MOSFET switches that alternate current in the
LED load from a fixed regulation level (110mA Nominal) to zero current. Variation of the duty cycle
of the switches results in reduction in current that is linear with respect to Duty Cycle. The nominal rate
of dimming is between 500Hz and 1.5KHz.
Dimming is done by opening the LED current path as well as the feedback path. Also switching is
halted to prevent additional energy during LED OFF periods.
The TTL level PWM dimming signal is passed through M6-A gate which feeds an inverting stage
comprised of M4-B and M4-B driver transistors. R17 prevents shoot through currents in the driver
stage. The inverted dimming signal turns off M2 which opens the LED current path. Simultaneously,
M5-A is turned OFF which prevents discharge of C10 during the open current loop stage. This stores
the feedback voltage level during the LED OFF period, thus preventing the error amplifier from railing.
The dim signal is also sent to the gate of M6-B which pulls the FF pin to Vref voltage. This disable
switching while avoiding fault conditions during the LED OFF period.
August 2009, Rev.1
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DN06062/D
Transformer Design
See Figure 6
Electrical Specificaitons
DC Resistance (25C)
2-6 (2+3,5+6)
12-14
2-6 (2+3,5+6)
12-14
2-6 (2+3,5+6), short 12-14
12-14 2-6, short (2+3,5+6)
2-6 (2+3,5+6)
(14-12):(2-5)
(14-12):(3-6)
Inductance
Leakage Inductance
Primary Saturation Current
Turns Ratio
0.018 Ohms Max
0.55 Ohms Max
5uH +/- 10%
320uH +/- 10%
400nH Max
8uH Max
9 A Min
8:1 +/- 1%
8:1 +/- 1%
100mVAC, 0 ADC
100mVAC, 0 ADC
100KHz, 100mVAC
100KHz, 100mVAC
<20% drop in Lpri
Figure 6 – Flyback Transformer Specifications
Protection Features Input UV Lockout
R19 and R25 and C2 set the Input Under Voltage Lockout level. UVLO is calculated by:
R19 ⎞
⎛
Vuvlo = Vuvref ⎜1 +
⎟
⎝ R 25 ⎠
Pulse by Pulse Current Limit
R12 is used to sense an limit primary peak current. R13 and C6 are used to filter leading edge current
spike created by leakage inductance and gate driver currents. The Iset voltage is set with the R21 and
R4 divider from Vref. The peak current trip point is calculated by:
Ipk =
Vref ( R 4) 1
( R 21 + R 4) R12
LED Open Circuit Protection
Since this is a current regulated loop, open circuit voltage could be a potential problem if it were not
specifically limited. This design utilizes the OV pin of the NCP1294 and the R27 and R26 divider ratio.
Vopen loop is calculated by:
⎛ R 27 ⎞
Vov = Vovref ⎜1 +
⎟
⎝ R 26 ⎠
August 2009, Rev.1
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DN06062/D
PC Board
Figure 7 – PCB Top
Figure 8 – PCB Bottom
August 2009, Rev.1
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DN06062/D
Qty
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Reference
T1
C5
C8
C16
C10
C6
C4
C2
C1
C9
C15
C3
C11
C12
C19
C21
C13
C7
C14
D1
D3
D4
Z1
U1
Q1
M1
M2
M4
M5
M6
R8
R6
R20
R22
R19
R27
R17
R10
R5
R21
R23
R4
R26
R13
R25
R14
R12
R1
R2
R3
R28
R15
R11
R32
Value
0.1uF
1.0uF
1.5nF
10nF
1nF
1uF
1uF
470pF
56nF
56nF
1.0nF
20pF
20pF
20pF
0.1uF
10uF
1000uF
1uF
7.5V
NPN
0
1
10k
10k
120k
1M
1k
20K
220
28.7k
41.2k
6.8K
7.32K
80
8K
0
0R05
11.5
150
150
150
1R0
4.99
open
Part Number
750311176
ECJ-1VB1C104K
ECJ-1VF1E105Z
GRM188R72A152KA01D
ECJ-1VB1C103K
GRM188F51H102ZA01D
ECJ-1VF1E105Z
ECJ-1VF1E105Z
ECJ-1VC1H471J
ECJ-1VB1C563K
ECJ-1VB1C563K
ECJ-1VC1H102J
ECJ-2VC2D220J
ECJ-2VC2D220J
ECJ-2VC2D220J
UVR2F010MED
GMK316F106ZL-T
ECA-1VM102B
UPW2V010MPH
MBRS3200
MBRS3200
MBRS3200
MMSZ4691T1G
NCP1294
NST489
NTD12N10T4G
STD7NK40ZT4
NTJD4105C
NTJD4105C
NTR4501
Description
Flyback Transformer
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Ceramic Chip Capacitor
Electrolytic Capacitor
Electrolytic Capacitor
DIODE, SCHOTTKY
DIODE, SCHOTTKY
DIODE, SCHOTTKY
Zener Diode
Enhanced Voltage Mode PWM Controller
General Purpose Dual NPN Transistor
Power MOSFET
Power MOSFET
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Resistor
Manufacturer
Wurth
Panasonic
Panasonic
Murata
Panasonic
Murata
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Murata
Panasonic
Panasonic
Nichicon
ON Semiconductor
ON Semiconductor
ON Semiconductor
ON Semiconductor
ON Semiconductor
ON Semiconductor
On Semiconductor
On Semiconductor
On Semiconductor
On Semiconductor
On Semiconductor
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
Vishay / Dale
PCB DECAL
750311176-CUT
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0603CAP
0805CAP
0805CAP
0805CAP
0805CAP
1206CAP
1210CAP
ECA_12.5D
ECA_10D
SMB
SMB
SMB
MMSZ11T1G_SOD123
SOIC16
TSOP-6
NTD12N10_DPAK
NTD12N10_DPAK
SOT-363
SOT-363
SOT-363
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
0603RES
1206RES
1206RES
1206RES
1206RES
1206RES
1206RES
1206RES
1206RES
1206RES
Table 1 – Bill of materials
August 2009, Rev.1
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DN06062/D
Measurements –
Dimming Curve
LED PWM Dimming Curcve
0.12
0.1
LED Current (A)
0.08
0.06
24Vin, 235Vout Current
26,4Vin, 235Vout Current
0.04
21.6Vin, 235Vout Current
24Vin,180Vout Current
26.4Vin,180Vout Current
21.6Vin,180Vout Current
0.02
0
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
Duty Cycle
Efficiency
Efficiency vs Dim Duty Cycle
90.0%
80.0%
70.0%
Efficiency
60.0%
50.0%
Efficiency
40.0%
30.0%
20.0%
10.0%
0.0%
0%
20%
40%
60%
80%
100%
120%
Duty Cycle
August 2009, Rev.1
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DN06062/D
1
© 2008 ON Semiconductor.
Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use;
nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is
provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all
liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON
Semiconductor may change any of its products at any time, without notice.
Design note created by Tom Duffy, e-mail: [email protected]
August 2009, Rev.1
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