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 www.onsemi.com 1 DN06062/D Figure 1 – LED Backlight Schematic August 2009, Rev.1 www.onsemi.com 2 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 www.onsemi.com 3 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 www.onsemi.com 4 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 www.onsemi.com 5 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 www.onsemi.com 6 DN06062/D PC Board Figure 7 – PCB Top Figure 8 – PCB Bottom August 2009, Rev.1 www.onsemi.com 7 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 www.onsemi.com 8 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 www.onsemi.com 9 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 www.onsemi.com 10