Application Note: AN-302 INTEGRATED CIRCUITS DIVISION IX9908 Design Considerations AN-302-R02 www.ixysic.com 1 AN-302 INTEGRATED CIRCUITS DIVISION 1 Introduction This application note provides general guidelines for designing an off-line LED driver using IXYS Integrated Circuits Division’s IX9908. The IX9908 can be configured to drive an external MOSFET device in a quasi-resonant flyback converter power stage that provides a constant current output to a LED string while maintaining a high power factor. Figure 1 This driver features a high voltage start-up circuit that eliminates the need for a VCC resistor, thereby improving overall power dissipation. In addition, there are multiple safety features such as under-voltage lockout, over-voltage protection, digital soft-start, foldback correction, and cycle-by-cycle peak current limiting. The IX9908 is an excellent choice for many phase-cut dimming and high power-factor correction LED lighting applications. IX9908 Block Diagram Vcc ZCV Ringing Suppress Blanking High Voltage Startup HV Gate Control GD Vcc Vcc Monitor Over - Under Voltage Lockout Reference Voltage Generator Over-Voltage Protection Leading Edge Blanking Foldback Sense Amp Control Logic Over-Temp Sensor Soft-Start Control Foldback Correction VR Shorted Winding Detection GND Leading Edge Blanking CS Leading Edge Blanking 2 LPF Analog Mux PFC www.ixysic.com R02 AN-302 INTEGRATED CIRCUITS DIVISION Figure 2 IX9908 Application Circuit DVCC RZCV1 AC - RSN DOUT COUT DSN CZCV CVCC + T1 CSN Aux CIN RZCV2 VCC Q1 ZCV HV GD LEDs IX9908 RIN1 RIN2 1.1 VR GND CS CC RCS DVR CVR LED Driver Specifications The following equations and component selections are based on the following LED driver specifications: Parameter AC Input Voltage Minimum Voltage Symbol Rating VAC_min 90 Maximum Voltage VAC_max 135 AC Input Frequency fAC 60 Units Vrms Hz Auxiliary Voltage Vaux_max 18 V Auxiliary Current Iaux_max 30 mA LED String Voltage VLEDstring 20 V LED String Current ILED_max 500 mA Estimated Efficiency Oscillator Frequency fS 85 67 % kHz Power Factor Maximum Duty Cycle PF Dmax 98 50 % % R02 www.ixysic.com 3 AN-302 INTEGRATED CIRCUITS DIVISION 2 Component Selection 2.1 DC Bulk Voltage at Low and High Line V DC_bulk_min = 2 V AC_min = 2 90V AC 127.3V V DC_bulk_max = 2 V AC_max = 2 135V AC 191V 2.2 2.7 Input Rectifier Bridge The breakdown voltage for this bridge is based on the maximum input voltage. P out = 20V 500mA = 10W Input Power Calculation P in 2.4 A small, 0.1F, 400V polyester film filter capacitor should be adequate for this application. Output Power Calculation P out = V LEDstring I LED_max 2.3 follows the rectified AC line voltage, enabling a very high power factor of up to 98% with low total harmonic distortion (THD). V bridge = P out P in = -------------- PF 10W = -------------------------- 12W 0.85 0.98 Select the next higher standard voltage, 400V. The rectifier bridge is exposed to high surge currents, so select a bridge that can handle at least five times the Irms input current. In this case, a 1A bridge would be sufficient. Primary Inductor Currents Calculation 2 V AC_min D max I pri_peak = -------------------------------------------------L pri f S 2.8 Calculate MOSFET Voltage Rating 127.3V 0.5 I pri_peak = ------------------------------------------ 0.522A peak 1.82mH 67kHz V DS = V DC_bulk_max + V ro + V spike For Lpri, see 3.1 Primary Inductance Calculation. I rms D max I rms = I pri_peak ----------3 = 0.522A peak 0.5 ------- 0.213A rms 3 V DS = 191V + 127V + 50V 368V 2.9 Reflected Flyback Voltage Calculation V LEDstring + V f 20V + 0.7V - = --------------------------- 127V V ro = ---------------------------------0.1626 N ------S N P See 3.2 Determine Transformer Turns Ratio for NS/NP ratio. 2.6 Assume that Vspike=50V. Due to the voltage spikes resulting from leakage inductance, the MOSFET should be selected with an adequate margin. A good choice would be between 500V and 600V. Note that duty cycle above 50% will result in converter stability issues such as sub-harmonic oscillations. 2.5 2 135V AC 2 382V Design RCD Snubber In a flyback topology, a snubber circuit is required to clamp the voltage caused by the leakage inductance, which is present in all transformers. Leakage inductance is highly dependent on the transformer construction, so care should be taken to keep it less than 2% of primary inductance. When no leakage inductance is known, a leakage inductance value of 37H (2% of 1.82mH) can be used as a starting point to calculate the snubber’s resistor, capacitor, and diode values. DSN is selected as 1N4007GP (1000V, 1A axial lead). Energy stored in the leakage inductance Le: 2 2 W1 = 0.5L e I rms = 0.5 37H 0.213 0.84J Input Capacitor, CIN The IX9908 is designed to operate without the need of a large bulk capacitor. This operating method enables the input current to form a triangular shape that closely 4 www.ixysic.com R02 AN-302 INTEGRATED CIRCUITS DIVISION Average power transferred from W1 to the snubber: Figure 3 P1 = W1 f S = 0.84J 67kHz 56mW 1.05 V max = V DC_bulk_max + V ro + V spike 1.00 V max = 191V + 127V + 50V = 368V 0.95 VCS vs. IZCV VCS (V) 0.90 • Vspike = 50V • Vro = 127V 0.85 0.80 0.75 Limiting the voltage to 368V. 0.70 0.65 V SN = 368V – 127V = 241V 2 0.60 200 2 V SN 241V - = ------------------ 1M R SN = ----------0.056W P1 TS 15s- = 0.015nF = -----------C SN = --------R SN 1M TS 1 1 - = ---------------------------------- 15pF C SN » --------= ------------------67kHz 1M R SN f S R SN 1000 1400 IZCV (µA) 1800 2200 The over voltage detection can be programmed by RZCV2 resistor. The output, VLEDstring, is 20V, so select an over voltage protection of 35V, and from the datasheet, VZCVOVP = 3.7V R ZCV1 V ZCVOVP R ZCV2 = -------------------------------------------V oovpth – V ZCVOVP For margin, CSN =1nF is selected. This value is a starting point, and might have to be adjusted to get the desired voltage spike suppression. 2.10 Over-Voltage Protection & Zero-Crossing Detection The application schematic in Figure 2 IX9908 Application Circuit shows that the voltage from the auxiliary winding is connected to the zero crossing pin (ZCV), via RC network resistor RZCV1, RZCV2 and CZCV. The circuit provides a delay so that switch-on can occur at the voltage valley, thus enhancing efficiency. The line voltage is sensed indirectly through the current in RZCV1. 18.7k 3.7V R ZCV2 = ----------------------------------- 2.210k 35V – 3.7V The delay capacitor can be approximated: R ZCV1 + R ZCV2 C ZCV = t RC ------------------------------------R ZC1 R ZC2 18.7k + 2.210k C ZCV = 1s ---------------------------------------------- 519pF 18.7k 2.210k Where tRC = 1s (see IX9908 data sheet). 2.11 Primary Peak Current Control The value of the current sense resistor (RCS) can be selected by using following equation: The current equation is given by: V CSmax R CS = ----------------I pri_peak V DC_bulk_min N A I ZCV = ------------------------------------------R ZCV1 N P 0.75V R CS = -------------------------- 1.44 0.522A peak Solving for RZCV1: R ZCV1 600 V DC_bulk_min N A = ------------------------------ ------I ZCV NP From the VCS vs. IZCV graph below we select IZCV = 1000A as the recommended current. For NA/NP value, see 3.2 Determine Transformer Turns Ratio. The RIN1 and RIN2 resistor values can be selected to scale the input voltage at the VR pin. The RIN1 resistor is selected with consideration of losses and high power factor correction. In this example we select RIN1 = 1046k. Select two 523k standard value resistors in the 0805 package. 127.3V R ZCV1 = ----------------- 0.1468 18.7k 0.001A R02 www.ixysic.com 5 AN-302 INTEGRATED CIRCUITS DIVISION See 2.4 Primary Inductor Currents Calculation. For adequate margin, select a 3Arms diode, such as STPS3150V. The RIN2 value can be calculated: R IN1 R CS G PWM I pri_peak R IN2 = -----------------------------------------------------------------------------------------------V DC_bulk_min – G PWM I pri_peak R CS RIN2 = 18.8k. Select a standard value resistor, 18.7k, where: PWM-Op gain, GPWM = 3 (from the IX9908 data sheet). Select the CVR capacitor to be 2.2nF and the DVR diode can be BAS16, 100V, or equivalent. The output capacitor can be selected based on the desired output voltage ripple. The dynamic resistance of the LED driven at a given current should also be considered, as this will determine the current ripple through the LED string. In this application, the LEDs have a dynamic resistance of 1 measured at 500mA operating current. 6 LEDs result in 6 of dynamic resistance. 2.12 Selection of VCC Capacitor and Auxiliary Blocking Diode In this application, a simple and inexpensive power supply for the IC is made from the auxiliary winding by using a blocking diode and a capacitor. V out_rip = I LED_max R dynamic V out_rip = 500mA 6 = 3V PP The output capacitor can be approximated: 2 I LED_max C out = -------------------------------------------------------V out_rip 2 120Hz The auxiliary VCC diode blocking voltage can be calculated: 2 500mA C out = ---------------------------------------------------- 442F 3V PP 2 120Hz NA V DVcc 2 V AC_max ------- + V aux_max NP V DVcc 191V 0.1468 + 18V 46V In this case, BAS16, 100V, or equivalent, would be selected. Select two 220F capacitors, EEV-FM1V221L. These capacitors are rated at 105°C for 4000 hours with 100kHz frequency. For a dimming application a 22F capacitor would be required: if there is no dimming requirement, then a 10F capacitor will suffice. 2.13 Output Schottky Diode and Capacitor The Schottky output diode is exposed to large currents when the converter is operated in critical conduction mode, CCM. Care should be taken to ensure adequate margins for the voltage and current ratings. The required blocking voltage for DOUT: NS V d_out 2 135V AC_max ------ + V LEDstring N P V d_out 191V 0.1626 + 20V 51V See 3.2 Determine Transformer Turns Ratio for the NS/NP ratio. Select a Schottky diode with a 150V rating. The output diode is exposed to large peak currents. V ro 1 – D max I d_rms = I rms -------------------- ----------------------------------V LEDstring + V f D max I d_rms 0.213A rms 1 6.135 = 1.31A rms 6 www.ixysic.com R02 AN-302 INTEGRATED CIRCUITS DIVISION 3 Transformer Design A step-by-step guide for transformer design will be presented here. Primary inductance and turns ratio will be calculated, and the appropriate wire size will be selected. The transformer core and coil former will be selected to support design power requirements, and a general guideline will be presented for transformer construction to achieve the best efficiency, and avoiding transformer saturation at higher temperatures. 3.1 Primary Inductance Calculation 2 L pri V DC_bulk_min D max = ----------------------------------------------------------------2 P IN F S 3.3 The first step is to find the RMS current through the primary winding, which can be approximated with the following equation: D max I rms = I pri_peak ----------3 I rms = 0.522A peak 0.5 ------- 0.213A rms 3 The primary winding wire area can be calculated: I rms 0.213A rms - = 0.0355mm 2 A w_pri = ---------- ----------------------J max 6A/mm 2 2 127.3V 0.85 0.5 L pri = ------------------------------------------------------ 1.82mH 2 12W 67kHz 3.2 Determine Transformer Turns Ratio D max NS V LEDstring ----------------------- = ------ --------------------N P 1 – D max V IN Where Jmax is the current density of a wire, which is stated by the wire manufacturer to be 6A/mm2. The diameter is determined by the following formula: AW - = Wire Diameter = 2 ------ Secondary to primary turns ratio: NS 20V + 0.7V ------ = --------------------------- 0.1626 NP 2 90V The auxiliary to primary turns ratio: V aux_max + V f NA ------- = ----------------------------------------------------------NP 1 – D max 2 V AC_min --------------------D max NA 18V + 0.7V ------- = --------------------------------------------- 0.1469 1 – 0.5 NP 2 90V ---------------0.5 The auxiliary to secondary turns ratio: V aux_max + V f NA ------- = ---------------------------------NS V LEDstring + V f AW AW --------- --------------4 0.7854 2 NS V LEDstring + V f ------ = ---------------------------------NP 2 V AC_min Where Vf is the voltage drop of the output diode. Determine Primary Winding Wire Size D w_pri 0.0355mm ---------------------------- 0.213mm 0.7854 From Table 1: Wire Gauge Table this is converted to 32 AWG. 3.4 Determine Secondary Winding Wire Size The peak current of secondary winding can be determined by the following formula: 2 I LEDmax 0.5A- = 2A I sec_peak = --------------------------- = 2------------------peak 1 – 0.5 1 – D max The RMS current can be found: 1 – D max - 2A peak 0.408 0.817A rms I sec_rms = I sec_peak -------------------3 Calculate the secondary wire size: I sec_rms 0.817A rms 2 A w_sec = ----------------- ------------------------ 0.136mm 6 6 N 18V + 0.7V ------A- = --------------------------- 0.9034 20V + 0.7V NS Calculate the secondary wire diameter: 2 0.136mm - 0.416mm D w_sec -----------------------0.7854 Convert using Table 1: Wire Gauge Table to AWG 26. R02 www.ixysic.com 7 AN-302 INTEGRATED CIRCUITS DIVISION 3.5 Determine Auxiliary Winding Wire Size The maximum current in the auxiliary winding can be calculated using: 2 I aux_max 30mA- = 0.12A I aux_peak = --------------------------- = 2---------------------peak 1 – 0.5 1 – D max Table 1: American Wire Gauge (AWG) Equivalent Wire Diameter (mm) Equivalent Wire Area (mm2) 40 0.0799 0.00501 The RMS current can be calculated: 1 – D max - 0.049A rms I aux_rms = I aux_peak -------------------3 The required wire area is given: I aux_rms 2 A w_aux = ------------------ 0.0082mm 6 Wire diameter: 2 ---------------------------- 0.102mm D w_aux 0.0082mm 0.7854 Convert using Table 1: Wire Gauge Table to AWG 38. 8 Wire Gauge Table www.ixysic.com 39 0.0897 0.00632 38 0.101 0.00797 37 0.113 0.01 36 0.127 0.0127 35 0.143 0.016 34 0.16 0.0201 33 0.18 0.0254 32 0.202 0.032 31 0.227 0.0404 30 0.255 0.051 29 0.286 0.0642 28 0.321 0.081 27 0.361 0.102 26 0.405 0.129 25 0.455 0.162 24 0.511 0.205 23 0.573 0.258 22 0.644 0.326 21 0.723 0.41 20 0.812 0.518 19 0.912 0.653 18 1.024 0.823 17 1.15 1.038 16 1.29 1.31 15 1.45 1.65 14 1.63 2.08 13 1.83 2.62 12 2.05 3.31 11 2.3 4.17 10 2.59 5.26 9 2.91 6.63 8 3.26 8.37 7 3.67 10.55 6 4.11 13.3 5 4.62 16.75 4 5.19 21.15 3 5.83 26.67 2 6.54 33.62 1 7.35 42.4 R02 AN-302 INTEGRATED CIRCUITS DIVISION 3.6 Calculate number of turns for the primary winding: Select Inductor Core & Calculate Turns The Ferroxcube catalog lists the following specifications for the E20/10/5 core set. L pri 2 1.82mH -------- ----------------------- turn 122 turns 121.99nH AL N pri = • (le/Ae) core factor = 1.37 mm-1 • Ve effective volume = 1340 mm3 • le effective length = 42.8 mm Calculate the maximum flux density: AL B max = N pri I pri_peak -----Ae • Ae effective area = 31.2 mm2 The air gap, G, of 0.3mm may be used for initial calculation. The effective permeability of the core can be calculated: B max i e = ----------------------G i 1 + --------------le 31.2mm 2 0.248T Note that Bmax for 1P2400, MnZn ferrite, is 0.36T at 100°C. Calculate number of turns for secondary and auxiliary windings: Where i = 2000 (for N27 material or 1P2400) and G=0.3mm. NS N sec = N pri ------ 122turns 0.1626 20 turns NP NA N aux = N pri ------- 122turns 0.1468 18 turns NP 2000 e = -------------------------------------------- 133 20001 + 0.3mm --------------------------------42.8mm Calculate Inductance Factor AL for this core: Calculate to make sure windings will fit the winding area of coil former. The Ferroxcube data sheet provides information for 8-pin coil former and specifies winding area of 27mm2. o e A L = ---------------le ---- Ae –7 121.99nH122turn 0.522A peak ----------------------2 turn ---------------------------------------------------------------------------------- –3 10 4 10 H 133 ---------- 121.99nH ----------------------A L ----------------------------------------- 2 2 1.37 m turn turn The fill factor has to be taken into consideration as this will affect winding area for this coil former. In general we can use fill factor Ku of 0.3 to 0.7 as a starting point. In this case 0.3 will be selected. Where 0 = vacuum permeability = 4 *10-7H/m. 1 2 Total Winding Area = ------- N pri A w_pri + N sec A w_sec + N aux A w_aux 27mm Ku 2 2 2 Total Winding Area 3.33 122 0.0355mm + 20 0.136mm + 18 0.0081mm 27mm 2 Total Winding Area 24mm 27mm 2 2 Note: The total winding area should not be exceeded due to the selection of approximate standard wire gauges from Table 1: Wire Gauge Table. R02 www.ixysic.com 9 AN-302 INTEGRATED CIRCUITS DIVISION 3.7 Flyback Transformer Construction The transformer block diagram shows the primary split into two sections of 61 turns each. The secondary and auxiliary are wound between the two primary windings. The transformer stack-up has three layers of insulation tape between the primary and the secondary side. This method enables the transformer to pass the safety standard for electrical strength requirement. In the 1 + Primary (NO0) 0.20mm 32 AWG 61T 6 2 Primary (NO1) 0.20mm 32 AWG + 61T Secondary (N2) 0.40mm 26 AWG 3 4 + Auxiliary (N1) 0.10mm 38 AWG Primary (NO1) Basic Insulation Auxiliary (N1) Basic Insulation Triple Insulation Secondary (N2) + 20T previous step, we selected an air gap of 0.3mm as our starting point. The air gap is critical because it allows the transformer to extend its maximum saturation; however, if the gap is larger, then it could contribute to higher leakage inductance. In this example, it is possible to achieve a leakage inductance less than 2% of primary. Triple Insulation Primary (NO0) 7 Bobbin 18T 5 Bobbin Primary (NO0) Coils wound on bobbin & insulated with XFMR tape Installation of ferrite core halves Core halves secured with XFMR tape For additional information please visit our website at: www.ixysic.com IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make changes to specifications and product descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated Circuits Division’s Standard Terms and Conditions of Sale, IXYS Integrated Circuits Division assumes no liability whatsoever, and disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any intellectual property right. The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division’s product may result in direct physical harm, injury, or death to a person or severe property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice. Specification: AN-302-R02 ©Copyright 2015, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 8/20/2015 10 www.ixysic.com R02

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