AND8136/D Offline LED Driver http://onsemi.com APPLICATION NOTE This application note provides a simple approach to designing an LED driver utilizing the ON Semiconductor NCP1014 self−supplied monolithic switcher. The easy−to−follow, step−by−step procedure guides the user into designing the different blocks that constitute the power supply, mainly the input block, the power stage, the magnetics, the snubber, the output block, and the feedback loop. The circuit diagram, bill of material, and PCB layout are also included at the end of the application note. This power supply is specifically designed to drive three LED’s. It meets IEC and UL requirements. EMI is minimal and a 70% achievable efficiency or greater is possible. The NCP1014 integrates a fixed−frequency current mode controller and a 700 V MOSFET. This device is housed in a PDIP−7 package and features soft−start, frequency jittering, short−circuit protection, skip−cycle, and a dynamic self−supply (no need for an auxiliary winding). Input Peak Current: Ipeak 5 · Iin(avg) 220 mA Circuit Description Input Block The input block of the power supply consists of a fuse, an EMI filter, a diode bridge rectifier, and an input bulk capacitor. Fuse The fuse F1 is protecting the circuit from current surges occurring at turn on. In this application, F1 is rated for 2.0 A, 125 Vac. EMI Filter The EMI filter suppresses common mode and differential mode noise and is very dependent upon board layout, component selection, etc. An X capacitor C1 and a common mode choke L1 are placed across the AC lines to attenuate differential mode noise, see Figure 1. The EMI inductor is slowing down any transient voltage surge to reduce high frequency noise. Both the capacitor and choke should be placed before the diode bridge and as close to the ac line input as possible to minimize RFI. Design Parameters The first step in designing a power supply is to define and predetermine the input and output parameters. Universal Input Voltage Range: Vac(min) 85 Vac, Vac(max) 265 Vac Output Specifications: Vout 11.75 V 2%, Iout 350 mA Diode Bridge Rectifier In order to choose the right diode bridge rectifier, the values of the forward and surge currents and DC blocking voltage must be considered. The surge current can reach values up to five times that of the average input rms current. It is therefore necessary to select a rectifier capable of handling such large currents. DC Blocking Voltage is calculated at high line: Input Power: P Pin out , where 78% estimated efficiency Pout Vout Iout 11.75 0.350 4.1 W Pin 4.1 5.25 W 0.78 DC Rail Voltages at Low Line and High Line: VR Vdc(max) 375 Vdc Vdc(min) Vac(min) · 2 85 · 2 120 Vdc Vdc(max) Vac(max) · 2 265 · 2 375 Vdc Forward Current: IF 1.5 · Iin(avg) 1.5 · 0.044 66 mA Surge Current: Average Input Current: Iin(avg) V Semiconductor Components Industries, LLC, 2005 March, 2005 − Rev. 1 IFSM 5 · IF 5 · 0.066 330 mA Pin 5.25 44 mA 120 dc(min) 1 Publication Order Number: AND8136/D AND8136/D Input Bulk Capacitor calculated will cause the power supply output to fall out of regulation. The purpose of the input bulk capacitor C2 is to hold up the rectified line voltage and also to filter out common mode noise. It is placed between the bridge rectifier output and ground. The size of the bulk capacitor depends on peak rectified input voltage and the ripple voltage magnitude. A larger capacitor will lower the ripple voltage on the dc input line, but will induce a larger surge current when the supply is powered up. Assuming a ripple magnitude of about 20% of the peak rectified voltage at low line, Cbulk can then be calculated using: Cbulk Let fop 100 kHz (operating frequency) max 48% (maximum duty cycle) Vin(min) Vdc(min) 20% 96 V (minimum input voltage) Pout 4.1 W (output power) 78% (estimated efficiency) Ipeak 220 mA (input peak current) Pin ( fac · Vpeak(min)2 Vin(min)2) Lpri 4.1 13 F 60 · (1202 962) Vin(min) · max Ipeak · fop 96 · 0.48 2.09mH 0.220 * 100 kHz Primary to secondary turns ratio: Npri Vin(min) · max N sec Vout VF · (1− max) In this case, we chose a 33 F aluminum electrolytic due to availability. Power Stage At the heart of the power stage is the ON Semiconductor NCP1014. The NCP1014 is a current−mode controller with a high voltage power MOSFET in a monolithic structure. The NCP1014 features soft−start, frequency jittering, short circuit protection, a maximum peak current set point, and a dynamic self−supply. It operates in skip−cycle mode below ¼ of the maximum peak current limit, thus no acoustic noise is present. For more information on this device, please go to www.onsemi.com. 120 · 0.48 7 turns 11.75 0.875 · (1−0.48) An easy way to check if the power capability of the transformer is large enough to supply the output is with the following equation: Magnetics Calculations The next step is the design of the flyback transformer. The design of the magnetics block is the most important and delicate part of the whole design process because it will determine how well the power supply will perform. The flyback−mode transformer functions by first conducting current in the primary winding, thus storing energy in the core of the transformer. The core energy is then transferred to the secondary winding when the primary side is turned off. The core and bobbin are standard EFD20 sizes. In order for the regulator to operate in discontinuous mode under worse case conditions and to maximize power, the maximum on time is 48% of the full period, therefore the maximum primary inductance is calculated based on a maximum duty cycle of 48%. Using a larger inductance than Pin(core) Lpri · Ipeak2 · fop Pout 2 Pin(core) 2.09 mH · 0.2202 · 100 kHz 5.05 4.1 W 2 Input Snubber Because of the high dv/dt characteristic of the power transistor drain voltage and of the transformer leakage inductance, voltage spikes and ringing occur at the drain when the power switch is turned off. Resistor R1, C3, D5 compromise an RCD snubber. In parallel to the primary winding are R2 and C4 which compromise an RC ringing damper which slows down the dv/dt and reduces the peak voltage therefore decreasing the ringing due to high frequency noise. Since i C · dv, increasing the dt capacitance will also reduce the magnitude of the voltage ripple. The snubber and ringing damper act together to protect the IC from voltage transients greater than 700 V and reduce radiated noise. http://onsemi.com 2 AND8136/D *D1 *D3 + C3 220p C2 33 when there are no LEDs connected or there is no LED current flowing. In order to limit the secondary voltage during this fault condition, and over−voltage zener diode, D9 is added. If the secondary flyback voltage rises above 47 V, D9 starts conducting and causes the optocoupler to conduct current as well, which then informs the NCP1014 on the primary side to reduce the energy transferred through the transformer. During this time, there is 47 V on C6 and 5.1 V on C5 which totals 52 V on the secondary side. Under a short circuit fault conditon, all LEDs are shorted. This can occur if only one LED is shorted or if the LEDs are supplied with current through a cable. The wires on the cable are either twisted together or are shorted together. The LED secondary current is limited by resistor R6 which develops a 1.25 V voltage drop. When the voltage across R6 is greater than 1.25 V, IC2 conducts and causes the NCP1014 to reduce the energy transferred to the secondary side. This is identical to the open circuit fault condition previously discussed. Pin 4 of NCP1014 R3 MUR120 1.0k 1N5338B IC2 D6 D7 C5 + 22 D8 Midcom SFH615A−4 3,4 7,8 C9 MMSD914 100p C4 47p 1,2 R2 2.2k + C6 100p R5 100 C7 0.001 TLV431 IC3 R6 3.6 D9 MUR160 *D4 R4 2.2k 5,6 T1 D5 1N5941B D11 LED1 1N5917 *1N4006 D12 IC1 5 7 1N5917 D13 NCP1014P100 X X + C8 10 4 10 H R1 91 k *D2 3 L1 8 C1 0.1 2 2.0 A Fuse 1 Connector 1 Output Block The output block or secondary side in Figure 1 consists of two main diodes, D6 (forward diode) and D10 (flyback diode), an optocoupler, resistors, zener diodes and storage capacitors. Diode D6 operates in the forward mode and conducts while the internal switch is turned on. Resistor R3 limits the forward current and diode D7 limits the voltage to 5.1 V. This also acts as the auxiliary supply on the secondary side and provides power to the optocoupler IC2 and the TLV431 labeled as IC3. During the flyback mode, the energy stored in the transformer T1 is released to the secondary load capacitor C6 via D10. Capacitor C6 smoothes out the current pulses and establishes an effectively constant dc voltage for the LEDs. The current is controlled and limited by using feedback. The LED current is converted to a voltage by using a 3.6 Ω resistor R6. The control reference is IC3. There are two fault conditions that can occur; open circuit and short circuit. An open circuit fault condition occurs Photo Transistor D10 MUR120 Figure 1. Circuit Diagram http://onsemi.com 3 1N5917 LED2 LED3 AND8136/D Table 1. Bill of Materials Ref. Component Value Qty Part Number Manufacturer IC1 450 mA, 100 kHz, PDIP−7 1 NCP1014AP100 ON Semiconductor IC2 Opto Coupler, Dip 1 SFH615A−4 Isocom IC3 1.25 V Shunt Reg., TO−92 1 TLV431ALP ON Semiconductor D1−4 1.0 A, 800 V, Gen Purp 4 1N4006 ON Semiconductor D5 1.0 A, 600 V, Ultrafast 1 MUR160 ON Semiconductor D6,D10 1.0 A, 200 V, Ultrafast 1 MUR120 ON Semiconductor D7 5.1 V, 5.0 W, Zener 1 1N5338B ON Semiconductor D8 1.0 V, Switching diode 1 MMSD914 ON Semiconductor D9 47 V, 3.0 W, Zener 1 1N5941B ON Semiconductor D11,12,13 4.7 V, 3.0 W, Zener 3 1N5917 ON Semiconductor T1 Flyback Transformer 1 31842 Midcom L1 Choke, Common Mode, 10 mH 1 40479 Midcom C1 0.1 mF, film, radial 1 R46104M275BIS Nissei C2 33 mF, 400 V, radial 1 KME400VB33RM16X31LL United Chem−Con C3 220 pF, 1 kV, 10%, disc 1 NCD221K1KVY5F NIC Components C4 47 pF, 1 kV, 10%, disc 1 NCD470K1KVSL NIC Components C5 22 uF,radial 1 ECA−1HHG220 Panasonic C6 100 uF,radial 1 ECA−1HHG101 Panasonic C7 0.001mF, ceramic 1 SR155C102KAA AVX C8 10 mF, 16 V, 20%, radial 1 SME16VB10RM5X11LL United Chem−Con C13* 1 mF, 16 V, radial 1 SR215E105MAA AVX C9 100 pF, 1 kV, 10%, disc 1 NCD101K1KVY5F NIC Components R1 91 kW, 1 W 1 RS−1W−91K−5 SEI R2 2.2 kW, 1/2 W, 5% 1 CF−1/2W−2.2K−5 SEI R3 1 kW, 1/4 W, 5% 1 CF−1/4W−1k−5 SEI R4 2.2 kW, 1/4 W, 5% 1 CF−1/4W−2.2K−5 SEI R4* 2 kW, 1/4 W, 5% 1 CF−1/4W−2K−5 SEI R5 100,1/4,5% 1 RN−1/4W−T1−100−5 SEI R7* 0.5 W, 1 W 1 RS−1−R5−5TR SEI R8* 1.2 W, 1 W 1 RS−1−1R2−5TR SEI R9* 22 W, 1/4 W, 5% 1 CF−1/4W−22R−5 SEI R10* 220 W 1/4W, 5% 1 CF−1/4W−221−5 SEI R6 3.6,1W,5% 1 CF−1W−3.6−5 SEI LED1−3 Luxeon Star 3 LXHL−MW1C Luxeon F1 2A, axial 1 251002TR1 LittleFuse 1 1715035 Phoenix Contact Connector 1 http://onsemi.com 4 AND8136/D Figure 2. PCB Metal Layer (front) Figure 3. PCB Metal Layer (back) http://onsemi.com 5 AND8136/D Figure 4. PCB Silk Screen References 1. Brown, Marty, Power Supply Cookbook, Butterworth−Heinemann, 1994. 2. Pressman, Abraham I., Switching Power Supply Design, Second Edition, McGraw−Hill, 1998. 3. Motorola, Inc., Handling EMI in Switch Mode Power Supply Design, AN−SMPS−EMI, Motorola, Inc., 1998. 4. NCP1014 Datasheet, ON Semiconductor, www.onsemi.com 5. Spangler, J., Hayes, L., AND8024 Application Note, ON Semiconductor, www.onsemi.com ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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