AND8226/D Nonisolated Positive Output Buck AC/DC Converter Prepared by: Jan Grulich ON Semiconductor http://onsemi.com APPLICATION NOTE This application note describes the way, how to easily design the simple, non isolated AC/DC converter for powering low voltage control part of mains applications with triac, or SCR power switch. Some examples are: dishwashers, microwave ovens, coffee machines, night illumination and so on. In comparison with resistive, or capacitive dropper is this solution more comfortable and features some advantages such as: • Wide Input Voltage Range 85 VAC – 265 VAC • Smaller Size, Lower Weight, Lower Total Cost • Good Line and Load Regulation, No Need of Additional Linear Regulators • Efficient Design with Up to 80% Efficiency • Overload, Short−Circuit and Thermal Protected • Simple for Low Cost Mass Production • Universal Design for Wide Range of Output Currents and Voltages The monolithic power switcher, used in this application, greatly simplifies the total design and reduces time to production. The new line of the Power Switchers, NCP1010 through NCP1014, is ideal for this purpose. This IC in the SOT−223 package reduces size and is suitable for mass production. The design consists of input filter, rectifier with filtering capacitor, power stage with switcher and inductor, output ultrafast rectifier, output filtering capacitor, feedback loop with zener diode and optocoupler and indicating LED. The only component necessary for proper powering of the IC is the VCC capacitor. The IC is directly powered from the HV Drain circuit via internal voltage regulator. To eliminate the noise at the feedback input, some small ceramic capacitor with value of around 1.0 nF is necessary to be connected as close to the FB pin, as possible. Schematic diagram C1 100 nF 2 E2 220 mF/25 V D1 MUR160 E1 + 10 mF/400 V 1 R1 1k + 1 2 CON2 ARK750/2 L2 1 mH + D2 1N4007 VCC HV GND FB L1 1.5 mH CON2 ARK500/2 LD1 GRN IO2 NCP1014ST IO1 PC817 E3 47 mF/25 V ZD1 11 V C2 1 nF Figure 1. Complete Schematic Diagram of the 12 V/0.2 A Converter © Semiconductor Components Industries, LLC, 2005 October, 2005 − Rev. 1 1 Publication Order Number: AND8226/D AND8226/D SELECTION OF CRITICAL COMPONENTS Inductor selection The average current through the inductor over one switching cycle can be expressed by Equation 4. For the selected output power need to be selected certain minimum value of the inductance. This value is dependent on the mode of operation. Reduced value results in Discontinuous Conduction Mode of operation (DCM). Practically was found, that the borderline between Continuous Conduction Mode of operation (CCM) and DCM is commonly set slightly below maximum output power. The result is low cost of the inductor, freewheeling diode (trr > 35 ns), higher efficiency and lower cost. The negative result is in lower output power and higher cost of the NCP101x Power Switcher. The current ripple in the inductor during the Ton time may be expressed by Equation 1. DIripple(Ton) + Ton @ Vds * VO) ǒ(V min *L min Ǔ Ic + fop_min @ ǒLVO Ǔ L min + Ǔ @ Ton ) ǒ2DI)ripple Ǔ @ ToffǓ Iinit (eq. 4) (2 @ VO @ IO @ (V min * Vds * VO)) (eq. 5) (DIripple 2 @ fop_min @ (V min * Vds)) IO = Output DC Current. The theoretical maximum output power will be shown in Equation 6. (eq. 1) Pout_max + L min @ (Iset 2 * Iinit 2) @ fop_min @ (V min *Vds) ǒ(V min Ǔ *Vds*VO) (eq. 6) 2 The current ripple in the inductor during the normal operation will be shown in Equation 7. DIripple + ((V min * Vds * VO) @ VO) (eq. 7) ((V min * Vds) @ fop_min @ L min) The output current will be shown in Equation 8. IO + fop_min @ (eq. 2) min DIripple 2 ) Iinit Ic = Inductor Operating Current, fop_min = Minimum Operating Frequency The theoretical minimum inductor value corresponds to Equation 5. Where: Ton = ON Time, Internal Power Switch in ON, Vmin = Minimum Rectified Input Voltage, Vds = Drain−to−Source Voltage Drop, Vo = Output Voltage, Lmin = Minimum Inductor Value. The current ripple in the inductor during the Toff time may be expressed by Equation 2. DIripple(Toff) + Toff @ ǒǒ ((Iset ) Iinit) @ Ton ) (Iset ) Iinit) @ Toff) (eq. 8) 2 Toff = OFF Time, Internal Power Switch in OFF. The current through the inductor at the beginning of the Ton time is shown by Equation 3. Iinit + Iset * DIripple (eq. 3) Iset = Peak Switching Current Set by the FB Loop. Table of Preselected Inductors (Vmin = 120 V, Vds = 9 V, VO = 12 V, Iset = 0.405 A, fop_min = 59 kHz) NOTE: Inductance (mH) Coilcraft Part Number (see appendix for address) DIripple (A) Output Current (A) 470 RFB0810−471 0.39 0.25 680 RFB0810−681 0.27 0.32 820 RFB0810−821 0.22 0.34 1000 RFB0810−102 0.18 0.36 1500 RFB0810−152 0.12 0.40 The output current is the theoretical value and need to be multiplied by the efficiency (~0.7). http://onsemi.com 2 AND8226/D Freewheeling diode selection The freewheeling diode needs to be selected accordingly to the mode of operation. For the CCM operation needs to be used the ultra fast diode with trr < 35 ns. For the DCM operation the standard ultra fast diode with trr < 75 ns is enough. TABLE OF PRESELECTED FREEWHEELING DIODES Part number VRRM (V) IF(AV) (A) trr (ns) Package MUR160 600 1.0 75 Axial Lead MURA160T3 600 1.0 75 SMD SMA MURS160T3 600 1.0 75 SMD SMB MURS260T3 600 2.0 75 SMD SMB Electrical specification of the example at Figure 1: Input: 85 VAC – 265 VAC Output: + 12 V / 200 mA NOTE: The polarity is proportional to common line. COMPONENT LAYOUT Figure 2. Component Layout – Top Side Figure 3. Component Layout – Bottom Side PCB LAYOUT Figure 4. PCB Layout http://onsemi.com 3 AND8226/D EMI TEST RESULTS Test Conditions: Input: 230 VAC Output: 11.7 VDC Load: Resistive 68 R Figure 5. Conducted EMI Contact Address of the Inductor Manufacturer: Coilcraft 1102 Silver Lake Road, Cary IL 60013 800−322−2645 847−639−6400 Fax 847−639−1469 21 Napier Place Wardpark North, Cumbernauld Scotland G68 0LL Telephone (Int) : 44 (0)1236 730595 Fax (sales) : 44 (0)1236 730627 www.coilcraft.com Email: fionas@coilcraft−europe.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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