AND8190/D Non−isolated Negative Output Buck AC/DC Converter Prepared by: Jan Grulich ON Semiconductor http://onsemi.com APPLICATION NOTE The monolithic power switcher used in this application greatly simplifies the total design and reduces time to production. ON Semiconductor’s NCP1010 – 1014 family, a new line of Power Switchers, is ideal for this purpose. The NCP101x is offered in a SOT−223 package for reduced size, and is suitable for mass production. The design consists of the input filter, rectifier with filtering capacitor, the power stage with switcher and inductor, output ultrafast rectifier, output filtering capacitor, the feedback loop with Zener diode and optocoupler, and an indicator LED. The only component necessary for proper powering of the IC is the VCC capacitor, since the IC is directly powered from the HV Drain circuit via an internal voltage regulator. To eliminate noise at the feedback input, a small ceramic capacitor of around 1 nF should be connected as close to the FB pin as possible. This application note describes how to easily design a simple, non−isolated AC/DC converter for powering the low voltage control portion of line−powered applications that use a triac or SCR power switch. Examples of such applications include dishwashers, microwave ovens, coffee machines, illumination, etc. Compared to passive solutions using resistors or capacitors to reduce the voltage, this design has significant advantages such as: • Wide input voltage range 85 – 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 protection • Convenient for mass production due to SMD devices • Universal design for wide range of output currents and voltages E2 220 F/25 V C1 100 nF 2 1 R1 1k5 D1 MUR160 E1 10 F/400 V 1 CON2 ARK750/2 L2 1 mH VCC HV GND FB 2 L1 1.5 mH CON2 ARK500/2 IO2 NCP1014ST D2 1N4007 E3 47 F/25 V C2 1 nF IO1 PC817 ZD1 LD1 11 V GRN Figure 1. Complete Schematic Diagram of the 12 V/0.2 A Converter Semiconductor Components Industries, LLC, 2004 December, 2004 − Rev. 0 1 Publication Order Number: AND8190/D AND8190/D SELECTION OF CRITICAL COMPONENTS Inductor Selection The current through the inductor at the beginning of the Ton time is The desired output power determines the minimum value of the inductance. This value is dependent on the mode of operation. A reduced inductor value results in Discontinuous Conduction Mode of operation (DCM). In practice, the switch−over point between Continuous Conduction Mode of operation (CCM) and DCM is commonly set to be slightly below maximum output power. This achieves a reasonable compromise between inductor size and ripple current, efficiency, and overall lower cost. The only significant negative aspect of this particular operating mode is a higher peak−to−average current ratio in the inverter circuit. The current ripple in the inductor during the Ton time may be expressed by the equation Iripple(Ton) Ton Iinit Iset Iripple Iset = Peak switching current set by the FB loop. The average current through the inductor over one switching cycle can be expressed by the equation Ic fop_min Iripple Iinit Toff 2 Ic = Inductor operating current fop_min = Minimum operating frequency. The theoretical minimum inductor value is given by the expression Vds Vo) (V min L min L min Ton = ON time, internal power switch is 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 the equation Iripple Iinit Ton 2 (2 Vo Io (V min Vds Vo)) (Iripple2 fop_min (V min Vds)) Io = Output DC current. The theoretical maximum output power will be Pout_max L min (Iset2 Iinit2) fop_min V min Vds) (V (min VdsVo) 2 The current ripple in the inductor during the normal operation will be Vo Iripple(Toff) Toff L min Toff = OFF time, internal power switch is off. Iripple ((V ((V min Vds Vo) Vo) min Vds) fop_min L min) The output current will be Io fop_min ((Iset Iinit) Ton (Iset Iinit) Toff) 2 Table of Preselected Inductors (Vmin = 120 V, Vds = 9 V, Vo = 12 V, Iset = 0.405 A, fop_min = 59 kHz) Inductance (H) Coilcraft Part Number (see appendix for address) Iripple (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 must be multiplied by the efficiency (~ 0.7). http://onsemi.com 2 AND8190/D Freewheeling Diode Selection diode with reverse recovery time trr < 35 ns must be used. For the DCM operation a standard ultra−fast diode with trr < 75 ns is adequate. The freewheeling diode needs to be selected according to the mode of operation. For CCM operation an ultra−fast 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 in Figure 1: Input: 85 – 265 VAC Output: + 12 V / 200 mA Note: The polarity is relative to the 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 AND8190/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 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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