advertisement Flyback Controller Simplifies Design of Low Input Voltage DC/DC Converters – Design Note 1016 David Burgoon Introduction Small, high efficiency DC/DC converters are critical to the design of leading-edge electronics. Achieving high accuracy and efficiency traditionally means adding extra components, complexity, and size. Not so with the LT3837. This flyback controller serves 10W to 60W isolated applications with high performance, simplicity, small size, and a minimum component count. blanking yields a high performance loop that is easy to compensate. High-Efficiency Controller Capabilities The LT® 3837 operates from a 4.5V to 20V input, but the converter input range can be extended upwards by using a VCC regulator and/or a bias winding on the transformer. It also provides a synchronous rectifier output with adjustable timing to optimize efficiency and enhance cross-regulation in multiple-output supplies. 3.3V, 10A Converter Operates from a 9V to18V Source The circuit shown in Figure 1 is a flyback design for a 3.3V, 10A output from a 9V to 18V input with a minimum of external components. The LT3837 samples the voltage on the primary winding during the flyback interval to provide superb regulation. Figure 2 shows ruler-flat regulation at 9V input, and a tight regulation window of ±0.7% over line and load. Synchronous rectification with adjustable The LT3837 eliminates the need for the traditional secondary-side reference, error amplifier, and optoisolator circuits by sampling the flyback voltage on a primary-side winding. Accuracy is enhanced with output resistance compensation. Current mode control with leading edge VIN 9V TO 18V The operating frequency is adjustable from 50kHz to 250kHz or can be synchronized to an external clock. Soft-start provides well controlled start-up with limited inrush current. Protection features include current limit with soft-start cycling for severe overloads, undervoltage lockout, and thermal shutdown. , LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. IHLP2525CZERR10M 0.1µH T1 2.2nF 22µF 25V • 47µF × 3 BAS16 22.1k 1% 10 • 1nF 2N3906 VOUT 3.3V AT 10A 220µF 6TPE220MI B0540W 10 Si7852DP 1µF 1µF SG 86.6k 1% 4.7 SG VCC FB UVLO 15k 1% PG SENSE+ SENSE– U1 LT3837EFE 12k 100k 1.37k 1% SG 330 VC PGDLY tON SYNC ROCMP ENDLY OSCAP SGND/PGND SFST CCMP 3.01k 1% Si7336ADP 0.008 0.5W 15 0.1µF 2.2nF 3.3nF • 150k 20k 47pF 0.1µF PA0184 1nF • 10k BAT54 DN1016 F01 Figure 1. Low Parts Count, 9V to 18V Input to 3.3V/10A Output Isolated Flyback Converter with ±0.7% Regulation 12/06/1016 timing yields excellent efficiency—88% over a wide range of operating conditions—as shown in Figure 3. 3.3V, 10A Converter Operates from a 9V to 36V Source Figure 4 shows an enhanced circuit that extends the input operating range of the LT3837 to 9V to 36V. Operation is converted to hysteretic start-up for efficient wide-range operation. Q1 provides a low-drop current source for start-up, and Q2 creates a suitable undervoltage circuit for VCC. These circuits, together with the VCC winding on the transformer, result in low VCC power at higher input voltages and low dissipation cycling when operating into a short circuit. This circuit is implemented in a 1.5in2 footprint. This circuit exhibits excellent regulation of ±1.2% over line and load, and efficiency of 88% over much of its operating range. Conclusion The LT3837 is part of a new class of flyback controllers developed by Linear Technology to satisfy the demand for economical, high performance power converters. It provides synchronous rectifier drive and eliminates the need for secondary regulation circuits and optoisolators. The LT3837 makes it easy to implement high performance Flyback designs that are cost effective, small and efficient. 90 3.6 9VIN 88 3.5 86 EFFICIENCY (%) OUTPUT (V) 3.4 9VIN 3.3 18VIN 3.2 84 18VIN 82 80 3.1 78 76 3.0 2 3 4 8 7 6 5 LOAD CURRENT (A) 2 10 9 4 3 5 6 7 8 10 9 LOAD CURRENT (A) DN1016 F03 DN1016 FO2 Figure 2. Regulation of the Converter in Figure 1 Figure 3. Efficiency of the Converter in Figure 1 4.7nF 250V VIN 9V TO 36V 1µH + 100µF 50V –VIN 9V TO 36V 0.1µH 3.3µF × 3 50V BAT43WS 100k • 316 470pF Q1 MBT3906DW1T1 VCC BAS21 121 100k 33 EFD20 LP = 5µH (N = 6 : 2) PA1835NL • 1nF SG VCC 100k 24.9k 0.1µF HAT2173H 4.64k SG 100k 21.5k UVLO MBT3904DW1 BZT52C6V2S Q2 17.4k 100k 470pF 47k • PA0184 1.07k • BAT54WS 10k SENSE– VC PGDLY tON SYNC ROCMP ENDLY OSCAP SGND/PGND SFST CCMP 3.01k 12.1k 220k U1 LT3837EFE 15 4.7nF PG SENSE+ 7m 1µF FMMT718 0.1µF SG VCC FB 10 FMMT618 Si7336ADP 4.7 120 33µF 16V BAT54WS CMPSH1-4 VOUT 3.3V AT 10A 220µF 6.3V –VOUT 3.3V AT 10A • VCC + + 47µF × 3 6.3V 147k 1.5nF 2.15k 47pF 0.1µF 220pF DN1016 F04 Figure 4. Wide Range, 9V to 36V to 3.3V/10A Isolated Flyback Converter with ±1.2% Regulation Data Sheet Download For applications help, call (408) 432-1900, Ext. 2134 www.linear.com Linear Technology Corporation dn1016f LT 1206 • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2006 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●