DI-148 Design Idea PeakSwitch ® Multiple Output Flyback Power Supply for Audio Amplifier Using Magnetic Amplifiers to Achieve Better Cross Regulation Application Device Power Output Input Voltage Output Voltage Topology Audio PKS607YN 75 W, 126 W Peak 195 - 265 VAC ±26 V, ±15 V, 5 V Flyback Design Highlights • Effectively replaces linear transformer-based power supplies in home audio applications • Dramatically reduces weight, allowing chassis cost reduction • Proprietary magnetic amplifier (mag amp) approach enables tight cross regulation of two main outputs in flyback topology • Low component count, compact and light weight. • Energy efficiency • >82% efficiency at full load • <800 mW no-load power consumption at 265 VAC • Reduces heat sink requirements and eliminates need for separate standby supply • Excellent transient response – improves audio quality • Integrated frequency jittering, together with a simple EMI filter, allows for CISPR-22/EN55022B conducted EMI compliance (see Figure 2) • Integrated safety/reliability features: • Accurate, auto-recovering, hysteretic thermal shutdown function maintains safe PCB temperatures under all conditions • Latching shutdown protects against output short circuits and C9 open feedback loops 2.2 nF Operation The universal input power supply shown in Figure 1 has multiple outputs, ±26 VDC, ±15 VDC, and +5 VDC. The ±26 V outputs can each deliver a minimum output current of 2.42 A peak (thermally limited) and 1.45 A continuous. Additionally, the voltage regulation and, importantly, transient response is excellent over the entire load range (no-load to full load). The controller in U1 skips switching cycles to regulate the output voltage, based on feedback to its EN/UV pin. When the current pulled out of this pin exceeds 240 μA, a low logic level (disable) is generated. At the beginning of each cycle, the EN/UV pin state is sampled, and if high, the power MOSFET is turned on for that cycle (enabled); otherwise the power MOSFET remains off (disabled). During start-up, switching is inhibited until the input voltage is above the under-voltage threshold, determined when a current >25 μA flows into the EN/UV pin. R16 68 7 1/2 W 250 VAC 15 V, 150 mA C23 330 pF IN 1 D7-8,D16-17 1N4007 C6 470 pF 1 kV C7 4700pF 1 kV VR2 P6KE91A R6 1507 1/2 W C45 100 MF 400 V 2,3 D6 UF4002 R52 L2 68 k7 1/2 W MP1305 10 11 VR3 P6KE91A R7 2.2 7 C49 470 pF 100 V N.C. 8,9 R53 68 k7 7 1/2 W 5 D3 1N4148 6 C47 1 nF 50 V RT1 O t T1 EER28 R4 2 M7 PeakSwitch U1 PKS607YN C1 330 nF 275 VAC D R5 2 M7 S R2 R1 1.3 M7 1.3 M7 C17 220 pF 250 VAC R51 100 7 1/2 W R3 4.7 k7 C18 220 pF 250 VAC R50 10 7 C48 33 pF 1 kV L PE D22 1N4148 D20 SB160 C20 680 nF 50 V D5 BYV32200 VR4 1N5253B 25V C34 100 nF 50 V R48 5.1 k7 1/8 W R24 510 7 5 V, 150 mA -15 V, 150 mA C28 100 MF 25 V C16 680 nF 50 V RTN -26 V, 1.45 A RTN Q2 MPSA42 C10 330 MF 50 V C11 C15 330 MF 680 nF 50 V 50 V 26 V, 1.45 A R21 130 7 D21 1N4148 R43 1 k7 U2A PC817D R23 510 7 C4 330 nF R46 5.1 k7 1/8 W D12 BAV20 D11 1N4148 D19 1N4148 D10 1N4148 Q1 2N5401 VR5 1N5254B 27 V R26 2 k7 N 195 - 265 VAC U7A LM358P R45 2.7 k7 D18 SB160 C35 220 nF 50 V 3 2 R44 100 k7 R27 1 k7 Figure 1. OUT U8 LM7915 GND C26 100 MF 25 V U2B PC817D Q3 2N3904 IN C14 330 MF 50 V C19 100 MF 25 V R47 2.7 k7 R49 10 7 1/2 W OUT U5 LM78L05 GND C13 330 MF 50 V D13 BAV20 C8 2.2 nF 250 VAC EN/UV GND L3 MP1305 C5 47 uF 35 V BP F1 3.15 A C50 470 pF 100 V IN D9 BYV32200 4 D2 UF4005 L1 10 mH OUT U6 UA7815 GND 12 VR1 P6KE91A C27 47 MF 10 V C37 10 nF 50 V R30 49.9 k7 1% R32 1 k7 C46 47 MF 10 V C39 1 nF 50 V PI-4884-021108 R31 49.9 k7 1% Schematic of a 75 W Continuous and 126 W Peak Audio Amplifier Power Supply. www.powerint.com February 2008 • • The core size and the winding wire diameter sizes (see Table 1) were chosen based on the average of the peak and the continuous output power. An RC snubber (R51and C48) was added between drain and source of U1 to reduce radiated EMI. 80 PI-4885-110707 The primary feedback circuit is closed across the ±26 V (52 V) outputs via VR4, U2A and VR5. This ensures that the sum of voltages across the ±26 V outputs is regulated. For improved cross-regulation, two separate magnetic amplifiers (mag amps) are used on the ±26 V outputs. A mag amp is a saturable reactor or inductor that uses a core material that has a rectangular B-H curve. Adjusting the current (magnetic flux) in the mag amp adjusts the period that it blocks current flow. 70 By adjusting the current in L2 and L3, the circuitry formed by U7A, Q1 and Q2 determines the proportion of the energy stored in the transformer that is delivered to the +26 V and -26 V outputs during the off time of the PeakSwitch internal MOSFET. At the extreme, with one output fully loaded and the other at no-load, almost all the energy is delivered to the output under full load. Importantly, both outputs cannot be simultaneously blocked, which would cause excessive clamp dissipation. Even with very large load transients 25%-100% that are common in audio supplies, the output regulation is maintained within ±5% on both outputs. Common mode choke, L1, and the two Y-capacitors, C8 and C9, form the common mode EMI filter. Common mode choke L1 also works in conjunction with X-capacitor C1 to provide differential mode EMI filtering. Key Design Points • The high crest factor of the music source allows physically smaller heatsinks. • The auxiliary ±15 VDC and +5 VDC outputs are obtained from a linear regulator. To limit the power dissipation in the (5 V and 15 V) linear regulators, a second 18 V output was added to transformer T1. • As the value of X capacitor C1 is above 0.1 μF, safety agency requirements specify that it R1 and R2 should be used to discharge C1. Resistors R1 and R2 are chosen such that (R1 + R2) × C1 ≤ 1 s. • To reduce grouping of switched cycles, a high gain optocoupler, U2, was used to drive small signal transistor Q3, which pulls current out of the EN/UV pin. Capacitor C34 adds high frequency gain to the feedback signal. • In a three wire system, placing Y capacitors (C17, C18) between line/neutral and earth helps reduce common mode EMI. QP 60 AV 50 40 dBµV Here two magnetic amplifiers (L2 and L3) are used for improved cross regulation. This is especially relevant for audio designs as power is drawn from the supply at audio frequencies. The highest output power component occurs at lower frequency where current is alternately drawn from each output but not simultaneously. 30 20 10 0 -10 -20 0.15 1.0 10.0 100.0 MHz Figure 2. Conducted EMI at 230 VAC With Output Grounded (Worst Case). Transformer Parameters Core Material EER28 NC-2H or equivalent, gapped for ALG of 139 nH/t² Bobbin EER28, 12 pin, Vertical Winding Details Add 3 mm margins on both sides of bobbin Primary: 16T × 2, AWG30, tape Shield: 1T × 1, foil 2 mils thick, 3 layers tape +26 V: 8T × 2, AWG23, 1 layer tape -26 V: 8T × 2, AWG23, 1 layer tape 18 V: 6T × 2, AWG28, tape Bias: 5T × 2, AWG30, tape Primary: 17 T × 2, AWG30, 2 layers tape Winding Order Primary (4–2), Shield (NC–1), +26 V (7–8), -26 V (9-11), 18 V (12-10), Bias (5-6), Primary (2-1) Primary Inductance 151 μH, ±20% Primary Resonant Frequency 2 MHz (minimum) Leakage Inductance 5 μH (maximum) Table 1. Transformer Parameters. (NC = No Connection, TIW = Triple Insulated Wire) Power Integrations 5245 Hellyer Avenue San Jose, CA 95138, USA. Main: +1 408-414-9200 Customer Service Phone: +1-408-414-9665 Fax: +1-408-414-9765 Email: [email protected] Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm. On the Web www.powerint.com The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, PeakSwitch, EcoSmart, Clampless, E-Shield, Filterfuse, StackFET, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. ©2007, Power Integrations, Inc. A 02/08 DI-148