L DESIGN IDEAS Simple, High Efficiency, Multi-Output, Isolated Flyback Supply with by Ryan Huff Excellent Regulation Introduction accomplished with inefficient, linear post regulators or efficient (but relatively expensive) switch-mode buck regulator ICs. All of these solutions fail the simplicity test in parts count and design complexity. Fortunately, a breakthrough IC makes it possible to achieve high efficiency and tight regulation while maintaining the simplicity typically Simplicity, tight regulation, and high efficiency are no longer optional features in isolated power supplies, but achieving all three is traditionally difficult. High efficiency often requires the use of advanced topologies and home-brewed secondary synchronous rectification schemes. Tight regulation for a multi-output supply is often L1-L2 DO1813P-331HC (COILCRAFT) L3: DO1813P-561HC (COILCRAFT) Q5: FMMT618 (ZETEX) Q6: FMMT718 (ZETEX) C1-C3: C3225X5R0J476M (TDK) C4: C3225X5R1A226M (TDK) C5-C6: 6TPE220MI (SANYO) C7: 10TPE220ML (SANYO) associated with a flyback supply. The LT3825 simplifies and improves the performance of low voltage, multioutput flyback supplies by providing precise synchronous rectifier timing and eliminating the need for optocoupler feedback while maintaining excellent regulation and superior loop response. PIN 1 TO 3, 16T OF 3 X 31 AWG PIN 9 TO 10, 4T OF 3 X 28 AWG PIN 4 TO 5, 16T OF 3 X 36 AWG PIN 7 TO 8, 3T OF 3 X 27 AWG PIN 11 TO 12, 6T OF 3 X 30 AWG PIN 1 TO 3, 16T OF 3 X 31 AWG VIN 36V TO 72V T1 T1 EFD20-3F3 GAP FOR Lp=27µH Pri2 3.3V AUX 2.5V 5.0V Pri1 TAPE 3.3V AT 3A L1 10 0.33µH C1 47µF 5 1nF 28.7k R1 47k 1/4W R2 20Ω 5.0V AT 2A L3 12 402k 3.01k Q3 Si4470EY 20Ω VCC 4.7Ω 1/4W 15k 2.5V AT 3A L2 PG UVLO SYNC 402k 1nF SENSE+ PGDLY 750Ω B0540W 7 15k 100k tON C5 220µF B0540W 8 FB + 11 Si4490DY C6 47µF 20V 0.56µH C4 22µF 3 220pF + 4.7nF 250V Q2 Si4470EY 4.7Ω 1/4W 1 D1 BAS21 C5 220µF 9 4 1µF 100V + 0.015 1W SENSE– LT3825 Q4 HAT2165H 10Ω 1/4W Q6 0.33µH C2-C3 2x 47µF + C6 220µF 47Ω Q5 RCMP 1µF 330 ENDLY GND 47pF OSC SFST CCMP 0.22µF SG VC 0.1µF 0.1µF 2.2nF 8 5 1 4 BAT54 15Ω 10k 1.5nF PA0184 Figure 1. Simple, high efficiency, 36VIN–72VIN to 2.5VOUT at 3A, 3.3VOUT at 3A, and 5.0VOUT at 2A synchronous flyback 30 Linear Technology Magazine • September 2006 DESIGN IDEAS L 48V Input to Triple Output: 5V at 2A, 3.3V at 3A and 2.5V at 3A 90 5.5 5.0 OUTPUT VOLTAGE (V) The circuit in Figure 1 shows an isolated, no-optoisolator, synchronous flyback, 48V to 5.0V at 2A, 3.3V at 3A, and 2.5V at 3A power supply. Figure 2 shows its efficiency. The converter’s efficiency of over 87% at the nominal input voltage of 48V and full, rated output current on each output approaches that of a higher parts count forward converter. This is primarily the result of a simple, well-controlled implementation of synchronous rectification. As a result of this high efficiency, the greatest temperature rise of any component is only 35°C above the ambient temperature with a paltry 100LFM of airflow. The feedback winding is used to regulate the output voltage instead of an optocoupler and secondary-side reference, with good results. The regulation curve shown in Figure 3 shows that ±1.6% is easily attainable when loading outputs proportionately. Even when the outputs are loaded in every possible 10% to 100% load current combination, the cross-regulation between all outputs is within ±3.6%. Figure 4 shows the supply’s transient response for a 1.5A-to-3A load step on the 2.5V output at 5A/µs slew rate and 36V input. With this 50% load step, all output voltages remain within ±2% of their set points. This circuit also has the advantage of having extremely low ripple on the output voltages; exhibiting less than 10mVP–P on all outputs at a switching EFFICIENCY (%) 85 80 75 VIN = 72V VIN = 48V VIN = 36V 70 65 10 20 30 40 50 60 70 80 90 100 5VOUT 4.5 4.0 3.5 3.3VOUT 3.0 2.5 2.0 2.5VOUT 10 20 30 40 50 60 70 80 90 100 LOAD (%) LOAD (%) Figure 2. Efficiency of circuit in Figure 1 Figure 3. Output voltage regulation of the circuit in Figure 1 frequency of 200kHz. This performance is attributable to the small, second stage, inductor/capacitor filter on each output. using Schottky diodes, which can vary by more than 0.25V over temperature and load. Under the same conditions, the voltage drop across the MOSFETs in Figure 1 vary only by 60mV, a factor of four better. The MOSFET-based topology tightly couples each output, thereby reducing voltage differences during extreme temperature and cross-loading conditions. Instead of using a parts-intensive, secondary-side voltage reference and error amplifier to drive an optocoupler, the LT3825 uses the primary bias winding on the flyback transformer, T1 (see Figure 1). The voltage on this winding during the flyback pulse is the average of all output voltages as reflected to the primary. The LT3825 feedback (FB) pin reads this voltage, which is then used to modulate the on-time of Q1 to regulate the output voltages. Cross-regulation performance is enhanced since the average of all outputs is presented to the controller as opposed to just one output voltage’s information as with an optocoupler. Another important benefit of this technique is that the output voltage information arrives at the controller immediately after the switching cycle is terminated. In a conventional optocoupler-based design, delays of tens to hundreds of microseconds occur in the optocoupler alone, severely limiting the converter’s transient response. LT3825 Operation The synchronous rectifier output (SG pin) of the LT3825 makes driving the synchronous rectifier MOSFETs (Q2–Q4) simple while maintaining a low parts count. Setting the dead-time of these synchronous rectifiers relative to Q1 only requires one resistor to program. Avoiding traditional, more complicated, discrete timing circuits allows the designer to set optimum dead-times since this timing is well controlled within the LT3825. The LT3825 also precludes the need for a secondary-side synchronous controller IC and its associated circuitry. The easy-to-implement synchronous rectification also has another advantage: it tightens the output cross-regulation. An alternative to synchronous rectification is IOUT 2.5V 1A/DIV VOUT 3.3V 50mV/DIV VOUT 5V 50mV/DIV VOUT 2.5V 50mV/DIV Other Features 0.2ms/DIV Figure 4. 1.5A to 3A to 1.5A load current step (top trace) on 2.5V output and output voltage responses (bottom traces) of circuit in Figure 1 Linear Technology Magazine • September 2006 An optional, resistor programmable, input undervoltage lockout is available. An optional soft-start capacitor 31 L DESIGN IDEAS controls the slew rate of the output voltage during start-up, which limits the inrush current of the input power supply. Since the LT3825 incorporates current-mode control, both shortcircuit behavior and ease of loop compensation are improved over voltage-mode controllers. The switching frequency can be set anywhere from 50kHz to 250kHz, making it possible to find the right balance of solution LTC4089, continued from page 22 High Voltage Buck Output Capacitor Selection All the ceramic capacitors used in the circuit are recommended to be X5R or better (X7R). However, be cautious about the claimed initial capacitance value (e.g., some 0805 size 22µF/6.3V X5R caps measure only 11µF at no bias) and derating with bias and temperature (some X5R caps derate to less than 20% of their initial values with full 6.3V voltage bias). It is critical to use a 22µF/16V X5R or better cap at the output of the LTC4089 buck regulator (connected to HVOUT), as low capacitance causes duty-jitter in certain conditions. The LTC4089-5 can operate with a 22µF/6.3V ceramic cap at the output. High Voltage Buck Current Limit As shown in Figure 7, the buck output current capability is a function of inductance and the input voltage. For most of the input range, the output current limit is 1A for a 10µH inductor size and efficiency for a specific application. The switching frequency can be synchronized to an external system clock for further flexibility. input voltage connected directly to the VCC pin, so several components are not needed to generate a bias supply, including D1, C6, R1, and R2. It Is Possible to Reduce the Parts Count Even More Conclusion and 1.1A for a 33µH inductor. When powered from the high voltage source, if the sum of the system load current at the OUT terminal and charge current (set by RPROG) exceeds the buck output current limit, the buck output voltage collapses to the battery voltage. connected to CLPROG pin. Figure 8 shows the schematic diagrams. 1.6 TYPICAL 1.5 L = 10µH IOUT (A) 1.4 1.3 1µF HVOUT 5V (NOM) FROM USB CABLE VBUS IN L = 10µH 0.9 5 10 15 20 VIN (V) 25 30 35 Figure 7. The high voltage switching regulator’s maximum output current for two different value inductors 32 HVPR LTC4089 4.7µF 1k 4.7µF OUT CLPROG IN-LMT 500mA 1000mA VOUT (TYP) VBAT +0.3V 5V 5V VBAT TIMER GND PROG 2k TO LDOs REGS, ETC. BAT 2k USB POWER 500mA ICHG 1.0 10µF HVEN 100k 0.1µF Li-Ion BATTERY + AVAILABLE INPUT HV INPUT (LTC4089) HV INPUT (LTC4089-5) USB ONLY BAT ONLY ICHG BAT D1 MINIMUM 1.1 10µH SW BOOST HVIN HIGH (6V-36V) VOLTAGE INPUT 5V WALL ADAPTER 850mA ICHG L = 33µH 1.2 The LTC4089 and LTC4089-5 combine a monolithic high voltage switching buck regulator, a full featured Li-ion battery charger, and a PowerPath controller in a tiny 3mm × 6mm DFN package. They solve many battery charging and power path problems and easily fits into handheld applications, such as portable GPS navigators and MP3 players, where a high voltage source and small PCB space are required. L 0.1µF ADPR LO HI L = 33µH Conclusion Accept USB and 5V Adaptor with Different Current Limits Like all other LTC PowerPath controllers, the LTC4089/LTC4089-5 can be configured to accept 5V adaptor/USB input in the same USB connector or different connectors with different current limits by changing the resistance ADPR (FROM SYS) 1.8 The LT3825 allows a designer to improve the performance of multioutput isolated flyback circuits while lowering parts count and simplifying implementation. L For lower input voltages (5V to 20V) and simpler designs, the LT3837 complements the LT3825. The LT3837 starts up and runs from the lower LTC4089 MP1 1k IN + PROG CLPROG MN1 2.87k 2k Li-Ion BATTERY 59k Figure 8. IN pin accepting USB and 5V Adaptor with different current limits Linear Technology Magazine • September 2006