DESIGN IDEAS Simple Isolated Telecom Flyback Circuit Provides Regulation Without Optocoupler by John Shannon Introduction Circuit Operation over time. They are also relatively slow. Optocoupler shortcomings add considerably to the total converter design time and ultimately limit performance. Consider instead the schematic of Figure 1. This is a flyback converter based on the LT1725. There are extremely few components and yet a high level of functionality. This design is short circuit proof and includes an input undervoltage lockout for increased reliability. The performance of this converter is shown in Figure 2. Output voltage is regulated to within 1% over a 2:1 input voltage range with 10% or greater load. No load regulation is within 2% over a 2:1 input voltage range. This is well within the typical requirement of 5% regulation. The LT1725 uses a proprietary technique to regulate an isolated output voltage without an optocoupler, thus greatly simplifying flyback converter design and reducing the component count. The result is reduced design time, smaller space requirements, lower cost, and improved performance. Traditional isolated flyback converters employ a secondary side voltage reference and error amplifier that drive an optocoupler, which sends the control signals back to the primary side. In addition to being parts intensive, this approach places an optocoupler in the feedback loop, which introduces a host of design problems. Optocouplers are poorly defined components—their gain is variable and subject to degradation The LT1725 flyback controller is a current mode control IC. Current mode operation provides for inherent line transient rejection and simple loop compensation. Current mode controllers have an “inner” fast current control loop and a slower “outer” voltage control loop. The inner current loop has immediate pulse-by-pulse control of the switching MOSFET M1. A normal switching cycle is as follows. The MOSFET M1 is turned on to begin the cycle. Once M1 is turned on, the current in the primary winding of the flyback transformer ramps up. When the primary current reaches a level determined by the value of the voltage on the VC pin, M1 is turned off. The voltage on the VC pin is set by the LT1725’s output voltage control loop—the outer loop. Once M1 turns continued on page 33 T1 D1 BAS21 • R13 820k R2 39Ω R1 47k C10 100pF • C7 470pF VOUT 5V 2A D7 12CWQ06FN R12 30Ω • VIN 36V TO 72V C9 100pF R4 33.2k + + R30 47k C3 15µF R14 33Ω C5 0.1µF • C8 100µF • C1 22µF 9 8 C13 0.47µF 7 R5 3.01k 1% –VIN R11 18Ω VCC FB GATE LT1725 ISENSE VC OSCAP SFST tON C2 1nF 50V 15 10 3VOUT UVLO 6 3 C14 33pF ENDLY MENAB ROCMP 14 13 R5 51k R33 47k 12 R32 75k 4 R9 3k RCMPC R10 16 39Ω M1 2 SGND PGND 11 5 1 C6 0.1µF T1: COILTRONICS CTX02-14989 C8: TDK C5750X5R0J107M C13: TDK C5750X7R2A155M M1: INTERNATIONAL RECTIFIER IRF620 R29 0.2Ω ISOLATION 1500V (561) 752-5000 (408) 392-1400 (310) 322-3331 Figure 1. –48V to 5V 2A isolated flyback converter 30 Linear Technology Magazine • May 2002 DESIGN IDEAS capacitors. This significantly reduces the power loss associated with the ESR of input capacitors. Figure 3 shows detailed current waveforms of this operation. CURRENT THROUGH Q1 5A/DIV CURRENT THROUGH Q3 5A/DIV Conclusion INPUT CURRENT FROM 3.3V SUPPLY 5A/DIV 1.25µs/DIV Figure 3. Each switcher has 5A peak current, but the total ripple at the input is still only 5A, minimizing CIN requirements. Design Example Figure 1 shows a design that provides 2.5V/15A and 1.8V/15A from a 3.3V input. Because the LTC1876 provides a 5V bias for MOSFET gate drive, a very low RDS(ON) MOSFET Si4838 (2.4mΩ typical) can be used to achieve high efficiency. Figure 2 shows that the overall efficiency is above 90% over a wide range of loads. Figure 2 also shows that the light load efficiency of this design is more than 84%. This is a direct benefit of the Burst Mode operation of the LTC1876. Further efficiency improvements come from operating the two step-down channels out-of-phase. The top MOSFET of the first channel is fired 180° out of phase from that of the second channel, thus minimizing the RMS current through the input The LTC1876 uses three techniques to efficiently power low voltage DSPs, ASICs and FPGAs from a low input voltage. The first technique uses an internal boost regulator to provide a separate 5V for the MOSFET gate drive. Secondly, its Burst Mode operation achieves high efficiency at light loads. Lastly is the out-of-phase technique which minimizes input RMS losses and reduces input noise. Complete regulator circuits are kept small and inexpensive, because all three switchers (one step-up regulator and two step-down controllers) are integrated into a single IC. For systems where a separate 5V is available or the input supply is greater than 5V, the internal boost regulator can be used to provide a third step-up output with up to 1A switch current. 5.25 100 5.2 90 5.15 80 5.1 70 VIN = 36V 5.05 VIN = 48V 5 4.95 VIN = 72V EFFICIENCY (%) OUTPUT VOLTAGE LT1725, continued from page 30 VIN = 72V 60 VIN = 48V 50 40 4.9 30 4.85 20 4.8 10 4.75 VIN = 36V 0 0 500 1000 1500 2000 OUTPUT CURRENT (mA) 2500 Figure 2. LT1725 regulation off, the current that had been flowing in the primary of the transformer begins to flow in the secondary. The voltage on the drain of M1 rises to a level determined by the transformer turns ratio and the output voltage. Similarly, the voltage on the feedback winding rises to a level set by the output voltage. The LT1725 reads the voltage on the feedback winding durLinear Technology Magazine • May 2002 0 500 1000 1500 2000 OUTPUT CURRENT (mA) 2500 Figure 3. Efficiency vs output current for the circuit in Figure 1 ing the flyback pulse using a proprietary sampling technique. This sampled voltage is then compared a precision internal reference and current is added to or subtracted from the capacitor on the VC pin. This has the effect of modifying the M1 turn-off current in such a way as to regulate the output voltage. An important benefit of this sampling technique is that output voltage information arrives at the controller about a microsecond 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 converters transient response. Additionally the LT1725 features internal slope compensation. This suppresses sub-harmonic oscillations that can occur with less sophisticated current mode controllers. Sub-harmonic oscillations increase output voltage ripple and increase switching stress. Conclusion The LT1725 isolated flyback controller greatly simplifies the design of isolated flyback converters. Compared to traditional opto-isolated designs, an LT1725 based circuit has far fewer components, superior transient response and is easier to stabilize. 33