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Three Output Isolated Flyback Regulator with LM2577

There are a many applications that require regulators with multiple outputs, such as a computer's power supply or a regulator used inside an automobile. Some of these applications require galvanic isolation between the regulator's input and output for protection and separate ground specifications.

The following simple flyback regulator using the LM2577 IC is designed with 3 outputs (5V@150mA, 7.5V@100mA, and -7.5V@70mA) and input-to-output isolation. The input voltage can be anywhere between 16V and 36V.

Dual output (5V to  15V) flyback regulator schematic with LM2577T-15

The galvanic isolation is achieved with the help of a 4N27 optocoupler. The LM385 adjustable voltage reference IC along with resistors R5 and R6 sets the main output voltage to 5V 4%.

The LM385 supplies a drive current to the optocoupler, proportional to the output voltage. Due to the high gain of the LM385, the LM2577's error amplifier is bypassed, and the feedback signal is fed directly to the compensation pin.

The flyback regulator's mode of operation is continuous, so a large primary inductance (Lp= 300H) is needed for the transformer. Using a Ferroxcube 812E250-3C8 E core for example, the primary winding requires about 50 turns. With the turns ratios as they are shown on the schematic and the small core size, the transformer windings must be wound tightly so that they fit the core windows. Interlaying the primary winding between the secondary windings will improve the coupling.

The Zener diode circuit (D3, R1 and R2) is added to provide the optocoupler transistor with about 20A of bias current, on top of the current sourced from the compensation pin (around 7A).

Since feedback is taken from the 5V output, its load and line regulation are better than that of the other two, which rely on feedback through the transformer coupling. The output ripple voltage of all three outputs is largely dependent on the filter capacitors used, and could be reduced by the use of additional high-quality, low ESR filter capacitors or an additional LC filter section.

Parts list:

  • IC1: LM2577
  • IC1: 4N27 optocoupler IC
  • IC1: LM385 adjustable band-gap voltage reference diode
  • D1: Fast recovery diode
  • D2: 51V Zener diode
  • D3: 4.7V Zener diode
  • D4, D5, D6: 1N5821 (Schottky diodes)
  • C1: 680F (Low ESR electrolytic)
  • C2: 0.1F (Low ESR capacitor)
  • C3: 0.47F
  • C4, C5, C6: 680F (electrolytic)
  • C2: 0.1F
  • R1: 10k resistor
  • R2: 100k resistor
  • R3: 2k resistor
  • R4: 47k resistor
  • R5: 15k 1% resistor
  • R6: 45.3k 1% resistor
  • R7: 220 ohm resistor
  • R8: 470 ohm resistor

Step-down (buck) regulators with LM2576
5V to 12V step-up regulator with LM2577
Simple flyback regulator with LM2577

LM2577T IC pinout (TO-220)