PB Fortich

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  • Isolated 24V TO 5V 2W Flyback Power Supply

  • Created: Jun 21, 2016

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Summary

A flyback power supply provides a good galvanic isolation that is quite useful in industrial control and automation. It secures test and measurement devices from possible faults. This isolated flyback power supply circuit can provide 5V output voltage at 400mA current, which is commonly used voltage and current rating in control system. The design circuit is compact since it has minimal external components. It has ±5% output accuracy for some sensitive application.


The MAX17498BATE off-line converter IC operates at a 500kHz operating frequency and it acts as the main component of this flyback DC-DC converter. It provides cycle-by-cycle, primary-side current-limit protection using the MAX14798BATE internal MOSFET switch, which is connected at the LX input. The resistor R6 sets the circuit peak-current limit to 1.3A using transformer T1's primary winding and the IC internal switch. This circuit also make use of RCD snubber network (R1, C3, and D1) in order to minimize leakage, energy ringing, and to clamp the voltage at the LX input. The T1 transformer provides galvanic isolation up to +1500V for the output. The isolated +5V output is set using the transformer secondary-side, optoisolator (U2), shunt regulator (U3), resistors (R9, R19, R20), and various other components. The circuit startup process begins when the voltage at VIN is greater than the circuit's +18V startup voltage threshold and a current-limit fault is not present. The resistors R3, R4, and R5 set the startup voltage and overvoltage-protection voltages to 18V and 36V, respectively. soft-start capacitor C10 programs the circuit soft-start period to 5ms. Upon the circuit rising above its startup voltage, the LX node begins switching and energy is transferred from the transformer primary side to the secondary side for the regulated +5V output. The secondary side of the circuit consists of a diode (D3), an inductor (L1), and a few output capacitors. When the MOSFET on the primary side is off, the transformer pushes the current to the load at the output and the remaining energy is stored in the output capacitors. When the MOSFET is on, the diode prevents the current flow back into the transformer and discharges the output capacitors.


This design is applicable in the development of an industrial automation especially to the control system. It is also applicable in telecommunication power supplies where majority of its devices is running 5V and 400mA supply. It can be part of some portable devices that requires compact power supply.

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