±340V DC Output, 15V DC Input, Isolated Flyback Converter

The project presented here is an isolated flyback converter featuring the LT3748 chip from Analog Devices. The circuit is designed for a +/-340V DC (2 x 340V) output from a 7V to 15V_DC input. The output current is up to 2 x 8mA. The board senses the isolated output voltage directly from the primary side flyback waveform during the off time of the power switch. No third winding, optocoupler, or signal transformers are required for regulation. A minimum load of approximately 2% of the maximum load is required on the output to maintain regulation.

Features

• Power Supply 7V to 15V DC @ 500mA
• Output ±340V DC
• Output Load Current 8mA + 8mA
• Soft start
• Under Voltage Lockout
• 5mm χ 4 Mounting Holes
• Screw Terminals for Easy Connection
• PCB Dimensions 78.90 x 22.23 mm

Output Short Circuits and SENSE Pin Over Current

The LT3748 has an internal threshold to detect when the primary inductor current exceeds the programmed range.  This can result from an inductive output short-circuit and an output voltage below zero, reflecting a voltage back to the primary side of the transformer which, in turn, causes the LT3748 to turn the external MOSFET on before the secondary current has discharged. When the voltage at the SENSE pin exceeds approximately 130mV—equivalent to 30% higher than the programmed ILIM(MAX) in the RSENSE resistor—the SS pin will be reset, stopping switching. Once the soft-start capacitor is recharged and the soft-start threshold is reached, switching will resume at the minimum current limit.

Soft-Start (Capacitor C8)

The LT3748 contains an optional soft-start function that is enabled by connecting an explicit external capacitor between the SS pin and ground. Internal circuitry prevents the control voltage at the VC pin from exceeding that on the SS pin. Capacitor C8 provides soft-start.

ENABLE and Undervoltage Lockout (UVLO)

A resistive divider from VIN to the EN/UVLO pin implements undervoltage lockout (UVLO). The EN/UVLO pin threshold is set at 1.223V. In addition, the EN/UVLO pin draws 2.4µA when the voltage at the pin is below 1.223V. Resistor R2 and R5 used to set the UVLO.

The LT3748 is a switching regulator controller specifically designed for the isolated flyback topology and capable of high power. It drives a low side external N-channel power MOSFET from an internally regulated 7V supply. No third winding or opto-isolator is required for regulation as the part senses the isolated output voltage directly from the primary-side flyback waveform.

The LT3748 utilizes boundary mode to provide a small magnetic solution without compromising load regulation. Operating frequency is set by load current and transformer magnetizing inductance. The gate drive of the LT3748 combined with a suitable external MOSFET allow it to deliver load power up to several tens of watts from input voltages as high as 100V.

Schematic

Parts List

LT3748 Block Diagram

Connections

Gerber View

Photos

Video

LT3748 Datasheet

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