LT8306

• Galvanically isolated 12V flyback DC output with up to 24 W of output power capability
• No Transformer third winding or Opto-Isolator for output regulation
• Supports IEEE802.3at (PoE+, Type 2) and IEEE802.3bt (PoE++, Type 3 and 4) interfacing and power supply control via a configurable jumper array
• Supports a wide input DC auxiliary power of 37V to 57V

The AD-PS3803-RD reference design is a power converter capable of harnessing any IEEE802.3at (PoE+, Type 2) compliant downstream Power-Over-Ethernet (PoE) supply to provide a galvanically isolated DC power.

At the core of this solution is the LT8306 60V No-Opto Flyback controller, stepping down the 37V to 57V PoE input voltage to a 12V, 24W DC output.

The on-board LT4275 LTPoE++/PoE+/PoE power delivery (PD) controller handles interfacing and power delivery to any PoE supply.

The LT4321 ideal diode bridge controller is used for improved end-to-end power delivery efficiency and eased thermal design by utilizing low RDS(ON) N-Channel FETs, replacing the eight-diode configuration typically found in passive PoE rectifier bridge.

The AD-PS3803-RD also has auxiliary DC power support for a wide input range of 37V to 57V.

APPLICATIONS

• Industrial Automation
• Surveillance Camera Systems
• VoIP Phones
• Payment Terminal Systems
• Wireless Access Points

The EVAL-LT8306-AZ evaluation circuit is a micro-powered no-opto isolated flyback converter that features the LT8306. This demo circuit outputs 12V and maintains tight regulation with a load current from 15mA to 2.0A and over an input voltage from 6V to 36V. The output current capability increases with the input voltage.

The EVAL-LT8306-AZ needs very small minimum load (15mA) to regulate the output voltage because of the LT8306 ultra-low switching frequency at very light load. To properly limit the output voltage and avoid preloading under a no-load condition, a 13V zener diode is placed between the V_OUT+ and V_OUT- pins.

Transformer leakage inductance causes a voltage spike on the primary side after the power switch turns off. To limit this leakage inductance spike within a MOSFET voltage rating of 120V, an RC snubber and a TVS clamp are installed to dampen the ringing and clamp the MOSFET drain voltage to a safe level. The RC snubber can be removed to improve the efficiency by 3–4% when the load is below 100mA at 24V typical input.

The Performance Summary table in the user guide summarizes the performance of the evaluation circuit at room temperature. The evaluation circuit can be easily modified for different applications with some predesigned transformers.

The LT8306 is a simple-to-use micropowered isolated flyback controller. By sampling the isolated output voltage directly from the primary-side flyback waveform, the IC does not require a third winding or an opto-isolator for regulation. The IC sets the isolated output voltage with a single external resistor. It integrates loop compensation and an internal soft-start circuit to reduce external components. Boundary mode operation provides a small magnetic solution with excellent load regulation. The low-ripple burst mode operation maintains high efficiency at light load while minimizing the output voltage ripple.

The LT8306 is packaged in a 6-lead ThinSOT^™. It is AEC-Q100 qualified for automotive applications. The LT8306 data sheet gives a complete part description, operation, and applications information. Refer to the data sheet in conjunction with the EVAL-LT8306-AZ evaluation circuit quick start guide.

• Flyback topology minimizes cost versus high-power alternatives, enabling a simpler and more economical power architecture.
• No‑optocoupler synchronous flyback topology for increased reliability and reduced component-related failures.
• Low heat dissipation - stable, reliable low‑voltage isolation with up to 94% peak efficiency for compact industrial designs.

Solution Requirements:

• Input Voltage: 18 V_DC – 32 V_DC (24 V_DC Nominal)
• Output Voltage: 11.80 V_DC – 12.27 V_DC (12.20 V_DC Nominal)
• Output Current: 6.25 A (max)
• Output Power: 75 W
• Output Voltage Ripple (Pk-Pk): 15 mV
• Load Transient Range (Pk-Pk): 2 V (1 A to 6.25 A)

Performance Requirements:

• 92% Efficiency: Vin = 18 V, Iout = 6.25 A
• 93% Efficiency: Vin = 24 V, Iout = 6.25 A
• 93% Efficiency: Vin = 32 V, Iout = 6.25 A

• Uses a no‑optocoupler synchronous flyback design for higher reliability and fewer failure points.
• Provides four output‑capacitor configurations for easy design tuning.
• Delivers >90% efficiency under typical load conditions.
• Supports up to 4 A peak, ideal for inverter AUX and control‑rail power.
• Compact isolated form factor that simplifies integration into inverter control boards.
• Lowers BOM cost and design complexity for renewable‑energy systems.
• Integrates seamlessly into a solar inverter’s auxiliary supply, powering DSPs, MCUs, sensors, and gate drivers.

Solution Specifications:

• Input Voltage: 20V to 28V (nominal: 25V)
• Output: 5V, up to 4A
• Topology: Isolated flyback controller with synchronous rectification
• Transformer: Shielded type (Würth Elektronik 750311589)
• Output Ripple: ≤ 250mV p-p (~5% of VOUT)
• Transient Response: Validated at 2A step load
• Key Attributes: Low cost, high performance