Abstract
This reference design is for a highly efficient, flyback, 12V, Class 3 powered device (PD). The design features the MAX5969B as its controller. The design also uses the MAX5974A, which controls current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE® 802.3af/at compliant and is a high-performance, compact, and cost efficient solution for a Class 3 PD. The design can also be customized to 12V/2A Class 4 PD
General Description
This reference design is for a highly efficient, flyback, 12V/1A Class 3 powered device (PD) that is IEEE 802.3af/3at complaint and cost effective. The reference design can be customized to a 12V/2A Class 4 PD.
The design features the MAX5969B and MAX5974A. The MAX5969B controller is fully compliant with the IEEE 802.3af/at standard in a power-over-Ethernet (PoE) system. The device can also get power from a wall adapter (WAD). The MAX5974A controls wide-input-voltage, active-clamped, inductive feedback, current-mode PWM converters and provides frequency foldback for PoE applications. Using these devices, this reference design is IEEE 802.3af/at compliant. It is also a high-performance, compact, and economical solution for a Class 3 PD or a Class 4 PD.
Specifications
The 12V/1A PD meets the following specifications:
- Input voltage: 36V to 57V
- WAD input voltage: 36V to 57V (10V to 57V with a different transformer)
- VOUT1: 12V/1A (up to 2A by optimizing the switching frequency)
- Output ripple: ±2.5% (can be reduced by adding more output ceramic capacitors or disabling the frequency foldback, FFB)
- Load transient VP-P: ±3% (25% step-load)
- Line and load regulation: ±2%
- Switching frequency: 300kHz (150kHz in FFB mode)
- Total efficiency with a load of 2A at 3V and 48V input: 90.3%, including input LAN transformer and diode bridge (93.3%, not including input LAN transformer and diode bridge)
Design Features:
- Inductive feedback: no optocoupler or shunt-regulator, so costs are lowered
- Self-driving input voltage: achieves synchronous rectifier, provides high efficiency and good load regulation
- Frequency foldback: 150kHz switching frequency when load is about 1W, provides high efficiency at light loads
Top view of the reference design. Bottom view of the reference design
Bottom view of the reference design. Bottom view of the reference design
Figure 3
Figure 4
Figure 5
Transient Response
- VIN = 36V
- VOUT = 12V/0.6A–0.9A–0.6A
- CH1: 200mV/div, 12V output voltage
- CH4: 500mA/div, output current
- Time base: 200µs/div
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