Circuit Design of DC/DC Converter for Power Supply System Based on MAX5941B

Photocoupler

Overview:
A DC-DC converter based on the MAX5941B controller was analyzed and designed. The converter uses a current peak-controlled flyback topology for use in powered devices in Power over Ethernet systems.

Power over Ethernet (PoE) technology is gaining popularity, eliminating the need to install many AC adapters and AC outlets for each end device.
A complete Power over Ethernet system consists of a Power Supply Device (PSE) and a Powered Device (PD). The two are based on the IEEE802.3af standard to establish information about the connection, device type, and power consumption level. Since the power of the power receiving equipment is low (≤12.95W), it is difficult to achieve high efficiency and high stability. This paper analyzes and designs a DC/DC converter with low cost, high efficiency (86%), output and input voltage isolation, good dynamic response, etc., and focuses on the detailed design of the control loop. The converter can be used in powered devices of Power over Ethernet systems and is compliant with the IEEE802.3af standard.

1 PD interface implementation A typical PD requires an IEEE802.3af-compliant Power over Ethernet interface and a DC/DC converter. The schematic is shown in Figure 1. Interface circuits include grading circuits, UVLOs, and current limiting circuits. If discrete devices are built, the implementation of the converter will be complicated and the cost may be higher than the integrated interface controller. The MAX5941A/MAX5941B integrate a PD interface controller and a PWM controller for Flyback and Forward to make design efficient and fast.



When a powered device is plugged into a PoE system, there are three phases that follow a certain sequence: detection, classification, and power-on. When the PSE output voltage is between 2.7 and 10.1V, the PD provides a 22.5kΩ input resistance in accordance with the 802.3af standard and a capacitance of 0.05μF to 0.12μF (eg 0.1μF). After successful detection, the PSE will classify the PD to determine how much power is consumed. The maximum power of class 0 (the default maximum power class) is 12.95W, and the detection resistance RCL=10kΩ at this time.

The power-on circuit isolates the PD from the PSE during the detection/classification phase, which is an undervoltage lockout mechanism. The MAX5941B operates up to 67V with a default UVLO function (starts when the input voltage is above 39V and turns off when it is below 30V). The threshold voltage value can be adjusted by adjusting the external partial resistor.

(Please read the PDF for details)