Determining Which PoE Transformer Is Best: Flyback or Forward?
September 15, 2020byAntonio Anzaldua Jr.With IoT on the rise, engineers are increasingly faced with options of transformer topologies in Power-over-Ethernet (PoE)-based designs.
The transformer is an electrical piece of equipment that has redefined power electronics, utility power, LED lighting designs, and now, the wireless LAN sector known as Power-over-Ethernet (PoE). There are two types of transformer topologies to consider for your next PoE-based design: forward and flyback.
POE变压器为低压设计提供了一种经济高效，小且节能的解决方案。图像用Eaton
PoE is a fairly new method of providing power without looking for a 120 V outlet or nearby circuit breaker to supply power to low-voltage devices. When this method was introduced, developers were only able to generate less than 5 W of power via PoE. Now, there is a way to provide power designs with up to 100 W of PoE.
The Struggles of Low-Voltage Power Design
在IT房间设计设备时，有很多需要考虑的事情。工程师面临的挑战之一是从哪里来源。而且，如果只有中型电压（例如480 V），是否有足够的空间添加降压变压器以获得低压电源？
这些问题以及电气安全代码，建筑代码和设备成本以及设计师可能会面临许多限制，以在共同数据室中创建一个完整的系统。
This is what makes PoE an attractive idea for such projects. It supplies low-voltage DC power directly to the device and eliminates the complexity of locating AC power sources.
The PoE method supplies the necessary power for designs that have video IP phones; multi-channel, wireless LAN access points; and multi-direction IP cameras.
Choosing the Right PoE Transformer
There aretwo types of PoE transformers that are heavily debated when used in DC-DC converters: flyback and forward topologies。向前和反式变压器可以将动力设备接口与其其余电路隔离，同时降低POE输入电压，从而为PD电路供电。
对于具有大于2.5 V的输出电压的设计基于反式转换器的变压器可能是最有用的选择。反式变压器是带有间隙芯的耦合电感器。这意味着在每个占空比中，将输入电压应用于初级绕组。
Eaton's flyback PoE transformers support 1500 Vac isolation in the power converter with up to 13 W of power. Image used courtesy ofEaton
Once the switch, bipolar transistor, or MOSFET opens and closes, the energy that is stored in the gap of the core flows over to the secondary windings of the transformer—delivering power to the load side. A disadvantage for flybacks is the distortion that will appear due to higher output current ripples. A required filter adds to the cost of the design.
对于通常需要小于12 V的输出电压的电源设计，向前POE变压器可能是一个富有成效的选择。飞回变压器和前进变压器之间的差异是附加的离散组件。由于Flyback Transformer可以存储能量，因此需要输出过滤器，能量存储电感和其他整流装置（例如二极管）才能达到相同的结果。
伊顿的前锋POE变形金刚可以用比回飞的更高的输入电压运行。图像用Eaton
飞回变压器都配备了多输出电压；但是，由于电流的巨大涟漪，在飞回变压器中有很多不必要的噪音。需要将较大的离散组件（例如电感器和电容器）添加到设计中，以实现与正向变压器相同的输出。
Neither transformer is inherently superior to the other。与大多数设计原则一样，工程师在飞回拓扑或前向拓扑之间的选择将在很大程度上取决于PCB的应用和足迹。
A Recent Transformer Example: Bel Fuse's SPoE
As an example of the burgeoning transformer industry, recently, Bel Fuse, a leading manufacturer in networking, telecommunications, and high-speed data transmission,announced a new series of surface-mounted transformers, dubbed SPoE。
贝尔保险丝的SPoEutilizes a flyback methodology. The company claims that this device is useful for high-density boards. With high-density PCBs, designers are able to use both sides of the board, which allows for additional components to be added.
Spoe Poe变压器。图像用贝尔保险丝
这是有益的，因为复杂的低压设计可能会产生不必要的失真，并且飞回拓扑需要其他组件。总体而言，SPOE可能是具有60 V的输入电压的设计候选。
Today the Internet of Things (IoT) is increasing the market of computing devices that are requiring more than 5 W to operate. Developers must stay ahead of the demand by continuing to present new power solutions for low-voltage, high-powered network devices. PoE transformers provide engineers with another tool to hook on their design belt.
