为配合PCB（印刷电路板）空间，我决定更改实现与外界隔离的6个变压器，从方形、双E核设计到更有效体积的圆形壶状核设计。公司以前设计开发时，设计文档不是强制的，所以我没有太多制作新变压器的基础。然而，只要我保持同样的转换率，适当的线规格和相似的流量密度，我认为风险就会很低。我精确地继承这个方法，也做了最糟情况的计算，由饱和度检验许多临界情况。我使用基于电压流量密度的变压器饱和公式(B=[V×108]/[4×N×A_C×f])，在这里B为流量密度，V为供电电压，N为转换次数，A_C为代表性区域，f为应用频率。它实现了设计具有超过50%的饱和极限。

　　为进一步减小风险，我们向海外经销商订购了推荐的若干低价格原型。由于原型工作的低数量，US工厂比常用的海外产品工厂更适合生产样品。原型运行正常，我们按时交付了新设计、低于预算

　　现场测试若干单元几个月后，我们接到报告称新设计在测试时间歇性失效。在测试场和实验室测试多次后，我们将问题定位于外界接口与另一个设备的相互作用。奇怪的是在发布之前我们已经多次测试过设备的兼容性。更失望的，失败的根本原因是新变压器设计的饱和。怎么会这样呢？我们已经进行了冗余设计，并测试了很多样品。我们找到最初设计资格样品变压器的老模块。当我们采用硬件测试设备时，不能复现问题。然而，使用测试场的硬件，问题就复现。现在我们不得不确定两板硬件之间是有差别的。

　　新变压器设计采用无缺口的壶型核，其价格低且比有缺口的壶型核供货容易。其在很多领域满足饱和需求。然而，我们在计算中没有考虑电流而不是电压引发的瞬态饱和。结果，圆形样品测试中，两个壶型核部分没有完全压紧，出现了不必要的空气缺口。因为壶型核沾了聚亚胺酯，不必要的缺口被永久固定了。这个无意识的空气缺口足够阻止样品变压器发生饱和。但我们将变压器产品移到海外工厂生产，工厂正确地使用没有空气缺口的变压器。这些变压器在现场测试饱和，我们不得不更换模块。好消息是我们没有很多产品在现场测试。

　　英文原文：

　　Mind the gap: What separates a failing transformer-isolated interface from a flawless protoype?

　　Tales From The Cube: Do your qualification testing on a design that best represents what will come off the actual production line.

　　By Jeff Fries, GE Transportation -- EDN, 12/3/2007

　　I recently had to revamp one of my company’s more-than-30-year-old designs. Like any project these days, the new design needed to be smaller, cheaper, include many more functions, and see completion in a relatively short time. To decrease risk, I chose to reuse much of the physical interface from the previous design, including a transformer-isolated interface to the outside world. However, the constraints of a smaller design, coupled with the new functions I was adding, did not allow for 100% reuse of the existing components for this interface.

Scratching for PCB (printed-circuit-board) space, I decided to modify the six transformers that provided the isolation to the outside world from a square-shaped, double-E-core design to a more volumetric-efficient, circular pot-core design. The company developed the previous design when design documentation was not mandatory, so I did not have a set of requirements on which to base the new transformer. However, as long as I kept the turns ratio the same, the wire gauge appropriate, and the flux density similar, I thought the risk should be low. I exactly followed this method and also ran a worst-case calculation to verify plenty of margin due to saturation. I

　　To further reduce risk, we ordered several prototypes from an overseas vendor that our supply chain recommended because of its low overall cost. Due to low volumes of the prototype run, a US factory rather than the regular overseas production facility produced the samples. The prototypes performed flawlessly, and we released the new design on time, under budget, and within scope.

　　A few months after fielding several units, we received a report that the new design was failing intermittently in the field. After visiting the field several times and running multiple tests in the lab, we attributed the failures to interactions with another piece of equipment at the outside-world interface. The puzzling thing was that we had tested compatibility with this equipment many times before release. Even more frustrating was that it appeared that the root cause was saturation of the new transformer design! How could this be? We had plenty of design margin and tested multiple samples. We found the old modules with the sample transformers with which we had initially qualified the design. When we applied this hardware to our test setup, we could not reproduce the failure. Using the hardware from the field, however, the failure was reproducible. Now we had to determine the difference between the two versions of hardware.

　　The new transformer design used an ungapped pot core that was less costly and easier to obtain than a gapped pot core. And it appeared to meet the saturation requirements with plenty of margin. However, we did not consider transient saturation due to current rather than voltage in our design calculations. It turns out that, in the prototype samples we tested, the two pot-core halves were not completely pressed together, thus creating an unwanted air gap. Because the pot core was dipped in polyurethane, the unwanted gap was permanently fixed. This unintended air gap was enough to keep the sample transformers from saturating. Once we moved production of the transformers to the overseas factory, that facility correctly built the transformers without an air gap. These transformers saturated in the field, and we had to change the modules. The good news was we didn’t have many units in the field.

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