6.6 KiB
为什么 RS485 差分总线电路中需要使用共模电感
这个问题来源于差分通信本身就可以抑制共模干扰，为什么还要“画蛇添足”的增加同样是抑制共模干扰的共模电感。
一个论坛上的回答比较中肯，原文如下：
Why common-mode choke on differential-signal serial lines?
I have 2 questions:
- Why using a common-mode choke on serial communication lines (like CAN, RS485) if the signal is differential? Wouldn't the common-mode noise simply be cancelled in the receiver?
- I reviewed a project where the CAN had a common-mode choke, but the RS485 did not. Is there a technical reason for that?
Thank you
Wouldn't the common-mode noise simply be cancelled in the receiver?
A lot of noise would be heavily cancelled in a differential receiver but, high frequency stuff can cause problems and, of course a CM choke acts like an inductor and hence increases its series impedance with frequency. High frequency noise beyond the bandwidth limitations of the receiver can still wreak havoc.
I reviewed a project where the CAN had a common-mode choke, but the RS485 did not. Is there a technical reason for that?
There may be but, the devil is in the detail (and the performance testing and the grounding of this and that and what spec it's being tested to and the data bandwidth and the chips in question.......)
What's your take on modern transceivers like MCP2561FD using a "SPLIT" output pin for the termination (see page 6), supposedly for common mode stabilization? Reading the datasheet they seem mostly concerned about radiated emissions at higher baudrates (CAN FD) and not so much about radiated/conducted susceptibility. –
Lundin
Sep 22, 2021 at 14:07
The link and the data sheet don't really say much about it. The data sheet covers two versions; one with and one without so there's not much to go on really @Lundin - maybe raise it as a new question? –
Andy aka
Sep 22, 2021 at 15:44
I posted a question about this pin over at Codidact here: CAN "split" pin, bus termination and common mode stabilization. –
Lundin
Sep 24, 2021 at 9:22
The differential receiver has a limited range of common mode rejection, on the order of volts. It cannot remove common-mode noise peaks whose voltage exceeds that level. The common mode choke transformer increases that range for high frequency pulses, using two techniques:
- It converts some of the high frequency noise (whether common mode or differential) to heat (unlike a transformer, the ferrite in a common-mode choke is purposely lossy
- It subtracts the common mode signal (as a coupled inductor) allowing 10's of volts of high frequency noise difference between the two grounds at the transmitter and the receiver
In answer to your question on why you saw a common mode choke in a CAN bus port but not in a RS-485 port: that's just a decision that those particular engineers made. It has more to do with economics and little to do with differences between CAN bus and RS-485.
A common-mode filter will block noise induced on the pair (and its reference ground) from making its way into your system, even if the differential receiver rejects it. In other words, the CM filter prevents EMI/ESD related system upsets in the presence of strong noise.
Even Ethernet, which not only uses differential signaling but also transformer isolation, can benefit from suppressing common-mode noise.
As to why it was applied to CAN bus and not RS-485 in that particular system, I suspect that the designer felt that the places the CAN bus connected to were more likely to have strong electromagnetic noise (like from ignition, fuel injection solenoids, and other power train sources) than the places RS-485 went (in-cabin only.)
总结来说就是增加共模电感可以更好的抑制干扰，比如说高频的干扰。使用共模电感的优缺点都比较明显，以下是对 CAN 通信中使用共模电感的一些说明：
总线固有的差分传输形式使得 CAN 对于共模干扰有很好的抑制能力，如图 4 所示。通过 CANH、CANL 相减可很好地消除来自外部的共模干扰，但 CANH、CANL 并非理想对称，快速上升的跳变沿，这些均会带来 EMC 问题。我们通过示波器看总线波形很完美，测试静电，EFT，浪涌，传导骚扰抗扰均无异常。但测试传导发射，则不能满足限值要求，看起来很正常的总线实际却向外在发送传导干扰。
对于 CAN 接口的 EMC 问题，除了选用更好性能，符合要求的 CAN 收发芯片，另一种简单的方法就是对 CAN 接口增加外围，共模电感是一种很好的选择。在现有汽车电子 CISPR 25 标准中，对传导骚扰限值有很严格要求。许多 CAN 收发器均会超过限值。如图 5 分别为按照车规限制测试增加和不加共模电感的 CAN 接口传导骚扰，共模电感值为 51μH，可以看到在各个频段下对噪声改善较为明显，测试结果仍有很大裕量。
共模电感对降低传导骚扰有明显作用（共模电感本质上是一个双向滤波器，一方面滤除信号线上的共模信号干扰，另一方面抑制信号线本身不向外发出电磁干扰），可帮助我们快速通过测试要求，满足现有汽车用要求，但总线增加共模电感也会带来两个问题：谐振和瞬态电压。共模电感不可避免地会有寄生电感，直流电阻，考虑总线节点数，通信距离等因素，会引起谐振，影响总线信号质量，如图 6，绿色波形为增加共模电感的总线波形，信号下降沿已有明显的谐振。另外，共模电感感量较大，且直接节在收发器接口，实际应用中出现短路，热插拔等状态会使共模电感两端产生瞬态高压，严重时会直接损坏收发器。
综上：共模电感用于总线的优缺点较为明显，它可以滤除信号线的共模电磁干扰，衰减差分信号高频部分，抑制 CAN 接口自身向外发出的电磁干扰，在传导骚扰方面有很好地改善作用，但应用仍要考虑其带来的谐振与瞬态电压，这些在长距离，多节点通讯中对总线信号质量是不利的，对于一般工业应用对传导发射并无严格要求，因此可不增加共模电感。