In my previous post, I explained why there are emissions in DC-DC converters that are insulated and how to minimize them.
I have also explained in detail the three main mechanisms that are responsible for emissions.
This has allowed us to see how we can reduce the emissions from these 3 different paths as much as possible.
There was one path in which emissions were caused by cables.
This has led to the conclusion that the cable needs to be connected to the GND plane, either in DC or AC.
When we perform this test, I have found that most of the time the value of the capacitor is not critical, as is also the value of the inductors, and we have found this to be true in most cases.
As long we break the antenna structure (wire, GND plane, feeder) we will reduce emission.
If you don't remember this, I would highly recommend that you go back and review my previous three posts if you don't.
As we will see later, however, the value of the inductor is critical for passing ESD, so we should pay attention to its value.
However, if you do this, sometimes you reduce the emissions, and then there will see a new problem. You may fail ESD.
I would like to discuss this issue with you today.
Let's assume that you have designed a handlet product that is powered by batteries.
In the context of this discussion, let's assume, just for the sake of the discussion, that you have an attached USB cable to your computer.
You will probably find that since the device is battery-powered, you have a DCDC converter to generate 3.3V, 1.8V, etc, and a microcontroller.
As we have seen in previous posts, emissions from an insulated DCDC converter use the cable as an antenna to transmit.
As we have seen, in order to weaken the emission, it is necessary to connect the antenna to the GND in order to damage the antenna structure.
For the purposes of this example, we will assume we have USB 1.1 or 2.0, which results in the following components: D+, D-, 5V, GND, and shield.
Consider the scenario where you connect the USB shield to GND and you pass the emission through!
That's great... however, one day later, you fail the ESD test.
I'd like to see why that is, and let's see what can be done to fix it!
By connecting the shield to the GND we have "helped" the ESD voltage to find a way to earth!
We can see from the drawing above that this is the case. When we connect the USB shield to the PCB GND, we are helping the ESD discharge to find its way to earth by "helping" it to find its way to it. As a result, your board will also be destroyed in this path.
In other words, do not do this!
There is a better way to do it.
In the above drawing, we break the Antenna, and we stop any high voltage going to the PCB
There are two things we want to accomplish:
Basically, what we want to do is break the antenna structure
We want to prevent an high speed signal from the shield from reaching the GND of the PCB
What can we do to make that happen? It is quite simple, simply connect the USB shield to the battery, and then use a high speed inductor between the battery return and the GND on the PCB, as shown above.
In my experience, I have used inductors whose values ranged between 22 uH and 100 uH. It should be noted that smaller values of inductors may not create enough impedance to stop high voltages from going to the PCB GND, while larger values may create inductors with poor impedance over a wider frequency spectrum.
In this post, you have been taught how to pass emissions on battery-powered devices with cables, and ESD on the devices with cables.
Thanks for reading! I hope you enjoyed it!
I look forward to seeing you in the next post.
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