There is a feeling in me that sometimes the old books are better than the new ones. As an example, a book by Hnatek called Design of Solid State Power Supplies might be one of those gems that shouldn't be missed in your library if you are interested in solid-state power supplies. This old book shows how switching power supplies were designed 30+ years ago, so it may not be "updated" with today's tech. However, I think it's a gem.
We are obsessed with efficiency when it comes to designing ACDC or DCDC power supplies. We do not want our power supply to get too hot since we do not want it to overheat. There is constant pressure on us to be as efficient as possible, because the more efficient we are the more ACDC or DCDC converters we are going to sell. In light of this, let's examine a pitfall that can arise when following this strategy without careful consideration. I would like to talk about the effect of fast recovery diodes on the conductive emission in this article.
As an example, let's take a look at what can happen in a typical ACDC or DCDC converter when we choose a fast recovery diode instead of a Schottky diode. Let us take a look at the following topology as an example:
Typical configuration of a switching power supply
The primary supply may either be an AC or a DC supply, it does not matter what it is. In contrast, on the primary side of the circuit, we usually have a higher voltage (RMS) than on the secondary side. The secondary rectifier we have instead is a full bridge rectifier. At the secondary, we usually use Schottky diodes. However, there are times when we use Schottky Fast Recovery diodes for the reason I mentioned above.
a Fast recovery type diode, does what the name indicates. It quickly turns off the diode! In order to understand how a fast recovery type diode affects the conductive emission of the diode, let's still look at figure 1.
Suppose that we are going to calculate the voltage on the anode side of a diode as soon as the diode switches itself off.
As a result, we can write Va = Ldi/dt. It is important to note that since we have chosen a fast recovery diode, the dT will be smaller than a normal diode, causing the voltage spike at the anode to be larger than that of a normal diode.
It is likely that this voltage spike resulting from the interwinding parasitic capacitor of the transformer will be coupled at the primary side, equally on both sides of the transformer, and will therefore become a common mode noise as a result.
A part of this common mode noise will fall within the 150 kHz - 80 MHz bandwidth and therefore, you may not pass your test as a result of this effect.
FIG.2 Common mode current due to the fast switching of the diodes.
The point is that every time we try to be efficient, we usually get unexpected problems, like higher emissions, as a result of our efforts. Is there any way we can solve this problem? Is there any way that we can have an AC/DC or DCDC converter that is efficient and yet minimize the emissions at the same time? This can be done in a few different ways. Adding a snubber in parallel with the diode is the simplest way to accomplish this. Normally, the capacitor should have a value around ten times that of the diode's capacitor at reversed voltages when switching, for example, if the diode has a 100 pF capacitor, then the capacitor should have a C value around 1nF (I know it sounds massive, but you should do your own test), whereas R should be sufficient to decrease the Q of the LC tank by 5-10 ohms, based on the diode.
FIG. 3 Is a simple way to reduce emissions. However, we lose efficiency.
The above shows how a seemingly complex problem has been analyzed and solved with the help of theory and what we've learned at university.
So, when you fail EMC. DO NOT PANICTherere is always an explanation. Sometimes it's hard to figure out the solution, but once you find the right model, you'll find it.
Good luck with your EMC!
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