A Common Mistake in WF3 Lightning Testing
Protecting against DO160G WF3 Lightning Induced Transient Susceptibility is not easy and can take years of experience. For example, a lot of engineers believe that filtering capacitors need to be rated at the lighting waveform voltage level.
Here is a SECRET
Capacitors can be UNDERRATED BY 3X the rating of DO160G WF3 Lightning level during section 22.0 Lightning Induced Transient Susceptibility and here is why.
Example: Given that you have a DO160 Lightning Induced Transient Susceptibility of B4G3L3 you will have the following lightning requirements for pin injection
WF3 Voc = 1500, Isc = 60
WF5a Voc = 750, Isc = 750
WF3 Lightning Design
One might think that based on the waveform 3 (WF3) voltage of 1500V you would need at least a 1500V capacitors at the location of C1, C2 and C3.
THIS IS NOT CORRECT FOR TWO REASONS!!!
1) MLCC can be overvoltage’d for short periods of time up to 3x there rating depending on the voltage rating of the capacitor.
During WF3 Lightning Testing, a good rule of thumb is a capacitor that is 500V rated (or less) can likely be rated for 3x its rating for a short period of time. Capacitors rated for 1500V (or higher) should only use a 1x rating unless you have data saying otherwise.
2) The source impedance WF3 Lightning Testing is 25 ohms! This means that at 1MHz a 0.1uf filtering capacitor looks like a 2-ohm resistance to the pulse creating a 2 to 25 voltage divider on the component.
WF3 Lightning Testing Results
Since I showed you have to simulate WF3 in LTspice at in this blog post, you can test this out in 60 seconds. Well here it is…
You can also see that if I remove the 100nf capacitor (C3), the voltage level will go up considerably.
This is because the impedance of a 10nF cap at 1MHz is considerable higher making the voltage divider less. See this example impedance vs. frequency graph of different value capacitors
If you do the math of the 10nF cap equaling about 13 ohms at 1MHz it seems to make sense.
RCAP / RCAP + RWF3 * VWF3 = VCAP
20 / ( 20 + 25 ) * 1500 = 666 V <- pretty close to the LTspice simulation RIGHT!
Understanding the two things discussed above can help you optimize your next or current design during WF3 Lightning Testing!
- Capacitor Overvoltage Ratings
- Waveform 3 Source Impedance
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