While an engineering student starts ones pursuit of graduation understanding the basics of passive elements like capacitors, inductors, while feeling comfortable understanding the properties, one doesn't understand how the same capacitors and inductors can be nightmares when they start designing the circuits.
The worst thing is that if there are any conductors or conductive paths separated by space/air, it acts as a capacitor and that is where the real problems start popping up if the designer understand this principle. Also, any over or under usage of capacitors in circuit design can be a major cause of design failure.
Let us list few scenarios in the circuit where capacitance/capacitor can give sleepless nights to the designers:
1. With lot of excitation, designers power up the new board, sometimes there can be cases like crystal doesn't work. while this can be attributed to many things to connections, improper layout, etc, one major problem could be that designer hasn't considered the PCB stray capacitance while calculating load capacitance or the load capacitors themselves are wrong values.
2. EMI/EMC is a tough subject and while designing your circuit it is always better if known sources of radiators are taken care of. The hidden radiators are sometimes difficult for designers to identify. Sometimes the unused pins of micro controllers can be source of EMI. The main reason being the capacitance coupling of noise to unused pins.
3. When your regulator is powered on and you observe unstable output, then there could be that your output capacitance could be an issue. Also, if the Output bypass capacitor of the regulator is not selecting to suppress noise it could lead to EMI problems. Also, if the output capacitors are not large enough to handle sudden load transients, there could be voltage dips, leading to unintentional rest of the circuit.
4. While we highlight the problems with output capacitors at regulators, the problem with capacitors at the input of regulator are plenty. Especially, with the switching regulators, the switching current transients are to be provided by the input capacitors. If the capacitors are not provided, the entire power topology which depends o that regulator might collapse.
5. Let us assume a designer is using TVS diode in his application to eliminate the ESD on the USB data lines. USB are high speed signals and if the TVS diode selected has high capacitance, you could see signal integrity issues due to impedance mismatch and could lead to collapse of communication mechanism.
6. Crosstalk we observe regularly between the traces on the PCB is due to the coupling capacitance between the traces. The inter layer capacitance also causes coupling between the layers. As we have dense board designs, one can imagine the coupling effects these capacitance can cause.
7. Limiting the load capacitance is very important for communication to happen properly (rise time and fan out). Designers use buffering mechanisms to limit the load capacitance.
2. EMI/EMC is a tough subject and while designing your circuit it is always better if known sources of radiators are taken care of. The hidden radiators are sometimes difficult for designers to identify. Sometimes the unused pins of micro controllers can be source of EMI. The main reason being the capacitance coupling of noise to unused pins.
3. When your regulator is powered on and you observe unstable output, then there could be that your output capacitance could be an issue. Also, if the Output bypass capacitor of the regulator is not selecting to suppress noise it could lead to EMI problems. Also, if the output capacitors are not large enough to handle sudden load transients, there could be voltage dips, leading to unintentional rest of the circuit.
4. While we highlight the problems with output capacitors at regulators, the problem with capacitors at the input of regulator are plenty. Especially, with the switching regulators, the switching current transients are to be provided by the input capacitors. If the capacitors are not provided, the entire power topology which depends o that regulator might collapse.
5. Let us assume a designer is using TVS diode in his application to eliminate the ESD on the USB data lines. USB are high speed signals and if the TVS diode selected has high capacitance, you could see signal integrity issues due to impedance mismatch and could lead to collapse of communication mechanism.
6. Crosstalk we observe regularly between the traces on the PCB is due to the coupling capacitance between the traces. The inter layer capacitance also causes coupling between the layers. As we have dense board designs, one can imagine the coupling effects these capacitance can cause.
7. Limiting the load capacitance is very important for communication to happen properly (rise time and fan out). Designers use buffering mechanisms to limit the load capacitance.
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