Today electronics industry and associated technology of PCB has advanced so much that more new challenges are arising in design and development phase of a high end electronic product. These challenges are specific to the high power high frequency switching devices which utilizes power electronic semiconductor components. The corresponding PCB layout and hence the circuit designer will have to face challenges while selecting appropriate components values and their placement on PCB. Yes it is correct, the placement of components on PCB matters. This is because of various issues like thermal management, high current flows, signal reflection problems, EMI/Cross talk issues and noise problems in high frequency and hybrid circuits containing both analog and digital circuitry on single board.
The Problem of Transient Current:
The major issue that many designers face is the abrupt or instantaneous current spikes that may occur in the PCB that is also called "load transients" because of variation in electrical connected to output ports on PCB. This transient current in PCB will cause imbalance in voltage distribution in many sections and components of PCB, thus causing the weak components to breakdown or even cause whole PCB to fail. This is mainly because the larger the current flow in a specific impedance the larger the voltage drop across it. Hence damaging the component with abnormal voltages. In order to normally distribute the voltages in PCB, it is highly desirable to keep the impedance of PCB as low as possible. So that in the case of transient current, the voltage drop will not matter much.
Apparently there is no such way to decrease the impedance of board using high power high frequency components except to use "Decoupling Capacitors".
What is Decoupling Capacitor.?
The decoupling capacitor is commonly used in power supply section of a PCB board. Particularly in Switch Mode Power Supplies (SMPS), due to high speed switch used in buck or boost converter, the resonance created by PCB is very large and hence it can generate voltage spike due to adequate impedance of PCB board and can be dangerous. Thus in order to decrease or restrict the impedance of PCB, a combination of parallel connected capacitors is usually used to contain or control the impedance of PCB.
An Example of PCB as a Capacitor:
Suppose we have a PCB with size 3inch x 2inch and thickness 20mils. Now we know that in a PCB the power and ground planes exist and in between the dielectric material is placed. So we can correlate this situation with the Parallel Plate Capacitor phenomena where the capacitance exist between two parallel conducting plates if a dialectic is placed in between them. Usually the FR-4 dielectric material is used in PCBs and that has dielectric constant 4.5. The formula for capacitance will be
C = Capacitance of PCB
k = 8.854 x 10-12 = 0.2249 inch
A = Area
d = PCB thickness or separation between two copper planes (GND and PWR)
r = dielectric constant of FR-4
We have A = 6 inch sq
d = 0.02 inch
r = 4.5
This can easily show that the capacitance so small as that calculated will result in large PCB impedance in the range of ohms hence voltage spikes in case of transient will be significant. A SIWAVE is simulation tool to check the impedance profile on various frequency ranges. Below frequency profile curve shows the simulation results on 5V power supply system with DCP010505bp chip implemented on PCB.
It can be clearly depicted from the above diagram that fluctuation in impedance starts to occur extensively from frequency 670 MHz to 905MHz.
Use of Decoupling Capacitor to Restrict Impedance
The selection of decoupling capacitor is not very difficult. Every capacitor has the non-ideality factors i.e. ESR and ESL (Effective Series Resistance) and (Effective Series Inductance) respectively. The inductance (ESL) reactance will increase with increasing frequency but resistance (ESR) is independent of frequency. Also there are parasitic capacitance and resistance of the capacitor itself which can also change the behavior of decoupling capacitor when used in power supply circuits.
The above formula is the frequency dependant reactance of capacitor which shows that the reactance of capacitor is inversely proportional to the frequency of system
The above formula shows that the reactance of inductance is directly proportional to frequency of circuit
Hence we can depict that the Impedance of Decoupling capacitor containing ESR and ESL will first be very high than as the frequency start to increase the impedance of decoupling capacitor decrease. Then at a particular frequency it will stop further decrease and start to increase again as shown in the below graph
The above graph shows the series harmonic frequency 410MHz where the capacitor is chosen to be 623pF. The dotted line shows the target impedance and the fluctuations above the dotted line are not desired. This is because of single decoupling capacitors. When multiple decoupling capacitors will connected in parallel this issue will be solved.
As we connected multiple (n) decoupling capacitors (of same value) in parallel then the equivalent capacitance will increase to n x C. However the ESR will decrease to R/n and ESL will also decrease to L/n. This result is highly desired because the self harmonic frequency point will remain unchanged.
Hence we can say that the selection criteria of decoupling capacitor is the consideration of self harmonic frequency point and also the parallel connected multiple decoupling capacitors can greatly decrease the impedance of circuit/PCB.
The above diagram shows 6 decoupling capacitors connected in parallel each 96pF. Also we can see that the desired impedance is below the target impedance.
Determining the Position of Decoupling Capacitor:
The position of decoupling capacitor on board is very important. There various techniques used some of them are
- 1- When placing the decoupling capacitor for power supply, you should try to place them on bottom side of PCB. If capacitor is needed to place on top side then place them as close to the power pins of IC/components as possible
- 2- When parallel connected decoupling capacitors are of different values, then place the smallest capacitor closest to the power pin of IC.
- 3- The tantalum or non-polar capacitors should be placed in ascending order close to the pins of device as shown in figure
- 4- A device or IC that has more than one power pins needs to have at-least one decoupling or bypass capacitor per power pin
A simulation tool discussed above name SIWAVE is usually used to perform the resonance analysis function on PCB. The result get from the simulation shows the specific areas where serious resonance were found at particular frequency values like 673MHz, 728MHz and 861MHz. So it is highly recommended to select the particular resonance frequency value as calculated and then place the decoupling capacitors on the areas of high resonance to mitigate that undesired resonance and restrict the impedance of circuit.
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