Circuit Design problem with Capacitor

Thread Starter

khatus

Joined Jul 2, 2018
84
Currently i am designing a PCB board for my new project. But the problem is, the chip or ic i'm using has the following connection for Supply in it's data sheet (it's common :p) the entire bottom layer is ground plane.


Three, same type of Ic will be used for my project.I connected them with 12V supply in the following manner as shown below



But later i thought another kind can also be used like this



But Now I am little confused which connection should i use?? What will be the difference if i use connection-2 instead of connection-1.

In the second connection i used a 0.3 uF capacitor with the supply. since the capacitor(0.1uF each of them) connected across VCC and ground of each individual IC's are in parallel.
3*0.1uF = 0.3uF.

Can Someone explain, which circuit will function properly?and why?

NB: the ic's are IR2110 Mosfet driver.
 

AlbertHall

Joined Jun 4, 2014
9,874
Connection-1 is the one to go for.
Each chip should have the capacitor connected with the shortest possible connections. This minimises the inductance in series with the capacitor and so minimises the impedance across the chip supply pins so when the chip draws pulses of current there is minimum disturbance of the chip supply voltage.
 

ebp

Joined Feb 8, 2018
2,332
The individual capacitors should be used, each placed as close as possible to the supply pin of the IC and each with a direct connection to the ground plane. Assuming they are surface mount, probably a single via is adequate but if you need to use very small ones it might be good to use two or three per capacitor.

Gate drivers "source" a lot of current to rapidly charge the capacitance of the FET's (or IGBT's) gate. Power distribution on a PCB always has inductance. Solid planes are very good, but individual tracks are not so good. You can't instantaneously change the current through an inductor. Without the caps, when the gate driver output goes HIGH and sources current into the gate capacitance, the sudden attempt to demand more current will make the voltage at the positive supply pin fall because of the inductance. The capacitor acts as a local charge reservoir that can supply current with only a small dip in voltage while the current through the inductance of the track "catches up." Though it doesn't usually happen with a gate driver, if the current very suddenly dropped the inductance of the track would cause the voltage to go very high - current stays constant, the load now looks like a higher resistance so by Ohm's law the voltage must go up. Now the cap acts as a local reservoir into which the inductively stored energy can be dumped. Obviously, for the cap to do its job well there must be as little inductance as possible between it and the driver.

These "decoupling" or "bypass" capacitors sometimes need to be higher in value but sometimes can be lower, depending very much on just what the circuit does. 0.1 µF is a very common value. At that capacitance, a ceramic capacitor is almost always the best choice. I recommend "X5R" or "X7R" types because the behave better than some others. Some types have a really big negative voltage coefficient of capacitance - as the voltage across them goes up the capcitance goes down. Some are so bad that if you operate them near their rated voltage the capactance can be down to 25% or less of the nominal value. XxR types do suffer from this, but not nearly as badly as types like "Y5V" or "Z5U." XxR types are a little more expensive.
 
Last edited:

Darkg

Joined Jun 1, 2018
10
Not much to add. Your confusion comes from assuming that a track is simply a conductor, like an ideal conductor. Often that is a useful assumption. In this case it is not!
 

crutschow

Joined Mar 14, 2008
24,977
I must have missed something. :confused:
You show Vcc connected directly to ground.
If Vcc is a plane it should not be identified with a ground symbol.
 

ebp

Joined Feb 8, 2018
2,332
I should have mentioned:
If the pinout of a device is such that the supply and ground pins are close together, the effectiveness of the decoupling caps can sometimes be improved by placing it between the supply and ground pins, even when planes are used for power. This may reduce inductance in the critical path by removing vias to the planes from that path.

A great many newer datasheets and even revisions of sheets for ancient parts now include a recommended layout drawing that will include decoupling caps.
 
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