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DPC ceramics, PCB proofing design of power integrity design need to do what thinking?

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DPC ceramics.In circuit design,generally we are very concerned about the quality of signal,but sometimes we often limit ourselves to the signal line for research,and treat the power supply and ground as an ideal situation,although this can simplify the problem,but in high-speed design,this simplification is no longer feasible.Although the direct result of circuit design is from signal integrity,we must not neglect the power integrity design.Because the integrity of power supply directly affects the signal integrity of PCB.Power integrity and signal integrity are closely related,and in many cases,the main reason for signal distortion is power system.For example,the ground bounce noise is too large,the design of decoupling capacitance is not appropriate,the loop impact is very serious,the cutting of multi power/ground plane is not good,the stratum design is not reasonable,the current is not uniform,and so on.

  1-decoupling capacitance

  We all know that adding some capacitors between the power supply and the ground can reduce the noise of the system,but how many capacitors can be added to the circuit board?What is the appropriate capacitance value of each capacitor?Where is the better position for each capacitor?We generally haven't considered these problems carefully,only relying on the experience of the designer,and sometimes even think that the less the capacitance,the better.DPC ceramics.In the high-speed design,we must consider the parasitic parameters of the capacitance,calculate the number of coupling capacitance and the capacitance value of each capacitance and the specific location of each capacitance quantitatively,so as to ensure that the impedance of the system is within the control scale.A basic principle is that the required decoupling capacitance should not be less than one,and the remaining capacitance should not be less than one.

  2-ground rebound

  When the marginal rate of high-speed equipment is less than 0.5ns,the data exchange rate from the negative capacity data bus is very fast.When it has strong ripple in the power layer that can affect the signal,the problem of power instability will occur.When the current passing through the ground loop changes,because the loop inductance will generate a voltage,when the rising edge shrinks for a short time,the current change rate increases,and the ground rebound voltage is added.At this time,the ground plane(ground wire)is not the ideal zero level,and the power supply is not the ideal DC potential.DPC ceramics.When the gate circuit of simultaneous switch is added,the ground rebound becomes more and more serious.With regard to 128 bit bus,there may be 50 u 100 I/O lines switching along the same clock edge.At this time,the inductance of the power supply and the ground loop fed back to the I/O driver switched at the same time must be as low as possible,otherwise,the static state connected to the same ground will present a voltage brush.Ground bounce can be seen everywhere,such as on chips,packages,connectors or circuit boards,which may cause power integrity problems.

  From the perspective of technological development,the rising edge of the equipment will only be reduced,and the width of the bus will only be added.The only way to keep the ground rebound acceptable is to reduce the power supply and ground distribution inductance.As for the chip,it means moving to an array chip,placing as much power and ground as possible,and connecting wires to the package as short as possible to reduce inductance.As for encapsulation,it means that the mobile layer encapsulation makes the ground plane of the power supply closer,for example,in BGA encapsulation.About connectors,it means using more ground pins or designing the connector from scratch to have internal power and ground planes,such as connector based ribbon cords.As for the circuit board,it means to make the adjacent power supply and ground plane as close as possible.Because the inductance is directly proportional to the length,it will reduce the ground noise if the connection between the power supply and the ground is as short as possible.

  3-power distribution system

  The integrity design of power supply is a very complex work,but how to control the impedance between power supply system(power supply and ground plane)is the key to the design in recent years.Theoretically,the lower the impedance between power systems,the better,the lower the impedance,the smaller the noise fluctuation and the smaller the voltage loss.DPC ceramics.In the actual design,we can determine the target impedance we want to reach by specifying the maximum voltage and power supply change scale.Then,we can adjust the relevant factors in the circuit to make the target impedance of each part of the power supply system(related to frequency)approach.




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