Tutorial 1.11: Calculating the Currents in the PCB:

The Circuit in LTspice

1.      The circuit in LTspice looks like shown in figure 1.11A below:

Fig. 1.11A: Circuit In LTspice

The Currents

1.    The drain current in U2 Si4128Dy is as shown in by the light blue trace 

       figure 1.11B below:

Fig. 1.11B: Drain Current in Si128DY Over 1ms.

2.     The drain current in Si4128DY in the time period of (60 – 280)us is shown in 

         figure 1.11C below.

Fig. 1.11C: Drain Current During Time Period of (60 - 280)us.

3.      The RMS value of the drain current during the period of (60-280)us = 1.7001A.

4.      The RMS value of the drain current in U2 over 1ms time period is 907.78mA as 

          shown in figure 1.11D below:

Fig. 1.11D: RMS Value of Drain Current of 1ms Time Period

5.      RMS Source current of U2 over a time period of 1ms = 907.66mA as shown in 

         figure 1.11E below:

Fig. 1.11E: Source Current U2 Over Time Period of 1ms

6.      The current in U1 LTC1624 pin SW up to 1ms = 63.165mA as shown in 

          figure 1.12F in the purple trace below:

Fig. 1.11F: Current in pin SW of U1 LTC1624 Over 1ms Time Period

7.      The current in inductor L1 is shown by the light blue trace in figure 1.11G below. 

8.      The RMS current in inductor l1 over a 1ms period = 1.9488A.

Fig. 1.11G: Current in Inductor L1 Over 1ms Period

9.     The current in the trace or wire between R2 and C1 as in the LTspice circuit in
        
         figure 1.11A above is shown in figure 1.11H below:

Fig. 1.11H: Current in Trace or Wire Between R2 and C1

10.    The current between R2 and C1 has an RMS value of = 1.9488A.

11.     From the above, we can conclude that the RMS value of the current does not 

          exceed 2A and reaches a peak value spikes of 6.4A over a period 

          of 280us - 60us = 220us.

Summary of Currents

A summary of current calculations from previous tutorials are shown in table 1.11A below:

Table 1.11A: Summary of Previous Current Calculations

A further summary of current calculations are shown in table 1.11B below:

Table 1.11B: Further Summary of Previous Current Calculations

KiCad Eeschema Circuit

The KiCad Eeschema circuit is shown for ease of reference in figure 1.11I below:

Fig. 1.11I: KiCad Eeschema Circuit

Input Current

1.     The input current in the conductor between IN pin V1 and capacitor C5 1000p in 
   

        the LTspice circuit is shown by the light blue trace in figure 1.11J below:

Fig. 1.11J: Conductor between IN pin V1 and capacitor C5 1000p Over Time Period of 1ms

2.     As can be seen from above the input current in the conductor between IN pin V1 

        and capacitor C5 1000p  over a time period of 1ms is 907.73mA

3.     The input current in the conductor between pin V1 IN and capacitor C5 1000p  over 

        a time period of 40us and 280us is shown by the light blue trace in figure 1.11K below:

Fig. 1.11K: Current In Conductor between pin V1 IN and capacitor C5 1000p Over Time Period 40us - 280us

4.       The RMS value of the current is 1.6329A.

5.        The results over 1ms period of 907.73mA are tabulated in table 1.11C below:

Table 1.11C: Input Current Pin V1 of IN Over 1ms Period

LTspice Capacitor C5

1.      The current in the LTspice circuit in capacitor C5 1000p over 1ms period is shown 

          in the purple trace in the figure below:

Fig. 1.11L: Current in LTspice Capacitor C5

2.      The current in capacitor C5 1000p over 1ms period is 3.404mA

3.      Capacitor C5 1000p in LTspice is C2 1000pF in KiCad Eeschema.

Conductor LTspice Capacitor C5 1000p and pin Sense-  of U1 LTC1624 and R4 .033

1.    The current in the LTspice circuit in the lower conductor between C5 1000p and 

        pin Sense- of U1 LTC1624 and R4 .033 is shown by the light blue trace in 

        figure 1.11M  below:

Fig. 1.11M: conductor between LTspice Circuit C5 1000p and pin Sense- of U1 LTC1624 and R4 .033

2.      As can be seen, it is described as I(R4)+Ix(U2D) by the LTspice software and is 

  

         written in light blue at the top of the graph in the figure above. 

3.       It is the current in R4 .033 and Drain current U of U2 Si4128DY. 

4.       It is in total 3.43mA. 

5.       In KiCad Eeschema it is the current in the lower top horizontal conductor between 

          pin 1 ISENSE of U1 LTC1624CSB and Drain current pins 5, 6, 7, 8 of U2

           SI4128DY_T1-GE3.

LTspice Pin Vin of U1 LTC1624

1.     In the LTspice circuit the current flows into pin Vin of U1 LTC1624 is shown in the 

        light blue trace in figure 1.11N below:

 

Fig. 1.11N: Current flowing Into Pin Vin of U1 LTC1624

2.      This current is 6.4948mA.

3.      In the KiCad Eeschema circuit, it is the current that flows into 

         pin 8 VIN of U1 LTC1624CSB.

4. A summary of the currents calculated above by LTspice are shown in table 1.11D below:

Table 1.11D

Current In LTspice Circuit Capacitor C2 100p

1.      In the LTspice circuit current in C2 100p capacitor is shown in the light blue trace in 

        the figure below:

Fig. 1.11O: Current In Capacitor C2 100p in LTspice Circuit

2.        In the LTspice circuit the current in capacitor C2 100p is 354.8nA. 

3.        In KiCad Eeschema it is capacitor C1 100pF.

Current In Capacitor C4 0.1u In LTspice Circuit

1.      In the LTspice circuit the current in capacitor C4 0.1u is shown in the light blue trace 

        in figure 1.11P below:

Fig. 1.11P: Current in Capacitor C4 0.1uF in LTspice Circuit

2.      The current is 54.69mA. 

3.       In KiCad Eeschema it is referred to as C4 0.1uF.

The Current In Diode D1 MBRS340 In The LTspice Circuit 

1.     The current in the LTspice circuit in diode D1 MBRS340 is shown in the light blue 

        trace in figure 1.11Q below:

Fig. 1.11Q: Current In Diode D1 MBRS230 In LTspice Circuit

2.       Current in the diode D1 MBRS340 is 1.7209A. 

3.        The Kicad Eeschema diode is referred to as D1 MBRS340.

Current In The Conductor Between Pin Tg of U1 LTC1624 And Pin G of U2 Si4128DY

1.      In the LTspice circuit the current in the conductor between pin Tg of U1 LTC1624 

        and pin G of U2 Si4128DY is shown by the light blue trace in figure 1.11R below:

Fig. 1.11R: Current Between Pin Tg of U1 LTC1624 and Pin G of U2 Si4128Dy

3.      In the LTspice circuit the current between pin Tg of U1 LTC1624 and pin G of U2 

         Si4128DY is 14.396mA. 

4.      In the Kicad Eeschema circuit the conductor is between pin 6 TG of U1 LTC1624CSB

         and pin 4 Gate of U2 SI4128DY-T1-G3.

5.      Summary of calculations done above are summarized as follows in table 1.11E below:

Table 1.11E

Current In The Conductor In LTspice Circuit Between Pin FB of U1 LTC1624 

And R2  35.7K And R1 20K 

1.       The current in the conductor in LTspice Circuit between pin FB of U1 LTC1624 and 

          R2 35.7K and R1 20K is shown in the light blue trace in figure 1.11S below:

Fig. 1.11S: Current In The Conductor  Between Pin FB of U1 LTC1624 and R2 35.7K and R1 20K

2.      The current in the conductor in the LTspice circuit between pin FB of U1 LTC1624 

         and R2 35.7K and R1 20K is 374.34nA. 

3.     The KiCad Eeschema description of this conductor is the conductor between 

         pin 3 SET of U1 LTC1624CSB and R2 35.7k and R3 20k.

Current In Capacitor C1 200u In The LTspice Circuit 

1.      The current in capacitor C1 200u in the LTspice circuit is shown in the light blue 

         trace in figure 1.11R below:

Fig. 1.11R: Current In Capacitor C1 200u In LTspice Circuit

2.     The current in capacitor C1 200u in LTspice circuit is 1.627A. 

3.      The KiCad Eeschema description of this capacitor is C5 200uF. 

4.      The calculations done above are tabulated in table 1.11F:

Table 1.11F

KiCad Eeschema Circuit With All The Relevant Currents 

The KiCad Eeschema circuit with all the relevant currents is shown in figure 1.11S below:

Fig. 1.11S: KiCad Eeaschema Circuit With Currents Indicated

Conclusion

From the above we can classify the currents into two groups:

1.       Currents that range from 354.8nA to 63.165mA and are below 100mA; and

2.        Currents that range from 671.75mA to 1.9488A  and are below 2A.

Previous: Tutorial 1.10: Assign Footprints To Switching Regulator and MOSFET Transistor.

Next: Tutorial 1.12: Trace or Width Calculation For PCB