Fermi3-Phase_Bus Presentation Measurement of Ripple Voltage, Capacitance Requirements.pdf

Using simulation for the design of amount of capacitance required for ripple requirement. This method avoid using cumbersome closed loop forms. It provides additional insights by providing frequency domain information which can be used for the design of EMI Filters.

1. BUS CAPACITOR DESIGN for 3 Phase
INVERTER
A presentation at FERMI National Lab

Hamid Kiabi

2. A short presentation about the utility of a simulation tool
in the design of BUS CAPACITANCE for a 3 phase Inverter
●Design of Bus Capacitance’s to meet the voltage ripple requirement is
cumbersome due to presence of many variables.
●Simulation is the best way to for the design of Bus Capacitance as it
involves many variables for ripple current like the switching frequency,
type of modulation, modulation index, bus voltage and phase current.
●And then very briefly examine the selection of CAPACITORS to meet
the CAPACITANCE requirement for the BUS Voltage Ripple.
●
●In the next slide will look at 3-phase inverter
●
●
●
●

3. 3
Voltage Source Inverter
V
V
Carrier Waveform
Va
Vcarr
Lbus
Cbus
V
VBUS
A
IBUS
SCHEMATIC for Generic 3-Phase Inverter
In the next slide will examine the result of the simulation for various currents
Load could be a motor like in electric
mobility (EV) or other similar applications

4. Iphase rms 85A Phase Current (Phase Current is the most dominant current)
IL rms 70A
ICAP rms 61A (Note no DC component for the Capacitor Current)
0 0.01 0.02 0.03 0.04 0.05
Time (s)
0
-50
-100
-150
50
100
150
I(Cbus) I(Lbus) I(RL1a)
OUTPUT WAVE FORMS
Capacitor current is due to the phase currents and not the DC input.

5. 0 0.01 0.02 0.03 0.04 0.05
Time (s)
0
20
40
60
80
VBUS
V BUS _RIPPLE without Filter Capacitor
In the next slide will find the frequency contents of this wave form

6. 40 60 80 100 120
Frequency (KHz)
0
5
10
15
20
I(Cbus)
Harmonics for the Bus Capacitor Current

1st
Harmonics at 40Khz
Higher Order harmonics will be suppressed by using
Ceramic capacitors which are effective
In the next slide will use the FFT result of the harmonics content
to estimate the impedance of the bus.

7. BUS IMPEDANCE
We purposefully used a large inductance value for the Lbus to separate the
DC and AC components of the Bus Ripple.
But a more reasonable value of Lbus of 1uH, result in the bus impedance
Of 0.25ohms at 40K. The impedance of bus capacitor will be much lower
as we will see. (It better be!)
In the next slide will look at the Capacitance value to meet a given
ripple voltage specification.
Bus Impedance refers to wiring from maybe a Battery to the Inverter

8. BUS Capacitance .vs. Ripple Voltage


Next solving for Zc; (the impedance of the capacitance needed)
Zc = Vripple/Irms = 1V/61A = 16mohms;

And solving for the capacitance value C
C = (1/ (2xpi *40x10^3 *16 x 10^-3) )= 250uF
The required capacitance is then 250uF (for the 16mohms impedance).
All the AC BUS current will go through the Bus Capacitance
In the next slide will examine the selection of the Capacitors to meet
the amount of the Capacitance required.
Would like to have a bus ripple voltage of about 1Vrms
Again the constraints are :
1. Vripple = Ir ms x Zc (Ripple Voltage depend on the rms of the current and the total
impedance of the capacitor); where Zc is the Impedance of the Capacitor Zc= 1/(wc)
2- Current Per capacitor should not exceed specification (how current shared
between capacitors)

9. Searching for Capacitors
* Choice of Capacitors will be either Ceramic or Electrolytic,
Selection criteria will be based on:
VOLUME & AREA
CURRENT HANDLING
COST

* Volume per one Ampere for the Ceramic and the Electrolytic Capacitors shown below.

1- Using a Ceramic Capacitor 4.7uf, 100V Volume is 10mm^3/A
2- Using a Electrolytic Capacitor 180uf, 100V Volume is 1200mm^3/A
Area per one Ampere
1- Ceramic 5mm^2/A (H=2.5mm)
2- Electrolytic 80mm^2/A (H=16mm)
3- Film Capacitors (only for HV applications)
* Cost
1- Ceramic ~ $ 0.25 /10K similar for both types
2- Electrolytic

10. ALL CERAMIC CAPACITORS Design
Using ceramic capacitors of 4.7uf/60V @ 1.5A (Samsung 1210 data sheet)
* Number of ceramic capacitors required to meet the 250uf for the filter capacitor bank is:

N = 250 uf/1.4uf = 178
*Current handling for using 178 ceramic capacitor is:(Samsung 1210 data sheet)

178 x 1.5A = 267A > 60A requirement
*Conclusion: Using ceramic capacitors far exceeds the requirement of
current handling specification of 60A .
Overkill but it is Okay.





(Is there a mistake here, where did 1.4uf come from See 1210 Samsung )

11. ALL ELECTROLYTIC CAPACITORS Design
Using the Electrolytic Capacitor of 180uf/160V
*Number of Electrolytic Capacitors required are:
N = 250uf/180uf = 2
*Current handling for using the Electrolytic Capacitor is:
2 x 1.8A = 3.6A < 61 A (too little we can not use)
Using 2 electrolytic capacitor meet the capacitance requirements but not the current
requirement.
Need to increase the number of electrolytic capacitors.
N = 61A / 1.8 = 34
180uf x 34 = 6.1mf
* Voltage ripple associated with a 6.1mf capacitor is :
Vripple = 61A/ (2.pi * 40khz * 6.1mf) = 0.40Vrms > 1Vrms

*Conclusion: Using Electrolytic Capacitors exceeds the requirement of
the Voltage ripple of 1Vrms.

Overkill but it is Okay.

12. VBUS with Filter Capacitor of 250uF
0.0165 0.017 0.0175 0.018 0.0185 0.019
Time (s)
58
60
62
64
VBUS
[0.0182341 , 61.0391]
[0.0186682 , 58.6949]
Vrms 0.9v

13. Conclusion
1- One type of caps (e.g. electrolytic plus few CC for HF),
Calculate Z, find the number of caps needed, check that the current in each is Smaller than maximum
allowable If not, add caps.
2- Mixed Types, making sure that the current and capacitance I/C is similar,
3- Use the simulation tools.
Thank you very much
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