bonitron M3460UC, Customer Reference Manual

Page: 48 / 56

M3460
48
7.2.4. C
APACITOR
S
PECIFICATIONS
At this point, a specific capacitor's characteristics can be used. It is best to use
the values that are listed at end of life for the capacitor to make sure that the
storage system is sized for the eventual degradation of performance over time.
The critical points to use are
𝐸𝑠𝑟
= Internal resistance at end of life, typically 150-200% of the initial
value.
𝐶
𝑒𝑜𝑙
= Capacitance at end of life
𝑉
𝐶𝑎𝑝𝑀𝑎𝑥
=
Maximum charge voltage for the capacitor. (A general rule is to use
95% of the value listed on the datasheet for a reasonable margin)
For our example, we will use the following values:
𝐸𝑠𝑟
=
.01Ω
𝐶
𝑒𝑜𝑙
= 132F
𝑉
𝐶𝑎𝑝𝑀𝑎𝑥
= 46V
7.2.5. M
INIMUM
S
ERIES
S
TRING
From
𝑉
𝑐𝑎𝑝𝑀𝑎𝑥
, we can calculate the minimum series string of capacitors that
will be required by the voltage rating. Below this voltage, there is an amount of
stored energy that cannot be used, and will remain in the capacitor bank. Since
there can be significant terminal voltage drop at the end of the discharge cycle
due to
𝐸𝑠𝑟
, it is best to use the terminal voltage of the capacitors (
𝑉
𝑇𝑒𝑟𝑚𝐸𝑛𝑑
)
for
this calculation.
V
TermEnd
= V
CapMax
− (I
peak
∗ Esr)
V
TermEnd
= 46V − (235A ∗ 0.01Ω) = 43.7V
For our example, the minimum number of caps in a series string would be:
n
series
=
V
end
V
TermEnd
=
320V
43.7V
= 8
7.2.6. A
VAILABLE
J
OULES
At this point, the available maximum joules for the string can be calculated.
V
Charge
= n
series
∗ V
CapMax
𝑉
𝐶ℎ𝑎𝑟𝑔𝑒
= 8 ∗ 46𝑉 = 368𝑉
The capacitor voltage at the end of the discharge
𝑉
𝐶𝑎𝑝𝐸𝑛𝑑
will be given by:
V
CapEnd
= V
end
− n
string
∗ I
peak
∗ Esr
V
CapEnd
= 320V + 8 ∗ 235A ∗ 0.01Ω = 338V