Aircraft electric power characteristics are critical in present-day designs. Design engineers of system blocks, ranging from central processing units to control surface actuators/motors, are increasingly concerned with voltage holdup and quality during transition times between external power aircraft electric system, emergency back-up power, or transient-induced voltage upsets.
One method used to prevent excessive voltage droop during transition of power sources is a hold-up capacitor placed on the Vcc line in question. Capacitor values are chosen based on the duration and magnitude of voltage drop-out, and calculations are provided for engineers to replicate when trying to determine the value and type of capacitor needed. The capacitor types best suited for such applications are super capacitors and tantalum or tantalum-polymer capacitors.
How to estimate capacitor values required for backup
Voltage droop can be calculated by using the following equation, assuming a constant current discharge method.
ΔV = ΔVESR + ΔVC = I * ESR + {I * Δt} / C
Hence, the capacitance value can be estimated using the equation:
C = I * Δt / (ΔV – I * ESR)
In this equation, ΔV (= VO-VW) is the total voltage droop designed for the circuit to operate at voltage VO and keep the voltage above the so-called cutoff voltage, VW, below which the circuit stops working. ΔVESR is the voltage droop due to equivalent series resistance (ESR); ΔVC is the voltage droop due to capacitance (C); I is the current required for hold-up or backup; and Δt is the duration, in seconds, for which this current is required for backup to be completed.
The energy required can be estimated using:
E = ½ C * {VO² – VW²}
As power requirements increase, applications demand higher instantaneous energy requirements and lower ESR. Larger value tantalum capacitors and high current, low ESR, pulse super capacitors are ideally suited for such applications.
Examples
There are several examples in which either Ta capacitors or low ESR pulse super capacitors may be utilized for such back-up applications, two of which are listed below.
Example 1: In an application with an operating voltage of 24 volts, and (for illustration purposes) I = 1 amp at V = 24 volts with a cutoff voltage is 9 V, and Δt (the amount of time required for backup)= 20 ms, C can be estimated using the aforementioned equation to be about 1 mF. Corresponding energy requirements are estimated to be about 275 mJ, and, for such an application, tantalum or tantalum-polymer capacitors would be an ideal solution.
Example 2: If the circuit is operating at 12 volts, and the circuit requirement for backup is I = 3 amps at V = 12 volts with a cutoff voltage of 8 volts, and Δt (the amount of time required for backup) = 100 ms, C can be estimated using the aforementioned equations to be about 75 mF, which translates to an energy requirement of 3 Joules. These requirements can be easily met by a non-polar, low ESR, 16 V rated, 120 mF, AVX BestCap pulse super capacitors, which require no balancing and are available in a 48 x 30 mm size part. Alternate solutions, like a module consisting of series or parallel combination of 2.5 / 2.7 V rated super capacitors, are offered by companies like Maxwell and Ness, but require balancing.
