Input Capacitors

Ripple Voltage

Ripple Voltage is the difference in voltage between “peak” and “valley” from our DC power supply.

When the motor is at rest and there is no power being drawn, it has a resting voltage. When you load the DC supply, it sags in terms of the voltage, and this is going to be measured right at the input side - ripple voltage. The BLDC motor controller delivers the power to the motor at very high frequencies. What happens is that you get some power that is delivered from the battery pack to the motor controller, and it then turns a coil within the motor on and off in a split second. This process repeats itself numerous times within a one second time span. During this time, the battery is being loaded and unloaded at a very high rate. This is where we see ripple voltage occurring.

The maximum ripple voltage that we have within our controller is 10% of the nominal voltage.

When the motor pulls current, it causes the DC Bus Voltage to drop.

Large amounts of current flowing back into the power supply can cause voltage spikes at the output terminals of the motor systems as well as on the power supply. The excess current can be produced by the motor acting as a generator or by the stored energy on the motor flowing back to the supply during fast decay. This occurs because the kinetic energy that the motor possesses when it is spinning has to dissipate - which occurs by energy being transferred back into the circuit. This voltage that is output will cause noise to the input. The goal is to minimize the voltage spikes by controlling and absorbing all of the excess energy.

Ripple Current

When driving the motor, the H-bridge alternates between drawing current to the different coils of the BLDC motor. This causes the current to fluctuate, thus causing a current ripple.

When driving the motor, the H-bridge alternates between drawing current from the power source and DC link capacitor, and cycling the current through the motor. This causes a current ripple.

Structure of a capacitor

A capacitor is a device that can store electrical energy. Taken simply, a capacitor consists of two conductive plates separated by an insulating material called the dielectric. The main structure is shown here:

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The capacitance of the capacitor describes how much charge it stores when there is a given potential difference across it. If each of the conductive plates has an area A and they are separated by a distance d, then the capacitance C is given by:

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where e is the permittivity, which is a property of the dielectric substance.

Equivalent Series Resistance (ESR)

The Equivalent Series Resistance is the internal resistance that appears in series with the capacitance of the device.  Almost all capacitors exhibit this property at varying degrees depending on the construction, dielectric materials, quality, and reliability of the capacitor. ESR is resistance from a combination of energy loss mechanisms under specific operating conditions.

Some energy losses within a capacitor can be attributed to the conductors while others involve the dielectric material. These losses vary mainly depending on voltage and temperature. The most common energy loss mechanisms include dielectric losses, ferroelectric losses, dielectric conduction losses, interfacial polarization, partial discharge losses, ohmic resistance losses, sparking between conductors, electromechanical losses, and eddy current losses.

Together with its capacitance value, ESR defines a time constant for charging and discharging of the capacitor and thus how quickly the capacitor react on voltage/current changes/ripple. In practical smoothing applications capacitor technologies are combined in parallel, where high capacitance parts are taking care of bulk filtering (aluminum or tantalum capacitors) and small MLCC capacitors with low ESR are taking care of fast, high frequency spikes.

Electrolytic Capacitors

In electrolytic capacitors, the two conductive plates are separated by an electrolyte and a metal oxide layer. Typically, aluminum, tantalum, or niobium acts as the conductive material. The dielectric in these capacitors is the oxide layer that forms on these metals.

Electrolytic capacitors have capacities from 100pF, and they are polar: a specific contact must be connected to the plus terminal. If they are connected with the wrong polarity, they may become very hot and even explode.

Electrolytic capacitors have high equivalent series resistance (ESR) making power loss high and transient response too poor for use with tough load-response requirements. However, electrolytic capacitors have a stable capacitance with high bias voltage and are inexpensive. If the ESR is too high, the current flow through the circuit would be too small. The figure below shows how the ESR (impedance) of a capacitor changes with resistance. Each curve shows different capacitance values.

Note that for each type of capacitor, there is a frequency at which the impedance is at a minimum. This frequency is the resonant frequency of the capacitor. Note that as the capacitance becomes larger, the resonant frequency gets smaller.

Ceramic Capacitors

Ceramic capacitors have very low ESR, but capacitance is reduced greatly with high bias voltage and can be expensive for large values.

In ceramic capacitors, the two conductive plates are separated by a ceramic material (which is the dielectric). The most frequently used type of ceramic capacitors are multi-layer chip capacitors (MLCC). In MLCCs, there are a number of conductive plates and a ceramic material sandwiched between each pair of plates. Effectively, they work as though they are many small capacitors in parallel, which gives a large combined capacitance.

Ceramic capacitors exhibit microphony: an effect where mechanical vibrations lead to electrical noise in circuits.

Ceramic capacitors are smaller in size and usually have a capacity of up to 1 uF; they do not care which of the pins is connected to the plus terminal and which to the minus.

Input Capacitor Selection

A bulk capacitor can ensure that the supply voltage and/or motor voltage does not drop too much incase of current spikes/surges.

We need to know what the inrush current/surge current is. Once we know the voltage drop we need, the capacitor size can be calculated by the formula: