What to Look for in On-Board Charger Design

 The first stage of the OBC is the EMI filter. During this stage AC power is filtered to remove any unwanted noise from the typical AC sine wave that is expected.  This noise is caused by a wide variety of different things including electric motors, cellular networks, transformers, and other variables traveling throughout the grid.  In order to protect the other OBC stages, the EMI filter limits voltage and current spikes that can be harmful.  Most commonly these filters are made up of X & Y Safety Capacitors, Common Mode Chokes, & AC Harmonic Filter Capacitors.  Many household devices contain EMI filters internally similar to this design (typically single-phase variations) in order to protect the components and the user. Shown in the figure below is the waveform before and after the EMI Filter.  This can reduce the amount of filtering needed in further stages.

 The second stage of the OBC is the Power Factor Correction Stage. Power Factor is a calculated ratio comparing the power input to the device to the power output from the device. This is an important value in terms of efficiency, however it also can cause issues to the grid, components, and user if not done properly.  

Poor power factor can be caused by two things, displacement or distortion. In each case, the voltage and current values must be adjusted in order to maximize the operating power of the circuit.  This can be done actively or passively. 

Displacement occurs when the circuits voltage and current sine waves are out of phase of one another.  Another way to look at this would be to look at the power equation P=IV.  If the peak voltage value is not at the peak current value in time, then the circuit is not achieving the maximum power it could.  The graphs below demonstrate the differences in peak power when voltage and current are out of phase.

Distortion on the other hand is described as changes in the waveforms original shape.  This is often caused by non-linear circuits, rectifiers, etc. 

In order to correct both of these losses, a Power Factor Correction stage must be implemented.  Both active and passive methods can be used. In the case of the shown circuit, switches convert the incoming waveform into a DC waveform.  In order to smooth the switching behaviors of the device, both capacitors and inductors are used in conjunction of one another to buffer the abrupt switching pattern.  Shown below is an example waveform outputted by the switching performance of the PFC stage (waveform shown in red).  To create a smoother signal, passive components are used to create the black waveform.  Once completed this stage creates a ripple voltage that can be filtered out in a later, more refined stage.

In the DC-DC Converter stage, input DC power comes in from the Power Factor Correction Stage and then is stepped up or down depending on the battery system that will be implemented.  In the case of most EVs, this voltage is stepped into the range of 400-1000 VDC. The step up/down voltage occurs at the transformer in the center of the circuit.  The surrounding active devices work in conjunction to perform more power factor corrections caused by the transformer.  

In the fourth and final stage of the OBC, an output filter is placed on the end of the stages in order to finely filter out any remaining harmonics. This is down with a simple RLC circuit in a passive context.  Then the newly adjusted power charges the battery of the device, in our case a battery in an electric vehicle.  Specific components are used to handle the extreme power conditions that occur in an electric vehicle output filter.  Special power resistors, high voltage capacitors, and high power inductors are designed to be able to not only withstand the electrical characteristics, but the physical characteristics required in vehicle operation.  Vibration resistance, temperature and humidity requirements, and space constraints are just a few of the characteristics engineers use to select components on an OBC.

Electrolytic DC-Link Capacitors

Film DC-Link Capacitors

Film Snubber Capacitors

Ceramic DC-Link Capacitors

Power Resistors

Film Snubber Capacitors

Ceramic Snubber Capacitors

Isolation Transformers

Film Resonant Capacitors

Ceramic Resonant Capacitors

Electrolytic Output Filter Capacitor

Film Output Filter Capacitors

Power Resistors