Component Layout

A Printed Circuit Board (PCB) design was created using Altium Designer software and professionally printed for the custom built motor controllers. The PCB design uses two separate boards which are stacked on top of one another. All of the power electronics of the inverter and associated

components allocated to one board, referred to as the “power PCB”. All other components are allocated to the second board, referred to as the “control PCB”. This scheme has numerous

advantages over a single PCB design. The overall footprint of the motor controller is reduced in size. It allows for separation of the small signal control electronics from the power electronics. It also allows for future customisation if a motor controller with different voltage and current ratings is desired; only a new power PCB is required as the control PCB can be used as a “universal” control board.

In general terms, the individual components are grouped into functional blocks. For example, all components associated with the 12V switching regulator power supply are physically positioned in a cluster with close proximity to one another. The cluster is then strategically positioned near the other functional which require input from and/or output to the power supply.

The PCBs are sized to be large enough for the components that are designated to each PCB. Additionally, the width of the PCB was dictated by the width of the heat sink. It was desirable to arrange the power MOSFETs so that they are spread evenly across the heat sink therefore the width of the PCB was selected to allow for this. A two layer PCB was selected and components were allowed to be positioned on both sides of the PCB. See Appendix C for the Altium Designer schematic diagrams and PCB design.

4.1.12.1.Power PCB

The power PCB is made up of the semiconductor switches and associated components which make up the three phase inverter. This includes the power MOSFETs, gate resistor networks, shunt resistor current sensors, bus capacitors, power connections for the battery and motor, and header pins for the interface with the control PCB.

A major concern with using surface mount components for the power electronics was the ability to remove heat from the components. The D2-Pak MOSFETs have the tab as the ‘drain’ pin which is soldered directly to the copper on the PCB. The majority of heat is conducted through this interface therefore a large amount of heat is transferred to the PCB traces. The layout was designed around optimising heat removal from the MOSFETs through these PCB traces. The drain pins of the high side switches have a common net, Vbatt which is connected to the positive terminal of the battery pack.

The low side switch drain pins have a separate net for each of the three phases. These nets are the phase outputs of the inverter and are connected to the motor terminals. This gives four nets which conduct heat directly to PCB traces from the MOSFETs.

All six MOSFETS are positioned on the top side of the PCB and a heat sink is positioned directly below. The four nets that require heat removal are brought through from the top layer to the bottom layer of the PCB through an array of ‘vias’ for each MOSFET. As the Vbatt net is the primary

means of heat removal for all three high side MOSFETs, it is allocated half of the area of the heat sink. A ‘power plane’ for this net is positioned directly below the high side switches for maximum heat transfer from the MOSFET drain pin through to the bottom layer then to the heat sink.

72 The remaining three nets are the primary means of heat removal for each of the three low side switches and therefore are allocated one sixth of the heat sink area each. A power plane for each net is positioned directly below each of the low side switches. The three nets are brought through from the top layer to the bottom layer through an array of vias to conduct heat from the MOSFETs. A thermal interface material is placed between the heat sink and the PCB. This provides the function of electrically isolating the power planes from the heat sink to avoid creating short circuits through the aluminium heat sink material. The thermal interface material also helps to increase the contact area between the PCB and heat sink as it is a flexible material which moulds to the shape of the surfaces to fill in small imperfections in the surfaces.

A second heat sink is placed on top of the MOSFET cases to allow for heat removal through the MOSFET bodies. A thermal interface material is placed in between for the purpose of increasing the contact area. Minor differences in MOSFET height can cause some of the MOSFETs to not have proper contact with the heat sink. The thermal interface material helps to minimise this issue. The two heat sinks are held in place with two M3 bolts between them, through the PCB. The MOSFETs and PCB are effectively clamped between the two heat sinks.

Figure 4.5 shows how the functional blocks are arranged on the power PCB.

Figure 4.5 - Functional block layout of the Power PCB top side (left) and bottom side (right).

Each of the six gate resistor networks are placed as close as possible to the corresponding MOSFET gate pins. The shunt resistors are placed directly in line with and in close proximity to the low side switch ‘source pins’. These resistors then connect to the power ground plane on the top side of the power PCB. The bus capacitors are positioned such that they are outside of the area covered by the heat sinks while still being in as close proximity as possible to the MOSFETs.

Custom made footprints were used for soldering power carrying wires directly to the top side of the PCB in a surface mount style to keep the bottom layer free of any protrusions. This leaves an empty surface on the bottom side for the heat sink to sit flat against the PCB’s heat conducting power planes. The MOSFETs were spaced a sufficient distance apart to allow for the phase wires to pass in between, and underneath the top side heat sink.

73 A photograph of the assembled power PCB is given in Figure 4.6.

Figure 4.6 - Photograph of the assembled power PCB. 4.1.12.2.Control PCB

The control PCB is made up of the remaining components required for the motor controller. Namely the microcontroller, power supplies, gate drivers, and all associated discrete components. The dimensions of this PCB were selected to allow for the control board to fit in the area above the power PCB, between the top side heat sink and the edge of the board. Figure 4.7 shows how the functional blocks are arranged on the control PCB.

Figure 4.7 - Functional block layout of the Control PCB top side (left) and bottom side (right).

The layout of the control PCB places functional blocks in logical order for the interconnections between one another. A power plane was placed on the bottom layer of the control PCB connected to the “power ground” net. This plane was extended to cover some of the top layer over the area occupied by the linear regulator power supply for heat sinking purposes. A second power plane was placed over the remaining area on the top layer of the PCB connected to the “digital Ground” net. The electrical connections (traces) between functional blocks are generally routed with vertical running traces on the top layer of the PCB and horizontal running traces on the bottom layer. This helped to minimise the number of times that a trace needed to jump between layers to get to its destination. If no such systematic approach is taken, the final traces may end up needing to jump

74 between layers many times to reach their destination. With this approach, a single trace should need to jump between layers no more than a few times. Photographs of the assembled control PCB top and bottom sides are given in Figure 4.8 below.

Figure 4.8 - Photograph of the assembled control PCB top side (left) and bottom side (right).