2) Power Distribution Board:



2) Power Distribution Board:

Overview

From an electrical standpoint, one of the main jobs of the robotic platform is the distribution of power from the platform’s battery sources to its many subsystems. These subsystems include the motor control module, motor module, data acquisition module, small board computer, and user accessories. Each subsystem has unique power needs and therefore a robust power distribution system must be developed to meet these unique needs. The power needs for the platform are summarized in table 2.1. The table also specifies which subsystems require a regulated (+/-5%) voltage input to operate correctly.

|Subsystem |Voltage |Current |Total Power |Regulated Source |

| |(V) |(A) |(W) |Needed? |

|Motor Control |12 |1.0 |12 |No |

|Motor Module |12 |2.0 |24 |No |

| |24 |4.0 |96 |No |

|Small Board Computer |12 |0.5 |6 |Yes |

| |5 |1.0 |5 |Yes |

|DAQ |12 |1.5 |18 |No |

|Accessories |5 |2.0 |10 |Yes |

| |12 |2.0 |24 |Yes |

Table EE2.1: Platform Power Needs, two motor module configuration

The platform is to have two sources of power; a 12V battery source for the electrical systems and a 24V batter source for the motor modules. Based on work done by the motor module teams (P07201 and P07202), a 12V sealed lead acid battery from B.B. Battery Co. (part number BP28-12) was chosen as the power source for the platform. Two of these batteries are to be stacked in series to provide the 24V needed to power the drive motors while one battery will be used to provide 12V to the electronics. Figure 2.1 shows a top level view of the platform’s power distribution needs.

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Figure EE2.1: Top level platform power needs

Design

For those modules requiring an unregulated input voltage, their supply voltage can be taken directly off the battery, as shown in figure 2.1. For the sources requiring a regulated input to function correctly, their supply voltage cannot be taken directly off the battery. This is because battery voltage is actually a function of current draw and remaining battery charge, as shown in figure 2.2.

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Figure EE2.2: Battery voltage as a function of current draw and battery charge.

From figure 2.2, at full charge, the battery voltage is actually slightly greater then 12V, depending on load current. As the battery becomes depleted, its voltage drops to a value below 12V. In order to provide the regulated input voltage required by the small board computer and user accessories modules, a DC-DC regulator must therefore be employed. For those sources requiring a regulated 5V rail, a simple Buck DC-DC regulator which takes 12Vin and outputs 5V regulated can be used. Based on work done by team P07202, a TPS5420 DC-DC regulator was chosen for this application. This regulator has many integrated features, including built-in power MOSFETS, and therefore requires only an external power inductor and a few passive components to operate. The application circuit for this part is shown in figure 2.3

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Figure 2.3: TPS5420 Application Circuit

For those subsystems requiring a regulated 12V rail, a Buck DC-DC regulator alone cannot be used. This is because a buck regulator takes an input voltage Vin and provides a regulated output Vout based on the formula.

(1)

In equation (1), D refers to the duty cycle of the switching MOSFETs. From the TPS5420 datasheet, D can be a maximum of .87. This brings the maximum regulated output to approximately .87*12V or 10.4V. From equation (1), a Buck DC-DC regulator is limited to providing a regulated output below its input voltage.

Another type of DC-DC converter, called a Boost Regulator, is able to provide a regulated Vout using the formula

(2)

Again, D is the duty cycle of the switching MOSFETs. Since D is once again limited to less then 1, a boost regulator cannot provide a regulated output voltage equal to its input voltage. From equation 2, a Boost regulator is able to provide a regulated output voltage greater then its input voltage.

From the preceding discussion, neither a Buck nor Boost regulator by themselves can be used to provide a regulated 12Vout from a 12V battery source. In researching this problem, two solutions emerged.

RP100 Solution

The first involves breaking the regulation needed for the 12Vout rail into two stages. The first stage uses a Boost regulator to boost the battery voltage up to approximately 15V. The second stage takes the boosted 15V, and uses a Buck regulator to knock that value down to a regulated 12V. 15V was chosen as an intermediate stage voltage for efficiency reasons; the smaller the difference between input and output voltage, the more efficiently a Boost or Buck regulator can operate at. Figure 2.2 shows a break down of the two stage 12V regulation idea.

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Figure EE2.4: Two stage 12V regulation

James’ part number and description to go here……

RP10 Solution

The second idea is to use one power management IC which is able to take 12Vin and output 12V, alternating between Buck and Boost modes as needed. After searching the major power management IC suppliers, the LTC3780 from Linear Technology meets these needs. This device uses proprietary technology to automatically switch between Buck, Boost, or Buck-Boost modes to provide a regulated output at, below, or above the battery voltage level at efficiencies up to 98%.

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Figure EE2.5: LTC3780 Application Schematic

The ability to operate as a Buck, Boost, or Buck-Boost regulator gives this part extraordinary versatility – it will allow the user to program the output voltage of this part anywhere from .8V up to 15V.

Simulation

RP100 Solution

RP10 Solution

Linear Technologies provides a way to simulate operation of the LTC3780 using a version of SPICE they call SWCad III. Using this simulation program, and the schematic shown in figure 2.5, the part was simulated to verify it would meet our regulation needs.

During simulation, the input voltage was varied between 13V (fully charged battery) to 9V (depleted battery). The output voltage for the part was set at 12V using resistors R7 and R8. A 5.5Ω resistive load was also placed on the device to simulate a worst case load of ~2A. All other components were picked based on Linear Technology’s application circuit. The results of the simulation are shown in figure 2.6.

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Figure EE2.6: SWCad III Simulation Schematic

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Figure EE2.7: LTC3780 Simulated 12Vout with 9V ................
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