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.

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.

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.

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

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.

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%.

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

Figure EE2.7: LTC3780 Simulated 12Vout with 9V<Vin<13V, 6.5ms run time.

From the simulation, the device is able to provide a regulated 12V output despite the drop in input voltage from 13V to 9V. Vout initially rises to slightly higher then 12V during the device’s startup period (known as soft start), but after the start up period, settles at 12V. From then on, the device maintains a regulated output voltage within +/-1% of 12V.

To show the versatility of the LTC3780, Vout was changed to .8V and then to 15V using R7 and R8. Both were then simulated with the results shown in figures 2.7 and 2.8.

Figure EE2.8: LTC3780 Simulated 15Vout with 9V<Vin<13V, 6.5ms run time.

Figure EE2.9: LTC3780 Simulated .8Vout with 9V<Vin<13V, 6.5ms run time.

As was with the 12V output, the output voltage settles on the programmed value after the startup sequence and remains to within +/-1% despite a change in input voltage from 13V to 9V.

To assist in part selection and to asses the performance of the regulator, the output ripple voltage across the output capacitor, the inductor ripple current through the inductor and the peak current load through the external MOSFETs was measured. This simulation was performed with a Vin of 12V, Vout of 12V, and a constant load of ~2A.

Figure EE2.10: LTC3780 output capacitor ripple voltge, Inductor ripple current

The output voltage ripple was found to be to be ~10mV, indicating that an output capacitor of 390uF with an ESR of ~50mΩ would be sufficient for proper regulator operation. The ripple current through the inductor was measured to be ~1.2A with a maximum current level of ~3A. Based on this data, an inductor of 4.7uH with a current rating of at least 3.5A can be used. The maximum current load through the MOSFETs was ~7A, indicating a power MOSFET with a current rating of 10A or greater could be used for all four external MOSFETs.

Finally, efficiency of the overall design was considered. Taken from the part’s datasheet, figure 2.11 shows estimated efficiency with 12Vin, 12Vout.

FigureEE2.11: LTC3780 Efficiency, 12Vin/12Vout.

From figure 2.11, the LTC3780 operates at efficiencies >90% at load currents greater then 200mA, which will be the typical operation conditions of the power supply.

Bill of Materials

Based on the platform power requirements presented in table 2.1, the power distribution board will have the following regulated outputs: two 5V rails with a maximum continuous current draw of 2A and two 12V rails, again with a maximum continuous current draw of 2A. One set of the 5V/12V rails will be used to power platform accessories; the other set will provide power to the small board computer. A block diagram of the power distribution board is shown in figure 2.6.

Figure EE2.12: Power Distribution Board – Block diagram

Based on this configuration, a preliminary BOM is shown in figure 2.7.

Figure EE2.13: Power distribution board - Preliminary Schematic.

The estimated parts cost for one power distribution board is ~80.00. Shipping costs are estimated to add an additional $25, bringing the cost of one power distribution board to $125. Additional funds will be required to have the printed circuit board manufactured. The estimated price for this, based on quotes from ______is approximately $300 for 5 boards.

Safety

Because the power supply will be connected to the main battery, precautions must be taken so the end user is not exposed to electrical shock. The power supply will also provide power to critical subsystems, and therefore must be robust and fully tested. It should also protect downstream electronics during an over current or over voltage fault. The power board as well as the enclosure it is to be placed in must be characterized so that it does not cause the power board to overheat.

To protect the user from electrical shock, the power board will be placed in a protective case. The enclosure will be electrically isolated from the power board and will have external connectors mounted to its end caps. These will allow the user to plug in the battery and external modules into the power board without having to open the case.

To make sure the power board will not overheat in its enclosure, the power output of the board can be estimated, and then an enclosure picked accordingly. From table 2.1, the total amount of power required by the small board computer and user accessories modules is ~50W. Assuming a worst cast efficiency of 80%, the power board will dissipate ~65W. Using this number, a fanless metal enclosure will be able to dissipate enough heat to prevent the power board from overheating. Actual hardware will have to be characterized to verify these results.

To make sure the power board is thoroughly tested, a test plan will be developed. A testing procedure, such as that outlined by the IECEE standards body, can be used to ensure the board is tested thoroughly.

Over current and over voltage protection will be handled by the DC-DC regulators.

TPS5420 Safety Features:

The TPS5420 Buck regulator uses the voltage drop across the upper MOSFET caused by the MOSFET’s Rdson to measure the current draw of the load. When the measured voltage drop exceeds a preset limit, it turns the upper MOSFET off for a cycle, preventing damage to components downstream. Over voltage monitors the voltage at the TPS5420’s sense pin – if it rises to 115% of its nominal value, the upper MOSFET is turned off. For both over current and over voltage protection schemes, regular operation resumes when the sensed voltage and current are within operating range.

RP100

LTC3780 Safety Features

The LTC3780 also employs over current and over voltage protection. Instead of monitoring the voltage drop across the upper MOSFETs, the LTC3780 throttles the current if a transient load (OC fault) causes the output voltage to drop to 70% of the nominal value. In an over voltage situation, switches B and D are kept on till the over voltage is over or the maximum negative current point is reached.

Risk Assessment

Contingency Plan

Pay off Dr. Walters and Jeff.