Output Modules

To simplify the design and potential troubleshooting, all outputs should be designed as a ‘module’ which has similar components and layouts. Each module must include current sensing and digital control to enable intelligent monitoring. Low side power switching to the

114 outputs has the potential to produce floating grounds. This leads to unpredictable voltage potentials as well as live circuits that could be hazardous or accidentally shorted to ground. Therefore, high side switching should be employed.

Protection circuitry should be included in the module so that devices without power protection can be safely connected. To assist with prototyping and development, it is desirable for each module to include manual control. Table 7.2 displays all the devices that are expected to be powered by the distribution board including their nominal and maximum current draw. An output module is designed for each, with components rated for the respective maximum power. The signals to and from each module should be routed to the embedded control circuit mounted above.

12V Rail

The 5 V and 12 V output modules use different high side switch ICs due to the minimum supply voltages. A BTS50055 Infineon IC is implemented on the 12 V rail, with 4.4 mΩ on- state resistance and current sensing capabilities. It features over-temperature and short circuit protection to prevent damaged to connected devices. A schematic for the 12 V output module is shown in Figure 7.6. The BTS50055’s enable line is active low, requiring an NMOS transistor to pull it to ground. The gain resistor on the current sense line converts current to voltage for an ADC.

115 5V Rail

The 5 V rail uses a Texas Instruments TPS2001C power distribution switch IC. It is one of the few affordable 5 V supply high side switch as capable of load currents up to 2 A. Intended for USB applications, it is rated for high current applications with short-circuit protection. It features a digital diagnostic fault line pin, which is asserted active low during over-current or over-temperature conditions. Figure 7.7 shows the 5 V rail output module. As per the TPS2001C datasheet [103] pull-up and pull-down resistors are placed on the FLT and EN lines.

Unlike the BTS50055, the TPS2001C does not include internal current sensing and therefore external circuitry is required. An issue for low side current sensing, placed on the ground return path, is its inability to detect a short-circuit which occurs before the sense resistor. Discrete current sensing is therefore placed on the high side of the module circuit.

The discrete current sense circuit uses an amplifier IC to measure a small voltage drop across a high precision series resistor, producing an output current proportional to the load current. As noise can affect the measurement accuracy, a bypass capacitor across the sense resistor filters it. To allow a microcontroller to read this measurement, a gain resistor is required to convert output current to voltage. Equations 7.3, 7.4 and 7.5, and The value of 0.004 is given in the current sense amplifier’s datasheet . Vsense is the voltage across Rsense and is selected as

it’s a small voltage drop with a 7.6 % error (as determined by the datasheet). Vout and Iout are

the maximum values produced by the current sense amplifier and Iload is the maximum load

current.

Table 7.4 and Table 7.5 are used to calculate Rsense and Rgain for each 5 V output module.

𝑰_𝒐𝒖𝒕= 𝟎. 𝟎𝟎𝟒 × 𝑽_{𝒔𝒆𝒏𝒔𝒆} (7. 3) 𝑹𝒔𝒆𝒏𝒔𝒆 = 𝑽_{𝒔𝒆𝒏𝒔𝒆} 𝑰𝒍𝒐𝒂𝒅 (7. 4) 𝑹_{𝒈𝒂𝒊𝒏}=𝑽𝒐𝒖𝒕 𝑰_𝒐𝒖𝒕 (7. 5)

116 The value of 0.004 is given in the current sense amplifier’s datasheet [104]. Vsense is the

voltage across Rsense and is selected as it’s a small voltage drop with a 7.6 % error (as

determined by the datasheet). Vout and Iout are the maximum values produced by the current

sense amplifier and Iload is the maximum load current.

Table 7.4: Current Sense Resistor Set Values Output Vsense (V) Iload (A) Vout (V)

Servo 0.03 2 4.5 On Board 0.03 2 4.5 LED 0.03 2 4.5 Sound 0.03 2 4.5 Aux5 0.03 2 4.5 ESCs 0.04 50 4.5 Network 0.03 0.75 4.5 12V DC 0.03 12.5 4.5 5V DC 0.03 10.25 4.5

Table 7.5: Current Sense Resistor Calculated Values Output Rsense (Ω) Rgain (Ω) Iout (A)

Servo 0.015 37500 0.00012 On Board 0.015 37500 0.00012 LED 0.015 37500 0.00012 Sound 0.015 37500 0.00012 Aux5 0.015 37500 0.00012 ESCs 0.0008 28125 0.00016 Network 0.06 37500 0.00012 12V DC 0.0023 37500 0.00012 5V DC 0.003 37500 0.00012 Shared Design

To ensure continuous overcurrent does not damage the high side switches, the power rails include series PCT fuses (F21). The same method of design outlined in Fuseology [105] was applied. If the output module load suddenly changes, a local current supply is required to prevent voltage droop. Input and output capacitors (C63, C62 and C64) decouple noise and

117

supply local current. Standard 100 µF and 100 pF capacitors are implemented to avoid parasitic inductance and cover a larger bandwidth. Sudden voltage spikes from inductive loads are suppressed by fly back diodes (D51). For diagnostic and testing, status LEDs (D52) are placed on the microcontroller’s switching signal and output lines. All signals to and from each output module are routed to a microcontroller through a central 40-pin board to board connector discussed in more detail in Section 6.2.3.

Figure 7.7: Schematic of a typical 5V output module

Digital monitoring and control of the distribution board is highly recommended to extend battery life; but manual circuit operation is possible. The manual override also ensures a particular output can be permanently switched on to simplify troubleshooting and fault identification. Two-pin jumpers (P29) pull the enable lines high, as seen above in Figure 7.7. Due to time restraints, digital control was not implemented and instead the jumpers manually enable the distribution board.

The distribution board is designed to power HADES’ existing components, but additional devices may be integrated in future. In anticipation of this, auxiliary output modules are

118 designed for the 5 V and 12 V rails. They are identical to the standard output modules in design, with 2 A of available current.