Notes for Load Controller design P. McGill Input Bus Voltage Monitoring ---------------------------- * To monitor input voltage up to 40 V, use a divide-by-ten attenuator. A 90.9 k in series with a 10.0 k would work. This would have a 9 k output impedance, which is greater than the 2.5 k max recommended for the 16F876A A/D converter, but a cap in parallel with the 10.0 k would lower the output impedance. * At 40 V input, this attenuator would dissipate 40^2/100k = 16 mW, which is too much. A VN2222 in the ground leg should switch the attenuator off when not in use. Even better, use a Fairchild FDV301N, which is a small, SOT-23 FET with an Rds of 4 ohms and an Id of 220 mA. * This attenuator would have a conversion factor of 40 mV per bit for a 10-bit converter. * A command in software could calibrate the bus voltage reading. You would issue the command, containing a parameter specifying the actual bus input voltage. Software would then calculate and store a multiplication factor to force the displayed voltage to the known value. To prevent accidental recalibrations, this command would only work if a calibration jumper was installed on the board. Load Voltage Monitoring ---------------------------- * To monitor input voltage up to 20 V, use a divide-by-five attenuator. A 40.2 k in series with a 10.0 k would work. This would have a 8 k output impedance, which is greater than the 2.5 k max recommended for the 16F876A A/D converter, but a cap in parallel with the 10.0 k would lower the output impedance. * At 20 V input, this attenuator would dissipate 20^2/50k = 8 mW, which is small compared to the dissipation of the DC-DC converter. * This attenuator would have a conversion factor of 20 mV per bit for a 10-bit converter. Ground Fault Sensing -------------------- * Use a 4.096 V voltage reference for the PIC 10-bit A/D converters. The 16F876A can accept an external reference. We have to use an external reference because the DC-DC converter supplying the isolated PIC is only regulated to 15%, i.e. its output can be 4.25 to 5.75 V, but the voltages we're reading are absolute and not ratiometric. * Use a MAX4081F Bidirectional High-Side Current Sense amplifier (x5 gain) to sense the ground fault current. The 4081 can sense bidirectional currents, with the output voltage at zero current set by an external reference. We want that reference to be half of the A/D reference, Vref, which we can provide by connecting REF1A to Vref and REF1B to ground. * Limit ground fault current to 1 mA, in case there's a hard, low-impedance fault to seawater. We want the 4081 output to swing 2 V above or below Vref/2 (i.e. 2.048 V) at 1 mA of ground fault current. * If we use the 4081 with 5V/V gain, we need to drop 2/5 = 0.4 V at 1 mA, which requires a current sense resistor of 400 Ohms. * These values will yield a conversion factor of 2 uA per count: (1000 uA / 1 mA) x (1 mA / 0.4 V) x (1 V / 5 V) x (4.096 V / 1024 cnt) = (2 uA / cnt) This will be offset by +512 cnts by the 4081, so 0 cnts = -1024 uA 512 cnts = 0 uA 1024 cnts = +1024 uA * We must have series resistors on each MOSFET relay in case two relays get activated at the same time, and, say, the V line gets connected to the -12 V line. The resulting current can't destroy other circuits or endanger people, and the two resistors in series have to withstand 60 V without burning up. These resistors should limit the maximum current flow to, say, 8 mA and will have to be selected and stuffed according to the voltage on each monitored line. For instance, the 28V line will need a 28/0.008 = 3.5 kOhm resistor rated at 0.008 x 28 = 0.224 Watts (use 0.5 W) For the comms lines I am assuming a maximum voltage of 15v, to limit current to 8mA that gives us 15 / 0.008 = 1.875 kOhm. The power rating should be 0.008^2 x 1875 = 0.12 so use 1/4 Watt. * The external resistor for the LM4040 voltage reference is determined by the equation Rs = (Vs - Vz) / (Il + Iz), where Vz is the reference voltage (4.096), Vs is the source voltage (5v - 0.5v = 4.5v), Il is the current into the PIC, and Iz is the current through the LM4040. Although the maximum operating current, Iz, for the LM4040 is 15 mA, we don't want to use this much current from our isolated 5v supply, so we will spec the resistor for Iz at 500uA. For Il at 150 uA, that gives us Rs = (4.5v - 4.096v) / (150uA + 500uA) = 622 Ohms. Input Bus Current and Load Current Monitoring --------------------------------------------- * Use a MAX4080T Unidirectional High-Side Current Sense amplifier (x20 gain) to sense both the input bus current and the load current. For max current of 5 A, we want to output 4.096 V to the A/D. The required shunt resistor is ((4.096 V) / (20 V/V)) / 5 A = 0.041 Ohms. Use the closest standard value of 0.04 Ohms (Dale part WSL2512R0400FEA). At 5 A, it will drop 5 A x 0.04 Ohms = 0.2 V and dissipate (5 A)^2 x 0.04 Ohms = 1.0 W. Bus Current Switching --------------------- * The Seimens BTS432E2 High Side Switch can pass a minimum of 9 A of current, nominally 11 A. * At the board design current of 5 A, the Ron = 38 milliOhm resistance will dissipate 5A^2 x 0.038 Ohm = 0.95 W. The BTS432E2 case thermal resistance from the junction to free air is 75 K/W, so this should cause an almost 75 degC rise in temperature at 5 A. At this higher temperature, the Ron will be higher, but the package should stay below the 150 degC shutdown temperature. A small heat sink may be required. Connectors ---------- * We want the input connector to be able to plug into a mother board, or to accept a header with crimp pins for individual wires if the mother board isn't being used. Size ---- * The space on the "lower deck" in the MVC sphere is 2.875 inches high and 10 inches in diameter. We will have to fit 20 load controllers in this space. There may be room for a vertically mounted LC motherboard on the top deck if it contains less than 10 cards. * Two 3/4-inch standoffs were added to the "upper deck", so the space between the two decks is now about 2.875 + 1.5 = 4.375 inches