Power controller

3D model of the Power controller from Altium Designer

Supply voltage

12–24 V DC

Fused circuits

9 (5 × 24 V outputs, fan outputs, digital outputs, 12 V circuit, internal electronics)

24 V outputs

5

24 V fan outputs

2 (shared fuse)

12 V outputs

2

Digital outputs (shared fuse)

2 × 12 V switched (5 A), 2 × 24 V switched (330 mA), 2 × PWM

Communication interface

CAN FD

Maximum data rate

1 Mbps

Table of contents

• • Table of contents
  • Pinout
    • Connector pinout
    • Microcontroller pinout
  • Distribution circuit
    • Green LED
    • Current sense
      • Shunt resistor
      • Amplifier
      • Filter
    • Power amplifier
    • Voltage measurement
    • 12 V supply (did not work)
    • 5 V, 3.3 V and CAN bus
  • Control logic
    • Sourced from the microcontroller
    • High-side switching
      • Buzzer and brake light
      • Pump
  • Known issues
  • Features to add
  • BOM (bill of materials)
• AR27 Concept
  • Changes
  • Supply table
    • Standard supply table
    • Autonomous supply table
  • Relevant rules

Pinout

Connector pinout

Microcontroller pinout

The power controller uses a microcontroller to control the pumps, fans, buzzer and brake light. It also handles current monitoring and the LED indicators. The microcontroller used in this system is the STM32G431CBT6.

Distribution circuit

The board has 9 distribution circuits, one for each fused output group. All 9 circuits use the same design, and the component reference designators follow the circuit number (circuit 1 uses R101–R104, C101, D101 and F101, circuit 2 uses R201–R204, and so on). Each circuit consists of:

• A fuse in a Littelfuse nano fuse holder, so the fuse can be replaced without soldering
• A 2 mΩ shunt resistor for current measurement
• A current sense amplifier channel and a low-pass filter to the microcontroller ADC
• An RGB LED, where the green channel shows that the fuse is intact and the red channel is controlled by the microcontroller

Green LED

The green LED indicates whether the fuse is intact. It is connected from the output side of the fuse to ground through a 4.7 kΩ resistor. When the fuse is intact, the LED lights up. If the fuse blows, the LED loses its supply and turns off. This makes it easy to find a blown fuse by looking at the board.

The LED is powered directly from the output voltage, so the current depends on the supply. At 24 V the current is roughly 4–5 mA, and at 12 V it is about 2 mA. This turned out to be brighter than necessary.

Current sense

The current in each circuit is measured by passing it through a shunt resistor with a very low resistance and measuring the voltage drop across it. This small voltage is amplified, filtered and read by the microcontroller's ADC.

Shunt resistor

A 2 mΩ shunt resistor is placed in series with the load. The voltage drop across it is 2 mV per ampere of current (0.002 V/A).

The shunt is a Vishay WSLF2512 metal strip resistor with 1 % tolerance and a 5 W rating. The low resistance keeps the losses small: at 8 A the voltage drop is only 16 mV and the power loss is about 0.13 W.

Amplifier

The amplifier is a quad-channel current sense amplifier with a gain of 200 V/V. When the measured voltage drop passes through the amplifier, the output is therefore 0.4 V per ampere.

The board uses three INA4180A4 amplifiers (U3, U4, U5) from Texas Instruments. This gives 12 channels, of which 9 are used. The INA4180 measures on the high side and accepts a common-mode voltage of up to 26 V, so it can measure directly on the 24 V outputs.

With the 3.3 V ADC on the microcontroller, the maximum measurable current is about 8 A (3.3 V / 0.4 V/A ≈ 8.25 A). Above this, the amplifier output saturates.

Filter

The filter is a low-pass filter with a cutoff frequency of 1.6 kHz.

It is an RC filter made of a 10 kΩ resistor and a 10 nF capacitor between the amplifier output and the ADC pin: f_c = 1 / (2π × 10 kΩ × 10 nF) ≈ 1.6 kHz. The filter removes high-frequency noise, for example from PWM-controlled fans and switching loads, before the signal is sampled. The capacitor also acts as a charge reservoir for the ADC's sampling capacitor.

Power amplifier

Voltage measurement

The supply voltage is measured so the microcontroller can monitor the LV battery voltage. The signal is read on pin PB2 (V-sense).

A voltage divider with a ratio of 1:11 is used, so 24 V corresponds to 2.182 V on the microcontroller pin.

The input voltage is calculated in firmware as V_in = V_pin × 11. With a 3.3 V ADC, the highest voltage that can be measured is about 36 V, which gives good margin above the 24 V supply.

12 V supply (did not work)

The 12 V supply is a synchronous buck converter that steps the 24 V supply down to 12 V for the pumps, inverter and data logger. It is built around an LM5148 buck controller (U801) from Texas Instruments, which drives two external N-channel power MOSFETs (IAUCN04S7L028, Q901 and Q902) as the high-side and low-side switches. The output inductor is a 0.65 µH Coilcraft XAL7070 (L901), and a 3 mΩ resistor (R910) is used for current sensing in the controller.

The buck converter overheated under load and made a whining sound.

A Mateksys PM20S-2 power module was used as a quick fix to provide 12 V for the competition.

5 V, 3.3 V and CAN bus

Uses the standard Align template.

• 5 V: An AP64060 synchronous buck converter (U6) steps the input voltage down to 5 V. It accepts 4.5–40 V in and delivers up to 0.6 A.
• 3.3 V: An AP2112K-3.3 LDO regulator (U7) generates 3.3 V for the microcontroller and the current sense amplifiers.
• CAN bus: An NCV7344 CAN FD transceiver (U2) connects the microcontroller to the CAN bus. The transceiver supports up to 5 Mbps, and the bus runs at 1 Mbps. A common-mode choke (L1) reduces noise on the bus lines, and a TVS diode (D1) protects them against ESD and voltage spikes.

The whole board is protected by a 400 W power TVS diode (D2) on the input, which clamps voltage spikes on the supply.

Control logic

Sourced from the microcontroller

The programmable red light is part of the same RGB LED as the green fuse indicator. It is controlled by the firmware and can be used to show the status of each circuit, for example an overcurrent or an output that is switched off.

Current is sourced directly from the microcontroller through a 350 Ω series resistor. When the pin is driven high (3.3 V), current flows through the resistor and the red LED to ground. The STM32 pins can supply up to about 20 mA each, so no transistor is needed. This gives an LED current of roughly 3–4 mA, which made the LEDs too bright. The resistor value should be increased on the next version.

High-side switching

The buzzer, brake light and pumps are switched on the high side. The load's negative side is permanently connected to ground, and the controller switches the positive supply. This means a short from the output wire to chassis cannot turn the load on, and the load is completely unpowered when it is off.

Each output uses a P-channel MOSFET as the main switch between the supply and the load. A P-channel MOSFET is used because it can switch the high side without a charge pump or gate driver. It turns on when its gate is pulled below its source (the supply voltage).

The microcontroller runs on 3.3 V and cannot pull the gate of the P-channel MOSFET far enough on its own. A small N-channel MOSFET is therefore used as a level shifter:

• Output off: The microcontroller pin is low, so the N-channel MOSFET is off. A pull-up resistor holds the P-channel gate at the supply voltage, so V_GS = 0 V and the P-channel MOSFET is off.
• Output on: The microcontroller pin goes high (3.3 V) and turns on the N-channel MOSFET. This pulls the P-channel gate towards ground, V_GS becomes negative and the P-channel MOSFET turns on.

A 1 kΩ series resistor on the N-channel gate limits the current from the microcontroller pin, and a 10 kΩ pull-down keeps the output off while the microcontroller is starting up or in reset.

Buzzer and brake light

The buzzer and brake light are switched on the high side using an N-channel MOSFET (BSS123NH6327XTSA1) that drives a P-channel MOSFET (BSS83PH6327XTSA1).

These outputs run on 24 V. The BSS83P is rated for -60 V and -0.33 A, which limits each output to 330 mA. This is enough for the buzzer and an LED brake light. The maximum gate-source voltage of the BSS83P is ±20 V, so the gate must not be pulled all the way to ground from 24 V. The two 10 kΩ resistors on the P-channel gate form a voltage divider that limits V_GS to about half the supply (≈ -12 V).

Pump

The N-channel MOSFET (BSS123NH6327XTSA1) is the same as for the buzzer and brake light, but the P-channel MOSFET (BSC084P03NS3GATMA1) has a much higher current rating.

The pumps run on 12 V and draw considerably more current. The BSC084P03 is a power MOSFET rated for -30 V and -78.6 A, with an on-resistance of about 8.4 mΩ. At 5 A this gives a loss of only about 0.2 W, so no heatsink is needed. Since the supply is 12 V, the gate can be pulled fully to ground without exceeding the ±20 V gate-source rating.

Known issues

• The 12 V supply did not work.
• The current measurement has not been tested.

Features to add

• Add MOSFETs to cut power if the LV battery voltage is too low, to save power.
• Add power distribution for the autonomous system.

BOM (bill of materials)

AR27 Concept

This section describes the concept for AR27.

Changes

• Connector
  • Change main connector
• Circuits
  • More dedicated supply circuits
  • Keep the same current sensing
  • Increase the signal LED resistor on the PCB (the LEDs were too bright)
• Microcontroller
  • Needs more inputs
• Digital signals
  • Increase the green signal LED resistor on the PCB (the LEDs were too bright)
• 12 V supply
  • Three separate 12 V supplies:
    • Pump 1
    • Pump 2
    • Inverter / other 12 V loads
• Autonomous
  • Supplied by ASMS
  • Potentially control ASSI
  • ASMS state detection

Supply table

Standard supply table

Autonomous supply table

Relevant rules