This form is intended to derive low level hardware & software specification from the Validated
High Level Requirements Capture Form [1]requirements_capture.md and
[2]system_optioneering.md
Its scope can be adapted to suit projects of varying complexity
System Specifications for Hardware & Software are derived from and traceable back to the High level Requirements. At this stage optioneering should be complete, and the design direction finalised.
Hardware specification should outline specific hardware devices, circuit design and hardware archetectures chosen to meet high level requirements.
Hardware Specification may contain the following subsections:
- Hardware Architecture
- Major Components - Can be specific or requirements set out for comparason of specific components
- Circuit Design
- Other
[HL.1, HL.2, HL.3]
The hardware will comprise of a single PCB to home the 2 DC/DC converter modules. These require local fan cooling for which +12v power and mounting holes will be provided.
[HL.7]
24v Power input will be via XT60 Connector mounted directly on PCB.
[HL.5]
MCU will be AtMega328p integrated to PCB with ESP32-WROOM-32UE as a WiFi tranciever. MCU will take ADC readings from 2 Allegro ACS712 current sensing modules, one between the DC/DC module and the 12v bus, the other between the 2nd DC/DC module and the 5v bus. Each MCU
[HL.5, HL.6]
A voltage divider will be used with an additional ADC input to monitor the voltage of the 12v bus.
- R1: 100k
- R2: 56k
This will allow measurements from 0v to ~14v to be measured by the microcontroller ADC, while limiting current draw to a few microamps. Input to the MCU will be protected by a 5.1v Zener diode, incase of voltage spikes greater than can be mitigated by the voltage divider.
[HL.9, HL.4, HL.13]
The 5v Bus will be distributed to 5 USB outputs via individual high side MOSFET switches for each channel, these will be connected source to 5v bus, drain to USB output, and gate to the logic circuit used to trigger a power shutoff condition.
Solder bridges will be provided on the PCB to bypass these MOSFETs, in the case they are not required.
[HL.4, HL.13]
Logic circuit to control power to each 5v USB output will follow the following logic, if:
A = microcontroller digital pin (1 per channel, logic level HIGH = channel deactivated)
B = Voltage Monitoring circuit (1 per +5v Bus, logic level HIGH = off nominal voltage detected)
C = Output to MOSFET gate (1 per channel, logic level HIGH = MOSFET OFF state)
IF A OR B == HIGH: C = HIGH.
[HL.13]
The voltage protection circuit will comprise of 2 TL431 Voltage reference ICs set as a window comparator. This circuit will output logic level LOW in the case +5V_BUS is pulled off nominal. (4.6v - 5.5v).
[HL.7, HL.8]
12v bus power will be distributed to 5 XT30 connectors, mounted directly on the PCB.
[HL.10]
Reverse voltage protection will be acheived via a P channel MOSFET1 at +Vcc in.
[HL.11]
The system will be protected from overcurrent conditions by a 20A fuse between Vcc in+ and 24v bus.
[HL.12]
PCB dimensions will be 100x120mm.
[HL.14]
LM3916 Dot graph drive IC will be used on each voltage bus, 5v, 12v and 24v. Some experimentation may be required to select the correct circuit layout & resistor selection for each application. 24v bus will require additional voltage divider,
R1: 100k
R2: 56k
to scale voltage to correct input for LED driver. 24Vin =~ 8.6Vout
Optional:
The PCB will contain footprints to allow 12v outputs to be switched via additional MOSFETs, as well as solder bridges to enable the PCB to be used without. [Dependent on overall size & space left after all other requirements have been met]
Non-Isolated DC-DC Converter, 3.3 → 15V dc Output, 20A
| Attribute | Value | Notes |
|---|---|---|
| Part Number: | I6A4W020A033V-001-R | |
| Supplier: | RS Components | |
| Vin: | 9 - 40v | |
| Vout: | 3.3 - 24v | |
| Power: | 250W | |
| Price: | £44.66 | |
| URL: | https://uk.rs-online.com/web/p/non-isolated-dc-dc-converters/1813289 | |
| Notes: | - | |
| Requirements Met: | (HL.1, HL.2, HL.3) |
Use: High side power switch
- Switching of 5v USB power from bus to individual outputs
- Switching of 12v power from bus to individual outputs (optional feature)
- MOSFET is ON when gate is @ 0v, OFF when gate is driven to VDD
Component Requirements:
| Attribute | Value | Notes |
|---|---|---|
| Vds | > (-)21V | Drain/Source Breakdown Voltage = Operating Voltage + 70% |
| Id | > (-)6A | Max Continuous Drain Current > Stall Current of Motor |
| Vgs | ~ -4.5 | Gate - Source Threshold Voltage[^Vgs] |
| Rds(on) | <2 ohm | Static Drain-to-Source-ON-Resistance2 @ Vgs |
In the case the requirements for a component are known, however the specific part is unknown, it would be best to use a spreadsheet to weigh up alternative options.
IRF5305PBF P Channel MOSFET
| Attribute | Value | Suitable | Notes |
|---|---|---|---|
| Part Number: | IRF5305PBF | ||
| Supplier: | Farnell | [x] | |
| Vds | -55V | [x] | |
| Id | -31A | [x] | |
| Rds(on) @ -10v | 0.06 | [x] | |
| Price: | £2.088 | [x] | |
| URL: | IRF5305PBF - Farnell | ||
| Availability | In Stock | [x] | |
| Notes: | |||
| Meets Requirements: | [x] |
Use: Reverse voltage protection
- MOSFET is ON when gate is @ 0v, OFF when gate is driven to VDD
Component Requirements:
| Attribute | Value | Notes |
|---|---|---|
| Vds | > (-)21V | Drain/Source Breakdown Voltage = Operating Voltage + 70% |
| Id | > (-)6A | Max Continuous Drain Current > Stall Current of Motor |
| Vgs | ~ -4.5 | Gate - Source Threshold Voltage[^Vgs] |
| Rds(on) | <2 ohm | Static Drain-to-Source-ON-Resistance2 @ Vgs |
In the case the requirements for a component are known, however the specific part is unknown, it would be best to use a spreadsheet to weigh up alternative options.
IRF5305PBF P Channel MOSFET
| Attribute | Value | Suitable | Notes |
|---|---|---|---|
| Part Number: | IRF5305PBF | ||
| Supplier: | Farnell | [x] | |
| Vds | -55V | [x] | |
| Id | -31A | [x] | |
| Rds(on) @ -10v | 0.06 | [x] | |
| Price: | £2.088 | [x] | |
| URL: | IRF5305PBF - Farnell | ||
| Availability | In Stock | [x] | |
| Notes: | DO NOT USE FOR REVERSE POWER PROTECTION ON 24V BUS. Vgs Max is 20v | ||
| Meets Requirements: | [x] |
Final Circuit Design, Schematics & Justifications for design
Specify the software requirements, functions, frameworks and tools required to meet the high level requirements
| Tool | Purpose | Justification |
|---|---|---|
| Arduino IDE | Software Development | Speed & ease of development for Arduino IoT functions |
| C++ | Programming Language | Native to Arduino environment |
| InfluxDB | Remote storage of power consumption data | Ease of setup & ease of posting data from remote devices |
| Grafana | Graphic display of power consumption via web interface | off the shelf solution that can integrate control methods for sending HTTP requests back to controller |
- ADC Samples of current sensor will be taken every 250mS
- ADC samples of 12v bus voltage will be taken every 250mS
- Power Channel MOSFETS are "Active Low" Therefore channels will be turned off driven by a HIGH pulse from microcontroller.
- Seperate API for each power channel "on", "off" and "restart",
When to review?
System Specification should undertake a review process, to ensure the design meets the clients needs before moving to Fabrication
Info On Requirements Matrix & Link to: Requirements Matrix Document
- Rugged Circuits: 10 Ways to Destroy an Arduino
- High Side vs Low Side Switch
- Important Stuff: MOSFET Specs You Need to Know
- Using the Atmel ATmega328P Analog to Digital Conversion Module
VGS & Rds Explanation
Assuming That
| Attribute | Value | Max Current Flow |
|---|---|---|
| Source Voltage | 5v | |
| Rds(on) @ Vgs -4.5v | 0.0133 ohm | 5/0.0133 = 376A |
| Rds(on) @ Vgs -10v | 0.0083 ohm | 5/0.0093 = 537A |
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[1v1]: Arduino: Analog Reference
Footnotes
-
Drain-to-Source On-Resistance
- If current required is 2.5A, then R=5/2.5: R=2ohm absolute max Rds during operation (in practice must be much lower)
- If Vcc = 5v, and gate is pulled to 0v GND, then Vgs ~-5v
- Datasheet shows Rds(on) @ Vgs -4.5 will be ~ 0.0133 ohm
- If Vcc = 12v & Gate is pulled to 0v GND, then Vgs ~ -10v therefore Rds(on) ~ 0.0093 ohm
- HOWEVER, controller can only provide 5v so will need drivers to fully turn off MOSFET, see3
↩ ↩2 -
Gate-Source Threshold Voltage
- Datasheet shows that Vgs must be >-2.0V to turn off device
- i.e. Id = -250 uA @ Vds (Maximum Drain - Source voltage differential)
- Therefore if VCC is 12v, Gate Voltage must be driven to >10v to turn off MOSFET
↩
