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MPPT 4.2v Li-Ion charger and SBC PSU
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README.md

MPPT 4.2v Li-Ion charger and SBC PSU

For use with Sensorable Open Source Camera v.1

The PSU board combines an MPPT (Multiple Point Power Tracking) solar charger, 4.2v battery IO port and a protected 4.2v output to power the main camera board.

This design has been assembled with PCBA and successfully used in temperate climate for over a year.

top-nocon bottom-nocon

Specification

  • Max input working voltage: 24V
  • Min input working voltage: 12v
  • Max input current: 2.5A
  • Max output current: 6.5A
  • Max operating current: 2A
  • Ripple: <50mVpp nominal steady state
  • Size: 65mm x 27mm x 15mm

Protection

  • Shunt Input protection: Bipolar Zener diode, 24V working voltage, clamps to 38.9V @ 15.5A; For ESD and shunt of high-voltage surges. First line of defence.
  • Series Input protection: 24V working voltage 31.6V clamp, 2.5A max current. Series FETs and associated circutry will: Block reverse voltage. Limit current during hot-plug (for no sparks) and during surges. If prolonged over-voltage surge is detected charger is disabled (w/ auto-restart after surge passes) to limit power dissipated in series FETs. Precision over-voltage clamp (31.6V) protects switching FETs from over-voltage.
  • Overload protection, Vbat: None, rely on BMS. Charge current and voltage is limited by charger.
  • Overload protection, Vout: 4A lim; 4.5ms time constant. Retry after >500ms. Short-circuit safe.
  • Reverse polarity protection, Vin: Safe DC voltage of +/-24V.
  • Reverse polarity protection, Vbat: None. Charger will not attempt to charge a reversed battery but other electronics would be damaged. Batt reverse protection must be placed on BMS.
  • Overheating protection: Charge only when safe for Li-Ion 0C-45C, and adjust current/voltage based on temp (LT8490 feature).
  • Electrostatic protection: ESD/Surge protection on Input voltage, Heartbeat input, Battery voltage, Device voltage.

Overview

The PSU is built around LT8490 buck-boost switching regulator battery charger with CC/CV modes and MPPT for solar input.

The purpose of the PSU is to provide a compromise between fast charging and efficient low light charging a 4P/5P/6P Li-Ion battery pack powering the main board. The PSU incurs more losses during charging (18v -> 4.2v) to minimizes losses during discharge (4.2v direct output). Vout port has an analog restart circuit for a hard reboot of the main device every 18 - 24 hrs.

photo

Heat dissipation

Switching MOSFETs, the main coil and other parts of the board require heat sinking. The board itself has more copper than the electrical design requires to help dissipate the heat, but it's not enough with passive cooling inside an enclosure. Additional heat dissipation was achieved by potting the entire assembly in a metal frame with Electrolube ER2074 epoxy resin. It produced a sealed unit that could be exposed outside the enclosure. The best result was achieved by potting the PSU inside a standard size Al U-channel.

Features

Pre-starter

Most battery protection systems require external voltage to be applied for their MOSFETs to open In/Out. On the other hand, LT8490 will not attempt to charge the battery until it measures the voltage. If the battery pack gets disconnected by its protection MOSFETs it will be restarted by a 3.3V trickle charge circuit. That voltage is enough to unlock the BMS.

prestarter

Under-voltage protection

G935 board can function with Vin > 3.4v. However, the initial draw from a 3.4v battery brings the voltage down to below that. This initiates a start-abort boot cycle that drains the battery and may even damage the board. This PSU has a special undervoltage control circuit built that disables the Vout to the device until the battery voltage reaches at least 3.7v. Once operational, the device will continue until the voltage drops to 3.20V. When off, the device is latched off until the restarter circuit triggers or the battery voltage rapidly rises (a freshly inserted battery, a freshly reset BMS).

Consider these scenarios:

  • battery connected, Vbat < 3.7v -> Vout = Vbat momentarily, then latches off to Vout = 0V.
  • battery connected, Vbat > 3.7v -> Vout = Vbat; operational
  • battery discharging, 3.2v < Vbat < 3.7v -> Vout = Vbat; operational
  • battery discharging, reboot, Vbat < 3.7v -> Vout = Vbat momentarily, then latches off to Vout = 0V.
  • battery charged to 3.7v, reboot -> Vout = Vbat; operational

undervoltage

Restarter

Given the remote nature of the PSU deployment a hard reboot on a schedule (approx every 27hrs) safeguards from the main board freezing for whatever reason. It is the equivalent of turning the power off for a few seconds.

A restarter heartbeat wire (P3.2) can be connected to an NFC pin on G935 boards to prevent the restart. When NFC is enabled G935 sends a short burst of pulses to the restarter resetting the timer.

Heartbeat signal requirements: Rising edge or edges (such as sine waves from NFC) with >2V positive peaks. Must be present for >1ms. Then, must relax for 2s < t << 27 hrs. For example, send a 1ms NFC pulse every 5s/5min/5hour. Can also be operated with DC with similar timing/voltage requirements.
Will not work with consistent output, must be pulsed.

Connectors

board-top board-bottom

IP68 USB-C connector

input from a solar panel or a wall charger. This connector provides a sealed connection up to 2A per contact and remains waterproof when decoupled. Amphenol LTW part number: UC-31PFFP-QS8001. Mates with UC-20AMM-QA8A01 or similar.

usbc pushup

Pushup board: the angle of the USB-C connector didn't allow mounting it directly on the PSU board, so we had to use a separate riser (push-up) board to align it with the enclosure.

We recommend using a simpler and cheaper external connector in your design.

Field notes:

  • the connector proved to be reliable over 2 years of use with low mating cycles
  • make sure the air was pushed out when inserting - it may push the male out
  • using a plug is recommended if not coupled to stop fouling the contacts
  • plugging in a USB-C charger won't work because the starting voltage is < 6v and the board don't have the means to negotiate a higher Vin.

Other connectors

4.2v battery contacts: use 17AWG - 13AWG wires or D < 1.8mm to connect the battery pack. Must be soldered.

4.2v device power: use 22AWG - 17AWG wires or D < 1.2mm to connect the main board. Must be soldered.

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