ON-socket

Drafts for a smart Extra Low Voltage Direct Current (ELVDC) socket.

Components

Voltage Reference

Needed for precise ADC measurements. 2.56V?

Switching

1. n-channel MOSFET switching the negative rail, while a common conductor is tied to the positive rail and possibly earth grounded. This will reduce the overhead for gate drivers, charge pumps or p-channel mosfets as switches. What are the downsides to this approach? Voltage sensing will be difficult as the MCU and ADC will be tied at the low side, not floating. A differential sensing with a common voltage higher than the maximum bus voltage is needed.

2. p-channel MOSFET on positive rail. More expensive, Rds worse, but common GND at 0V.

Potential MOSFETs:

n-Channel
Name, Rds, Vds, Remark/Price
IRFS7530-7PPbF, 1.15 mOhm, 60V
FDMT80060DC, 1.1 mOhm Rds, 60V
NTMFS5C604NLT1G, 1.2 mOhm, 60V
NTMFS5C612NLT1G, 1.5 mOhm, 60V, $1.30
IRLS3036-7PPbF, 1.5 mOhm, 60V
IRFS7730-7PPbF, 1.7 mOhm, 75V
CSD18540Q5B, 2.0 mOhm, 60V
STH315N10F7-2 2.1 mOhm, 100V
CSD18542KTT, 4.0 mOhm, 60V
CSD19502Q5B, 4.0 mOhm, 60V
IRFP4310Z, 4.8 mOhm, TO-247AC
IRF3205, 8 mOhm, TO-220, cheap

Potential gate drivers:

LM5109B, external bootstrap diode, $0.60
LM27222, low Iq but max 30V
TPS7A6350-Q1, 10µA Iq,
UCC27201A-Q1, 1µA Iq, 120V, $1.60 
MIC4605

Current Sensing

1. MOSFET Vds sensing. Only valid if the consumer is a pure current source or current sink. What happens if the gate is discharged and Vds rises to 50V? The following amp and ADC will go into saturation, this needs some more thought. High impedance line from the amp, limited with a diode (crowbar)? Possible if the input impedance is high enough. Temperature drift.

2. PCB shunt. Inexpensive. Needs calibration. Quite high real estate on the PCB. Lowest part count, bidirectional sensing no problem.

3. SMD/THT resistor. Inexpensive but rather large for a high current rating. No calibration needed.

4. Integrated hall effect sensor, e.g. Allegro ACS712. Good efficiency. High price. Sourcing and sinking will both be covered (still needs antiparallel MOSFETs though). (heavy idle: 5V supply, 10mA Iq)

ADCs with integrated amps: INA220: 12bit Bi-Directional, Zero-Drift, Low- or High-Side, I2C Out Current/Power Monitor, max 26V bus voltage. 1mA Iq ADS1115: 16bit 4channel 8-800sps ADC with 16x PGA, i2c, differential mode, 150µA Iq HX711: 24bit (less than 20bit usable) i2c ADC with amplifier, commonly used in weight scales, 1.5mA Iq

Voltage Sensing

Via high impedance voltage divider into ADC: input voltage, output voltage.

Arc Sensing

Unplugging a consumer with an inductive load or a high current flow will result in arcing. Can this event be mitigated? How to identify arcing? The resulting plasma will be a large voltage drop right at the plug.

Micro Controller

8bit with integrated ADC should be sufficient. ESP8266 with integrated WiFi is inexpensive but has high power demands.

Power Supply

For logic power a low Iq buck converter. Best with synchronous rectification, few external parts. Potential candidates: TPS54061 (0.09 mA), LM5156 (0.01 mA), LM5009A (0.5 mA), all types up to 60 Vin and about 150mA output. If the n-channel MOSFET is switched at the high side a gate driver is needed. Power supply for the gate driver needs a separate 10V rail. An LM5007, LM5010 (or an alternative with integrated synchronous rectification and enable input) can be used to form a step down converter to 10V. This step down converter should be shut down with a GPIO pin from the MCU if no load is present, so it needs an enable input. Although a driver might charge and discharge the FET gate with 1..2A, the charge time is sufficiently low and can be buffered by caps. For a very low switching frequency (ie. using the FET as a power switch) this is fine. When the FET is used to throttle the output (e.g. via BAM / bit ampltitude modulation), the gate driver needs more power.

Communication

Wireless: NRF24L01+ or WiFi with ESP8266 However, everything should be passive first and work plug and play without having a communication channel.

Optional Display

Small OLED

Faults and Errors

1. Overload: Hiccup pattern with recovery time depending on input voltage drop.
2. Short Circuit: Switch output off. Probe current for 1ms every 500ms. Activate output again if current is below threshhold. Needs a more sophisticated algorithm for plug-in events when input capacitors are charged, soft start could be implemented with PWM.
3. Polyfuse?
4. Diode over output against back-EMF.