I have a number of Linux single-board computers and gadgets that take a well-regulated 5 VDC at various currents, say up to 6A or so (although typical is 0.5 - 3A).
A year ago David Hess helped me with the current measurement part (
in this thread), but now I'd like to incorporate a power on/off switching too. I'd appreciate any comments or suggestions on achieving this.
The core idea is to use a 5VDC switchmode supply, say a
Meanwell RS-50-5 that can provide up to 10A at 5VDC, to provide a slightly higher voltage, say 5.2VDC to 5.3VDC. This is split into two, with simple LC filters as these SBCs are usually too small to include sufficient filtering in them. One rail is always on, and provides power to a Teensy LC or 3.2 (via internal LDO, as it uses 3.3V logic level; unless powered by USB. The two MBR130 Schottky diodes are needed as the Teensies do not have those themselves). The other rail is switchable on/off, and should be able to provide up to say 6 or 8 A at 5 VDC to an SBC. This high power rail is monitored for voltage and current glitches by the microcontroller. The other rail will have a much smaller load, 500mA or less.
Here's the schematic I've got thus far:

The filtering section splits the incoming 5.25VDC into two, and applies some filtering. L1 is a high-current choke (something like max. 15A, 5mOhm; at this voltage and current levels, it'll have about 0.1uH of inductance), and C1 a bulk electrolytic capacitor, something on the order of 1000uF. L2 and C2 are similar but smaller/cheaper, as the maximum load on the other rail is much less. The 470kOhm resistors are just to bleed off any charge from the capacitors when the power is turned off, for safety; I'm thinking especially accidental discharges when connecting new USB devices, or touching USB connectors. None of the values for these components are decided yet; any suggestions? I'd especially appreciate any practical observations.
The on/off control section uses a P-channel MOSFET (
Infineon IP50P04P4L-11). (I do need to switch the positive rail on and off; the microcontroller and the SBC share the ground, so the switch must be between positive rail and the load.) Although I could probably drive this MOSFET using a 3.3V pin from the Teensy at the target currents, I think using an NPN transistor (2N3904) to drive the MOSFET using the 5.25VDC rail is better. Note that this is essentially steady state operation, not PWM. In order to ensure the MOSFET is off during power-on (when the ENABLE pin is an input to the MCU; floating), there is a 47kOhm pull-down resistor; the 3.92kOhm to the base of the transistor is for current limiting the microcontroller output pin to roughly 1mA.
Note that if the load is inductive, and generates a voltage spike at switch-off, I'd rather have the MOSFET take the hit rather than pass it through to the power supply; that's why I did not include a flyback diode to the MOSFET. (I could add an MBR130 Schottky or something though, if there were a reason to.)
The current sensing section is the same as
in this thread, so you can probably ignore that. The voltage sensing section uses a voltage divider to halve the voltage, and uses the second half of the TLV2372 rail-to-rail in/out opamp to buffer it, so that the Teensy can measure the analog signal (and it'll be in 0 to 3.3V range). Again, the current and voltage are measured for glitches/spikes, not necessarily for their exact absolute value. (Both current and voltage will have to be calibrated using a test load and a DMM anyway.)
The MCU section is just a note that the upper (lower-current) 5V rail is also used to power a microcontroller. However, the total current draw for the upper rail will be less than 500mA.
Suggestions? Criticisms? Does this look sane to you?