I am planning a system that needs to maintain low power draw (especially when in its standby state), which means I need to use some switches to turn off parts of the system when not needed. The main power supply in the system is a lithium ion battery which has a voltage range of 50-84V DC from 0 to 100% SOC.
Initially it seemed like the obvious choice for a switch would be a logic level N channel power mosfet in low-side configuration. This is because they do not draw current in order to keep them switched on or off, have a very low resistance so waste a low amount of power, and in low-side config can be driven by either the ESP32 GPIO or via a level shifter to 5V to give better Rds(on).
In comparison, mechanical relays draw constant power to stay on, and can have contact resistance 10X the Rds(on) of a mosfet in some cases. It appears they also wear out much quicker. And solid state relays that I have found so far are generally very large compared to a mosfet or relay and drop a constant voltage up to 1V across them, which would be a big problem for high side switching of say 5V.
However, the key problem with the low side switching NMOS solution is that if you switch off the ground to a part of the circuit, for example switching off the ground to the 5V LDO that powers the PIC in my diagram, then that part/power domain will now float at whatever the Vcc is (unless it is isolated), so in this case up to 84V. For a simple load this might not be an issue, but in my case the PIC microcontroller takes 1 digital (D0) and 1 analog (A0) input from the ESP32 (highlighted in red), so it would ground itself through those data lines into the ESP's power domain which would apply 84V onto the PIC/ESP which would cause damage.
Therefore, high-side switching seems more appropriate as that means all power domains will float at GND when turned off, so leaving data lines connected between domains would at worst put 5V onto an IO pin when the MCU is powered off in another domain, which isn't as dangerous, and this can also be mitigated by setting the outputs going into a domain to low when that domain is powered off. This is what I have shown in a generic way in the diagram with Q1, Q2 and the two gate drivers.
It seems the workaround for low-side switching is isolated communication between domains, which is commonly done with opto-isolators for digital signals, but these draw current when being used. In addition, from my research it seems passing analog signals between domains (which is required) in an isolated manner would require some complex circuitry such as an analog to frequency modulator and demodulator on the other side.
I have been having trouble identifying the best way to do high side switching with either PMOS or NMOS at 84V. I tried looking at gate drivers, or load switches (which it seems are mosfets + gate drivers combined for convenience) to do the heavy lifting for me, but it has been very difficult to find any drivers rated for a max Vcc of 84V or higher on UK sites such as Rapid Electronics, RS Components or Farnell, especially in through-hole format.
From doing some research, it seems there are some high side gate drivers that do not have the full Vcc across them and instead derive a separate voltage from the main power supply, so they do not need a Vcc rating of 84V.
For PMOS, one example is using a zener diode and some resistors to create a defined voltage drop to create the required negative Vgs, as just pulling the gate to ground would mean Vgs is -84V which is far too high.
For NMOS, some methods I found to generate a voltage higher than Vcc to turn on the gate include:
- Bootstrap supply using a capacitor, diode and some resistors
- Charge pump
- Magnetically isolated/floating supply derived from Vcc
All of these methods have tradeoffs of course, so my question is between all the options (including any better ones I haven't listed), what is the most power efficient and low component count gate driver + gate driver supply + mosfet combination for simple on/off switching? Alternatively, lowest power + component count load switch if applicable. This includes:
- The quiescent/leakage power used by the gate driver and its supply in either the mosfet's on or off state
- If a PMOS high side driver + supply is much more efficient than any of the NMOS options, then increased power wasted due to higher Rds(on) of PMOS compared to NMOS should be factored in too.
- Low component count is simply to keep the gate driver + supply combination small and easy to assemble by hand on a through-hole solution such as a stripboard since it will be need to be duplicated once for every mosfet switch used in most cases. It is possible that I could design and order a PCB to mitigate this issue if it's necessary to use surface mount or a lot of components, but I would like to avoid that if possible as it would add time and complexity.
- The solution must be able to switch up to 3A without a heatsink. For a gate driver this is irrelevant as it depends on the mosfet, but for a load switch with the mosfet integrated this spec would be more important.