Battery switchover circuit and parts cemetery - Page 1
Forum section: electronics / projects

author: SwordFish
author_rank: Newbie
timestamp: July 06, 2022, 02:32:42 pm
content: I am designing a DC UPS for personal use. It will be in a rack. Loads are router, modem, POE switch, cameras, ... I tried to optimise the design for low power usage and mostly discrete parts what i already have at home or easy to source locally. (Hard to get specialized parts and hate to fight with the couriers.)I am trying to make it work for the first time. So hoped for same constructive opinion. Mostly i am shure it will work.1) I am unable to simulate (in crap online Java simulator) the charge pump. Is it feasible like this? Or shoud i go for a differential drive, double cap, graetz bridge solution?See dashed line "Power switch" D11,D12,C12Q12 and Q13 might disturb its working. After the turn on it looks stable to me, but before that i don't really see how the voltage will stabilize while the power mosfets are off.(If you say isolated power supplies. I only find unstocked or high quiescent current ones locally.)1.1) Picture bottom right corner "Possible workaround" Maybe this will startup. Any better idea? I will try it on a breadboard. But my orders are late (low stock) and I won't have time for it in the next 10 days. I'll get to it eventually.2) I have a big ~1kWh battery. It is better to not work the circuit than to over back charge or undercharge the Lithium battery. So I doubled the switching elements. See dashed line "Battery switch 1" and "Battery switch 2" I know not everything is doubled like the comparator, but it is way better than one mosfet away from catastrophy. Does anyone has a safety double series connected similar circuit? I don't find many on the internet.3) See dashed line "Power / Battery select" What would be an appropriate reverse polarity protection?- A P-mosfet between Q1 and R3/IC2A. Not shure about possible Q1 gate voltage overload.- A double diode like BAV99 connected to GND, 5V and the comparator input.(Q2 is to switch off the load on the battery when everything is off, Q1 is for symetry)4) See dashed line "Power / Battery select" MCP6542 comparator has an internal 1,5 to 6,5 mV hysteresis. Should i get more with a hysteresis resistor (Pin 3 and 1) ? Any advice? Maybe a filter cap parallel to R3?The Mean Well LRS-150-24 main power supply (+24V) according to the datasheet has RIPPLE & NOISE (max.) 200mVp-p. At normal operation the battery 16,5V and 24V has a lot more difference. And a power failure should be faster. Like a Power outage. Am I Right? My designs tend to work, but fails like this with an external unknown not working like i expect...Edit: Sorry the picture has big resolution. Need to zoom.

author: SwordFish
author_rank: Newbie
timestamp: July 11, 2022, 02:45:19 pm
content: 1), 1.1)I have done some breadboarding. See hand drawn picture. It is interesting that with or without the marked diode between 5V and the charge pump. This crude circuit worked.Without the marked diode, i can measure 4,5V on the capacitor (connected to both mosfet's source).If i tried to measure relative to ground. I can see that the voltage fals to zero in a blink of a time. As i see, the multimeter's impedance ties it to ground. And now i remembered Kirchhoffs Current Law and now i am shure i need the diode for the final circuit to be able to turn it on.With Diode i measure 9,2V on capacitor. Relative to the ground is 9,57V and if i turn on it is 21,37V.(Comment: This oscillator with these values gives bearly enough power to turn on the mosfets. Less then 4V gate voltage)3)Done some calculation. BAV99 should be enough.4)Still don't know. But if i leave space on the PCB. Later i can increase the hysteresis if i need it...2)Noone is doing safety related mosfet switching element doubling?