Purpose of an extra transistor for battery monitoring - Page 1
Forum section: electronics / projects

author: rgujju
author_rank: Newbie
timestamp: June 05, 2021, 06:45:40 am
content: Hello folks,What is the purpose of the extra transistor Q1A? The circuit would work with only Q1B to enable/disable the voltage divider.Is it just to invert the signal or does it serve some additional purpose?Also why is Q1B a P-channel? I have almost always seen a P-channel MOSFET used to enable/disable a circuit. Why arent N-channel MOSFETS used for this?

author: n5al
author_rank: Contributor
timestamp: June 05, 2021, 08:26:33 am
content: I'm not so great at explaining things, but here's a quick answer:1. The configuration of Q1B in your battery monitoring circuit is called a "high-side switch", either PNP bipolar transistors or p-channel FETs are typically used in high-side switching. Conversely, "low-side switching" usually involves NPN or n-channel FETs. Your high-side switch circuit conveniently allows the monitoring point, called "BATTERY", to be referenced to ground (through R21, of the R20/R21 voltage divider).2. The main function of the Q1A driver transistor is to allow a lower voltage control signal to switch a higher voltage source.In your circuit, imagine that the "BAT_MON_EN" control signal is a 0V to 5V digital signal. Furthermore, assume that "VLi-Ion" is at 6 Volts. If Q1A was absent and the "BAT_MON_EN" signal was directly driving the Q1B gate, the Q1B transistor would always be conducting -- that is, you couldn't turn it fully off, because the Q1B Vgs would be switching between 6 - 0 = 6 Volts and 6 - 5 = 1 Volt, as the "BAT_MON_EN" control signal switched between 0 Volts and 5 Volts.If you want a much better answer, that is still a fairly quick read, try this link:https://www.baldengineer.com/low-side-vs-high-side-transistor-switch.html

author: Simon
author_rank: Unknown
timestamp: June 05, 2021, 09:38:43 am
content: A P channel will switch from a positive supply with a low voltage on it's gate. It will be switched off by taking the gate to the same voltage the source is connected to which is typically the +supply.An N channel will switch to a negative (GND) with a high voltage on it's gate. It will switch off when the gate is taken to the same voltage as the source with is 0VThe controlling signal if from a µC cannot be higher than 5V. If the positive supply of the battery is more than the µC supply then the P channel mosfet cannot be turned off by a micro controller. a lithium cell will be up to 4.2V so even with 1 cell if the µC is run off 2.7-3.3V it's can't control the P MOSFET and could be damaged if connected directly.So an N channel MOSFET that is rated for at least the supply voltage is used to pull the gate of the P MOSFET down to 0V. When the N channel MOSFET is turned off the R3 pullup resistor will pull the gate to the positive supply where the source is connected which puts 0V between the P channel's gate and source.The N channel MOSFET can be controlled by a 0-5V signal from a µC, when turned on it will pull the gate voltage of the P MOSFET more negative than the source and turn it on. When turned off it will let the gate be pulled to the supply turning the P channel off.R2 is a pull down for the N MOSFET as the gate is so high impedance that it can be charged by "anything" and be turned on if the µC is not powered and has it's output in high impedance. It keeps the gate discharged unless positively powered by a signal.