NPN and PNP to gate of N-channel

[voltagedrop]

I am attempting to reverse engineer a fairly expensive marine "smart" battery charger. At the target battery - connection on the PC board is an N-channel mosfet, which I realize is where the "throttling" to the target battery happens. However the gate of this mosfet is connected to both a PNP and NPN transistor, in parallel to each other. The base of both BJT's are connected together and go to an I/O of the microcontroller. If the Mosfet gate must be pulled high to conduct, what would the point be of the BJT that can pull the gate low when the same is accomplished by turning off the BJT pulling it high. Seems like a complicated way of having a pull down resistor on the Mosfet gate. I can clean up my hand scratched schematic and post it if my explanation is confusing.

 

[Bluejets]

Show diagram with part numers of all discrete devices.

 

[Harald Kapp]

in parallel to each other.

Really?

I'd expect something like this:

Electronic – Two BJT transistor vs driver for PWM MOSFET – Valuable Tech Notes

 

[kellys_eye]

The base of both BJT's are connected together and go to an I/O of the microcontroller.

Just because the devices may have identical packages doesn't necessarily mean they have the same pin-out.

 

[voltagedrop]

Really?

I'd expect something like this:
View attachment 62851

Electronic – Two BJT transistor vs driver for PWM MOSFET – Valuable Tech Notes

Yes. This is how they are connected. I will get my schematic cleaned up and posted. But my hand scratch is here, with the portion I am referring to in the upper left corner. I state "in parallel" as Q1 and Q2 provide alternate paths for the gate of Q3.

In the schematic you posted, I am not clear of the purpose of Q2. If the gate of Q3 needs to be pulled high to "turn on" Q3, Q1 will take care of that. Would turning off Q1, and even adding a pull down resistor between R2 and gate to Q3 for good measure, not "turn off" Q3 by pulling the gate low, so what is the point of Q2. Obviously there is a point, or it wouldn't be there, I'm just not seeing it.

With relation to my chicken scratch. Q1 = P60NF06L, Q4=2X, Q6=2T, U3 = K131 (can not find a specific datasheet yet, but I think it is an op-amp used to monitor current sense across R1). Microcontroller is an STM8 - 20 pin.

 

[bertus]

Hello,

Your scribble is very hard to understand.
Please use the correct schematic symbols, in stead of the blocks.
In this picture you will find a lot of symbols that you can copy and paste in to a drawing program:

Bertus

 

[Harald Kapp]

I state "in parallel" as Q1 and Q2 provide alternate paths for the gate of Q3.

That is where you managed perfectly to get us confused as this is not what is commonly understood as "parallel" in the community. This is what a parallel circuit looks like:

what is the point of Q2.

Fast pull down of the gate of the MOSFET. Much faster than by a pull-down resistor. This in turn will turn off the MOSFET faster and consequently reduce switching losses.

 

[voltagedrop]

Though Harold Kapp already answered my question (thanks for that, now I want to build a couple of circuits and compare the differences on the oscilloscope with just the pull down resister vs with the "pull down" BJT). Here is the cleaned up schematic. Gave me a great opportunity to play with KiCad.

The issue with the charger is the STM8 has failed. The charger is 2 banks, and has 2 identical circuit boards for each bank, each fed by their own center tap, step down transformers. 1 bank works fine, the other did nothing, and had all LED illuminated at all times. I was hoping to copy the firmware from the good STM8 to burn it to a new IC, but alas, the manufacturer has enabled read protection. Thanks for the help.

 

[Delta Prime]

Looks like you are a natural
From scribble to readable schematic you're on your way kiddo! Good job.

 

[danadak]

You might compare pin V between good and bad board, is part being held
in reset due to external component gone bad. Or another pin a short condi-
tion, regulator fried .....A bit unusual a processor part goes bad.

There is some unusual constraints in spec section on external capacitor (VCAP)
needed on processor.Depending on aging and its technology its changed its
specs...

Do a resistance check board to board comparison...


Regards, Dana.

 

[voltagedrop]

You might compare pin V between good and bad board, is part being held
in reset due to external component gone bad. Or another pin a short condi-
tion, regulator fried .....A bit unusual a processor part goes bad.

There is some unusual constraints in spec section on external capacitor (VCAP)
needed on processor.Depending on aging and its technology its changed its
specs...

Do a resistance check board to board comparison...


Regards, Dana.

I plan to do just that, thanks. I did take the "bad" processor and put it on the "good" board and it acted the same way (returned good processor to make sure I didn't damage board). I didn't learn anything doing that though, except practicing some SMD soldering. I plan to do the checking you suggested to find out if there is a "why" did the processor fail before putting the "good" IC on the "bad" board, so I don't kill it to.

 

[roughshawd]

I don't know how many times I have told people this. Batteries are a type of capacitor, not just a capacitor. It requires 1 hit to charge a cap, it takes Voltage Ohmage and .002 Amperage to trickle charge a battery. If you want to think in 6 figure values, I will call my shrink for you. Do the math. Not the thinkin'!!!!

 

[roughshawd]

So, you can't hope to know whether the battery is charging or not, and it will take some time to really understand what is going on when you apply signal. From here education dictates that you learn what signal is, and how it works to create networks that can do almost anything that power circuits can do. But that's electronics, and I'm stuck with it. !!!
Here is a hint on signal, it's the frequency that is required to run the network....

 

[Harald Kapp]

Batteries are a type of capacitor, not just a capacitor.

Not at all. The principle of operation differs widely between capacitor and battery.
In a capacitor energy is stored in an electric field between two plates.
In a battery energy is stored by a chemical process and also released by a chemical process.

It requires 1 hit to charge a cap, it takes Voltage Ohmage and .002 Amperage to trickle charge a battery.

Not completely wrong, but not right either.
You can't charge a capacitor without any kind of current limiting aka a series resistor, even if it is only the resistance of the wires leading from the voltage source to the capacitor. Without resistance, the charge current would be infinite:
[math]I = C × \frac{dV}{dt}[/math]With [math]\frac{dV}{dt} = \infty[/math] when a voltage source were applied to a capacitor without current limiting.

Also

.002 Amperage to trickle charge

is an oversimplified generalization. The trickle charge depends on the capacity of the battery. The higher the capacity, the higher the trickle charge. Plus not all battery types can be trickle charged.

Do the math. Not the thinkin'!!!!

@roughshawd : good tip. Why don't you follow your own advice?