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# Automotive Generator Universal Digital Voltage Regulator

#### Diver300

##### Well-Known Member
Also, you can put more MOSFETs in parallel to reduce the voltage drop and the heat dissipation. The MOSFETs a very small, so you could easily have several and get a very small voltage drop.

#### CK3

##### Member
...

You should use a much larger Gate-Source Voltage than the threshold voltage. The threshold voltage is the voltage at which the MOSFET starts to turn on. To get the best resistance you need more voltage. The NTMFS5C612NL has a threshold voltage of 1.2 - 2 V, but that is at a drain current of 250 μA. To get the typical 1.2 mOhm you need 10 V on the gate.

If it's a negative-earth vehicle, when the dynamo is off, the MOSFET source will need to be a 0 V, so the gate will have to be at 0V as well. When the dynamo is generating, the drain and source will be about 7 V, and the gate will have to be at 17 V to get the best resistance. You don't really need any current at 17 V and you will have the big advantage of little heat being generated, but you certainly won't be able to use the 3.3 V output of the GPU to drive the gate.
Thank you for that lucid explanation! When you put it that way, it all makes perfect sense.
There is an IC designed to drive an N-MOSFET in this way. It's an LM5050 (https://www.ti.com/lit/ds/symlink/lm5050-1-q1.pdf). One of those and an N-MOSFET would probably work as a cut-off with no further connection to the regulator part.

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#### CK3

##### Member
The LM5050 data sheet says it's a high side driver, but could it also be used as a low side driver? I'm thinking of the field current switch. In my 6v automotive voltage regulator application, a "B" circuit regulator energizes the generator field by grounding its end of the field coil. If I want to control that with a digital signal, I again have the situation of the signal not being high enough voltage to fully enhance the power MOSFET. If I use a simple gate driver, I'll get 6 or 7.5 volts at most from the car battery circuit; maybe less depending on state of charge. Not really enough, right? Is there a way to use the charge-pumped LM5050 to solve that? Or a more appropriate charge pumped low side driver? I've looked at a bunch of low side gate drivers and they all seem to be limited to (almost) rail-to-rail, which would be an improvement over a 3.3v logic signal, but probably still marginal in a 6v system.

#### rjenkinsgb

##### Well-Known Member
The simplest things would be either photovoltaic FET drivers (though they are slow switching), or integrate a small boost converter such as a "simple switcher" type IC that could be used to power the FET gate drive circuits & allow fast drive for PWM on the field control.

Though the LM5050 or 74700 do look to be perfect for the "cutout" control!

#### CK3

##### Member
I've played around with the IRL40B209 HEXFET® Power MOSFET and I can use it to turn a 6v headlight on and off with a 3.3v logic signal from the STM32 MCU directly on the gate. So, a rail-to-rail driver might be fine for that MOSFET (see Fig. 1 in the data sheet). But, I can't find an equivalent surface mount part.

#### CK3

##### Member
They have arrived!

#### Buk

##### Active Member
Very nice. I love the way the large diameter, plated through holes have come out.
Did it cost much extra to have the boards cut to that very particular shape?

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#### Nigel Goodwin

##### Super Moderator
Very nice. I love the way the large diameter, plated through holes have come out.
Did it cost much extra to have them cut to that very particular shape?
There are no extras for holes with JLCPCB.

#### CK3

##### Member
Very nice. I love the way the large diameter, plated through holes have come out.
Did it cost much extra to have the boards cut to that very particular shape?
No extra charge at all for the shape. I paid extra for some SMT assembly (just LEDs and their resistors), and got hit with "extended" parts charges, which are parts that have to be retrieved for assembly, I guess. You really have to hunt around to find parts that are not "extended". For example, if I'm looking for a 47 nF capacitor, there are 644 choices in stock, and it seems to be a guessing game to figure out which ones are "extended". I have gone through that exercise on the "power" board that I'm working on: https://oshwlab.com/carlk3/volt-reg-1-1_copy_copy_copy_copy_copy_copy. For that one, I plan to use their assembly services as much as possible, since it is all surface mount (and needs to be for the thermal design to work [if it works]).

But, can't complain, for the price (\$20.24, including not-the-slowest shipping, for five of them)!

#### Diver300

##### Well-Known Member
How do you get the oval holes for F1? I'm using DesignSpark and I don't think that oval holes are available on that.

#### CK3

##### Member
How do you get the oval holes for F1? I'm using DesignSpark and I don't think that oval holes are available on that.
I just picked up footprint KEYSTONE_3568 from the EasyEDA User Contributed library.

#### Nigel Goodwin

##### Super Moderator
How do you get the oval holes for F1? I'm using DesignSpark and I don't think that oval holes are available on that.

There's an explanation of how to create them on the DesignSpark website.

#### CK3

##### Member
I'm finally about done with assembly and bench testing:

Wiring the power electronics by soldering point-to-point was a pain:

I am looking forward to replacing that with a board in the next iteration.

Next, onto the car!

#### CK3

##### Member
Did my first on the car test today:

The basic function seems to work well, so that's good news. However, there is something very wrong with my tach input. The car wouldn't even start until I disconnected it from the ignition points. Back to the drawing board.

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