# 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.

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#### Diver300

##### Well-Known Member
For high current traces, you can just have no solder resist on the bits that take high current, and add solder when the components are fitted.

2 oz copper is 70 µm thick, or 0.07 mm. Tin has a lot more resistance than copper, about 5 times, but you can easily get 0.5 mm of solder onto a wide track, which will reduce the resistance by a factor of more than 2 compared to 2 oz copper.

You could even solder down bare copper wire along the track. A bit of 2.5 mm2 solid wire* is about 1.8 mm diameter, so can be soldered along a 3 mm wide track quite easily. A 3mm wide track on 2 oz copper is 0.21 mm2 so the copper wire on top of it reduces the resistance by over 10 times.

A thinner board will transmit heat from front to back better, but it won't make any real difference in conducting heat across the board, as the copper does that. If the heat is conducted from front to back, unless there is something on the back of the board to take heat away, it's not going to make any difference.

Tracks inside the PCB don't add to the thermal problems. They can make it a bit better by conducting heat sideways better. Redundant tracks don't help much if the copper is only 1 oz and the heat dissipation from an internal track is worse than from a surface track.

*2.5 T+E cable (Translation to American:- 14/2 Romex) is a good place to get solid wire like that.

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#### CK3

##### Member
For high current traces, you can just have no solder resist on the bits that take high current, and add solder when the components are fitted.

2 oz copper is 70 µm thick, or 0.07 mm. Tin has a lot more resistance than copper, about 5 times, but you can easily get 0.5 mm of solder onto a wide track, which will reduce the resistance by a factor of more than 2 compared to 2 oz copper.

You could even solder down bare copper wire along the track. A bit of 2.5 mm2 solid wire* is about 1.8 mm diameter, so can be soldered along a 3 mm wide track quite easily. A 3mm wide track on 2 oz copper is 0.21 mm2 so the copper wire on top of it reduces the resistance by over 10 times.
Some great ideas there!
A thinner board will transmit heat from front to back better, but it won't make any real difference in conducting heat across the board, as the copper does that. If the heat is conducted from front to back, unless there is something on the back of the board to take heat away, it's not going to make any difference.
I'm planning to bolt the board down to a steel plate with a 0.5mm piece of something like "SIL PAD" in between. (I didn't know it was called that until I watched this video yesterday:
.) Hopefully, that will take some heat away.
Tracks inside the PCB don't add to the thermal problems. They can make it a bit better by conducting heat sideways better. Redundant tracks don't help much if the copper is only 1 oz and the heat dissipation from an internal track is worse than from a surface track.

*2.5 T+E cable (Translation to American:- 14/2 Romex) is a good place to get solid wire like that.
The guts of the old regulator are a mine of good solid wire:

That current coil is really thick stuff. Probably too thick to lay on a PCB track. But I have lots of Romex scraps around, too. I will have to reconsider my board layout (again). I put the 100+ mil tracks on the back side, but I want to keep that side smooth. So, I need to get some big, straight tracks on the top side. And then, maybe I won't have to go so crazy with vias, which I have been liberally sprinkling around, hoping they can take the current.

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