# MOSFETs and current output

#### Schidtztorm

Joined Oct 1, 2019
77
I have a dilemma! I'm building a phone charger. I'm using two small npn for a differential type regulator that is controlling a mosfet for current. I'm trying to get this thing to put out 1 amp, or at least 700 mA. It's only putting out around 200mA.
I'm currently using a 2sk3568. I posted schematic below.
What am I doing wrong?LG

#### AlbertHall

Joined Jun 4, 2014
10,777
What is the supply voltage to this circuit?

#### crutschow

Joined Mar 14, 2008
26,002
Try reducing the emitter resistor from 25k to 10k.

#### ronsimpson

Joined Oct 7, 2019
1,123
Vin (input voltage) needs to be 5 volts or more above Vout, which is +5V.
What is the input voltage?

Think: What is the G-S turn on voltage for the MOSFET at 1A?

Last edited:

#### Schidtztorm

Joined Oct 1, 2019
77

#### Schidtztorm

Joined Oct 1, 2019
77
Try reducing the emitter resistor from 25k to 10k.
Will do! Thank you.
I had a question for you as well. They've been upgrading AAC so you probably didn't see it.
if I have an identical FET, could I run them parallel on the same circuit? And would it double the current? Or would it stay the same? I never tried such a thing.

#### Schidtztorm

Joined Oct 1, 2019
77
Vin (input voltage) needs to be 5 volts or more above Vout, which is +5V.
What is the input voltage?

Think: What is the G-S turn on voltage for the MOSFET at 1A?
Sorry bout that! It's 12.5 V
What is the supply voltage to this circuit?
12.5V

#### Dodgydave

Joined Jun 22, 2012
9,854
I have a dilemma! I'm building a phone charger. I'm using two small npn for a differential type regulator that is controlling a mosfet for current. I'm trying to get this thing to put out 1 amp, or at least 700 mA. It's only putting out around 200mA.
I'm currently using a 2sk3568. I posted schematic below.
What am I doing wrong?LG

View attachment 189540

Your circuit is useless, just use a simple Emitter follower transistor TIP41 with 6V zener on the Base , this will give you 5.3V on the battery and as much current as the supply and transistor will give...

#### ronsimpson

Joined Oct 7, 2019
1,123

#### DickCappels

Joined Aug 21, 2008
6,790
You really should consider some current limiting for your circuit -there is no "intentional" current limiting in the circuit, so you have no control of how much current this circuit will try to shove into a battery if you connect a dead battery to it.

This circuit may look familiar. When the current through R2 develops enough voltage across the base-emitter of Q2 to make it draw current, the transistor will begin conducting, starving the base drive of pass transistor Q1 thus limiting the current to the load. This works with MOSFETs too.

#### Schidtztorm

Joined Oct 1, 2019
77
Your circuit is useless, just use a
You really should consider some current limiting for your circuit -there is no "intentional" current limiting in the circuit, so you have no control of how much current this circuit will try to shove into a battery if you connect a dead battery to it.

View attachment 189575
This circuit may look familiar. When the current through R2 develops enough voltage across the base-emitter of Q2 to make it draw current, the transistor will begin conducting, starving the base drive of pass transistor Q1 thus limiting the current to the load. This works with MOSFETs too.
The phone, as well as the lithium ion battery both have current limiting of 500mA. I want more because I want to use it while charging at the same speed. Fast chargers from the manufactures are 1000mA. I'm just trying to get close to that with what I have. Which are small L6 npn's and assorted N channel mosfets.
simple Emitter follower transistor TIP41 with 6V zener on the Base , this will give you 5.3V on the battery and as much current as the supply and transistor will give...

#### Schidtztorm

Joined Oct 1, 2019
77
You really should consider some current limiting for your circuit -there is no "intentional" current limiting in the circuit, so you have no control of how much current this circuit will try to shove into a battery if you connect a dead battery to it.

View attachment 189575
This circuit may look familiar. When the current through R2 develops enough voltage across the base-emitter of Q2 to make it draw current, the transistor will begin conducting, starving the base drive of pass transistor Q1 thus limiting the current to the load. This works with MOSFETs too.
I do like this circuit. It would stay the same for fets? Could I get an amp out of this using what I have? You don't think what I'm using will work?

#### ronsimpson

Joined Oct 7, 2019
1,123
5k and 2.5k where?
In place of the 50k and 25k. Either way it should work. But I can't trouble shoot with out knowing the voltages.
I just built it in SPICE using your values and it works for me.

Last edited:

#### Schidtztorm

Joined Oct 1, 2019
77
In place of the 50k and 25k. Either way it should work. But I can't trouble shoot with out knowing the voltages.
I just built it in SPICE using your values and it works for me.
I'm going to rebuild this thing with some variations. First I'm going to eliminate the 5V zener. It's not 5.1.
I'm going to put 14400 ohms above 10k to make 5.12V which pushes the max base V over by 120mV. I'm hoping the little L6's have at least that much tolerance. Then swap out the 25k for a 10k as Crutchow recommended.
What do you think?

#### crutschow

Joined Mar 14, 2008
26,002
f I have an identical FET, could I run them parallel on the same circuit? And would it double the current?
No, because there can be a significant difference in the gate-source threshold voltage even between two MOSFETs of the same type.
The one with the lower Vgs(ths) will hog most (or all) of the current.

#### crutschow

Joined Mar 14, 2008
26,002
And what's the basic for your sweeping statement?

#### ronsimpson

Joined Oct 7, 2019
1,123
5V zener. It's not 5.1
5.0V Zeners are not common. 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6 are common 5% parts.

#### Schidtztorm

Joined Oct 1, 2019
77
5.0V Zeners are not common. 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6 are common 5% parts.
Yeah! I'm also aware of that, but it's 5V just the same. It's a little glass one.
I built the one I previously mentioned above, with the 5.12V voltage divider, and swapped out the 25k for the 10k. Still no change. Still at around 200mA.?
Any suggestions that will make this circuit work better? Maybe a coupling cap? My supply pulses along side of a week DC signal.

Last edited: