# mosfet current limit circuit

#### ilginsarican

Joined Jul 13, 2017
96
Hello,
I use this circuit to limit supercap charging current.There are 10 supercaps(each one is 300F 2.7V) in series.
According to this circuit, the maximum current that capacitors draw is about 1.3A.
Although I use a cooler, p channel mosfet(Q6) is getting very hot.(160C) because it is in active region.
To solve this problem I've reduced the value of R3.
I thought that if the mosfet approaches the saturation zone, it will heat up less.
But the liimitted current did not changed so overheating problem continues.

#### ericgibbs

Joined Jan 29, 2010
9,384
hi ilg,
What is the Gate to Source voltage of P MOSFET, when its running hot.?
E

#### ilginsarican

Joined Jul 13, 2017
96
hi ilg,
What is the Gate to Source voltage of P MOSFET, when its running hot.?
E
It is about 2.6V.I am trying to increase this voltage because of heating problem.

#### ericgibbs

Joined Jan 29, 2010
9,384
hi,
As you say with only Vgs= 2.6v the FET is only just On, I make it around 36Watts dissipation, for completely discharged caps. == 10F
How long is the full charge period.?
E

#### ericgibbs

Joined Jan 29, 2010
9,384
hi ilgin,
This is a close approximation to your circuit.
The charge time approx 250secs, max heat in MOS 33 Watts
E

#### Attachments

• 8.5 KB Views: 2

#### ilginsarican

Joined Jul 13, 2017
96
hi,
As you say with only Vgs= 2.6v the FET is only just On, I make it around 36Watts dissipation, for completely discharged caps. == 10F
How long is the full charge period.?
E
It takes 15 minutes fully charge.
hi ilgin,
This is a close approximation to your circuit.
The charge time approx 250secs, max heat in MOS 33 Watts
E

View attachment 180875
Hi Eric,

hi ilgin,
This is a close approximation to your circuit.
The charge time approx 250secs, max heat in MOS 33 Watts
E

View attachment 180875
Hi Eric,
It takes 15 minutes fully charge,I use supercap,total capacitiy is 30Farad.
The capacitors draw 1 amp for 15 minutes.
How do you calculate power dissipation?
The mosfet is not fully open,because of that it is getting hotter than fully open situation.

#### ericgibbs

Joined Jan 29, 2010
9,384
hi,
Using Q=C.V , where Q= I.t , gives.... t= (30* 28v )/1.2 = 700Secs.
You say 900 Secs at 1A.

E

EDIT:

Have you considered 'burst' control from the MCU drive, ?
How quickly must the cells be recharged.??

#### Attachments

• 430.2 KB Views: 6

#### Sensacell

Joined Jun 19, 2012
2,544
This is a linear current regulator, the only thing you can do to reduce the heating is:

1) Reduce the current

2) Use an inductive switching circuit, instead of a linear regulator.

http://www.ti.com/product/BQ24640

#### ronv

Joined Nov 12, 2008
3,770
Hello,
I use this circuit to limit supercap charging current.There are 10 supercaps(each one is 300F 2.7V) in series.
According to this circuit, the maximum current that capacitors draw is about 1.3A.
Although I use a cooler, p channel mosfet(Q6) is getting very hot.(160C) because it is in active region.
To solve this problem I've reduced the value of R3.
I thought that if the mosfet approaches the saturation zone, it will heat up less.
But the liimitted current did not changed so overheating problem continues.

View attachment 180871
You could use a power resistor to drop some of the voltage and thus some of the power in the FET.

#### crutschow

Joined Mar 14, 2008
24,130
Besides the options that Sensacell and ronv mention, you could also use a better heatsink (possibly with fan cooling) for the MOSFET.
You say you use a "cooler" but what does that consist of?

#### BobTPH

Joined Jun 5, 2013
2,207
A current controlled buck converter is the only way to go, IMHO.

The bang bang style converter would be appropriate.

Bob