How much headroom for a power transistor?

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I am trying to choose a transistor capable of driving LED's at 700mA - 800mA. Would a metal can transistor or T-220 transistor rated for 1A be enough? The lights will not be on for much more than 3min routinely and perhaps 40min - 1hr occasionally.

Or........is a MOSFET a better choice & why?
 

crutschow

Joined Mar 14, 2008
23,526
Is the BJT transistor just being used as a switch or to control the LED as a constant-current source?

A MOSFET (which also is a transistor) requires no gate current, whereas a BJT requires base current that can be up to 10% of the collector current if it's being used as a switch.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
Is the BJT transistor just being used as a switch or to control the LED as a constant-current source?

A MOSFET (which also is a transistor) requires no gate current, whereas a BJT requires base current that can be up to 10% of the collector current if it's being used as a switch.
It would be used solely as a switch. I assumed that the BJT(you are right, they are both transistors!) would add to total current consumption.
So adding another 80mA to 800mA brings the total current to the doorstep of 900mA. Is this too close to an "Ic" of 1A rating of the transistor. Seems like it is.
 

KeithWalker

Joined Jul 10, 2017
459
I am trying to choose a transistor capable of driving LED's at 700mA - 800mA. Would a metal can transistor or T-220 transistor rated for 1A be enough? The lights will not be on for much more than 3min routinely and perhaps 40min - 1hr occasionally.

Or........is a MOSFET a better choice & why?
It really depends on what you are switching the transistor with.
A junction transistor would need a switching voltage >0.8V and a current of up to 80 mA to saturate it. The Vce sat. (on-voltage drop) will be around 200mV. The total power dissipation will be the collector current x the collector voltage, plus The base current x base voltage: ( 0.8 x 0.2 ) + (0.08 x 0.7 ) = 216 mW
Most power MOS FETs need a switching gate voltage of >6V. A very small amount of current will flow to charge up the gate capacitance at turn-on. They are available with Rds (on-resistence) as low as 28 mOhms which would give negligible power dissipation at 800mA.
 
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Thread Starter

iONic

Joined Nov 16, 2007
1,650
It really depends on what you are switching the transistor with.
A junction transistor would need a switching voltage >0.8V and a current of up to 80 mA to saturate it. The Vce sat. (on-voltage drop) will be around 200mV. The total power dissipation will be the collector current x the collector voltage, plus The base current x base voltage: ( 0.8 x 0.2 ) + (0.08 x 0.7 ) = 216 mW
Most power MOS FETs need a switching gate voltage of >6V. A very small amount of current will flow to charge up the gate capacitance at turn-on. They are available with Rds (on-resistence) as low as 28 mOhms which would give negligible power dissipation at 800mA.
What would be switching the transistor will be a PIR sensor that provides 3.3V....not sure on the current. You are talking about the base current being "up to 80mA correct? What datasheet parameter is this?
 

KeithWalker

Joined Jul 10, 2017
459
What would be switching the transistor will be a PIR sensor that provides 3.3V....not sure on the current. You are talking about the base current being "up to 80mA correct? What datasheet parameter is this?
The base current will depend on the base emitter saturation current (Vbe(sat)) at the required collector current.
It is usually about 10% of the collector current.
A 180 ohm resistor connected between the 3,3V and the base should supply enough current to saturate the transistor, It all depends on whether the source can supply 80mA.
 
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crutschow

Joined Mar 14, 2008
23,526
You are talking about the base current being "up to 80mA correct? What datasheet parameter is this?
You look where they list the collector-emitter saturation voltage (typical shown below).
Note they use a base current that is 10% of the collector current.
In most cases you could probably get by with 5%, at a small increase in saturation voltage.

upload_2019-2-21_20-5-0.png
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
So, all told, then with an Ic of 1A and a Ib of 100mA, and a load current of as much as 800mA I am at 900mA. Should I get a Transistor with a greater Ic? A logic level MOSFET as I am not sure of the current of the PIR Sensor.

EDIT:
Quiescent Current - 50uA
Power Consumption - 65mA
 
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Thread Starter

iONic

Joined Nov 16, 2007
1,650
However, I believe I was incorrect when adding Ib to Ic. Ic would be a maximum of 800mA, regardless of Ib.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
Yes, the collector current rating is just for the collector current, not the emitter current (which obviously is more).
Well all is taken care of. It may be overkill, but it will never burn up! I had a couple dozen FQPF13N06L(60V, 10A N-Ch MOSFETs)....not sure why I had so many...but I used one of these and all is good.

Thanks for the insights.

i
 
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