BJT selection for the reference circuit

[little0192]

Hi,
I have simulated the below circuit in LTspice and found that the BJT (Q203) is having less power dissipation (~1mW) then the Q202 BJT.
However, the designer selected a 200mW package for the Q203 and 100mW for the Q202.

Can anyone help me understand what am I missing here? As per my understanding even if I select the 100mW BJT part then also the circuit will be okay as per the LTspice simulations.

In scenario 1, When Motor is ON, Q203 (ON), Q202 (OFF)
In scenario 2, When Motor is OFF Q203 (OFF), Q202 (ON)

Q202 MPN: RN1102
Q203 MPN: RN1402

LTspice file are attached for the reference.
Thank you.

 

[danadakk]

Hi,
I have simulated the below circuit in LTspice and found that the BJT (Q203) is having less power dissipation (~1mW) then the Q202 BJT.
However, the designer selected a 200mW package for the Q203 and 100mW for the Q202.

Can anyone help me understand what am I missing here? As per my understanding even if I select the 100mW BJT part then also the circuit will be okay as per the LTspice simulations.

In scenario 1, When Motor is ON, Q203 (ON), Q202 (OFF)
In scenario 2, When Motor is OFF Q203 (OFF), Q202 (ON)

Q202 MPN: RN1102
Q203 MPN: RN1402
View attachment 182789

LTspice file are attached for the reference.
Thank you.

Q202 only shows the 1.6 x 1.6 in its datasheet.

Q203 only shows the 2.9 x 2.5 in its datasheet.


Regards, Dana.

 

[little0192]

Q202 only shows the 1.6 x 1.6 in its datasheet.

Q203 only shows the 2.9 x 2.5 in its datasheet.


Regards, Dana.

Hi Dana,
My question is why to use two different packages for the bjt. Why not select the same bjt for both the places here.
Thanks

 

[danadakk]

Not sure......maybe someone else can spot the reason.

 

[FvM]

Both digital transistors are functional compatible, probably it's the same chip in different packages. There may be "historical" reasons, e.g. a previous revision used a different type for Q203.

 

[dick_freebird]

I find the selection odd. The bigger is put to the internal pullup
at ~1mA (but Pdiss maybe 1mW and only for a turnon pulse?),
while the little is put to the external sense line of unknown
attributes.

I'd call that the greater thermal threat / fault challenge not to
mention the wisdom of a small-die bjt up against the outside
world alone. Only if "VS" is guaranteed to be safe and weak
does using the smaller make sense to me. Unless maybe it -is-
a weak chopper signal from an opto and can't stand any excess
Cstray, and half the Cjc is a 20% increase in RPM range....