Transistor specs for sub in switching supply?

[frontline@nospam]

I have a switchmode power supply in a car amplifier that uses 8
matsushita 2SD1445A-Q transistors unable to find these I want to sub
2SD2151-Q that is supposed to be a direct sub and available however
some specs are not in the datasheet - Cob and hFE3 also
Pc is 30W and not 40W as the D1445A is.
Well thats the info I have, my question is what should I be concerned
with and how will these differences effect operation at higher
currents?
Jeff

 

[David J. Starr]

frontline@nospam wrote:

I have a switchmode power supply in a car amplifier that uses 8
matsushita 2SD1445A-Q transistors unable to find these I want to sub
2SD2151-Q that is supposed to be a direct sub and available however
some specs are not in the datasheet - Cob and hFE3 also
Pc is 30W and not 40W as the D1445A is.
Well thats the info I have, my question is what should I be concerned
with and how will these differences effect operation at higher
currents?
Jeff

They will most likely work. Cob (Cbc?) is one of those "smaller is

better" parameters. The ideal transistor would have zero. Newer
transistors usually have less and so even if the data sheet fails to
spec the parameter, the newer device will likely be better than the old
device. Hfe has never been tightly controled, so all circuits intended
for mass production are designed to work with transistor hfe the varies
by a factor of three from lot to lot of devices. Pc (power,collector)
is a function of package style. Transistor power dissapation is limited
by heat. Heat is developed in the device junctions and is conducted to
the ambient air thru the case. Larger cases have a lower thermal
resistance than smaller ones and so can dump more heat. If the case is
the same, the thermal resistance is the same and thus the power handling
capability is the same.
Question. How did the 8 orignal transistor happen to fail?
Transistors don't wear out, they get blown out by a circuit fault. If
the circuit fault that took out the original transistors is still
present, the new transistors will fail just like the original ones did.

David Starr

 

[frontline@nospam]

The first failure was due to capacitors these were replaced with a type
I have used before in the same model amplifiers with no problems.
I have replaced these transistors before also no problems, this is the
first time with this replacement part and in two amps both have been
returned blown, also both were tried in the same vehicle so it can also
be a problem with the vehicle but I do not have it here to check it out
and have to rely on someone else if that was done.
They checked out fine on the bench after repair.
Thanks for the info, I hope they did not leave the specs out because
they are worse!
Jeff