Re: Recommend transistor?

From: Ban (bansuri_at_web.de)
Date: 11/03/04 

Date: Wed, 03 Nov 2004 18:35:12 GMT

Clarence wrote:
>>> I need a PNP transistor for switching 5 A/20 V at audio frequencies
>>> or a bit higher. I've been through a dozen parametric searches and
>>> downloaded scores of datasheets - there are too many candidates with
>>> nearly identical characteristics, I can't try them all.
>>>
>>> How do you go about picking the right power transistor? Anyone have
>>> a favorite with a low Vce(sat) and a DC gain >30?
>>
>> You cannot switch AC with a bipolar junction transistor, it has a
>> built-in diode function, that will rectify and induce a horrible
>> distortion. However you can try two Mosfets in series, because they
>> have a parasitic diode from drain to source. But best will still be
>> a relais, and that is what is done normally.
>>
>> If it is pulsed DC then you can do what you want. Have a look at
>> switching times too, because BJT tend to be pretty slow. They will
>> need a lot of current drive to achieve low Vce, so a good driver
>> stage is a must.
>> If the transistors have almost identical data, they will also behave
>> similar, so probably all of your candidates will work nicely.
>> Please be a little more specific about what you want to do, you will
>> get better advise then.
>> --
> Ban
>
> She said nothing about switching AC.
> "switching 5 A/20 V at audio frequencies "
> Is still a DC signal.
> The level is 20Volts. The rate is "Audio frequencies."

How do you know? To me 5A/20V sounds more like the output of an audio power
amp with 100W into 4 Ohms, but if it is supposed to be a switched power
supply or something similar you might be right. A low Vce_sat supports your
understanding. That is why I wrote the last sentence above.
The transistors for both applications might be quite dissimilar, in fact. 

-- 
ciao Ban
Bordighera, Italy
Quantcast