power disspation on fan motor driver

[chu]

disspation

hi everybody,

please help me on some questions. thanks in up-front.

1) in some of the datasheets, i saw "low Vce(sat)", as what i understand is this is Vce of BJT connected to output pin. if normal BJT is used, the Vce(sat) should be 0.7V, the point it starts saturate (because Vbe is 0.7V), if Vce lower than this value, then collector current Ic will drop extensively which we don't want. am i correct on this ? please correct me.
are there any chips with Vce(sat) lower ? example 0.3V ?

2) for some of the 12v fan driver chip, Icc=20mA but Io=500mA, i though Io is coming from Icc (kirchoff current law) ? how this happen ? please explaint.

3) ba6809 datasheet calculation on chip power dissipation is :
total power P-total=Pcc + Po ; Pcc=Vcc*Icc ; Po=Vo(low)*Io
i though whole chip power dissipation should only be Pcc ?? why is ba6809 calculated this way ?
is the Vo(low) means Vce ?? please explaint why.
if Vo=Vce true, then Pcc should have include Po. why not correct ?

4) does power disspation in absolute max rating means the max power the packaging can take ?
it doesn't related to IC power dissipation but IC power cannot more than it. true ??

5) when we were to use a fan motor driver, what item on power dissipation should we take care ? how about the load (fan) ?

thank u very much.

_________________
rgds,
chu

 

[huojinsi]

hi everybody,

please help me on some questions. thanks in up-front.

1) in some of the datasheets, i saw "low Vce(sat)", as what i understand is this is Vce of BJT connected to output pin. if normal BJT is used, the Vce(sat) should be 0.7V, the point it starts saturate (because Vbe is 0.7V), if Vce lower than this value, then collector current Ic will drop extensively which we don't want. am i correct on this ? please correct me.
are there any chips with Vce(sat) lower ? example 0.3V ?

2) for some of the 12v fan driver chip, Icc=20mA but Io=500mA, i though Io is coming from Icc (kirchoff current law) ? how this happen ? please explaint.

3) ba6809 datasheet calculation on chip power dissipation is :
total power P-total=Pcc + Po ; Pcc=Vcc*Icc ; Po=Vo(low)*Io
i though whole chip power dissipation should only be Pcc ?? why is ba6809 calculated this way ?
is the Vo(low) means Vce ?? please explaint why.
if Vo=Vce true, then Pcc should have include Po. why not correct ?

chu

1. About ur the first question,When the Vce is lower than 0.7V,the transistor will be in the saturation region and doesn't lose amplify ability,so the collector current cann't drop! Theoretically,the Vce of BJT saturation voltage can reach 0.3V.But for large output BJT, it is very difficult to reach 0.3V,because Vce is restricted by process yet!
2. About ur the second question, u can simply understand like this. Assume Vin*Iin=Vo*Io (not care for the dissipation of IC),when the Vin*Iin is constant and the Vo drops, the Io will surely become large!
3. the total power include the dissipation of IC and load power,it is Ptotal=Vin*Iin=Vcc*Icc+Vo*Io. "i though whole chip power dissipation should only be Pcc ??" is error!

 

[chu]

hi huojinsi,

thank u so much for your reply. i need more advice from u.

1) on question 1, if i look into the Ic vs Vce graph, when Vce saturated (on left of graph) Ic change/ drop a lot when Vce reduce a bit. please kindly explaint to me again, thx.

2) why use Vce(sat) for driving huge load(fan) ? i thought normally use Active Fwd Region of the transistor.

thanks again.