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BU208A 参数 Datasheet PDF下载

BU208A图片预览
型号: BU208A
PDF下载: 下载PDF文件 查看货源
内容描述: NPN硅功率晶体管 [NPN SILICON POWER TRANSISTOR]
分类和应用: 晶体晶体管功率双极晶体管
文件页数/大小: 6 页 / 205 K
品牌: ONSEMI [ ONSEMI ]
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BASE DRIVE  
The Key to Performance  
By now, the concept of controlling the shape of the turn–off  
This shows the parameter that really matters, dissipation,  
whether caused by switching or by saturation. For very low  
base current is widely accepted and applied in horizontal  
deflection design. The problem stems from the fact that good  
saturation of the output device, prior to turn–off, must be as-  
sured. This is accomplished by providing more than enough  
L
h
a very narrow optimum is obtained. This occurs when I  
B
B1  
I
, and therefore would be acceptable only for the  
CM  
FE  
“typical” device with constant I  
curves become broader and flatter above the I . h = I  
point as the turn off “tails” are brought under control. Eventu-  
. As L is increased, the  
CM  
B
I
to satisfy the lowest gain output device h  
at the end of  
B1  
scan I  
FE  
B1 FE CM  
. Worst–case component variations and maximum  
CM  
high voltage loading must also be taken into account.  
If the base of the output transistor is driven by a very low  
impedance source, the turn–off base current will reverse  
very quickly as shown in Fig. 3. This results in rapid, but only  
partial collector turn–off, because excess carriers become  
trapped in the high resistivity collector and the transistor is  
still conductive. This is a high dissipation mode, since the  
collector voltage is rising very rapidly. The problem is over-  
come by adding inductance to the base circuit to slow the  
base current reversal as shown in Fig. 4, thus allowing ac-  
cess carrier recombination in the collector to occur while the  
base current is still flowing.  
ally, if L is raised too far, the dissipation all across the curve  
B
will rise, due to poor initiation of switching rather than tailing.  
Plotting this type of curve family for devices of different h  
,
FE  
essentially moves the curves to the left, or right according to  
the relation I = constant. It then becomes obvious that,  
h
B1 FE  
for a specified I , an L can be chosen which will give low  
CM  
B
dissipation over a range of h  
and/or I . The only remain-  
FE  
high enough to accommodate the  
B1  
ing decision is to pick I  
B1  
lowest h  
part specified. Neither L nor I are absolutely  
FE  
critical. Due to the high gain of Motorola devices it is sug-  
gested that in general a low value of I be used to obtain  
B
B1  
B1  
optimum efficiency — eg. for BU208A with I  
= 4.5 A use  
CM  
1.2 A. These values are  
Choosing the right L Is usually done empirically since the  
I
1.5 A, at I  
= 4 A use I  
B
B1  
CM B1  
equivalent circuit is complex, and since there are several  
lower than for most competition devices but practical tests  
have showed comparable efficiency for Motorola devices  
importantvariables(I  
to plot fall time as a function of L , at the desired conditions,  
for several devices within the h  
FE  
mative method is to plot power dissipation versus I  
,I ,andh atI  
).Onemethodis  
CM B1 FE CM  
even at the higher level of I  
.
B
B1  
specification. A more infor-  
An L of 10 µH to 12 µH should give satisfactory operation  
B
for a  
of BU208A with I  
2 A.  
of 4 to 4.5 A and I  
between 1.2 and  
B1  
CM  
B1  
range of values of L .  
B
TEST CIRCUIT WAVEFORMS  
I
I
B
B
I
C
I
C
(TIME)  
(TIME)  
Figure 3  
Figure 4  
TEST CIRCUIT OPTIMIZATION  
The test circuit may be used to evaluate devices in the  
conventional manner, i.e., to measure fall time, storage time,  
and saturation voltage. However, this circuit was designed to  
evaluate devices by a simple criterion, power supply input.  
Excessive power input can be caused by a variety of prob-  
lems, but it is the dissipation in the transistor that is of funda-  
mental importance. Once the required transistor operating  
current is determined, fixed circuit values may be selected.  
4
Motorola Bipolar Power Transistor Device Data  
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