Help with rise/fall time of BC548

[Sim Joo Khai]

Does anyone know the approximate rise/fall time (in microseconds) of On
Semiconductor's BC548 under conditions of Ic = 162 mA, Ib = 20 mA and Vce =
6V? Their datasheet does not provide it and technical support takes about a
week or more to reply.

Please reply, thanks in advance.

Joo Khai
[email protected] (ReMoVe tHe nospam_ if you want to e-mail me.)
Singapore

 

[Winfield Hill]

Sim Joo Khai wrote...

Does anyone know the approximate rise/fall time (in microseconds)
of On Semiconductor's BC548 under conditions of Ic = 162 mA,
Ib = 20 mA and Vce = >6V? Their datasheet does not provide it
and technical support takes about a week or more to reply.

Joo Khai, a transistor's rise / fall time generally isn't provided
in data sheets because you'll have to calculate the answer, which
depends completely upon the circuit conditions. Sometimes you'll
see a value with a specific set of measurement conditions listed,
but this cannot be relied upon if you change any of the circuit's
component values, etc.

In addition to current, two of the most important issues are input
impedance and output load. For example, you mention 20mA of base
drive, but what constitute's base shutoff, open-circuit 0mA? Do you
have a negative shutoff drive (good, even even if for a short time)?
Or a shorted base? Is the collector load resistive, 6V/162mA = 37
ohms? No added capacitive load?

Turning to your BC548, it doesn't appear to be recommended for use
above 100mA. Beta drops to less than 40% of the value at 10mA (at
10V, worse at lower voltages), the gain-bandwidth product, f_T, is
plummeting, and the Vce(sat) voltage is heading for the sky. :>(
You'd be better off with one of Zetex' super-E-Line transistors.


Thanks,
- Win

whill_at_picovolt-dot-com

 

[CBarn24050]

your operating way out the device specs. redesign your circuit.

 

[Paul Burridge]

Does anyone know the approximate rise/fall time (in microseconds) of On
Semiconductor's BC548 under conditions of Ic = 162 mA, Ib = 20 mA and Vce =
6V? Their datasheet does not provide it and technical support takes about a
week or more to reply.

Are you seriously saying you can get a Vce of 6V at 160mA of Ic with
this particular device??

 

[Watson A.Name - Watt Sun]

Does anyone know the approximate rise/fall time (in microseconds) of On
Semiconductor's BC548 under conditions of Ic = 162 mA, Ib = 20 mA and Vce =
6V? Their datasheet does not provide it and technical support takes about a
week or more to reply.

You're forcing the beta to be only 8, which shows that the transistor
is having a really difficult time with that much current. If you read
the Fairchild data sheets, you would see that the transistor has a
maximum Ic of 100 mA, so you've exceeded that by a wide margin. Don't
expect to get an answer to a question that's violating the maximum
manufacturer's specifications!

Use a transistor that's made to handle that much current. The BC337
is a good, cheap choice.

Please reply, thanks in advance.

Joo Khai
[email protected] (ReMoVe tHe nospam_ if you want to e-mail me.)
Singapore

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[Watson A.Name - Watt Sun]

Sim Joo Khai wrote... [snip]

Turning to your BC548, it doesn't appear to be recommended for use
above 100mA. Beta drops to less than 40% of the value at 10mA (at
10V, worse at lower voltages), the gain-bandwidth product, f_T, is
plummeting, and the Vce(sat) voltage is heading for the sky. :>(
You'd be better off with one of Zetex' super-E-Line transistors.

Exactly. But the pinout of the Zetex may not be the same. A couple
things I don't like about those Zetex transistors. One is they're not
second sourced, so your source may dry up when they go out of business
or get bought out (like much of the semiconductor industry lately).
And I've found the prices to be much higher than other industry
standard transistors.

I recommended a BC337 in another thread.

Thanks,
- Win

whill_at_picovolt-dot-com

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[Sim Joo Khai]

Then may I know if the BC337 will comfortably switch a (square wave)
frequency of 500 kHz at Ib = 20 mA, Ic = 162 mA and Vce = 6V without a
significant rise/fall delay that affects the waveform?

Please reply, thanks in advance.

Joo Khai
[email protected] (DeLeTe ThE NoSpAm_ 2 RePlY)
Singapore

 

[Paul Burridge]

Then may I know if the BC337 will comfortably switch a (square wave)
frequency of 500 kHz at Ib = 20 mA, Ic = 162 mA and Vce = 6V without a
significant rise/fall delay that affects the waveform?

Anyone got a few minutes for a Spice simulation? I don't happen to
have a model handy for this excellent transistor, sadly.
To the OP: what are the load characteristics of the circuit you have
in mind?

 

[Winfield Hill]

Sim Joo Khai wrote...

Then may I know if the BC337 will comfortably switch a (square wave)
frequency of 500 kHz at Ib = 20 mA, Ic = 162 mA and Vce = 6V without
a significant rise/fall delay that affects the waveform?

Comfortably? The answer is a conditional YES. Or use a 2n4401, etc.

It's a conditional answer, because you now introduced the aspect
of delay, and a thoroughly-saturated transistor switch will have
considerable storage delay. This will distort your squarewave by
stretching the ON time, but not the OFF time, so it'll no longer
have an exactly 50% duty cycle. Why don't you tell us more about
what you're trying to do, we may be able to suggest improvements.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[CBarn24050]

your values dont make sence. this transistor has a minimum gain of around 100
at this Ic, so your base current would only need to be around 1mA. I think you
have not expressed your question correctly.

 

[Watson A.Name - Watt Sun]

Then may I know if the BC337 will comfortably switch a (square wave)
frequency of 500 kHz at Ib = 20 mA, Ic = 162 mA and Vce = 6V without a
significant rise/fall delay that affects the waveform?

You won't need as much base current for the BC337, because it was made
to handle higher currents. You say significant, but that gives us no
indication of what it is. I would try some to find out, they should
do much better than a BC548. There are other switching transistors
such as the PN2369 which have very low rise and fall times.

Please reply, thanks in advance.

Joo Khai
[email protected] (DeLeTe ThE NoSpAm_ 2 RePlY)
Singapore

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[[email protected]]

Anyone got a few minutes for a Spice simulation? I don't happen to
have a model handy for this excellent transistor, sadly.
To the OP: what are the load characteristics of the circuit you have
in mind?

*
* BC337 Spice models by Philips:
*
..MODEL QBC337-16 NPN
+ (IS=4.887E-14 NF=1.002 ISE=2.552E-15 NE=1.65 BF=207.2
+ IKF=0.902 VAF=184.8 NR=1.002 ISC=7.006E-14 NC=1.25
+ BR=21.85 IKR=0.1 VAR=20 RB=50 IRB=0.0002 RBM=2
+ RE=0.119 RC=0.25 XTB=0 EG=1.11 XTI=3 CJE=4.217E-11
+ VJE=0.6596 MJE=0.3434 TF=6E-10 XTF=2.1 VTF=2.4
+ ITF=0.79 PTF=86 CJC=1.734E-11 VJC=0.1419 MJC=0.3484
+ XCJC=0.455 TR=3.5E-08 CJS=0 VJS=0.75 MJS=0.333 FC=0.652)
*
..MODEL QBC337-25 NPN
+ (IS=4.13E-14 NF=0.9822 ISE=3.534E-15 NE=1.35 BF=292.4
+ IKF=0.9 VAF=145.7 NR=0.982 ISC=1.957E-13 NC=1.3
+ BR=23.68 IKR=0.1 VAR=20 RB=60 IRB=0.0002 RBM=8
+ RE=0.1129 RC=0.25 XTB=0 EG=1.11 XTI=3 CJE=3.799E-11
+ VJE=0.6752 MJE=0.3488 TF=5.4E-10 XTF=4 VTF=4.448
+ ITF=0.665 PTF=90 CJC=1.355E-11 VJC=0.3523 MJC=0.3831
+ XCJC=0.455 TR=3E-08 CJS=0 VJS=0.75 MJS=0.333 FC=0.643)
*
..MODEL QBC337-40 NPN
+ (IS=7.809E-14 NF=0.9916 ISE=2.069E-15 NE=1.4 BF=436.8
+ IKF=0.8 VAF=103.6 NR=0.991 ISC=6.66E-14 NC=1.2
+ BR=44.14 IKR=0.09 VAR=14 RB=70 IRB=0.0002 RBM=8
+ RE=0.12 RC=0.24 XTB=0 EG=1.11 XTI=3 CJE=3.579E-11
+ VJE=0.6657 MJE=0.3596 TF=5E-10 XTF=2.5 VTF=2
+ ITF=0.5 PTF=88 CJC=1.306E-11 VJC=0.3647 MJC=0.3658
+ XCJC=0.455 TR=2.5E-08 CJS=0 VJS=0.75 MJS=0.333 FC=0.843)
*

Martin.

 

[Winfield Hill]

CBarn24050 wrote...

your values dont make sence. this transistor has a minimum gain of
around 100 at this Ic, so your base current would only need to be
around 1mA. I think you have not expressed your question correctly.

He's asking about risetime and falltime, so that implies switching.
The implication is that he's switching a load (resistance?) tied to
+6V (and with a +6V output when off, no bias current). The circuit
draws 162mA with Vce(sat) = close to 0V output, when turned on with
20mA of base bias current (to insure switch collector saturation).

Thanks,
- Win

whill_at_picovolt-dot-com