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Old 17th January 2013, 01:57 PM   #41
wahab is offline wahab  Algeria
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The VBEM device must be selected to provide low VCEsat as well
as very high beta.

Sanken produce a dedicated transistor for this application with
a 3A rating but still a remarkable beta of more than 500 and
typicaly in excess of 1000.
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Old 17th January 2013, 02:27 PM   #42
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Quote:
Originally Posted by wahab View Post
The VBEM device must be selected to provide low VCEsat as well
as very high beta.

Sanken produce a dedicated transistor for this application with
a 3A rating but still a remarkable beta of more than 500 and
typicaly in excess of 1000.
That device is available in the US, through Allegro or one of their affiliated parties.
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Old 12th September 2014, 10:20 PM   #43
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I want to thank everyones contributions. I am still trying to get the bias right. I called Douglas self and he told me to start with a 100 ohm compensation reisistor. That seems very high to me.
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Old 14th September 2014, 02:34 AM   #44
wahab is offline wahab  Algeria
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Quote:
Originally Posted by homemodder View Post
I think the main issue is the choice of the transistor which leads some to use more complex multipliers which is unnecessary like cfp. Cfp is prone to oscilation, Darlington is not and has about the same gain and same packages as single tranasistor. In fact using just a single super beta transistor is more than enough for 98 percent of designs and has a positive benefit above the others. The higher the hfe the more temperature sensitive the transistor, as is a lower voltage type to a lesser extent.
Good point HM, indeed there s transistors specialy designed for VBEM purpose and as you point it they have high gain but also low VCE sat, high current capability and consequently low re, re = emitter internal resistance.

Other than that i must admit that the VBEM is quite an annoying part in a design, no wonder that i personaly went lateral fets as often as possible.

Edit , i just noticed that i already posted the datasheet...

Same problems same solutions....
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Last edited by wahab; 14th September 2014 at 02:39 AM.
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Old 17th September 2014, 05:50 PM   #45
jpc2001 is offline jpc2001  United States
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You need a solid connection to the sensing transistor. If you have a flaky connector and it opens up (on the base or emitter, say) the VBE multiplier will conduct zero current. Then current at the outputs will rise to several amps. If the outputs don't blow right away, a positive feedback loop occurs as the outputs heat up and the current through them increases over time.

If you're lucky, overcurrent protection kicks in or a fuse blows before damage occurs.

Maybe that goes without saying. Carry on with the discussion about compensation resistors and bypass caps.
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Old 17th September 2014, 08:22 PM   #46
jpc2001 is offline jpc2001  United States
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Quote:
Originally Posted by keantoken View Post
transistors commonly used for the Vbe multiplier typically have low Hfe of 100 or so.
Why is that?

If you replace a Vbe transistor with one of higher beta, is it going to cause trouble?
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