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"Forced Beta" of an NPN BJT in SMPS PWM control chip?

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grizedale

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Do you know what is the "Forced Beta" of an NPN BJT?

On page 4 , part "(2)" of the MC33063 datasheet, it refers to the "Forced Beta" of the on-chip NPN.

...But i do not know what is "Forced Beta"?


MC33063 DATASHEET
https://www.ti.com/lit/ds/symlink/mc33063a.pdf
 

Forced beta simply refers to those MC33063 circuit variants, where the output transistor's base current is set by a resistor, e.g. figure 6, in contrast to darlington operation with pin 1 + 8 connected. An Onsemi application note explains more about what they call "forced gain" operation: https://www.onsemi.com/pub/Collateral/AN920-D.PDF

In my opinion, the term doesn't apply strictly, because the circuit uses a fixed Ib despite of a varying Ic. True forced beta operation can be e.g. achived with a feedback current transformer, that set's a fixed ratio between Ib and Ic, it can be found in some self oscillating inverters.
 
Thanks,

Page 4 of the MC33063 datasheet states that the hfe of the transistor is 50 (min).

Page 13 of the AN920-D application note shows an example where they are forcing the gain to be 20, by simply driving too much base current into the switching transistor.

This seems pointless, since there is an available gain of 50.....so why use 20, -it just means youre running too much current in the drive circuit?

(I wonder if the hfe of 50 at 25 deg C alters for other temperatures?)
 

I fear, you are reading too much into a potential misleading term. The datasheet as well as the application note clearly describes, what the circuit does, driving a fixed base current into the output transistor, that assures saturation under the respective conditions. That's all. I already mentioned, that the term forced beta or forced gain isn't very descriptive in my view.

This seems pointless, since there is an available gain of 50.....so why use 20
Click to expand...
If you forgot about the questionable term forced beta for a moment, you are describing what most BJT switch drivers do to assure satured operation, and that's far from being pointless.
 
In a saturating BJT switch you run excess Ib to get a superior Vce. Vce(sat)
times current times duty is your inefficiency power term for that switch. Up
to some point you are better burning base current than taking a high Vce.
"Force beta" is a way to look at whether you are getting well into saturation.
Of course the more you bury it, chasing Vce, the more you are taking a
recovery-time switching loss as a penalty (especially if that recovery time
turns into cross-conduction, ouch).
 
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