# How calculate PNP turn OFF resistor in BuckBoost converter?

#### treez

##### Advanced Member level 5
How do they arrive at the equation for the RBE resistor on (half way down) page 21 of the following app note.
RBE is the resistor needed to help sweep out the minority carries after the PNP is turned OFF.
The converter is a BuckBoost converter with external high side PNP Power switch.

#### FvM

##### Super Moderator
Staff member
I don't see a substantiation for the magic number 10 with unit volts used in the calculation. The same is used on page 17.

### treez

Points: 2

#### treez

##### Advanced Member level 5
Yes, the LTspice simulator offers nothing in the choice of RBE either.(as attached)..no matter what RBE you choose in LTspice, it makes virtually no difference.....and the PNP has high gain at high current, which it doesnt in reality
--- Updated ---

Another point (with MC33063 app note) with the schematic on page 20, would you agree, is that both Q1 and Q2 need to have the same beta. If they dont, then the one with the higher gain, will get over-saturated at the base drive current chosen, and so will be very slow in switching off.

This is a problem because the schem on page 20 shows that the base current of Q1 is in fact the same as the base current of Q2, and is determined by the value of RB.
Would you agree with this?...and that it is a potential drawback of this circuit.

#### Attachments

• buckboost.zip
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#### FvM

##### Super Moderator
Staff member
34063 is one of the cheapest switchers on the market, you don't use it if you intend high efficiency.

treez

### treez

Points: 2

#### treez

##### Advanced Member level 5
it looks like the RBE resistor is calculated thus....

Current required to sweep out minority carriers from BC region is "CE current at turn-OFF" / (beta x 10)

...they then assume a VBE of 1V.

Then use ohms law with the above current and voltage to get RBE
Woudl you agree?

#### FvM

##### Super Moderator
Staff member
Why beta x 10, why VBE = 1V? These are just arbitrary assumptions without substantiation.

treez

### treez

Points: 2

#### treez

##### Advanced Member level 5
I see your point.
Incidentally, on page 27 of the MC34063 app note...

...they have a buck with output of 15V/3mA, but a sense resistor of 22 milliohms. Also, the base drive resistor is 51 Ohms and would have approx 34V across it.
Do you agree that this schematic on page 27 is nonsense?
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>>>>>>>>>>>>>>>>>o<<<<<<<<<<<<<<<<<<
Page 8 of the MC33063A datasheet…

…states that if a non Darlington drive configuration is used in situations where i(base) > 30mA and switch current <300mA, then instead, an external PNP transistor should be used as in fig 7b on page 9.

The thing is , that external PNP is in the non-darlington configuration, so surely this just means that the 2us extension of ON time just happens to the external PNP transistor instead , and you’re just as badly off as before?
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>>>>>>>>>>>>>>>>>>>0<<<<<<<<<<<<<<<<<<<<<<<<<
hi....
Page 3 of SLVA252B, about the MC34063A PWM control chip, states that the oscillator charges from 0.75v to 1.25v...

SLVA252B:

...However, page 4 of the MC34063 datasheet shows the oscillator charging from 0.432V to 1.264V...

MC34063A datasheet:

..do you know which one do we believe?

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#### Easy peasy

##### Advanced Member level 5
There appear to be quite a few typo and other errors in that app note, but not :

" ...they have a buck with output of 15V/3mA, but a sense resistor of 22 milliohms. Also, the base drive resistor is 51 Ohms and would have approx 34V across it.
Do you agree that this schematic on page 27 is nonsense?"

as the output is supposed to be 3.6A ave max, ( not 3.0mA as typed ) so the 706mA in the base is in fact correct ( 36V/ 51 ohms ) to drive the xtor fully on - although a better xtor may allow less base current.

there is another error in a boost converter on page 12, fig. 11 the o/p is stated as 5V for a 9V input - when clearly the ckt is a booster ( 28Vout in text )

there are bound to be other errors too ...
--- Updated ---

same base drive in fig.34 again because the ckt is drawing 3.6A ave and the xtor needs plenty of base drive for the assumed Beta ( Hfe ) of 35.
--- Updated ---

again, 1A, in fig 31
--- Updated ---

plenty of errors in the text of fig. 27
--- Updated ---

again the 550mA in Fig. 22 seems about right given the xtor has to supply more than 5A to the buck ckt ...

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