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Miller Capacitence PNP transistor - Beware of SATURATION

MacIntoshCZ

Member
Hello,
I would like to ask how can i get rid of saturation. PNP transistor is turning off slowly. I want to invert logic level of timer 555 to have regulation from 0-50% duty cycle and control mosfet.
I would like to also ask how can i contruct graph like this here . There is no ICE/VCE char in datasheet of 2n3906.
I tried higher base resistor but it makes pnp to turn on slower.
Thanks a lot
***
When measured on osciloscope, mosfet is disconected, probe at collector.
This is my circuitry:
1594380582038.png
 

ronsimpson

Well-Known Member
Most Helpful Member
Several things to look at.
1) remove the diodes and start over. Put a small capacitor across the base resistor. I don't know the actual value but you want the base to go up to 10 to 13 volts when "555timer" rises.
2) There are large capacitors inside the MOSFET. 1k is not enough current to turn off the transistor. Play with reducing the 1k to 330 or some thing.
1594382408704.png
This circuit will drive the Gate on/off. It makes the "1k" a transistor and will pull down much harder. Also it will keep the little transistors out of saturation.
1594382696126.png
 

MacIntoshCZ

Member
Without diodes its little worse than before.
I dont have problem with turn on. I tried cap parrelel to base resistor but it do not affect turn off delay.
I dont have mosfet connected. It makes things worse - bigger delay.
I dont know if that second circuitry you posted works as inverter.
 

alec_t

Well-Known Member
Most Helpful Member
If the 555 supply is 9V (the same as the collector voltage), then the 555 output voltage will only pull up to about 8V so won't turn off the PNP fully in the first circuit in post #5.
 

MacIntoshCZ

Member
Not a inverter.

Cap will not work with diodes. Cap usually really helps.
Thanks for help.
My circuitry at the moment only contain base resistor 10k with pararrel cap "102". 100k resisitor on BE. 220R resistor at collector to ground.
Works flawlessly.
 

MacIntoshCZ

Member
Is there any other way to speed up transistor? When resistor at collector is 220r its getting little warm. Could it be adjusted with base resisitor?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What are all the diodes for?. And what is the supply voltage for the 555?, is it the same 9V rail?.

Why do you think it's slow turning off?, can you post a scope diagram of the waveform on the collector?.

As for how to construct the graph above (why?) you do it with a multimeter, plot and measure the different points, and then manually draw the graph from the data.
 

MacIntoshCZ

Member
Diodes? Dont know. Created from some schematic on internet when googling for "pnp baker clamp".
555 has 9v vcc.
Delay was something to 10u seconds with 1k res. With 220r it is something around hundreds of nano seconds. So its fine.
 

ronsimpson

Well-Known Member
Most Helpful Member
With 220r it is something around hundreds of nano seconds.
TI.com data sheet for NE555: output rise and fall time is typically 100nS but could be as slow as 300nS.
If you think things are slow now add the capacitance of a MOSFET Gate.
2N3906: Under conditions different than you are using but..... switching time (not delay time) is best at 50mA of collector current.
1594396178240.png
This is some thing I typically do. Inverts! Pull down is active not 1k or 220 ohms. Will pull down hard. It also pulls up hard.
1594396404313.png
 

MacIntoshCZ

Member
TI.com data sheet for NE555: output rise and fall time is typically 100nS but could be as slow as 300nS.
If you think things are slow now add the capacitance of a MOSFET Gate.
2N3906: Under conditions different than you are using but..... switching time (not delay time) is best at 50mA of collector current.
View attachment 125867
This is some thing I typically do. Inverts! Pull down is active not 1k or 220 ohms. Will pull down hard. It also pulls up hard.
View attachment 125868
Is there any way to invert just logic level? Not voltage itself?
 

MacIntoshCZ

Member
Here are some measurements on oscilloscope.
Yellow are variant with 2n3906 100k, 220r, parralel cap with 10k base res.
purple is yours invertot with 2 npn transistors. Its software inverted to positive level.
quickprint 17/18 is without mosfet attached
19/20 mosfet is attached
Any sugestion to improve?
I would try this:
 

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ronsimpson

Well-Known Member
Most Helpful Member
then the 555 output voltage will only pull up to about 8V so won't turn off the PNP fully in the first circuit in post #5.
This maybe the answer. But I would think a speed up Base capacitor would have helped greatly.
The B-E resistor=100k, change to 10k or even down to 1k, but not less. With 100k the Base might have 0.5V B-E. Not good. The Base speed up cap should have pushed the Base above the 9V supply for a short time.
 

unclejed613

Well-Known Member
Most Helpful Member
Diodes? Dont know. Created from some schematic on internet when googling for "pnp baker clamp".
looks like more diodes than necessary for a Baker clamp. [this description] might explain better how a baker clamp is supposed to work.
 

unclejed613

Well-Known Member
Most Helpful Member

ronsimpson

Well-Known Member
Most Helpful Member
This is getting hard to follow. Too many circuits.
Please show the output of the 555. Is the rise and fall time about 100nS?
 

crutschow

Well-Known Member
Most Helpful Member
You might try a fast switching transistor such as the 2N5771.
It has a much shorter saturaturation turn-off delay as compared to most general-purpose BJTs.
It's main limitation is the 15V max collector-emitter voltage rating.

LTspice switching comparison simulation below:
Notice that the 2N5771 has a very small delay (<100ns), compared to the 3us of the other two transistors.

1594440282999.png
 

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