Logic NPN switching PNP at 90 to 100 volts

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DonaldL

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I have a PNP switching 90 to 100 volts at 2 to 3 amps and a NPN at logic level. the PNP is a IXTH48P20P that has a Gate-Source Threshold Voltage of -4 volts.
what i would like to know is what type of logic level NPN should i use to power on the PNP at say 10 volts and should i have opto isolation between the 100 volts and the 5 volts.


Donald.
 

Your schematic is a little confusing as the IXTH48P20P is a MOSFET but you show it as a BJT.

However, the question seems to ask what transistor type to use for Q1. Almost any type that can handle 100V should be OK as the collector current need not be more than a few mA. Assume that when the Q1 is turned ON, it's collector voltage goes to 0V then calculate the values of R1 and R2 to give the gate voltage you need on Q2.

As Q1 is an NPN bipolar type, the base current decides whether it conducts or not so assuming 0.65V Vbe, and it has a minimum hfe of say 50 you can calculate R1 from Ic/hfe than reduce it's value by say 10 times to be sure Q1 goes fully into saturation.

To advise whether opto-isolation is required, we need to know the application. It would only be for safety reasons, the circuit will function electrically without one.

Brian.
 


I thank you for your reply and yes i know i show a bjt as i was in a hurry. the mosfet is as stated a PNP mosfet being switched by a cheap NPN from logic level 5 volts. so even if i am switching the PNP at 10 volts the NPN has to handle 100 volts also.

the circuit will be switching a dynamic inductor at 60 cycles per second and will not have Bemf but will have timing overlap as in two on at a time in a make before break set up.
thanks for the on the functionality.

Donald
 

Hi,

Do you need to connect the load to GND?
I'm asking because low side switching is more simple...

I don't understand how "make before break" relates to the driving circuit..

Klaus
 

If you use something like a MPSA42 high voltage switching transistor for Q1, it should not be a problem. You may need the additional circuitry like in my example if the switching speed is a factor for you.
 

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KlausST said:
Hi,

Do you need to connect the load to GND?
I'm asking because low side switching is more simple...

I don't understand how "make before break" relates to the driving circuit..

Klaus
Click to expand...
The high side switching is a must because i am trying to mimic a rotating positive brush and the make before break would be the brush making the next contact as it rotates so i will have to have a make before break circuit capabilities.

i was going to switch the the PNP Mosfet with a 200volt logic level NPN like a IRA640A Mosfet but i don't think i need that heafty of an NPN since the PNP can be fully on with -10 volts. i was wondering if the TK30A06N1,S4X NPN Mosfet will switch the PNP ok and what would be your resistor recommendations .

E design,
I will be switching At 60 CPS so i don't think switching speed will be a factor.
 

DonaldL said:
E design,
I will be switching At 60 CPS so i don't think switching speed will be a factor.
Click to expand...

I wasn't referring to the switching frequency, but rather the rise/fall time of the switching device to minimize losses.
 

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