Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

What is the output resistance of totem pole (NPN/PNP) FET driver stage?

Status
Not open for further replies.

Flyback

Well-Known Member
Hello,
What is the output resistance (high and low) of the PNP/NPN ‘totem pole’ driver stage in the attached schematic?
I take it that “R(out) high” is RB/hfe = 1000/200 = 5 ohms?
The problem is that the datasheets only quote hfe at 25degC and our ambient temperature is 170degC, how will this affect the hfe value?
FMMT619 NPN datasheet
**broken link removed**

FMMT720 PNP datasheet
**broken link removed**
 

Attachments

  • Totem pole fet driver.pdf
    16.3 KB · Views: 149
The gain is dependent on temperature; see the three curves. You can guess what 170C will do.
Also the gain is dependent on collector current. This transistor peaks at 120mA.
This is for the transistor working in the linear regoin. As you approach Vsat other factors come in.
upload_2016-1-1_9-26-1.png

upload_2016-1-1_9-31-58.png
 
The transistors are rated only to +150C. What is the application that has continuous operation at +170C?

ak
 
its measurement while drilling for fracking. I appreciate that the datasheet says that 150degC is max, but the customer says that these bjt's have done the job at 170degc ambient.
 
The Q2 and Q3 bases see two different driving point impedances, R15, and R15 in parallel with M3's Rdson. Also, there is a period of time when both transistors are off, for an infinite output impedance. So it looks like there are three distinct output impedances.

Also, unless the circuit is potted in thermally conducting material, potting moves less heat away from the device than does air.

ak
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top