How can I find NPN's and PNP's that match Hfe?
lardom
Posts: 1,659
I built a triple half H-bridge to experiment with a sensorless BLDC. I can make the motor spin but I can't make it run closed loop yet. One of the problems I'm having is that the BEMF from the three phases don't match. In closed loop the BEMF from the floating phase is fed to a comparator and is compared to a common virtual ground.
At the moment I'm trying to match NPN collector current and match PNP collector current by swapping resistors individually. For example, I'm using pn4401 small-signal transistors to drive the bases of three TIP31's and three TIP42g's. One pn4401 has a gain of 139. Another pn4401 has a gain of 226.
I don't have a lot of experience biasing transistors so that's why I'm asking if variation in transistor Hfe is common. Are mosfets easier to balance?
At the moment I'm trying to match NPN collector current and match PNP collector current by swapping resistors individually. For example, I'm using pn4401 small-signal transistors to drive the bases of three TIP31's and three TIP42g's. One pn4401 has a gain of 139. Another pn4401 has a gain of 226.
I don't have a lot of experience biasing transistors so that's why I'm asking if variation in transistor Hfe is common. Are mosfets easier to balance?
Comments
EDIT: Now that I have time to actually read your post I would off the top of my head suggest that you might handle this with opamps regulating the current so then it doesn't matter that the driving transistors are mismatched. Variation in Hfe is very normal, and no, mosfets will not be easier to balance and they are very different beasts.
That's great news. (that made my day!) I just bought a supply of LM339's to experiment with. I'd heard about opamps being used as constant current devices but it didn't occur to me that I could use them to balance mismatched transistors.
So the opamp with feedback will automatically compensate for the transistor which provides the actual drive current. If you tie the inverting input of the opamp back to the emitter of the drive transistor you will have unity gain where the voltage on the + input of the opamp will be matched on the emitter. For PNP, just think of the voltage rail as being the "ground" so that voltages are relative to that.
To save me having to explain it all, here is a quick link
"Warning: hFE is not a "good" transistor parameter; for instance, its value can vary from 50 to 250 for different specimens of a given transistor type. It also depends on collector current, collector-to-emitter voltage, and temperature. A circuit that depends on a particular value for hFE is a bad circuit."
-Phil