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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Hey... when you see all these people building motor drivers (steppers in particular) all you hear is mosfet mosfet mosfet. Being the newer tech, and in many applications a superior device, people tend to use them for this application. I am building a couple stepper motor drivers to handle 24v and 2A pulses spread over 8 bjt's. Based on extremely simple calculations, the bjt would dissipate 3W if held at 2A constant with the 24v supply. But the mosfet dissipates 48W under the same conditions. Plus because of the low dv/dt, I was reading that voltage spikes aren't as bad because it doesn't turn on as fast as a mosfet. And due to the fact that the two h bridges would only have 2 devices on at a time, and the step pattern is such that in some cases only one h bridge with 2 devices on at a time, that means average power would be much much lower.
My question is why? Why are mosfets so popular where bjt's have 1600% higher efficiency (again, constant power calculation)?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
well first i question your calculation, but second, when buiding my own vertical CNC; i found that MOSFets did run cooler, and were cheaper to buy then the large BJT, generally you can get ultra low resistance for Rds for MOS's of 60 volts or less. and NPN's vs PNP is also a concern (having high current needs for the gate and high voltage drop 0.6-0.8) ...
Use the L298H, it will save your soul and sanity....
and still better, use the L297 ic to control the L298H, then youll have full implemanetation with minimum fuss.
when you say "constant power" switching needs to be considered, because steppers are not continuosly rotating unless power is chopped, and switching losses are therefore added as well.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
how would you calculate the switching losses? i plan to use the bd681 and bd680 pnp and npn transistors. at 2 amps, the vsat is ~1v. and at 2 amps, 2A*1v=2W(oops).
one of the reasons i like to use the npn/pnp combo is that i can just feed them the same signal with no inverters. this exact design powered the same motor at 5v and only started heating when running at higher speeds. and this was with 3904 and 3906.
and yes, i did use a mosfet with .5 ohm on resistance for my calculations, which would in fact bump up losses to a larger degree. what would you say?
edit: what i just realised is that the type of stepper i use can be driven in"puch pull" where you simply power the center tap and drain the outside connections. i have been using bjt's so i don't have to deal with driving a high side n-channel mosfet. but still, you dont think that a bjt might be more efficient? i dont see anything in the datasheet's that indicate switching losses. with igbts it's just turn off energy, and seeing as they are basicly just a mosfet controlling a bjt, i would think that a bjt datasheet would have a similar turrn off energy rating.
well first i question your calculation, but second, when buiding my own vertical CNC; i found that MOSFets did run cooler, and were cheaper to buy then the large BJT, generally you can get ultra low resistance for Rds for MOS's of 60 volts or less. and NPN's vs PNP is also a concern....
Use the L298H, it will save your soul and sanity....
and still better, use the L297 ic to control the L298H, then youll have full implemanetation with minimum fuss.
when you say "constant power" switching needs to be considered, because steppers are not continuosly rotating unless power is chopped, and switching losses are therefore added as well.
deal extreme has this super cheap l298 driver. of course it would probably take 3 months to arrive
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yep there are many sources, some real cheap, but as for bens point about thermal and effciency issues...
youll need four heatsinks, electrically isolated (if i remember right, ive been drinking the tequila) but with the L288 (and others) youll get a half bridge (2 transistors) all in one packegaE with the metal tab at ground potential... so in vending machines or medical equipement you can have 10 half bridges all on one large piece of aluminum. you cant do that with discretes without a mica/GrEase/washer type deal wich adds cost complexity and potential failure points.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
yep, i just bought some 100v fets with .0085 ohm on resistances. less than a watt! But does anyone know what connector these things use? It's a 23km-k240-11v from minebea co ltd. I have looked at the diagrams, but a cant tell what connector is used. I have some connectors, but they only have crimps in the otside leads of each coil. The center taps are unavailable.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... edit: what i just realised is that the type of stepper i use can be driven in"puch pull" where you simply power the center tap and drain the outside connections. i have been using bjt's so i don't have to deal with driving a high side n-channel mosfet. but still, you dont think that a bjt might be more efficient? i dont see anything in the datasheet's that indicate switching losses. with igbts it's just turn off energy, and seeing as they are basicly just a mosfet controlling a bjt, i would think that a bjt datasheet would have a similar turrn off energy rating.
If you want easy high-side driving without a gate/base voltage above the input voltage then either a PNP BJT or P-channel FET will do. NPN BJT's and N-channel FETs both require a voltage higher than the input for high-side switch driving, so BJT's don't really have an advantage here. I think the BJT's and IGBT's switching energy is associated with the charge stored in the depletion region(s) and this dominates the switching loss, but as FETs don't have a depletion region switching loss is smaller and depends on how fast you can drive it on and off because while it's between the two states it's dissipating power. But a stepper driver is low speed I'm guessing, and at low switching frequencies switching loss can be less important than the on-state conduction loss.
I know that IGBT's win pretty handily over MOSFETs at very high voltage ratings when operating above 600V or so because to get a MOSFET with that voltage rating would require a higher on-resistance, but IGBT's are less inefficient at lower voltages because they basically have a built in diode drop. But I'm not too sure about where (or if) people still use power BJT's. I know I've seen many switching IC and gate driver datasheets with integrated BJT switching transistors though I wonder if the analog portions of those chips might drive that decision or if there's a usage range where BJT's still win.
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stepper driving
