Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: corrado33 on October 21, 2009, 09:08:32 PM
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Hey guys, so I've decided to buy the electronics of my project before I buy the hardware for it (cause if I can't get the electronics to work, then my project will never get off of the ground).
Anyway, I've been sourcing out parts for my H bridges, and I've been looking on DigiKey.com, but they have so many parts. I've found the capacitors just fine, but I'm having trouble finding the right mosfet to use. I know on the H Bridge tutorial, there was a part number, but I can't quite read (I'm guessing admin's) handwriting to figure it out.
So here are my motors, and I'm running them at 24v. http://www.robotmarketplace.com/products/0-BHG31.html (http://www.robotmarketplace.com/products/0-BHG31.html)
What transistors should I use? The tutorial mentioned something about PWM optimized mosfet's as well? Should I be looking for those since I'm obviously going to be using PWM? Could someone point me in the right direction?
Also, the tutorial mentioned something about amplifying the signal coming from (most likely the microcontroller) to the transistors themselves... However, in the power amplification tutorial, he said that those circuits should only be used for amplifying low voltages, like a sensor or something. So, correct me if I'm wrong here, but most microcontrollers will output a 5V signal. So what should I amplify that 5V to? 9V? 12V? And also, how do I do it? I did find a 5V to 9V voltage converter on http://www.goldmine-elec-products.com/prodinfo.asp?number=G16902 (http://www.goldmine-elec-products.com/prodinfo.asp?number=G16902) however, it has a max amperage of.. well, something that's lower than my motors will be drawing, but does that matter since this'll only be attached to basically the "switch" of the transistor? What do you guys suggest in the voltage amplification department?
Thanks for your help.
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Your question is not easy to answer as there are several H-bridge configurations. Also, H-bridges are simple in principle but not so much in practice. Do you have a schematic of the H-bridge you wish to build? This would be helpful for us to suggest parts.
As to which MOSFETs to use. Yes, there are many to chose from. Since they will be used basically as switches look for ones with a low Rdson (the resistance between the drain and source when the MOSFET is fully turned on). Then the Vdss (voltage between drain and source at saturation) which should be a couple times the voltage used (50V minimum for a 24V supply). The Id is the drain current and should be greater than the motors stall current. Finally the Vgs which is the gate to source voltage to fully turn on the MOSFET.
Power MOSFETs have a good amount of gate capacitance and may require lots of current to turn on the gate. So a MOSFET gate driver may be required, this can be a chip made for this purpose or a bi-polar transistor or a smaller Mosfet. Therefore a logic level gate is a mote point and the Mosfet with a higher Vgs is fine.
Lastly is the MOSFET's power dissipation or how much heat can be removed. The Rdson is part of this, from Ohms laws and the power equations Mosfets with a very low Rdson don't dissipation much heat IF they are switched full on but when switching on/off at PWM speeds they dissipation much more heat. So look for a mosfet package that can be heat sunk like TO-220s (these are leaded parts that are easier to use by a hobbyist).
I tried to explain the hows and whys of MOSFET selection. I looked at the H Bridge tutorial but could not find the Mosfet part number on-line that is written in the tutorial. Also the tutorial is to show the basics of how on H-bridge works and has a few issues in practice. Here is a better circuit:
http://www.cadvision.com/blanchas/hexfet/np-s.htm (http://www.cadvision.com/blanchas/hexfet/np-s.htm)
The Digi-Key PNs for the Mosfets are: IRF9Z30PBF-ND & IRFZ40PBF-ND and will work for the motors and voltage you chose.
Do be prepared to burn up a few parts, I did when learning how to build H-bridges. When some thing burns up analyze what was wrong and fix it before doing it again. When it all works though it is a great satisfaction to have built it yourself.
Another way is to use one of the many Dual H-bridge chips. These have all the extra stuff inside making them very easy to use.
Good luck, have fun and come back here to ask more questions.
wr
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Finally the Vgs which is the gate to source voltage to fully turn on the MOSFET.
Wrong!
Vgs is the point where (weak) conduction starts.
To use a MOSFET at "logic level" (voltage-vise), the Miller plateau (MP) needs to be quite a bit lower than Vdd, as the low R_ds_on lies on the right side of the MP (ie. Q_gd must be charged).
This illustrates the V_gs/Q_g relationship
(http://That.Homepage.dk/Img/MOSFET_Charging.png)
BUZ11 is a well known power MOSFET, utilized in numerous circuits (probably the most used MOSFET in schematics floating around the net), but nobody in their right mind would ever call the BUZ11 a logic level MOSFET.
Here is V_gs and R_ds_on of the BUZ11:
(http://That.Homepage.dk/Img/BUZ11_Vgs_and_Rds_on.png)
This shows that you need (at least) 6V to even touch the MP:
(http://That.Homepage.dk/Img/BUZ11_Gate_Ch_Vs_Vgs.png)
If you wanna know whether a MOSFET is logic level or not, look at the voltage of the MP, or better - look for the term "logic level" in the header of the datasheet.
Even a logic level MOSFET needs lots of current to switch quickly (reducing power losses) however, so for efficient switching, a MOSFET should never be connected to an I/O port or a logic gate, although at fairly low frequencies (< 5..10 kHz), penalties are usually bearable.
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Søren,
Thanks for the correction. I deleted that statement from my post above to prevent any later confusion.
Your explanation is not only correct but well illustrated.
wr