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Author Topic: High current mosfets for H bridge with ESD protection.  (Read 1763 times)
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I plan on making an dual half H bridge for a robot, but i have been having a hard time finding the right N-Channel and P-Channel Mosfets.
 I plan on using this schematic ---> http://www.electronics-tutorials.ws/transistor/tran60.gif     to keep things simple.
Right now i am using a TI L293D which internally is the same the schematic except its doubled. That chip is made to only run 1.0 A through it and the motors im using are pulling 2.2A, and i am not using flyback diodes soooooooo, its time to design an H-Bridge.  I need a mosfet that can run 3 amps continuously at 12V and it has to be ESD protected cause im most likely to be stupid and break one thats not ESD protected.
i was thinking this mosfet would be good but im not entirely sure.
http://datasheet.octopart.com/BTS117-Infineon-datasheet-62262.pdf
Sorry guys im just an electronics newb. I know how the circuit just not how to read data sheets -.-
your help is greatly appreciated, i have already spent hours and hours looking so accuse me of not trying lol. i need to get back to home now. thanks )_)


* tran60.gif (9.35 KB, 355x284 - viewed 44 times.)
« Last Edit: February 14, 2011, 06:37:49 pm by rvasquez6089 » Logged

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Quote
I plan on using this schematic -
Can't read it, the link gives a broken file. Why not attach it to the post?
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wierddd, let me attach it.
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i was thinking this mosfet would be good but im not entirely sure.
Well the main snag is that it isn't a mosfet. The clue is in the title "Smart low side power switch", it is as the title suggests a switch for power switching the 0v or low side of the supply. Don't even consider making a bridge out of this, because you will need a high sided power switch as well and they won't turn on and off at the same time and you get shoot through, which is where you get a dead short from both sides turning on for a small amount of time.
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well i know its for the low side. I haven't been able to find the matching high side, or P-channel mosfet to go with the N channel mosfet. And im not even sure my arduino can switch the mosfet on. the data sheet says the input threshold voltage is 2.2 volts. Is that the amount of voltage needed to switch the gate?
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I have a whole tube full of high sided switches that match this but you need a FET not this device.

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the input threshold voltage is 2.2 volts. Is that the amount of voltage needed to switch the gate?
No it is the voltage when the gate starts to turn on. At this point it will be operating in the linear mode and so will get very hot. You need to look at the fully saturated gate voltage. That is normally given when they quote the On resistance between source and drain.
For use directly with an arduino look for a "logic level" FET like the IRF7862PbF or the IRLI640G.
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have a look here :

http://www.laboelectronique.be/english/ardponth_ang.html
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The problem with these circuits is that both n-channel and p-channel MOSFETs will be on simultaneously during the switch over, leading to massive rapid current spikes (10's to 100's of amps potentially).  This called 'shoot-through' and is to be avoided in high-power circuits as it can  cause all sorts of mayhem.

If you are very careful with the choice of threshold voltages and overall power supply voltage it could be made to work, but be aware that the spread of threshold voltage is large in MOSFETs.  If you are using 12V supply you have the problem that typical threshold voltages are 2 to 4V or so for non-logic MOSFETs, so when the gates are between 2V and 10V both devices will be on, and hard on at 6V... 

High current MOSFET H-bridges normally use n-channel devices throughout and special driver chips to generate the gate drive to the high-side device.  Many have provision for programmable delay between upper and lower device switching to prevent shoot-through.  One example H-bridge driver is the HIP4081.  Having both top and bottom MOSFETs off briefly is not a problem as one of the built-in diodes will take over the current.
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