(This might be better suited to a different newsgroup, but I know the embedded people know what they're doing ;)
I'm looking at half-bridge designs for 3-phase motor driving.
If I have the direction right, some of these designs use P-channel MOSFETs on the top and N-channel MOSFETs on the bottom. However, I've also seen a few that use N-channel MOSFETs for both the top and bottom.
Is this correct, and what are the advantages of one design over the other?
Thanks! -Mike G.
It's easier to drive the gate on a high-side P-channel FET; it's easier (and less expensive) to find really high performance N-channel FETs.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Great explanation, thank you very much!
-Mike G.
Hello Mike,
It's not just easier and cheaper, it's also reducing inventory from two types of power FETs to one and the price break for the n-channel will come at half the number of units produced.
Regards, Joerg
Consider opto-isolating the motor and power transistors from the digital side. Whatever way you may explore, remember that you can hold one side (high-side, probably) on while you PWM the other (low) side. That will simpify the driving circuitry.
BTW 1) N-Ch has lower Channel resistance than P-Ch ( 2.5 times ) 2) You need 2 series MOSFETs , both driving an inductor get into trouble . If you drive and Inductor with 1 MOSFET the diode will stops any flyback . 3) MOSFETS work fine ( same Rds) if Drain is + or - , but Diode will clamp Voltage swing . Used in active rectification to reduce diode drop to 1/10 volt at high amps . 4) it may take very high power to drive . The Gate will draw high current when gate reaches 7 vdc ( discharge Crss) . This means a bipolar may have the same input power . But at lower frequencies the powr MOSFET has lower input power and draws none in any static position ( full on or off )
It is a shame that werty could not be a little more coherent in his response (but I am aware that English may not be his first language).
Yes, most MOSFET or IGBT based three phase bridges will feature N-channel devices more than the complementary pairings. This is because the N-channel devices are often cheaper than the P-type complements but you also tend to end up specifying the next device up the range as the complementary P-channel devices will not handle quite as much current or power as the N-channel devices. There are, of course, complications with an all N-type design. You need to generate a voltage for the gate of the upper device that will reach beyond the bridge supply rail (remember you need the voltage between the gate and the source connections of the device and as the source floats with the driven output voltage this can introduce matters of gate drive isolation).
I either make a small multi-output switch mode power supply and use opto-isolators to switch the gate voltage on to each device, or I expect a pulsed gate drive signal and use that to drive the gate through a small transformer.
IR have a number of application notes that relate to proper driving of MOSFET and IGBT's. These are well worth the read.
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