grege@gold.gvg.tek.com (Gregory Ebert) (02/27/91)
2hneirony@kuhub.cc.ukans.edu writes: > > I am attempting to rig up a MOSFET H-bridge to run a bidirectional DC >motor from a 24 volt source; > [...] International Rectifier (213-772-2000) has devices which allow you to sense current without a resistor in series with the load; the devices are called 'HEXSense' mosfets. The data sheet I have on the IRC150 says it's 100V @ 30A, but is a preliminary data sheet as of 1987. The diode you mention is to provide a return path for inductive loads. You dont need them in HEXFETs because they are an inherent device. In fact, almost all MOSFET and bipolar power devices have these 'anti-parallel' diodes in place because they are imperative. The zener across the gate-source is to limit Vgs, but is totally unnecessary with good design techniques. A low-impedance gate driver protects you from high dV/dt at the drain. I assume your H bridge is something like this: V+ o-------------------*---------* | | MOSFET 1 MOSFET 3 | | *--motor--* | | MOSFET 4 MOSFET 2 | | GND 0------------------*---------* MOSFETs 1 & 2 are both on, or 3 & 4 are both on, or all are off. Oy Veh ! DONT turn on 1 & 4 or 2 & 3 at the same time ! The gate drive for MOSFETs 2 & 4 is trivial: just give them about 8-10 volts and they're turned-on rock solid. For MOSFETs 1 & 3, use separate pulse transformers. I've even used the 1:1 audio transformers from Radio Snack with perfect results. The only trick is that you need to drive the transformers with pulsating DC (10Khz is great). I put a full-wave bridge rectifier at the gate-drive end, with a parallel RC network whose RC time constant is about 5 times greater than the period of your pulsed-DC. Adjust the duty cycle so that you just start to get little 'dips' in the gate-source voltage; this assures there is no DC component in the 1:1 transformer. DC and transformers don't mix. ===> make sure your 'dead time' is greater than 5*R*C <=== (dead time is the time interval between MOSFETs 1&2 truning off, and MOSFETs 3&4 turning on, etc) Finally, make SURE you minimize inductance in the source lead. You can generate enough Ldi/dt to zap the device with excessive Vgs. Always use a separate wire from the HEXFET device to the ground of the gate-driver. ##### ## | ## # | # # /|\ # #/ | \# #######