> > For safety I learned relays fail open circuit while MOSFETs fail short > > circuited, I also experienced MOSFET short circuit failure. Happen to > > know anything more about this? > > > It is for this reason that higher current dc loads, which will often be > inductive are often of the t-bar design. The armature, when released > will rise and hit the crossbar contact with enough force to break the > microweld, but even that isn't trusted, so I've seen circuits that used > a 4 pole contactor with all 4 poles in series.
Tried two in series with buzzing sound sometimes but do not care to much about it since it's only temporary. Maybe AC is a better choice then using relays? > I'm sure you've all seen the familiar flywheel diode placed across the > relay coil to prevent the inductive spike when turning off the relay, > from rising more than a volt or two above the supply voltage, it is > absorbing the relay coils currant during the turn off period, preventing > the transistor from being over voltaged by the inductance and punched > thru for a permanent short. Yes I also added this diode for myself. > This also delays the relays release by a > few milliseconds. If controlled by a relay, the circuit should be > expected to have a few milliseconds of lag anyway. How fast current decrease depend on resistance and should be possible to make this time shorter by for example adding zener diode or resistor in series with diode with a higher but limited turn off voltage spike. > Modern mosfets can often withstand 1000 volts or more for short periods > of time, so a lot of research has gone into mosfet _drivers_ to do 2 > things, 1st being to switch it as fast as possible to reduce its heat > dissipation during the transition, This is indeed important for low switching losses. > and 2nd to get the opposite side of a > switching circuit turned on just slow enough you don't have a short > because both sides are on at the same time, but you must get the > opposite side turned on with a minimum of dead time between them to > short circuit the voltage stress on the one just turned off. 5ns can > make or destroy in this case. The diff is that it works for decades, or > you break the mirror and let all the smoke out. The dead time may add some voltage distortion and some devices have lower voltage drop then turned on, otherwise current is thru free wheeling diode. Nicklas Karlsson _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
