I will have to drive 25 Mosfets in parallel. Using each driver for each device is one way. Other than that, is there any other method to drive all parallel connected MOSFETS without having any time delay between them so that all devices turn on and off at the same time?
Dear Hayakrysh,
what is your requirement regarding speed?
MOSFETs are in principle voltage driven (at not too high frequency), so it appears very possible to let one driver drive all of them - in parallell. A basic advantage with MOSFETs is that they have increased channel resistance with increasing temp, so basically there is no need for ballast resistors to avoid thermal runaway (you can google ballast resistor, they must be used with BJT, standard transistors).
I think I would have started with a couple of MOSFETs in parallell driven by the same driver and checked for performance....
Hope this helps!
Valter
Dear Hayakrysh,
what is your requirement regarding speed?
MOSFETs are in principle voltage driven (at not too high frequency), so it appears very possible to let one driver drive all of them - in parallell. A basic advantage with MOSFETs is that they have increased channel resistance with increasing temp, so basically there is no need for ballast resistors to avoid thermal runaway (you can google ballast resistor, they must be used with BJT, standard transistors).
I think I would have started with a couple of MOSFETs in parallell driven by the same driver and checked for performance....
Hope this helps!
Valter
But we must pay attention to the overall capacity of the gates. Fast switching may require considerable currents.
There will inevitable be delays. The most intuitive way would be to use a bunch of logic gates, all driven by a single signal. The logic gates must probably be configured in such a manner that the required modulation is produced at the output. This becomes difficult as the number of mosfets increase. First try with 4 mosfets.
In a simple case of two mosfets, logic gates can make the mosfets work in an opposite manner (on and off) or they can be both on or off. This can be done using OR AND gates. As you can see, this becomes increasingly difficult if you want the whole logic gate network to do some modulation for your mosfets. Good luck :)
As multiply stated above, there will always be different delays to the different MOSFETs. The use of separate drivers local to the MOSFETs will reduce the capacitive load on drive line, and this in turn will reduce the delays. However, it is also possible to control the routing to the MOSFET gates so that the series inductance of each path is very similar (wide tracks are better here, because they reduce the individual inductance). The delay difference will then be dominated by the mismatch between the input capacitance of the MOSFETs, but this mismatch can be quite small if the MOSFETs come from the same batch. Note that the series inductance on the outputs can in some circumstances be equally important - and separate drivers do not help here.
If you don't need fast edges on the switching you can also use a resistor at the driver to slow down the transition time beyond the delay mismatch. (You may in any case wish to control the transition time to avoid generating high-frequency electrical interference). However, the transition current will still not match between the different devices due to threshold mismatch; the mainproblem here would be hot carrier injection in the first device to switch on, though this too is less of an issue of the MOSFETs come from the same batch. The final issue to consider when lengthening the transition time is the effect of dissipation during the transient.
Regardless of whether you use separate drivers, simulations that include board parasitics are really helpful here, and (naturally) the MOSFET manufacturer's data sheets should be consulted for illumination on all these matters.
- Badges
- Science topic
- Similar topics
- Engineering
- Electrical Engineering