1. Home
  2. Electronics Design
  3. parallel fets

parallel fets

Hi,

I was thinking about how to minimize switching losses and also on state resistance, as well as having reduced gate driver requirements, and was thinking this would be a good idea:

use 1 fet with very fast rise/fall times as well as low gate charge (but high on-state resistance)

then the rest of the parallel fets rise/fall times as well as gate drive requirements are almost irrelevant. As long as they switch within say 100ns they will relieve the single fet which hopefully fully switched within 10ns, and then had normal Rds(on) losses for 90ns. Then for 100kHz switching, the other lower Rds(on) fets would take all the load for 10us, while the other "pulse" fet has a chance to cool down until the next 100ns switch point.

So the benefits are reduced gate drive current requirements, and not having to find fets that have BOTH low Rds(on) as well as low gate charge and fast rise/fall times.

Another idea I was thinking, to use a zero ohm resistor on the single fast fet, as the gate ringing may not matter on that fet as long as it is able to stay turned on for at least 100ns before the ringing turns it off.

Would these ideas be practical?

cheers, Jamie

Reply to
Jamie Morken
Loading thread data ...

It's called 'assisted switching'. Your version would transfer all switching losses to the faster part. Switching losses can be a significant portion of the total, in a switching element. Gate drive loss due to CRSS in the the slower elements can be reduced, if a suitable path for the current is provided.

Most assisted switching circuits attempt to reduce losses dissipated in the main switches by storing/recovering the energy involved. If this can be done without producing extreme dv/dt or di/dt, so much the better.

RL

Reply to
legg

Warning: I'm a newbie in this area...

The idea sounds ok. One mosfet acts like a sword and another mosfet finishes of the switching like a sledge hammer.

For mismatched parallel mosfets (single wimpy mos driver), the mosfet with the heaviest gate capacitance may rule. Other parallel Mosfets with lighter gate capacitances follow the retarded rate of the driver. Assuming Cg1>>Cg2. Fixable I guess with multiple drivers.

I've seen some articles where power mosfets and bipolars are combined in parallel in an effort to reduce heat.

D from BC British Columbia Canada.

Reply to
D from BC

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.