Paralleling Mosfets in smps for Cooler Operation

Oscillation derives from rapid transients which bring about interactions between L,C parasitics in the circuit and a tendency of a MOS to get more current then others and hence be switched ON. The voltage dependent capacitance of the Zener works against that at least at the lower frequencies after which it becomes inductive and makes things worse. They are some skeptics though who claim it always makes things noisier. However its presence is a bonus as it protects against high voltage transients.

Andy

Well actually we only have RDS losses since they are used to switch on a mains load. We have 4 MOSFETs in parallel and another 4 MOSFETs in parallel to act as two big MOSFETs in series to allow for bi- directorial switch (ac load). One cluster of MOSFETs share the same

100k gate resistor (we turn on in about 1ms), the other cluster share another 100k resistor. We have seen no problems, but I must admit that due to schedule problems we have not yet have time to investigate fully. The reason for having two gate resistors is to avoid fast pulsen to migrate through the DS path to the other MOSFET cluster gates.

Regards

Klaus

Wow, 100K sounds really high. I don't know the application but couldn't it use a triac, or two SCRs if there is lots of inductive load?

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Regards, Joerg

http://www.analogconsultants.com

I started off with the TPS2817 mosfet driver (2A peak output) with a SPA21N50 Rds(0.19) Qgt=95nC (I have to admit, I tried to skip the math and just picked a low Rdson mosfet..I later learned that this may not be best for my app.) Calculated total power loss is 3.7Watts...

Mosfet Conditions Id=2.4Apeak Vd=270Vpeak f=100khz, D=40% Continuous mode Cuk topology

I was thinking of going "Rambo" :) and using

2 parallel Fairchild Superfets FCPF7N60 (rds=0.53, Qgt=25nC) with a IXYS IXDD414 (12A peak output) driver... Cooler?? This is more in the interest of how cool can I go.. :)

Just about all fets in the VDMOS library of LTspice have Rg=3ohms? Is this accurate? Is it all the metal leading up to the gate?. If so, this limits the peak current from those impressive high Iout mos drivers.. I looked at a few mosfet datasheets for Rg...haven't found it yet. D from BC

Cool...(I mean literally cool... :) I'll certainly check it out... D from BC

A previous design used a triac, but I saw the possiblility to make the power supply extremely simple by using a MOSFET (zero static current consumption) and changing the system to let the micro run only periodically and otherwise be in sleep mode. Its 100k because the state of the MOSFET gate must be valid even in sleep mode (micro holds the outputs in the correct state), but I need to draw minimum power from the power supply. The micro runs for 30ms at 10mA and sleeps for about 250ms. So the average consupmtion of the 5V node to the entire system is below 2mA

Regards

Klaus

Yes, the drive level can be taxing with triacs. But MOSFETs should never draw any gate current so I guess you could use much less than 100K.

I once saw the opposite strategy. Someone had cost-reduced the power supply to the bones by dropping any kind of regulation. No regulator, no zener, just one resistor. Problem was nobody told me. So when something didn't work I probed the uC crystal to see whether it was still humming along. Phssst ... poof. They had obviously made sure that the uC workload and thus its current draw was very even all the time so VCC would fall between min and max datasheet values. Touching the oscillator pin must have made it stop.

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Regards, Joerg

http://www.analogconsultants.com

see what you can get at the recycling place, before the invention of the RCD/GFCI isolating transformers were an essential piece of contractor equipment and they were built to last.

Last year I got a 1500VA one for $12,

check what's being chucked first.

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Bye.
   Jasen

I've been poking at the idea of getting a Tektronics digital phosphor oscilloscope.. But.... I can get a iso transformer off Digikey for about $50.00CAD. (Not sure if there will be added shipping charges due to weight.. I added to my Digi order to see but no change..still $8.00CAD..?? Suspect transformer price has been bumped to compensate for shipping.) But first, I'll check my local electronics surplus depot for a transformer. Bring a cart with wheels to carry it? :) D from BC

safety,

That sure sounds like a design asking for trouble. But on the other hand I often think about some of our designs in which we use six-sigma WC calculations and so on, when a simpler design could work ok with some figure of failure rate

Regards

Klaus

safety,

It sure does but supposedly it worked with a low failure rate. Don't remember if it was this one but I've seen a uC fed via an input pin, using the substrate diode as a rectifier. Probably to save another cent. That reduces the number of components that could fail by one so I guess it could fall under Taguchi's "creativity" function. Of course, the core motivation could also have been plain old greed ;-)

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Regards, Joerg

http://www.analogconsultants.com

Actually it might even be cheaper since the qoute now goes 8up, the package is a more common one (high runner) avoiding an expensive bulky MOSFET and lastly if you need you can just remove half of the MOSFETs if you have a lower current product variant.

Regards

Klaus