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NMOSFET

Hi,

I have following questions

  1. I am currently using NMOSFET transistor IRF540N. Its input capacitance is approximately 1000pF. I was wondering that is there NMOSFET out there that have the same or less input capacitance but higher power rating like between 350 to 450 watts, preferably greater than 400 watts with lagre Id current and Vdss voltage between 300 volts to 500 volts. Desirable Vgs is +/- 20volts. Cananyone suggest one?

  1. I am using Linear Technology DC to DC converter LT3970. The link to the data sheet is as follows

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a. My question is that if I increase the sampling rate than noise at the ouput goes higher or it will get lower. b. How does sampling frequency of the DC to DC converter introduce noise?

  1. I am using the HIP4080 chip to drive the H bridge made of four NMOSFETs IRF540N. The brdige is driving an inductive load. I am driving the HIP chip with a micro using PWM. The frequency of the PWM is 100KHz at various duty cycles. The HIP chip requires two input PWM signals. Now I saw that switching of the transistors and the load is introducing noise into my digital circuit. The HIP chip application notes discusses layout issues and solutions but its not effective. So, I am thinking that what if I optically isolate the micro PWM signals from the HIP. An optoisolator between the HIP chip and the micro. But I do not know how should I calculate the bit rate or bandwidth of the optoisolator.Any suggestions?

Is it possible to effectively isolate the digital power from the analog supply? What would be the best solution.

Thanks jess

Reply to
Jessica Shaw
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Not sure what you mean. Switched FETs aren't usually measured in hundreds of watts. What matters is to switch them fast and remain well in the safe area of the SOA.

  1. I am using Linear Technology DC to DC converter LT3970. The link to

You mean clock rate and ripple? Ripple will become worse with a lower clock rate unless you provide a (larger) inductor with more uH and larger capacitors.

Mostly as ripple onto the input rail (VIN) and the output rail (VOUT). This can be reduced by larger low-ESR capacitors. Some noise also leaks out of the inductor as a magnetic field. The term "shielded" is usually a marketing term and only means that they are not quite as leaky as a can after uncle Leroy shot it off the fence post a dozen times :-)

You can isolate it with these logic output optocouplers, for example:

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They are not too expensive but fast. The HIP4080 appears to have Schmitt inputs altuogh the datasheet is vague. The transition times wouldn't be so critical if you want to use another optocoupler,.

Check that HIP4080 for long term availability, stock levels are a bit low.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Well, the HIP4080 is good for about 60 V in real-world applications, despite their published ratings that their own apps people admit are entirely bogus.

So, why are you talking about 300 - 500 V? If you want to do a higher voltage bridge, take a look at IR2113 and similar drivers. These put out 2 A of gate drive, so Cgs is less of a concern. There ARE newer MOSFETs available with a lot less Cgs and other better features, the IRF540 is a 20 year-old part. Such units as the IRFB31N20D are faster and have less Cgs.

Jon

Reply to
Jon Elson

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