Heatsink and mosfet Insulation

[tictac]

Hi
there is one mosfet that switches with 50khz
how can I omit stray capacitance betwean mosfet and heatsink?
I use one insulator betwean them. where do I connect heatsink? to GND or Device body? Is there any way to omit EMI publishing from heatsink?

regards.
Hamed

 

[karesz]

Hi,
In the way that you dont have it ...(omits all stray caps)
Device body is usal on some potential, heath sink on that can be problematics & is a transmittre antenna for switching transients :-(...
I would isolate it and connect to GND.
K.

 

[tictac]

hi karesz
and how can I make earth connection for my circuit?

 

[karesz]

hi,
I mean the GND of your circuit(low potential), not the mains protection earth.
You?
Earth is to isolate from your electronics GND, but your metal case (if you have it) is to connect to that_earth.
K.

 

[tictac]

lets talk a little about my circuit.
its one flyback smps dc to dc .
Vin=24v (2 X 12v battery ) and output =1300v - 40 mA
and there is no isolation (such opto) betwean primary and secondary ground.

now, where do I connect heatsink ? to GND or Device chassis ?

Can I connect secondary ground to chassis?

regards

 

[karesz]

Hmm, good question because Im not a HV expert...
I would have the complete electronic isolated from chassi & protection earth(heath sink on GND)_its for me a must!!
I beliv hadle to correct if I has GND (& HV) isolated from all, from chassi too & chassi is with a 1 MOhm parallel to100nF/1uF on the GND to prevent some chargings, but maybe a HV-person makes it others?.. Do you share a schematic pls?

Experts: pls comment it! Is mortal danger here?

K,

 

[tictac]

yes , here it is
https://www.4shared.com/file/237163930/c36e505c/Schematic14.html

please help me about good grounding and making earth connection.

regards

 

[IanP]

Back on earth ..
You are talking about switching 50kHz, at that frequency stray capacitance means almost bugger-all .. incomparable with the snubber capacitor (5.6nF) in parallel with the MOSFET ..
Just put a mica or silicone rubber insulating washer (and bush) between the MOSFET and the heatsink, connect heatsink to GND (chassis should be connected to EARTH) and stop split hairs ..

Rgds,
IanP

 

[tictac]

Hi IanP

incomparable with the snubber capacitor (5.6nF) in parallel with the MOSFET ..

I think the value of this cap is too high. what is your oponion?I thnik it should be about pico farad. can you suggest the value for it?

I put complete insulator betwean mosfet and heatsink.

I will put one metal chassis for it . but my circuit works with 2xBattery and doesnt use 220v-50hz.

how can I make earth for my circuit and which point of my circuit I should connect to chassis? some one tell me connect your secondary GND of your transformer to chassis and some one tell me that connect two 1nf series capacitor betwean 1300v and GND of your output voltage and connect the middle pin of these capacitors to chassis.
I dont know what can I do for it?

regards

 

[karesz]

Yes the so called "Y-Capacitors" are common to use, but I know it only from primary GND & Secondary GND to earths to connect at the middle point...
You have both GNDs common, these is the reason why I wrote over GND to earth connection with a 100nF capacitor (I agree; its too much, common are values of 2.2 or 3.3nF!) -parallel resistor complex(these resistor can be not overall standadrs conform!)...
Anyway; select pls capacitors as well isolated types for these parts, their makes for "smart meter vendors" very much problems too...
K.

 

[tictac]

no ,its different from Y-Cap . these 2 capacitor use in betwean of output voltage and common pin of these 2 cap connected to chassis.

 

[karesz]

yes & no!
These "Y-Cap" is meaned as "CATEGORY of safety" for capacitors, used as Y-Cap too, but in that case what I did described; you can see, its realy not a cap(or two) working in Y-circuit, but same what you need too...
In my firm (2-3 years ago) it was a usual description for that aart of capacitor too, their are "Smart meter global vendor"...).
K.

 

[tictac]

now, whats your oponion? how can I make earth and can I connect earth to chassis?

regards

 

[karesz]

Tictac,
As I wrote at 12:24 too; I would apply a 2.2-3.3 nF, AC capacitor between your "common GND" & the chassis & finished., IanP wrote it too: "(chassis should be connected to EARTH) and stop split hairs" ..
I think IanP can agree with me?
K.

 

[tictac]

thank you for your good replies .
now I want to have a atmega16 to sample output voltage and current with ADC of micro and display on one lcd.
in which way can I sample with adc that micro doesnt reset because of output noise? my output is varing voltage from 100v to 1300 v .
regards

 

[karesz]

What about instrumentations amps as preamps/signal conditioners_with some LP-filterings? AD8224 or the INAxxx family...
I think; you have for both sensings resistors: pls. bypass their voltages directly at these resistors first & after that use serial-RC filters too the diff-amps would be good direct at the sensing to have_maybe some away from your uP, but so are the disturbances more lower...
K.

 

[tictac]

Can I use auxiliary winding to get lower voltage proper for giving it to the adc and its safe. or use linear optocoupler such LOC110 ? which is better?

regards

 

[IanP]

The simplest option is to employ the basic voltage divider, similar concept as is used in high-voltage probes (see: https://www.repairfaq.org/sam/hvprobe.htm ), add low-leakage capacitor to smooth the divided voltage and perhaps add a high-impedance buffer, if the ADC input impedance is to low ..
A pair of diodes connected to [signal +Vcc] and [signal GND] somewhere between the voltage divider and buffer may be beneficial ..

Rgds,
IanP

 

[tictac]

IanP:
incomparable with the snubber capacitor (5.6nF) in parallel with the MOSFET ..


I think the value of this cap is too high. what is your oponion?I thnik it should be about pico farad. can you suggest the value for it? in one app note I read that the value of capacitance you use in parallel of drain-source must be 3 times of inherent capacitance of mosfet D-s . is it true?

regards.

 

[IanP]

C25 together with R10 form a snubber network to protect the power MOSFET against voltage spikes ..
The values are OK .. Don't change them ..

Rgds,
IanP