so if i had to switch the mosfet on and then keep it switched on say for two secs then the heat dissipation would be 2.43 watts even though the passive load is drawing 3 amps at 300vdc ?
---------- Post added at 17:28 ---------- Previous post was at 17:17 ----------
The 2.43 W calculation is basically correct. There are also other kinds of transistor losses involved, see my post #10.
The 280 W maximum rating is a more theoretical value, that won't be achievable in most cases, e.g. because you can't provide a respective heatsink size.
So apart from the other losses are you saying that only 2.43 watts is dissipated by the mosfets therefore i should be able to have quite a small heatsink say 60mm wide would give me 12degrees per watt therefore the heatsink would go upto say 25 degrees plus ambient temp?
would my reasoning be correct
would this be enough heatsinking for the mosfets???
---------- Post added at 17:39 ---------- Previous post was at 17:28 ----------
The bridge rectifier power dissipation will be in fact twice the value of a single diode. According to a 8A datasheet that I have (KBU8), it's 2*3A*0.9V = 5.4W, resulting in about 100 K temperature rise with 18 K/W thermal resistance.
please explain 100 "K" does this = kelvin or centigrade
1 Kelvin =273.15 degrees celcius then 100 x 273.15 = 27315 degrees centigrade (about the same as a large furnace).
---------- Post added at 17:42 ---------- Previous post was at 17:39 ----------
would the bridge alone need a 12.5 cm heatsink and the mosfets a 6.0 cm heatsink
but note that this is the case only when the mosfet is turned on and stays in that state.so if i had to switch the mosfet on and then keep it switched on say for two secs then the heat dissipation would be 2.43 watts even though the passive load is drawing 3 amps at 300vdc ?
Imagine the behavior or the mosfet like a resistor in series with your load, the power on each resistor will be V*I or I²*R or V²/R where R is the mosfet resistance, V is the voltage across drain-source and I is the common current,
all these represent the same equation and give exactly the same result.
Alex
No. You should review this in a physics book or at Wikipedia.1 Kelvin =273.15 degrees celcius
how long do you keep the mosfets on for? after a while the boot-strap caps will discharge and eventually the UVLO on the IR2110 will turn the high side off - can you post a picture of your circuit (photo) - is there an inrush current with your load?
OK - had a look at the timing - putting an isolated scope on the high side gates will tell you if the bootstrap supplies droop too far. Even with only a few watts in the mosfets and bridge rectifier you will need a reasonable heatsink to keep the sink temp to 60 deg say (this is too hot to touch for more than a second or two) Regards, Orson Cart.
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