I want to design h bridge using mosfet gate driver ir2112 and transistor IRF3205. I want to calculate the dissipated heat in the Mosfet in order not to use heat sink. according to the datasheet of both devices I calculated the dissipated power. the power supply is 24 volt, current 10A,frequency 20khz.
Rds(on) at temperature 25 C is .008 ohm. at 125 C (hot ) RDs(hot)= RDson (at 25)[1+.005x(T125-T25)=0.008[1+.005(125-25)]=0.012 ohm or 12 mohm.
so to calculate PD RESISTIVE = [ILOAD² × RDS(ON)HOT] × (VOUT/VIN) = 100*.012*0.94=1.128 watt.
from the ir3205 datasheet CRSS=211pf, ir2112 datasheet the gate driver current is almost 350 mA so
PD SWITCHING = (CRSS × VIN² × fSW × ILOAD)/IGATE = 211*^-12*24*24*20*1000*10/.350 = 0.069 watt
so the total power dissipated = 1.128+0.069= 1.19 watt.
is this right. if so will it need heat sink?
reference attached

 

The MOSFET dissipated power is largely the ON resistance times the current when on, plus any MOSFET switching losses.
Vout/Vin is not a factor in that.

I usually calculate the switching losses by using a triangular linear estimation, which is multiplying the switch output rise time and fall time by 1/2 the voltage times times the ON current times the switching frequency (1/2 τrise * Iload*V*fSW + 1/2 τfall * Iload*V*fSW).

The gate power is largely dissipated in the gate driver not the MOSFET gate (which looks mostly like a capacitor which dissipates no power).

 

The datasheet quotes junction to ambient at 62°C/W so 1.19W will give a junction temperature 74°C above ambient. The maximum junction temperature is given as 175°C so it should be OK up to an ambient temperature of 113°C.

[EDIT] Subject to modification of the 1.19W by crutschow's formula.

 

Vout/Vin is not a factor in that.

I usually calculate the switching losses by using a triangular linear estimation, which is multiplying the switch output rise time and fall time by 1/2 the voltage times times the ON current times the switching frequency (1/2 τrise * Iload*V*fSW + 1/2 τfall * Iload*V*fSW).

The gate power is largely dissipated in the gate driver not the MOSFET gate (which looks mostly like a capacitor which dissipates no power).

Vout/Vin means the duty cycle which i think it's an important factor. if it is .5 so half the current will pass through the mosfet and so on.

Do you think that it doesn't need heat sink?

 

Duty cycle as far as I know it is the relationship between two periods (time/time). Amplitudes are not involved.

 

oh, sorry i went to dc-dc converter relations.

 

Vout/Vin means the duty cycle which i think it's an important factor

Okay, that makes sense.
You are using Vout/Vin as an approximate value for the duty cycle of the MOSFET (it will be somewhat greater than that due to losses).

 

I tried this mosfet Irf3205 with the mentioned data but it got warm at current 5 A,20khz however I used heatsink.
I have ready made dc driver board in its datasheet it can hold up to 10 A with this mosfet https://www.google.com.eg/url?sa=t&...P2q1FERJq8sQn-ykKJHw6g&bvm=bv.137132246,d.d24
without heat sink but it is soldered on the pcb, how could it survive however it has almost the same Rds on