Switching Losses Calculation - Power Electronics

                             

                           

We had Detailed Lecture  on Switching Losses in previous posts, we discussed Switching Losses in detail and derived the formulas for Switching Losses for Resistive Load, in this post we will solve the actual example of switching Loss in Resistive Loads using that formulas.

Question:

Calculate the switching energy loss during Turn on and turn off periods as shown below in the graphs. Calculate the average power loss if switching Frequency is 10 kHz and what is the max instantaneous power dissipated?

In following steps we will calculate the Average power loss and Instantaneous power Loss first of all


1.Calculate Woff and Won

As we know that          
     
                                          Vbus x Imax x Toff
                              Woff= -------------------------
                                                         6

Now look at graph of Turning off profile, the one with Toff
Vbus= 200  ( The one with green line)
Imax= 100  (The one with Red line)
Toff= 5 uS

               Put in above equation
             
                                         200 x 100 x 5us
                              Woff=---------------------
                                                    6

               Woff= 0.0166666666666667 W

 
Now calculate Won that is actually switching loss when device is turning ON.


                                          Vbus x Imax x Ton
                              Won= -------------------------
                                                         6


Vbus= 100v (Green Line in graph 2)
Imax= 50A 
Ton= 2uS


                                          50 x 100 x 2us
                              Won=---------------------
                                                    6

                            Won=0.0016666666666667 W


2.Calculated average switching power loss: Ps (x by fs)

Ps= Total switching Loss= Won+Woff


                           Ps=0.0183333333333334 W


P average= Ps x Fs (Switching frequency)

          

   Pav= 183.333333333334 W

3.Max. Instantaneous power dissipated


a)Instantaneous power expression



               P instantaneous=  Vmax(1- t/Ton) x Imax (t / Ton)



Now for above equation we know all parameters except variable 't' , so we will find it in following step



b)For Max. Instantaneous power: Derivate expression found in (a) and equate to zero to find an expression for ‘t’. Put ‘t’ in step (a) to find max. Instantaneous power dissipated.


                          dP/dt instantaneous=  0   


so first derivate the Power and then equate that expression to find variable 't'.

 after taking derivative of Pinst w.r.t dt the following equation was formed


                               Vm x Im  x   (1 - 2t/Ton)
               dP/dt=  --------------            
                                 Ton


Note: While taking derivative treat Vmax , Imax and Ton as constant.

now put dP/dt=0 and find 't'



                            Vm x Im  x   (1 - 2t/Ton)
                      0 = --------------            
                                 Ton


After simplification

                           t = Ton/2
Ton= 2 uS

             

                         t=1 uS



Now put this value of 't' in equation of  P instantaneous in step (a)


           P instantaneous=  Vmax(1- t/Ton) x Imax (t / Ton)




            After putting values of Vmax, Imax, t, Ton from Turning ON profile P instantaneous was found out to be:



           P instantaneous= 



Kindly give feedback about the lecture and ask here if you couldn't understand anything.