buck-boost converter in dcm mode

[dor8]

hi
i need help in buck-boost converter in dcm mode:
1.I want to solve exercises of this converter(in dcm) so I search questions and solutions - because it's easier for me to understand things through exercises , So if someone please could send me it would be nice.

2. How I calculate switching losses and conduction losses in MOSFET and how I calculate losses in the diode ?

3. If someone could please explain to me the currents the current on diode ,current onmosfet - I cant understand how to calculate the RMS current- i think i need to know how calculate the RMS current to qustion number 2

 

[BradtheRad]

The diode changes its own resistance, to create a voltage drop.
At 10 mA the voltage drop might be 0.6V.
At 1 A it might be 0.8V

If you measure current and voltage during the time it's conducting, then you can calculate power loss.

Taking an average reading is okay, although the results may be incorrect on account of dead time during the cycle. That is why you should try to get the reading for the time the diode is conducting.

A similar concept can be used to measure power lost by the mosfet.

 

[dor8]

Thanks, how I calculate the RMS current on mosfet?
Does anyone have formulas to calculate the losses on the switch?

I think that this whole subject will be easy for me to understand through the exercise of the converter at DCM, if someone could please send me it will help me a lot

 

[BradtheRad]

Observe the waveform on an oscilloscope. Measure voltage across the mosfet during operation. (This is one part of the calculation.)

To get amperage readings, insert a low ohm resistor near the mosfet. Observe the waveform across this resistor. Measure the voltage. Calculate V/R to get amperage.

From these readings you can determine power loss in the mosfet.

 

[goldsmith]

2. How I calculate switching losses and conduction losses in MOSFET and how I calculate losses in the diode ?

Hi to all
I think "Bradtherad" simplified things as you can have a basic understanding about the issue .
But if i try to add some mathematical equations i can write :
For mosfet if we neglect gate dissipation , and zero cross loss we can say that the dissipation will given by :
1/T integral over Rdson*Id^2 dt from zero up to the ton .
For diode the same is applied :
Ploss = 1/T integral over VDmax*Idmax dt from zero up to the ton . for Vdmax Bradtherad gave a good understanding method for that . it has a minimum value and maximum value instead of different currents . for reaching to the maximum value of loss power across it , we must consider the maximum voltage instead of maximum current through it .

Best Wishes
Goldsmith