HELP! Power Electronics Basics

[dave_dj88]

Hey,
I did well in power electronics and converters in my first year at uni, but i'm worried about some of my basics:

1. I dont understand how operating transistors in saturation region is more efficient than operating them in their linear region, when the current in saturation region is higher.

2. We were told that there was a relationship between input Vi and output Vo and this depended only on duty cycle d and transformer turns ratio.
What effect does load have on the output voltage? So will the basic transfer equations, Vo/Vi = d/(1-d) etc..., be incorrect when considering load values?

I would really appreciate some help here.
Thanks!

 

[barrybear]

Without going into a lot of maths and physics very simple explanation.
1 When a transistor is saturated it is in a fully conducting mode and there fore will not offer any resistance to the current path ( or very very little).
2 Once you exced the quoted VA from a transformer the voltage will not follow the standard load calculations.

 

[TekUT]

1. Saturated transistor
When a transistor (BJT) going into the saturation region a injection of charge taken to start into the emitter region, due to the high numbers of minority charge into the emitter region you've also a high injection of charge into the collector region and the final effects is to collapse the resistance of the collector region, as a final result the voltage across the transistor into the saturation is more smaller than the other region. This phenomenon are know as "modulation of the conducibility".

2. Swith Mode Power Supply
Analysis of switch mode power supply into the steady-state condition can be did by using the fact that average voltage on inductor must be equal to 0 and average current into capacitor must be qual to 0 otherwise you aren't into the steady state condition.
After this assumption is easy found the dc-dc transfer function of every converter.
Dynamical analysis is another story and can be carried in several ways, from state space averaging (Middlebrook) or by using the switch averaging way (Vatché Vorpérian), of course dynamical analysis will show same result of steady-state analysis when you look the relation for a frequency equal to 0.

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Basic forumula is just a approximation of the real converter behaviour, as example try to derive the simple Buck formula for the duty-cycle ration by considering also the effect of the diode voltage drop, you'have another formula with inside the averaged diode forward voltage drop and so on... basical formula is widely used to design the power cell under the assumption that the parasitics components of the converter will be minimized by carefully choice of converter components.

Hope it help
Best regards

Powermos

 

[dave_dj88]

Thanks for all the help!
My project for next year is gonna be power electronics related, so i thought i should be well versed from the beginning.
Cheers all!

 

[TekUT]

If you like to start also think to have a budget to buy this book:

**broken link removed**

Bye
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