Output voltage of equation of step-up chopper is always derived using conservation of energy. Is there any derivation using simple KVL and KCL ?
Is there any limitation on load type ?? Its hard for me visualize how voltage can be stepped up in R-load.
for DC-DC converters prefer "Fundamentals of Power Electronics" by Ericson.
It will clear all your doubts
Yes,As Mr.Hari said you can get the output voltage equation by using KVL and equating the average voltage across the inductor to zero.
This is what i was trying to do for step up chopper for duty cycle of 50%(.5 sec ON and .5 sec OFF)
1) When thyristor is ON :-
V=L(di/dt)
change in current during ON period = V/2L
output voltage=0 for this period as load is in parallel to thyristor
2) When thyristor is OFF :-
output voltage= V + L(di/dt)
since average current across an inductor is 0. Therefore, rise in current during must be equal to fall in current
Now, output voltage = V+L*V/2L*1/.5 = 2V
now average output voltage for one period = (0+2V)/2 = V
but i am not getting 2V
I know i am not much clear but sorry i didnt find any good editor where i can sketch circuit diagrams and write equations properly.
Dear Sarvesh,
I hope this will help,
Rise in current when switch turned on, di = V*T/2L,,T=time period...
Fall in current when switch turned off ,di = (Vo-V)*T/2L,
Equating these two eqns, Vo =2V
when switch is off Vo=2V , and when switch is ON Vo=0
Now if we find average output voltage for one time period
then output average voltage = (2V*Ton)/T
and surely from this equation output average voltage is not 2V but less than 2V as Ton/T is less than one.
Generally,when the output is DC,a capacitor designed to have minimum ripple is placed at the output.So in the above derivations it is assumed that the output voltage Vo is constant all the time.Therefore when switch is ON Vo is not Zero and a diode is placed to make sure that the capacitor doesn't discharge through the switch.
By using closed loop control and PWM switching this output voltage is maintained at a set value depending on the load.
I hope this will be helpful.....
Capacitor can be placed only in parallel to the load as the output is DC.
If the load is purely resistive then when thristor is ON , output voltage must go to zero so that KVL is applicable. So if we are assuming output voltage to be constant then we must have specified that load is highly inductive and even if load is inductive if we consider inductor+back EMF as load then also output voltage must go to zero in order to satisfy KVL. This was my question is there any condition on load ?
Anyways you may be right but my question was the derivation. If i get the derivation using KVL then i can clear all my doubts . Wherever i searched for derivation i found derivation using conservation of energy.
When i tried to derive by KVL and KCL then i found that voltage cann't be stepped up in R-load so i want to know practical results that in R-load is the voltage really stepped up or all our experiments are for DC-motor control.
I hope i am clear about my doubts and questions .
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