Exp-10

Simulation of Step Up Chopper using MATLAB

Theory:

A step up chopper provides a minimum output voltage equal to the DC supply voltage and the output voltage higher than DC input can be obtained.

Fig. 1 Step-up chopper

As shown in the circuit diagram, in this chopper, a large inductor (L) in series with the source voltage V_S is essential to get the voltage higher than the supply voltage.

Mode-1: Chopper is ON

When switch SW is ON, the closed circuit path is shown below. V_S(+) – L – SW – V_S(-)

Fig.2 Operation of step-up chopper when switch SW is ON

During this period (T_ON) the inductor stores energy in it.

Now the load current will increase from I_min to I_max and supply voltage is applied to the load V_O = V_S.

Mode-1: Chopper is OFF

When the switch is turned off, the inductor current is forced to flow through the diode and load.

Now the closed circuit path will be Vs+ - L – D – Load - Vs-.

Fig.3 Operation of step-up chopper when switch SW is OFF

During this period (T_OFF) the polarity of the emf induced in L is reversed as shown.

(Remember that it is inductor property to oppose sudden change in voltage).

Thus the voltage across load is

V_o=V_s+L(di/dt)

Which is more than supply voltage.

With switch SW is OFF, the current would fall from Imax to Imin.

Applying KCL during this condition,

In summary during T_ON, energy stored in inductor & during T_OFF stored energy is released to load.

Fig.4 Output waveform of load current

During T_ON, energy input to the inductor is

Win = (Voltage across L) (average current through L) T_ON

Won=Vs((Imax+Imin)/2) Ton

During T_OFF, energy released by the inductor is

Woff = (Voltage across L) (average current through L) T_OFF

Woff=(Vo−Vs)((Imax+Imin)/2) T_OFF

considering the lossless system, these two energies will be equal.

Vs((Imax+Imin)/2) Ton =(Vo−Vs)((Imax+Imin)/2) T_OFF

Vo=Vs.(T/(T−Ton))

Vo=Vs (1/(1−α))

Form the above equation, the average voltage across the load can be stepped up by varying duty cycle.

If switch SW is always OFF, α =0 and V_O=V_S.

If switch SW is always ON, α =1 and V_O = ∞.

When the SCR is kept ON and OFF for equal amount of time (α = 0.5), the load voltage is 200% of the input DC voltage.

Simulation:

The simulink model of step-up chopper explained above is made in MATLAB software. It is shown in Fig. 5.

Fig. 5 Simulink model of step-up chopper

After running the simulation the waveforms obtained are presented in Fig. 6.

Fig. 6 Output load current (in Ampere) and load voltage (in Volt)

Conclusion:

In the simulation it is clearly shown that input DC voltage is 100 V. Output voltage is 1001 V. Here duty cycle set at the value of 90 %. Theoretical output voltage should be 1000 V. But here it is more. In practical case it depends on value of L and parameters of load.

Prepared by: Shri H I Joshi (M.Tech - Electrical, PhD - Electrical) LEE, G P Himatnagar