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Tampere University of Technology Switched-Mode Converters DEE-33106 12.5. 2015
Electrical Engineering

Teuvo Suntio

Programmable calculator allowed

Note: The answers can be given either in English or Finnish.

Ouestion 1.

Ouestion 2.

Ouestion 3.

Define shortly the following terms or concepts relevant to switched-mode converters a) Vs
balance, b) As balance, c) M(D), d) Kerit e) DCM and f) DDR control (Note: Direct English
language translation is not enough but the meanings are reguired to be explained!) Each
subguestion gives 1 pt.

Fig. 1 shows a sample of the inductor current of an ideal boost converter. In addition with the
induotor-current waveform (Fig. 1), we know that its output voltage is 30 V. a) What is its
operation mode (CCM, BCM, DCM)?, b) What is its average output current?, €) What is its
input voltage?, d) What is its duty ratio?, e) What is the value of its inductor L, and f) What
is its average output capacitor current? Each subguestion gives 1 p.

     
 

3A

  

0 6 us 10 us 16 us 20 js

Fig. 1

Fig. 2 shows the power stage of a boost converter having switching freguency f; of 150 kHz.
Its critical Kori value eguals DD”. Assume the diode D) to be ideal. a) Choose the value of
the inductor L;, such that the converter operates in DCM when the output voltage and output
power are 50 V and 50 W and the input voltage varies in the range of 12-40 V (3 pts). b)
Define of the value of pulse ratio when the output voltage and output power are as defined
above and the input voltage eguals 30 V (2 pts). ) Define average current of diode D, at the

operating point of b) (1pts). D=4/KM(M-1), K D M(D)=1/D'.

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Fig. 2

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Tampere University of Technology — Switched-Mode Converters DEE-33106 12.5. 2015
Electrical Engineering

Teuvo Suntio

Ouestion 4.

Ouestion 5.

Programmable calculator allowed

The switching freguency of the flybcak converter in Fig. 3 is 150 kHz, and it is operating in
DCM. Its input voltage Vn =350 V, output voltage V,= 48 V, transformer turns ratio n,/n; =
0.21, Lv = 2 mH, C, = 330 uF, and output power P, = 40 W. The duty ratio is defined
byD =/K-M and the critical K by D?. Assume the output diode D; to be ideal. Define 2)
the maximum voltage of the switch S, , b) the maximum voltage of the diode D,, c) the K
value of the converter, d) the duty ratio, e) the peak current of S, , and f) the average current
of the diode D;. Each subguestion gives 1 pt.

 

Fig. 3

An active reset Forward converter is shown in Fig. 4. The converter works in such a way that
the main switch S, conducts during the on time, and the auxiliary switch S, during the off
time, respectively. The input voltage is 200 V, the switching freguency is 100 kHz, and the
magnetizing inductance is 4 mH, respectively. The active reset capacitor is large enough so
that its voltage can be assumed constant.-The absolute maximum voltage of the MOSFET
S, is 600 V. Derating of 15 % in voltage is reguired for reliability reasons. This guestion
gives maximally 8 pts.

a) Compute the steady-state voltage v,, as a function of the input voltage and the duty ratio
D of the MOSFET S, applying Vs concept to the magnetizing inductor. (3 pts)

b) What is the maximum allowed duty ratio D to satisfy the maximum allowed voltage of
S,? (2 pts)

c) Sketch the waveform of the magnetizing current i,,, and compute its peak value when the
duty ratio is the allowed maximum. (3 pts)

 

 

Fig. 4

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