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Tampere University of echnology Switched-Mode Converters DEE-33106 16.12. 2013
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,K), d) Korit e) BCM and f) PFC (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 buck converter. In addition with the
inductor-current waveform (Fig. 1), we know that its input voltage is 50 V. a) What is its
operation mode (CCM, BCM, DCM)?, b) What is its average output current?, c) What is its
output 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.

 

0 6 js 10 js 16 us 20 MS

Fig. 1

N Yet = R N SSNSTIS.t<!!

Fig. 2 shows the power stage of a boost converter having switching freguency f; of 150 kHz.
Its critical Kori value is DD. Assume the diode D) to be ideal.(2) Choose the value of the
inductor Lin 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),

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). (6) Define the peak-to-peak ripple current of

 

the inductor at the operating point of b) (lpts). D=J/KM(M-1, K - ;
vas

M(D)=1/D'.

Fig. 2

 

102)
Tampere University of Technology — Switched-Mode Converters DEE-33106 16.12. 2013
Electrical Engineering
Teuvo Suntio Programmable calculator allowed

Ouestion 4. — The switching freguency of the Flybcak converter in Fig. 3 is 150 kHz, and it is operating in
DCM. Its input voltage V;,=350 V, output voltage V,=48 V, transformer turns ratio 12/1; =
0.21, Lu =2 mH, C, = 330 uF, and output power P, = 40 W. The duty ratio is defined
by D=/K-M and the critical K by D. Assume the output diode D, to be ideal. Define a)
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, €) the peak current of S; , and f) the average current
of the diode D). Each subguestion gives 1 pt.

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Fig. 3

Ouestion 5. — 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

k 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
Sis 600 V. Derating of 15 % is reguired for reliability reasons. This guestion gives
maximally 8 pts.

 

a) Compute the steady-state voltage Vcr 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? (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)

tn i 1:n D a

 

 

 

 

 

Fig. 4

 

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