Exam text content

 

 

 

Tampere University of Technology — Switched-Mode Converters DEE-33106 1.3. 2016
Electrical Engineering

Teuvo Suntio Programmable calculator allowed

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

Ouestion 1.

Ouestion 2.

The next six subguestions are related to certain relevant terms in power electronics and the
inductor current shown in Fig. 1. a) Explain the meaning of Vs balance, b) Explain the
meaning of As balance, c) What is the operating mode of the converter having inductor
current as shown in Fig. 1 (CCM, DCM or BCM) ?, d) What is the switching freguency of
the converter? e) If we assume that the converter is a Buck converter, what is its average

output current? and f) what is its average input current? Each subguestion will give one
point.

 

Fig. 1

Fig. 2 shows the sample waveforms of the input and output currents as well as the capacitor
of voltage of an ideal basic second-order switched-mode converter. The output voltage of the
converter is 75 V. a) What is the name of the converter (buck, boost or buck-boost)?, b)
what is its operating mode (CCM, DCM, or BCM)?, c) what is its duty ratio?, d) what is its
input voltage?, e) What is the value of its inductor L, and 1) What is the value of its output
capacitor C? Each subguestion gives 1 p.

 

uput Current (A) "Put Current (A)
= a W
sä ja
Ue = x
|

 

 

 

s

2

E

s

5

5 0 34 10 134 20 234 30
Time (445)

Fig. 2

16)
 

 

Tampere University of Technology — Switched-Mode Converters DEE-33106 1.3. 2016
Electrical Engineering

Teuvo Suntio

Ouestion 3.

Ouestion 4.

Programmable calculator allowed

Fig. 3 shows the power stage of an ideal buck converter having switching freguency f; of 150
kHz. Its Korit value eguals D'. a) Choose the value of the inductor L such that the converter
operates in DCM when the output voltage and current are 50 V and 1 A as well as the input
voltage varies in the range of 60-150 V (3 pts). b) Define of the value of duty cycle (D) when
the output voltage and current are as defined above and the input voltage eguals 80 V (2 pts),
and c) Define the peak-to-peak inductor current at the same operating point as in (b) (1 pt).

D=M. | =
1-M TiRa

i;

 

Fig. 3

The switching freguency of the Flyback converter in Fig. 4 eguals 150 kHz, and the
converter operates in DCM. Its input voltage Vi, = 350 V, output voltage V, = 48 V,
transformer turns ratio n,/n; = 0.21, Lv=2 mH, C, = 330 uF, and output power P, = 48 W.
The duty ratio is defined by D=M JK 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, e) the peak current of S; , and
1) the average current of the diode D;. Each subguestion gives 1 pt.

i D

   

Fig. 4

23)
 

Tampere University of Technology — Switched-Mode Converters DEE-33106 1.3. 2016
Electrical Engineering

Teuvo Suntio

Ouestion 5.

Programmable calculator allowed

An active reset Forward converter is shown in Fig. 5. 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 % is reguired for reliability reasons.

8) 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. (2 pts)

b) What is the maximum allowed duty ratio D? (2 pts)

o) Sketch the waveform of the magnetizing current Try And compute its peak value when the
duty ratio is the allowed maximum. (2 pts)

 

Fig. 5

3(3)