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Tampere University of Technology — Switched-Mode Converters DEE-33106 25.4. 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? €) If we assume that the converter is a Boost 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 power stage of a certain current-fed converter. The MOSFET gate control
scheme is defined in the figure. The converter operates in CCM. a) Define M(D) of the
converter in terms of 1, /1,, (3 pts) b) Define V, /V, (1 pt), €) What is the type of the converter

(buck, boost or buck-boost) (1 pt) ,and d) Define the average inductor current in terms of input
current (1 pt).

J

 

Fig. 2

1(3)
Tampere University of Technology — Switched-Mode Converters DEE-33106 25.4. 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 Kc 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).

 

Fig. 3

Fig. 4 shows a certain buck-derived converter, where the switching freguency of
V, and V, eguals 100 kHz, and the converter operates in CCM. Its input voltage Vin varies from
200 V to 400 V, output voltage V, = 48 V, C,=330 uF, L, = 100 uH and output power P, = 48
W. The primary-side capacitors can be assumed to be high enough so that the midpoint voltage
stays constant. Assume the output diodes V3 andV4 to be ideal: a) define the reguired
transformer turns ratio (Ny/N;) for obtaining the stated output voltage when the maximum duty
ratio can be only 90 % of its theoretical maximum value (1 pts), b) define the maximum reverse
voltage of diode V3 (1 pt) c) define the maximum voltage of MOSFET V, (1 pt), d) what is the
operational freguency seen by output inductor L, (1pt),and e) define the maximum peak-to-
peak ripple current of the inductor when the operation point of the converter varies as stated
above (2 pts).

 

Fig. 4

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Tampere University of Technology — Switched-Mode Converters DEE-33106 25.4. 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.

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. (2 pts)

b) What is the maximum allowed duty ratio D in terms of MOSFET specification? (2 pts)

c) Sketch the waveform of the magnetizing current i,,, and compute its peak value when the
duty ratio is the allowed maximum. (2 pts)

 

 

Fig. 5

3(3)