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24.02.2020

N) Tampere University Joni Markkula

Exam of the course DEE-33106 Switched-mode converters, 5 cr.

Please answer all of the five (5) guestions. The use of calculator is allowed. Please answer in Finnish or English.

Ouestion 1: Answer TRUE or FALSE (+0.5 for correct, -0.5 for incorrect, 0 for empty, total max 8 p.)

mann o

Tos Sn

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p.

In steady state sum of ampere-seconds in capacitor is zero during one switching period

In a given circuit increasing the inductor value decreases current ripple

Unit of inductance (H) can also be presented in SI units as Vs/A

Ideal capacitor would create an ideal current source

Converter goes from CCM to DCM when load resistance increases significantly

Capacitor parasitic inductance starts to dominate the impedance value on very high freguencies
Mosfet is controlled by voltage signal to transistor gate

Ceramic capacitors have higher ESR then electrolyte capacitors

Inductor AC losses can be calculated if numbers of turns and switching freguency are known
Doubling switching freguency doubles the switching losses in transistor

Faster transistor rise time causes less current harmonics

Capacitor has both AC and DC losses

. When inductor is saturated, inductance value decreases with increasing current

Power factor correction means having more accurate sguare waves
Normally it is best to build tailored control mechanism instead of using existing ICs
EMC immunity means that device is not emitting electromagnetic waves

Owestion 2 (6 p.)

 

 

 

Figure 1: A non-inverting buck-boost converter

Fig. 1 shows a non-inverting buck-boost converter, which can be operated at three different modes: boost,
buck-boost, and buck.

Mode 1: The switch S2 is constantly OFF and switch S; is operated normally with duty ratio D.

Mode 2: The switch S is constantly ON and switch S? is operated normally with duty ratio D.

Mode 3: The switches are operated together with duty ratio D (that is, both switches either conduct or not
conduct in unison)

PLEASE TURN THE PAGE!
Draw on-time and off-time circuits for each operation mode (2 pts)
a) Define symbolically the conversion ratio M(D) from input to output voltage for each operation

mode (2 pts)
b) Identify the operation modes (1 pts)
c) Briefly discuss the downsides of using this converter topology (1 pts)

Ouestion 3 (6 p.)

 

 

 

 

 

 

 

Figure 2: A non-ideal flyback converter

Vin = 340V W, = 60V f; = 10 kHz I, =25A
Vp = 0.7V Tas = 100 m) 1, = 100m9 n= = = 0.26
1

Fig. 2 shows a flyback converter with non-ideal magnetizing inductor, switch, and diode. The transformer and
capacitor can be assumed ideal. Take the diode voltage drop, switch resistance and inductor resistance into
account in calculations.

a) Calculate the voltage over the transistor during off-time (1 pts)
b) Solve the duty ratio and average inductor current (3 pts)
c) Design output capacitor so that the output voltage peak-to-peak ripple is 5 V (2 pts)

Ouestion 4 (6 p.)

A boost converter (such as one used in a PFC stage) is designed with the following parameters:

 

 

 

 

 

 

 

Henan
Input voltage 325VDC
Input current 10.0 A DC va
Output voltage 420 VDC son da a
Tu 100 kHz e
Vas 10V
Tunnen | 50 Rc Raza 3.0 *CW
Diode heatsink Rin 3.0 *C/W Pa" Rao Paon

 

 

 

 

PLEASE TURN THE PAGE!
Following components are used (datasheets included, please check carefully per component!):
MOSFET IPW65R045C7

Diode IDH16G65CS5
Inductor 500 uH
Output capacitor 1500uF, 450V electrolytic

a) Calculate MOSFET and diode conduction losses in worst case
b) Calculate other types of diode losses, and diode die temperature, worst case
c) Aftera failed EMC test, you have to add gate resistance Rg=10ohms. Calculate MOSFET switching loss.

Ouestion 5 (6 p.)

a) Explain in sufficient detail how inductor works in switched mode converters. Include picture(s) and
eguations of basic principles and describe also coupled inductor.

b) You have been asked to design a dc-dc converter for a small solar installation where a battery system
would be charged. Describe the strategy on how to make a usable switch mode converter. What needs to
be taken into account?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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