SMPS Design

ET4384

Design of Low power Supplies

Assignment on

DESIGN OF DIFFERENT TOPOLOGIES OF

SMPS

Submitted on 13.03.2014

By

Rizwan Rafique Syed (4333861)

Shihabudheen Kavungal Kolparambath (4310411)

2

Table of Contents

Problem Content Page No

Introduction 3

Problem Statement 3

A Possible topologies 3

A1 Lowest Frequency in BCM Mode 4

B Calculation details for different topologies 4

C Preferred topology when 28

D Preferred topology when 28

Conclusion 30

References 30

List of Tables

Table Content Page No

1 Summary of Down converter parameters when 27

2 Summary of Up converter parameters when 27

3 Summary of Flyback converter parameters when 28

4 Summary of Down converter parameters when 29

5 Summary of Up converter parameters when 29

6 Summary of Flyback converter parameters when 30

3

INTRODUCTION

There are several topologies associated with SMPS. However this assignment discusses three basic

topologies. i.e., Buck (Forward) Converter, Boost (Downward) Converter and Buck-Boost (Fly back)

Converter. Each topology can be operated in different modes according to the requirement/

application.

To select the best topology for a given requirement, it is essential to know the operation, pros &

cons of a particular topology. Following factors are considered for selection of SMPS.

1. The output voltage is lower/higher than input voltage

2. The magnitude of output current

3. Switching at fixed/variable frequency

4. Cost

5. Size of component

6. Isolation requirement

7. Losses & Efficiency

8. EMI Level

9. Output Ripple

In the assignment, we will investigate the three topologies in continuous, discontinuous and

boundary condition mode as per the customer requirement considering the above factors and hence

a choice will be made for selection of the SMPS for the problem requirement.

PROBLEM STATEMENT

Design a switched mode power supply rail with , given that the available input voltages

are . The limit of output current is given as .

Assumptions

Series resistance of coil is related to self-inductance as √ , where in and

in when varies very much within one switching cycle,

Diode is assumed to have no forward voltage drop and no losses

Ringing time is neglected

Fixed frequency operation,

SOLUTION

A. The possible topologies

The required voltage is 12V while the input voltages available are 5V & 48V. As a preliminary survey

without analysing any selection factors, following combinations are possible.

Step up 5V to 12V using a Up converter-CCM, DCM & BCM

Step down 48V to 12V using a Down converter- CCM, DCM & BCM

4

Step up 5V to 12V using a Flyback converter- CCM, DCM & BCM

Step down 48V to 12V using a Flyback converter- CCM, DCM & BCM

A1. Lowest frequency in BCM mode

The main advantage of using SMPS in BCM mode is that the frequency can be varied according to

the variation in the input voltage and load. It is also important to mention that the switching losses

and core losses increases with increase in frequency. Therefore, designer will have a tendency to

decrease the frequency at High power and low input voltage. However, the reduction in frequency is

restricted to audible frequency (20Hz-20 kHz). BCM should be kept above the highest audible

frequency value. Therefore, In order to make switching inaudible to humans at a safe side, the

lowest frequency should be well above 20 kHz, so that component tolerance should not bring the

frequency to or near to the highest audible frequency. In this assignment, we have considered 25kHz

as the lowest possible frequency.

Note: If the designer is using components with small tolerance limit, the lowest frequency could be

selected near to highest audible frequency accordingly.

B. Calculation details for different topologies

This part of the assignment investigates the topologies discussed in A with facts & figures. Following

parameters are calculated for these topologies in different conduction mode.

a. Inductance value, L

b. Series resistance of coil, RL

c. Effective value of current through the coil,

d. Losses in the inductor, and

e. Effective value of current through the switch,

f. Losses in the switch,

g. Total Losses in the converter and efficiency

Given data

Available FET’s 50V/RDSon-0.02Ω, 100V/RDSon-0.05Ω, 200V/RDSon-0.1Ω

Assumptions

Series resistance of coil is related to self-inductance as √ , where in and in when varies very much within one switching cycle,

Diode is assumed to have no forward voltage drop and no losses

Ringing time is neglected

5

Fixed frequency operation,

No correction required for inductor value.

In case of voltage inversion, sign of voltage may be neglected.

FET losses-Only conduction losses are considered

COMPONENT SELECTION

The main components in the Converter considered in this assignment are Diode, Switch, Inductor

and Capacitor. Since design and selection of diode and capacitor are not covered in this assignment,

we will focus only on Inductor and Switch. Inductance of Inductor is calculated based on formula,

which will be calculated in the subsequent session. The available switch is FETs 50V/RDSon-0.02Ω,

100V/RDSon-0.05Ω, 200V/RDSon-0.1Ω. The selection of switch is based on maximum voltage that

will appear across the switch during turn off. Therefore the switch selection is as follows

a. Down Converter- FETs 50V/RDSon-0.02Ω , since Vin =48V is the maximum voltage across

switch

b. Up Converter- FETs 50V/RDSon-0.02Ω , since Vout =12V is the maximum voltage across

switch

c. Flyback Converter with input voltage as 5V- FETs 50V/RDSon-0.02Ω , since Vin+Vout

=5+12=17V is the maximum voltage across switch

d. Flyback Converter with input voltage as 48V-FETs 100V/RDSon-0.05Ω , since Vin+Vout

=48+12=60 V is the maximum voltage across switch

I. DOWN CONVERTER

a. CCM Mode

The duty cycle in continuous conduction mode is given by

The minimum value of inductance so that current will not go to discontinuous conduction mode is

(

)

The series resistance of the inductor is approximated as

√

√

6

The size of the coil is determined by inductance value and the maximum energy that has to be

stored. Therefore, which determines the storage capacity is calculated as

The variation of current in one switching cycle is given by

Since the losses in the circuit are analysed using the RMS values, RMS value of current through the

coil can be found using

Similarly, current through the switch is calculated as

Now the losses can be calculated from RMS current

Since the variation of current in switching cycle is very less, core losses are neglected.

Now the conduction losses in the switch is calculated as

Total Converter losses with 50V FET

7

Efficiency

b. DCM Mode

The relationship between input and output voltage in DCM mode is given by

Assuming we get

The maximum value of L is calculated as

( )


√

, which determines the storage capacity is calculated as


The effective value of current through inductor is given by

8

The RMS value of current through switch is calculated


Since the variation of current in switching cycle is 8A, core losses = winding losses



Efficiency

c. BCM Mode

Neglecting dead time, duty cycle under all circumstances is

9

Since , maximum current through coil can be calculated as

In BCM mode, is varying according to the input voltage. , at any is given by

The minimum value of frequency is given by

( )

Considering , the maximum inductor value can be calculated as

( )


√

The effective value of current through inductor is given by


The RMS value of current through switch is calculated

10





Efficiency

II. BOOST CONVERTER

a. CCM Mode

The duty ratio is given by

The minimum value of L in order to guarantee continuous mode of operation is given by

11


√

The peak inductor current is calculated as


Since losses are our main concern, the RMS value of coil current can be calculated using the below

equation.

For the switch, calculation of RMS current yields,


12

Since the variation of current in switching cycle is very less, core losses is neglected.



Efficiency

b. DCM Mode

In DCM mode, the duty cycle is given by

Assuming

The maximum value of L is calculated as


√

The peak value of current through the coil is given by

√

13


The RMS value of current through the inductor can be calculated using

The effective value of current through the switch is calculated using the formula

(

)

(

)

Therefore,


Since the variation of current in switching cycle is 19.2A, core losses = winding losses



Efficiency

14

c. BCM Mode



The maximum inductance value corresponding to the lowest frequency is given by


√




15





Efficiency

III. FLYBACK CONVERTER (Input is 5V, while output is 12V)

a. CCM Mode


16


(

)

(

)

(

)


√




(

)

(

)

(

)

(

)

The RMS value of current through the switch can be calculated using

(

)

(

)

17

(

)

(

)

Now the coil losses can be calculated from RMS current

Since the variation of current in switching cycle is less, core loss is neglected.

While the conduction losses in the switch is calculated as


Efficiency

b. DCM Mode

The duty cycle in DCM mode is given by

The maximum value of value of inductance is given by

(

)

18

(

)


√


√

√


The RMS value of current through coil is

The RMS value of current through switch is


19




Efficiency

c. BCM Mode

The duty ratio in BCM mode


(

)


(

)

(

)


√


20

(

)








21

Efficiency

IV. FLYBACK CONVERTER (Input is 48V, while output is 12V)

a. CCM Mode



(

)

(

)

(

)


√



22


(

)

(

)

(

)

(

)

The RMS value of current through the switch can be calculated using

(

)

(

)

(

)

(

)

Now the coil losses can be calculated from RMS current

Since the variation of current in switching cycle is very less, core losses is neglected

while the conduction losses in the switch is calculated as

Note that here for FET 100V as voltage appearing across switch during turn off is

60V and hence available option is 100V FET


Efficiency

23

b. DCM Mode

The duty cycle in DCM mode is given by

The maximum value of value of inductance is given by

(

)

(

)


√


√

√



24








Efficiency

c. BCM Mode

The duty ratio in BCM mode

25


(

)


(

)

(

)


√


(

)




26







Efficiency

The table below depicts the summary of above calculations.

27

Table 3: Summary of Down converter parameters when

Down Converter

CCM DCM BCM

Duty Cycle 0.25 0.25 0.25

Lmin for CCM and Lmax for DCM (µH) 225.00 11.25 45.00

RL (ohm) 0.071 0.016 0.032

Ipeak (A) 4.20 8.00 8.00

Ripple Current d(IL) 0.40 8.00 8.00

IL,RMS (A) 4.00 4.62 4.62

Isw,RMS 2.00 2.31 2.31

Pcoil,loss (W) 1.14 0.34 0.68

Psw,loss (50V)-R=0.02 0.08 0.11 0.11

Psw,loss (100V)-R=0.05 0.20 0.27 0.27

Psw,loss (200V)-R=0.1 0.40 0.53 0.53

Total converter Loss with 50V FET 1.22 0.79 1.46



Efficiency in % 97.52 98.39 97.04

Table 3: Summary of Up converter parameters when

Up Converter

CCM DCM BCM

Duty Cycle 0.58 0.58 0.58


RL (ohm) 0.026 0.006 0.012

Ipeak (A) 10.08 19.20 19.20


IL,RMS (A) 9.60 11.09 11.09

Isw,RMS 7.34 8.47 8.47

Pcoil,loss (W) 2.41 0.72 1.44

Psw,loss (50V)-R=0.02 1.08 1.43 1.43

Psw,loss (100V)-R=0.05 2.69 3.58 3.58

Psw,loss (200V)-R=0.1 5.38 7.17 7.17




Efficiency in % 93.23 94.36 91.77

28

Table 3: Summary of Flyback converter parameters when

Up-Down Converter for 5V Up-Down Converter for 48V

CCM DCM BCM CCM DCM BCM

Duty Cycle 0.71 0.71 0.71 0.20 0.20 0.20

Lmin for CCM and Lmax for DCM (µH) 25.95 1.30 5.19 192.00 9.60 38.40

RL (ohm) 0.024 0.005 0.011 0.066 0.015 0.029

Ipeak (A) 25.16 27.20 27.20 5.25 10.00 10.00

Ripple Current d(IL) 1.36 27.20 27.20 0.50 10.00 10.00

IL,RMS (A) 13.60 15.70 15.70 5.00 5.77 5.77

Isw,RMS 11.43 13.19 13.19 2.24 2.58 2.58

Pcoil,loss (W) 4.47 1.33 2.67 1.64 0.49 0.98

Psw,loss (50V)-R=0.02 2.61 3.48 3.48 0.10 0.13 0.13

Psw,loss (100V)-R=0.05 6.53 8.70 8.70 0.25 0.33 0.33

Psw,loss (200V)-R=0.1 13.06 17.41 17.41 0.50 0.67 0.67

Total converter Loss with 50V FET 7.08 6.15 8.81 1.74 0.62 1.11



Efficiency in % 87.14 88.65 84.49 96.20 97.34 95.44

C. From the table 1,2 &3 shown above and the calculation, for the given problem/application,

Buck converter operating in DCM is preferred over any other topologies. This is due to the fact

that the losses are minimum/efficiency is maximum for this topology. The higher efficiency is

due to the following reasons.

a. The inductor value is less as compared to other modes in Buck Converter. This results in

reduced series resistance and hence reduced coil loss.

b. The voltage appearing across the switch during turn off is 48V,neglecting transients/over

voltage condition, FET of 50V can be selected for the design. Besides, The resistance of

this switch is less compared to other switch reducing the conduction losses.

c. Although core loss is neglected in CCM mode, despite the addition of core losses in

DCM, DCM mode is still having the highest efficiency due to the fact that the inductance

and hence series resistance is much lower in DCM mode compared to CCM mode

D. When the minimum output current is changed from 0.2A to 1A, the preferred topology would

be down or forward converter in CCM mode. Table 4,5 &6 gives the complete details when

This is again due to the higher efficiency/low losses. The change in efficiency when

changes are due to the following reasons.

a. The inductance value is reduced from as is a function of

in CCM mode given by the formula

In this formula, we can find that

29

Since coil resistance is related to inductance by √ , thus coil resistance

reduces, which in turn reduces the coil losses ( . Therefore overall converter

efficiency is increased.

b. In CCM mode, is very less compared to other modes.

Therefore core loss is assumed to be zero which in turn reduces the total loss and hence

increases the efficiency.

Table 4: Summary of Down converter parameters when

Down Converter

CCM DCM BCM

Duty Cycle 0.25 0.25 0.25


RL (ohm) 0.032 0.016 0.032

Ipeak (A) 5.00 8.00 8.00


IL,RMS (A) 4.04 4.62 4.62

Isw,RMS 2.02 2.31 2.31

Pcoil,loss (W) 0.52 0.34 0.68

Psw,loss (50V)-R=0.02 0.08 0.11 0.11

Psw,loss (100V)-R=0.05 0.20 0.27 0.27

Psw,loss (200V)-R=0.1 0.41 0.53 0.53




Efficiency in % 98.76 98.39 97.04

Table 5: Summary of Up converter parameters when

Up Converter

CCM DCM BCM

Duty Cycle 0.58 0.58 0.58


RL (ohm) 0.012 0.006 0.012

Ipeak (A) 12.00 19.20 19.20


IL,RMS (A) 9.70 11.09 11.09

Isw,RMS 7.41 8.47 8.47

Pcoil,loss (W) 1.10 0.72 1.44

Psw,loss (50V)-R=0.02 1.10 1.43 1.43

Psw,loss (100V)-R=0.05 2.74 3.58 3.58

Psw,loss (200V)-R=0.1 5.49 7.17 7.17




Efficiency in % 95.62 94.36 91.77

30

Table 6: Summary of Flyback converter parameters when

Up-Down Converter for 5V Up-Down Converter for 48V

CCM DCM BCM CCM DCM BCM

Duty Cycle 0.71 0.71 0.71 0.20 0.20 0.20

Lmin for CCM and Lmax for DCM (µH) 5.19 1.30 5.19 38.40 9.60 38.40

RL (ohm) 0.011 0.005 0.011 0.029 0.015 0.029

Ipeak (A) 71.40 27.20 27.20 6.25 10.00 10.00

Ripple Current d(IL) 6.80 27.20 27.20 2.50 10.00 10.00

IL,RMS (A) 13.62 15.70 15.70 5.05 5.77 5.77

Isw,RMS 11.44 13.19 13.19 2.26 2.58 2.58

Pcoil,loss (W) 2.00 1.33 2.67 0.75 0.49 0.98

Psw,loss (50V)-R=0.02 2.62 3.48 3.48 0.10 0.13 0.13

Psw,loss (100V)-R=0.05 6.55 8.70 8.70 0.26 0.33 0.33

Psw,loss (200V)-R=0.1 13.10 17.41 17.41 0.51 0.67 0.67




Efficiency in % 91.21 88.65 84.49 97.95 97.34 95.44

Conclusion

This assignment investigates the variable topologies in Switched mode power supply. The design of

SMPS for this assignment was based on factors like component selection and Converter losses, with

many factors neglected for the ease of calculation and to reduce the complexity. However, actual

design involves many factors, some of which are listed below.

a. The design of each component, including Capacitor, Diode, Switch and Inductor

b. Level of EMI

c. Output ripple and ripple rejection

d. Switching losses in the Switches, Diode losses etc.

e. Ringing time

f. Transient conditions like variation in input voltage, load variation etc.

g. Turns ratio in case of fly back converter

h. And most importantly the cost

References

1. Ir. Frans Pansier (2014). Lecture Notes, Introduction to SMPS topologies, chapter 1,2,3,4 & 5.

ET 4384, Design of low power supplies

2. Mohan Undeland and Robbins. Power Electronics-Converters, Application and Design(2nd

edition), Wiley Publications

Documents

SMPS Design