Transcript Document 7514439
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here Switched Mode Power Supplies Add Instructor/Instructors name here Prof. Vivek Agarwal
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Definitions and Keywords
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Duty cycle/ratio-
The fraction of time for which the switch is ‘ON’ in one complete switching cycle
DCM
(Discontinuous Conduction Mode)- The operating mode of the converter when the current through the inductor is discontinuous with respect to time i.e. it is zero for some finite duration in a switching cycle.
CCM
(Continuous Conduction Mode)- The operating mode of the converter when the current through the inductor is continuous with respect to time i.e. it is never zero during the switching cycle.
Critical Conduction Mode-
The operating mode of the converter when the current through the inductor is just continuous (or just discontinuous) with respect to time
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5 1 3 4 2 INSTRUCTIONS SLIDE
Concept details
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Step number 1 2 3
Concept details
Details of the step Image / Diagram Text to be displayed Action / Motion in the step
Circuit diagram of a boost type dc dc converter Operation of the circuit when the switch ‘S’ is closed V i I G L S D C R V O As the switch ‘S’ closes and opens, the inductor stores and releases energy giving a boosted output voltage (V O V ) i The switch ‘S’ should open and close with a time period of 0.5 sec V i I L L I O S C V i I L I p V L T ON I p + Show this as rising current V i R V O L di dt =V i Hence current through the inductor increases. The diode D is reverse biased during this interval and does not conduct The thickened arrows should continuously move in the direction of the “arrow heads” as shown in both the loops The current I L should rise slowly from its initial value (Ip ) to its final value (Ip + ) Operation of the circuit when the switch ‘S’ is open V i V i I L Ip V L L I L S C I O T ON T Ip + Show this as falling current T OFF V i - V o R V O During this interval the diode ‘D’ is forward biased L V O di dt =V -V i O V i di is negative dt current through the inductor decreases The thickened arrows should continuously move in the direction of the “arrow heads” as shown in both the loops The current I L should fall slowly from Ip + to Ip value in the slot T OFF
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Concept details
Step number 4 5
Details of the step Image / Diagram Text to be displayed Action / Motion in the step
Overall operation of the circuit The output voltage expression: V o 1 V i
d
where, d lies between 0 and 1.
Hence, the output voltage gets boosted up Case I Effect of duty cycle on the operation of the circuit keeping inductor value constant a) CCM operation I G I L I p V i Combination of operation when the switch is ‘ON’ and when the switch is ‘OFF’ as given previously Average voltage across the inductor during a cycle = 0. Therefore: ON = o OFF V o V i OFF OFF 1 1
d
V o This is the output voltage expression in case of CCM operation D L S C R For a given inductor value and reasonably large duty cycle, the current through the inductor is continuous (as shown) i.e.
CCM
operation T ON I p + T OFF Sequentially show the animation of both modes i.e. when the switch is ‘ON’ and when the switch is ‘OFF’ as in the previous two circuits – i.e. the animation toggles between the two states The duty cycle needs to be animated here. Starting with a reasonably high duty cycle (as shown in the figure), the inductor current should rise from the initial value Ip and reach a peak value Ip + at the end of the ON period then slowly decrease back to Ip at the end of the OFF period as shown in the figure, thereby resulting in a continuous current mode of operation T b) Critical Conduction Mode of operation I G I L T ON I p T T OFF With the same inductor value, the duty cycle is now reduced such that the current through the inductor becomes just continuous –
Critical Conduction Mode
The duty cycle should then be slowly reduced till a point when the inductor current starts from the initial value of 0, reaches a peak Ip at the end of the ON period and then decreases slowly back to 0 exactly at the end of the OFF period as shown in the figure, thereby resulting in a just continuous (or critical) current mode of operation
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Concept details
Step number 6
Details of the step
c) DCM operation b) Critical Conduction Mode of operation I G I L
Image / Diagram
T ON I G I p + (new) T OFF T Case II L Big sized inductor S D C V i Effect of inductor value on the operation of the circuit keeping duty cycle fixed a) CCM operation I G I L I p I G T ON I p + T T OFF V i L Normal sized inductor S D C I G I L T ON I p T T OFF
Text to be displayed
Without changing the inductor value, the duty cycle is further reduced till the inductor current becomes fully discontinuous i.e.
DCM
operation.
Ip +( new) >Ip> Ip+ Action / Motion in the step
Next, the duty cycle should be further reduced till a point is reached when the inductor current starts from the initial value of 0, reaches a peak Ip + (new) at the end of the ON period and then decreases slowly back to 0 before the end of the OFF period as shown in the figure, thereby resulting in a discontinuous current mode of operation R V O R V O For a given (fixed) duty cycle and a reasonably large inductor value, the current through the inductor is continuous i.e.
CCM
operation Here, the inductor size needs to be animated giving the waveforms according to the size of the inductor used in the circuit.
Starting with a high value of inductor (shown in the figure with a bigger size), the inductor current should rise from the initial value Ip and reach a peak value Ip + at the end of the ON period then slowly decrease back to Ip at the end of the OFF period as shown in the figure, thereby resulting in a continuous current mode of operation With the same (fixed) duty cycle, the inductor value is now reduced till the point when the inductor current becomes just continuous –
Critical Conduction Mode
The inductor size should be slowly reduced till a point is reached when the inductor current starts from the initial value of 0, reaches a peak Ip at the end of the ON period and then decreases slowly back to 0 exactly at the end of the OFF period as shown in the figure, thereby resulting in a just continuous (or critical) current mode of operation
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Concept details
Step number
Details of the step Image / Diagram
c) DCM operation V i L Small sized inductor S D C I G I G I L I p + (new) T ON T T OFF
Text to be displayed Action / Motion in the step
R V O With the same (fixed) duty cycle, inductor as value the is reduced there comes a point when the current through the inductor becomes discontinuous -
DCM
operation
Ip+(new) >Ip> Ip+
Next, the inductor size should be further reduced (as depicted in the figure) till a point when the inductor current starts from the initial value of 0, reaches a peak Ip + (new) at the end of the ON period and then decreases slowly back to 0 before the end of the OFF period as shown in the figure, thereby resulting in a discontinuous current mode of operation
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Interactivity and Boundary limits
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In this section provide, interactivity options for all the parameters/components of the concept.
For example: Numerical values to change the state of the component: By providing input boxes Drag and drop of components: To test the comprehension of the users Movement of objects: To explain the action of the components
Interactivity option
number
enable correctness of the results of the experiment.
move the ‘balloon’ of gas in the resonator When the balloon is placed in high pressure region, it would shrink and its temperature will rise (it will become red). When moved to low pressure region, it expands becoming cold (blue) ....
gas
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Interactivity and Boundary limits
Interactivity option number 1 Details of interactivity Image / Diagram Text to be displayed Boundary limit 4 3 2 3
5 1 2 3 4 INSTRUCTIONS SLIDE
Questionnaire
to test the user
A small, (5 questions) questionnaire can be created in the next slide, to test the user's comprehension.
This can be an objective type questionnaire.
It can also be an exercise, based on the concept taught in this animation.
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Questionnaire
1. For a Boost type DC-DC converter, V O (output voltage) is: Answers: a)
d)
for DC-DC converters is: Answers: a) 0 0.5
d
1 0 1 3. For a given power output, the peak current in the inductor of a DCM operated DC-DC boost converter is: Answers: a) less than the peak current in case of CCM b) equal to the peak current in case of CCM current in case of CCM c) greater than the peak d) none of the above
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Questionnaire
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4. The ripple in the inductor current of a CCM operated DC-DC boost converter is: Answers: a) greater than the current ripple in case of DCM b) equal to the current ripple in case of DCM c) less than the current ripple in case of DCM d) none of the above 5. The expression of the output voltage of the Boost DC-DC converter operating in DCM is : Answers: a) and
d V o V
o
1
V
i d
c) 1
d
d V i
b)
V o
is a complex expression of d) none of the above
R, L
Answer key: 1(b), 2(d), 3(c), 4(c), 5(b)
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Links
for further reading
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Links
for further reading
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[1] “DC-DC Switching Regulator Analysis”, by Daniel M. Mitchell, McGraw-Hill Book Company, 1988.
[2] “Power Electronics: Converters, Applications and Design” , by Ned Mohan, Tore M. Undeland and William P. Robbins, Third edition, John Wiley & Sons.
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