Offline Switch mode power supplies

Question 1

Why can buck, boost and inverting converters not be used directly from the mains

Answer 1

They require a DC mains where the mains is AC

They do not have isolation between input and output

Question 2

When creating a flyback converter from a basic circuit what will you have to change

Answer 2

Replace the single inductor with 2 windings on the same core

Question 3

What happens in the flyback circuit when the switch is closed

Answer 3

current flows through the primary coil and charges the inductor and the diode prevents any current from flowing into the 2nd winding

Question 4

What happens in the flyback circuit when the switch is open

Answer 4

No current flows through the primary coil but the polarity of the secondary voltage reverses and current flows into the capacitor and load

Question 5

What is the Vo/Vin equation for a flyback controller

Answer 5

Vout /Vin= -D/(1-D) x (Nout)/(Nin)

Question 6

For a flyback converter how does the inductor work in relation to the windings

Answer 6

Current only flows in one winding at a time and in only 1 direction so one winding charges inductor and one discharges

Question 7

What is true about both input and output currents of flyback converters

Answer 7

Both pulsed and require heavy smoothing

Question 8

What can a Flyback converter be used for

Answer 8

Can be used as the core of a complete power supply unit to give a low DC voltage from AC mains (often called off-line)

Question 9

In the AC Mains line what does the EMI filter do

Answer 9

Protects against noise

Question 10

In AC mains line what does the Power factor correction do

Answer 10

Smooth’s the input current

Question 11

In the AC mains line what regulates the system output voltage

Answer 11

Controlling the PWM Duty Cycle

Question 12

What do practical products often require protection against

Answer 12

Over load or short circuit

Question 13

Describe the AC Mains voltage graph

Answer 13

It is an ideal sin wave

Question 14

Describe the voltage of AC mains after rectification

Answer 14

It is an Ideal but rectified sin wave

Question 15

Describe the voltage of AC Mains after rectification and smoothing

Answer 15

Capacitor smoothed voltage leaving ripple

Question 16

Describe the graph of the AC mains current after Rectification and smoothing

Answer 16

Input is not a sign wave and the rectified form only flows around the peak of rectified voltage

Question 17

What do peaks of current cause

Answer 17

Large voltage drop which clip the peaks of the mains voltage so it is not a perfect sin wave which lead to losses in the mains distribution system

Question 18

What is passive power correction

Answer 18

Passive power correction uses filters to reduce the higher harmonics and leave the current closer to a sin wave

Question 19

What is active power correction needed for

Answer 19

More powerful loads

Question 20

What does active power factor correction involve

Answer 20

Rectifying inputs and feeding into a boost convertor to make it close to a sin wave

Question 21

What happens to the output from the power factor correction after it has been fed into the boost converter

Answer 21

It enters the flyback converter

Question 22

What is the main advantage of a switch mode power supply

Answer 22

smaller and lighter than linear supply as the inductor works at switching frequency

Question 23

What sort of values would you expect for the frequency of a switch mode power supply

Answer 23

MHz