Power Electronics

Question 1

What are the different types of Linear Regulators?

Answer 1

Series, Low drop out and Shunt

Question 2

How does a series type regulator work?

Answer 2

It uses a BJT like a resistor, because by controlling the base current we can control the amount of voltage that is dropped across the BJT allowing us to control the output voltage.

Question 3

How do we maintain the output voltage in a series regulator?

Answer 3

Using a feedback system.

Question 4

What happens if the output resistance decreases in a series regulator?

Answer 4

The output voltage will decrease due to this. This will be detected by the feedback and will increase Ib to increase Ic since Ic is controlled by Ib and Ic = Iout. This will return the output voltage to its nominal value.

Question 5

What happens if the output resistance increases in a series regulator?

Answer 5

The output voltage will increase. Feedback will decrease Ib to decrease Ic and voltage will return back to its nominal value.

Question 6

In a BJT what is the equation for Vce?

Answer 6

Vce = Vin - Vout

Question 7

What are Load dropout Regulators good for?

Answer 7

Providing a regulated output voltage when there is only a small difference between the input and output voltages.

Question 8

How is a Load Dropout Regulator different to a Series regulator?

Answer 8

It is the same except it uses a MOSFET instead of a BJT.

Question 9

How is a load connected to a Shunt Regulator?

Answer 9

The load operates in series with a voltage source and resistor and in parallel with the Shunt Regulator.

Question 10

How does a Shunt Regulator work?

Answer 10

Voltage across the load is kept constant by having the transistor conduct just enough current to the ground.

Question 11

Why are Shunt Regulators inefficient?

Answer 11

No matter what the load current is, the Shunt Regulator will always draw the max current.

Question 12

What are the advantages of Linear Regulators?

Answer 12

Simple and low cost.

No EMI interference.

Question 13

What are the limitations of Linear Regulators?

Answer 13

Low efficiency as power rating increases.
Transformer required for isolation.
Can’t have higher output voltages than input voltages.
Can’t have multiple outputs.
Heat Generation.

Question 14

How does a Switch mode Regulator work>?

Answer 14

It uses a switching element to convert the incoming voltage into a pulsed voltage. It will then use a Capacitor | Inductor circuit to smooth the pulsed signal.

Question 15

What can we use as a switch in a switch mode regulator?

Answer 15

MOSFET. Turn the mosfet on until the desired voltage is reached then turn it off. We can repeat this at high frequencies to achieve the desired voltage with less heating.

Question 16

What are the advantages of Switch Mode Regulators?

Answer 16

High efficiency compared to Linear Regulators.
Transformers for high frequency isolation are compact.
Can produce multiple outputs.

Question 17

What are the limitations of Switch Mode Regulators?

Answer 17

They have a complex design.
Increased noise due to the switching element.
produces EMI

Question 18

How does a Switch Mode Regulator have greater efficiency?

Answer 18

Because of the Mosfet, when this is on the voltage drop is at a minimum and when it is off no current will flow through it.

Question 19

In a switch mode regulator what is the equation for the average output voltage?

Answer 19

Vout = Ton / Ts * Vin.

Or Duty cycle * Vin.

Question 20

What is a buck converter?

Answer 20

A DC-DC converter that will step voltage down while stepping current up.

Question 21

Why is a Buck converter better for stepping down voltages than a voltage divider?

Answer 21

It doesn’t use resistors which lose power as heat.

Question 22

What is the working principle of a buck converter.

Answer 22

We use the MOSFET as a switch to continuously open and close the circuit. A capacitor is used to maintain the power to the load when the switch is open. When we close the switch, we try to change the voltage across the capacitor instantaneously which is impossible so we use an inductor to control this current. The diode gives the inductor current a path to flow when the switch is open as a force in the inductor will maintain the flow of current.

Question 23

What is the voltage across the diode when the switch is closed and open?

Answer 23

When the switch is closed the voltage across the diode is Vin. When it is open the value is 0.

Question 24

What is the value of the voltage across the inductor when the switch is open and closed?

Answer 24

When the switch is closed, the voltage is Vin - Vout. When the switch is open the voltage is -Vout.

Question 25

What is the value of the average inductor current?

Answer 25

The output current.

Question 26

When is the buck converter in continuous conduction mode?

Answer 26

When the buck converter has reached a steady state value.

Question 27

What happens in continuous conduction mode?

Answer 27

There will be continuous inductor current, meaning that it will never equal zero. The integral of Vl is zero over one period.

Question 28

What is the relationship between Vin, Vout and the Duty cycle in continuous conduction.

Answer 28

D = Vin / Vout

Question 29

What is the equation for inductor ripple current in continuous conduction?

Answer 29

Iripple = Vin - Vout / L * D * Ts OR Iripple = Vout / L * (1-D) * Ts

Question 30

How can we reduce the inductor ripple current in continuous conduction?

Answer 30

By increasing the switching frequency (since it is inversely proportional to Ts) or by increasing the inductance.

Question 31

What happens to the buck converter if we increase the load resistance in continuous conduction?

Answer 31

The output current will reduce so that the output voltage remains constant. Since the average inductor current = output current this will also decrease.

Question 32

What is the critical load in a buck converter?

Answer 32

It is the necessary resistance needed at the output to maintain continuous conduction.

Question 33

What happens to the inductor current at the critical resistance?

Answer 33

It reaches zero at the end of the switching period.

Question 34

What is the equation for the critical current in a Buck Converter?

Answer 34

deltaI / 2 OR Vout * (1-D) * Ts / 2L

Question 35

What is the equation for the critical resistance?

Answer 35

Vout / Iout(critical) OR 2L / (1-D) * Ts

Question 36

When will discontinuous conduction occur in a Buck Converter?

Answer 36

When the load resistance is greater than the critical load resistance.

Question 37

What happens to the Buck converter if it is in discontinuous conduction mode? (4)

Answer 37

The inductor current will reach zero before the end of the switching period. The diode will stop conducting. Inductor current will remain zero for the rest of the period. Inductor voltage will fall to zero through this time.

Question 38

What is a Boost converter?

Answer 38

It will generate an output voltage that is greater than the input voltage.

Question 39

What is the working principle of a Boost converter?

Answer 39

When the switch is closed energy will build up in the inductor in the form of a magnetic field. No current will flow to the output at this time. When we open the switch the magnetic field will collapse and induce a large voltage. A force will keep current flowing with this large voltage. This will charge up the capacitor. Every time we open the switch the large voltage spike will increase the voltage across the capacitor.

Question 40

What happens to the inductor current since the inductor voltage is much higher than the input voltage?

Answer 40

The inductor current will be much lower than the input current.

Question 41

What is the relationship between Vout, Vin and the duty cycle?

Answer 41

Vout / Vin = 1 / (1-D)