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MECHENG 370 > Power Electronics > Flashcards

Power Electronics Flashcards

1
Q

What are the different types of Linear Regulators?

A

Series, Low drop out and Shunt

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2
Q

How does a series type regulator work?

A

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.

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3
Q

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

A

Using a feedback system.

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4
Q

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

A

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.

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5
Q

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

A

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

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6
Q

In a BJT what is the equation for Vce?

A

Vce = Vin - Vout

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7
Q

What are Load dropout Regulators good for?

A

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

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8
Q

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

A

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

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9
Q

How is a load connected to a Shunt Regulator?

A

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

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10
Q

How does a Shunt Regulator work?

A

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

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11
Q

Why are Shunt Regulators inefficient?

A

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

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12
Q

What are the advantages of Linear Regulators?

A

Simple and low cost.

No EMI interference.

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13
Q

What are the limitations of Linear Regulators?

A

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.

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14
Q

How does a Switch mode Regulator work>?

A

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.

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15
Q

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

A

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.

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16
Q

What are the advantages of Switch Mode Regulators?

A

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

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17
Q

What are the limitations of Switch Mode Regulators?

A

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

18
Q

How does a Switch Mode Regulator have greater efficiency?

A

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.

19
Q

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

A

Vout = Ton / Ts * Vin.

Or Duty cycle * Vin.

20
Q

What is a buck converter?

A

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

21
Q

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

A

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

22
Q

What is the working principle of a buck converter.

A

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.

23
Q

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

A

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

24
Q

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

A

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

25
Q

What is the value of the average inductor current?

A

The output current.

26
Q

When is the buck converter in continuous conduction mode?

A

When the buck converter has reached a steady state value.

27
Q

What happens in continuous conduction mode?

A

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

28
Q

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

A

D = Vin / Vout

29
Q

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

A

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

30
Q

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

A

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

31
Q

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

A

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

32
Q

What is the critical load in a buck converter?

A

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

33
Q

What happens to the inductor current at the critical resistance?

A

It reaches zero at the end of the switching period.

34
Q

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

A

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

35
Q

What is the equation for the critical resistance?

A

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

36
Q

When will discontinuous conduction occur in a Buck Converter?

A

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

37
Q

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

A

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.

38
Q

What is a Boost converter?

A

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

39
Q

What is the working principle of a Boost converter?

A

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.

40
Q

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

A

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

41
Q

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

A

Vout / Vin = 1 / (1-D)

MECHENG 370 (8 decks)

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