EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Converters – general diagram
DC/DC converters are relatively new device due to the invention of power electronics.
What is the input to the converter?
What is the output to the converter?
How do we control the DC/DC converter?
1
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Converters – classifications
If we want to increase the voltage we use a,
If we want to decrease the voltage we use a,
If we want to be able to decrease or increase the voltage we use a,
Another specialized version of the above converter is a,
2
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Converters – switching mechanism
One of the easiest ways to change the voltage is to simply use a switch to,
We can quantify the time the switch is on vs off with a duty cycle, D
3
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter design
Another way to call a step-up converter is a __________________________.
What can we control in the boost converter to change the output voltage?
The characteristic of buck converters is,
•
The output voltage is _________________________ compared to the input voltage.
•
The output current is _________________________ compared to the input current.
•
The output power is _________________________ compared to the input power.
4
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter switch ON mode
Let us analyze what happens when the switch is ON.
When S = ON, what happens to,
•
Diode
•
Inductor
•
Capacitor
Draw the current loop(s).
5
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter switch OFF mode
Let us analyze what happens when the switch is OFF.
When S = OFF, what happens to,
•
Diode
•
Inductor
•
Capacitor
Draw the current loop(s).
6
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter inductor’s contribution
How does the inductor help the circuit?
When does the inductor charge?
When does the inductor discharge?
7
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter capacitor’s contribution
How does the capacitor help the circuit?
When does the capacitor charge?
When does the capacitor discharge?
8
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Converters – Step-down converter diode’s contribution
Why is it necessary to have the diode?
When does the diode conduct current?
When does the diode block current?
9
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter output voltage
We can control the output voltage and current by changing the duty cycle,
What is the range limit (min,max) of the output voltage?
With the power being calculated on both sides,
10
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter sizing components
First off, we choose the frequency of switching the MOSFET, f. Typical values would be,
For design purposes, we need to find out the size of the inductor, L
Next, we need to size the capacitor with,
This capacitance is a function of, the output voltage ripple, which is expressed as a percentage,
11
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter design
Another way to call a step-up converter is a __________________________.
What can we control in the boost converter to change the output voltage?
The characteristics of a boost converter is,
•
The output voltage is _________________________ compared to the input voltage.
•
The output current is _________________________ compared to the input current.
•
The output power is _________________________ compared to the input power.
12
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter switch OFF mode
Let us analyze what happens when the switch is OFF.
When S = OFF, what happens to,
•
Diode
•
Inductor
•
Capacitor
Draw the current loop(s).
13
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter switch ON mode
Let us analyze what happens when the switch is OFF.
When S = ON, what happens to,
•
Diode
•
Inductor
•
Capacitor
Draw the current loop(s).
14
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter inductor’s contribution
How does the inductor help the circuit?
When does the inductor charge?
When does the inductor discharge?
15
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter capacitor’s contribution
How does the capacitor help the circuit?
When does the capacitor charge?
When does the capacitor discharge?
16
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter diode’s contribution
Why is it necessary to have the diode?
When does the diode conduct current?
When does the diode block current?
17
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter output voltage
What is the duty cycle “D” of a switch?
We can control the output voltage and current by changing the duty cycle,
With the power being calculated on both sides,
18
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-up converter sizing parameters
First off, we choose the frequency of switching the MOSFET, f. Typical values would be,
For design purposes, we need to find out the size of the inductor, L
Next, we need to size the capacitor with,
This capacitance is a function of, the output voltage ripple, which is expressed as a percentage,
19
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC converter example A (part 1)
We want to build a DC-DC converter for the system above, with the PV module = 72VDC and
the DC bus = 48VDC. We can model the load on the DC bus as R = 10Ω.
1. What type of DC-DC converter should we use?
2. What would be the duty factor for the converter?
20
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC Step-down converter example A (part 2)
We want to build a DC-DC converter for the system above, with the PV module = 72VDC and
the DC bus = 48VDC. We can model the load on the DC bus as R = 10Ω. Additionally, we want
to have a ripple of less than 5%,
Calculate the required inductance. To be safe, let us oversize the minimum inductance by 25%.
Calculate the required capacitance. (no need to oversize this)
21
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC converter example B (part 1)
We want to build a DC-DC converter for the system above, with the PV module = 48VDC and
the DC bus = 125VDC. We can model the load on the DC bus as R = 25Ω.
1. What type of DC-DC converter should we use?
2. What would be the duty factor for the converter?
22
EEE146 Chapter 5 – DC/DC Converters (part 1)
DC/DC converter example B (part 2)
We want to build a DC-DC converter for the system above, with the PV module = 48VDC and
the DC bus = 125VDC. We can model the load on the DC bus as R = 25Ω. Additionally, we
want to have a ripple of less than 8%,
Calculate the required inductance. To be safe, let us oversize the minimum inductance by 25%.
Calculate the required capacitance. (no need to oversize this)
23