switching power supply regulators - university ofcallen58/501/switchingpsnotes.pdf · switching...

1Material originally prepared by Prof. Prescott

13 Sept 2010

Switching Power Supply Regulators

Recall the linear power supply concept:

A linear control element is series with the unregulated DC is used, with feedback, to maintain constant output voltage.

For the linear power supply regulator:

• The output voltage is always lower than the input voltage

• The control element acts as a variable resistance to keep the output voltage constant under changing load.

• The control element is always dissipating power, so this causes loss in efficiency – a characteristic of this kind of regulator.

Here is an alternative approach to the regulator problem – a Switching Regulator:


13 Sept 2010

Switching regulators operate by rapidly switching the pass transistor between two efficient operating states:

1. Cutoff – where there is a high voltage across the pass transistor, but no current flow.

2. Saturation – where there is a high current through the pass transistor, but a very small voltage drop.

Characteristics of Switching Power Supplies:

• Since the control element is either on or off, there is little power dissipation.

• Can generate output voltages higher than the unregulated input.

• Can generate output voltages of opposite in polarity to the input.

• Can be smaller and lighter weight than a linear PS regulator.

• Problem: NOISY and Complex to build

Many applications require both types to be used. For example, aswitching regulator may provide the initial regulation, then a linear regulator may provide post-regulation for a noise sensitive part of the circuit, such as a sensor interface or A/D converter.

Here is a comparison of linear versus switching power supplies


13 Sept 2010

The switching power supply is an improvement (in many ways) overthe linear power supply. Here is the concept:

Here are the essential waveforms that describe the operation of this system.

loadcurrent

Switching Power Supply Block Diagram

Switching and Control Waveforms


13 Sept 2010

Before going further, let’s review how inductors charge and discharge.

There are three types of switching power supplies:

1. Step down regulator – sometimes called a “buck” regulator.

2. Step up regulator – sometimes called a “boost” regulator.

3. Inverting regulator – sometimes called a “flyback” regulator.

In Project #2 we will focus on buck and boost regulator design.

L

di t IV L L

dt T

recall:

Switched Inductor Action


13 Sept 2010

Here’s a closer look at the control element, plus capacitor and inductor, showing how these are configured for the three types of regulators:

Make sure you understand the principles described by these diagrams

Diode D1 is often called the “catch”diode. It is usually a Schottky-barrier diode, having low forward voltage drop and fast switching speed.


13 Sept 2010

Let’s analyze this circuit and learn how it works:1. The filter averages the voltage square waves which are created by switching

Vin on and off. The filter averages these waveforms to produce a dc voltage.

on ono in in

on off

t tV V V

T t t

duty cycle

2. When Q1 switches ON, the inductor charging current is:

in sat oLL

V V VVI t t

L L

Vsat is the voltage dropped across Q1 when it is saturated.

3. When Q1 switches OFF, the inductor current is:

1( )

o DL L peak

V VI I t

L

See and on the next page

Step-Down Switching Regulator

toff


13 Sept 2010

Voltage across switch Q1

1in DV V

inV

inV

ontofft

T

satV

in satV V

1DV

pkI

pkI

pkI

( )in C AvgI I

1( )D AvgI

( ) 1( )2pk

o C Avg D Avg

II I I

Voltage across Diode D1

Switch Current Q1

Diode D1Current

Capacitor Current

Capacitor Ripple Voltage

2pkI

2pkI

2ont 2offt

12 2pkI

( )ripple p pV

o pkV V

o pkV VoV

Inductor Current

A closer look at the waveforms in this system:


13 Sept 2010

Here’s how to configure the switching regulator for step-up and inverting operation.


13 Sept 2010

Now look at the components of the control circuitry


13 Sept 2010

Tying it all together, we have the following step-down switcher

The control circuitry can be implemented using individual devices or you can make use of an integrate controller such as the 74S40, as shown below.

However, we will not use a device such as this. Instead we willimplement the functions using a 723 and a dual 555 timer.


13 Sept 2010

Here is a schematic of the way we will implement a switching power supply for Project #2.


13 Sept 2010

It is critically important that you have a complete understanding of how this circuit operates before you try to build it. You must also understand how the 723 and 555/556 devices function.

Now let’s look at some design considerations. There are three primary subcomponents to this project. They are:

1. The control element and charging circuitry

This schematic shows the TIP-42 used as the control element. You are not constrained to using a PNP device here. You can also investigate the use of the TIP-41, which is an NPN transistor. You also might want to try using a power field-effect device. Try the IRF511.

Can the 556 provide sufficient drive current to use this kind of implementation?

TIP-41What is necessary for the 556 to drive this device? Is it much easier to use than a BJT?

IRF-511


13 Sept 2010

2. The oscillator and pulse width modulator

Oscillator (astable multivibrator)

(20 kHz – 100 kHz)

C

B

A

Pulse-Width Modulator

(bi-stable mutivibrator)


13 Sept 2010

3. The voltage sensor and error amplifier

B

Here we have the familiar 723 – you should understand all about this device at this point.

Output voltage is sensed here

In this application, the 723 is operating in a circuit that has an output voltage that is lower than the internal reference voltage. Recall this diagram from Project #1:

Error voltage output to the PWM

maybe you should use one of these?


13 Sept 2010

Design Steps for the Step Down Switching Regulator

Here are some suggested steps to designing the output circuitry for the switching regulator. These are to be used as guidelines to get you in the ballpark. Use these in conjunction with the document entitled “Step Down Switching Regulator Operation & Design.” Don’t hesitate to experiment –there is plenty of latitude in the design.

Step #1 – State your “givens”

OUT

max

D1

ripple(p-p)

DC input voltage

V DC output voltage

Maximum load current

V Forward voltage of catch diode

V Saturation voltage of switching transistor

V Peak-to-peak ripple (~0.5%)

IN

OUT

sat

V

I

Step #3 – Determine the ratio of on/off time for the switching transistor

Step #2 – Determine the peak output current

( ) 2L pk OUTI I

1on OUT D

off IN sat OUT

t V V

t V V V

Step #4 – Select a switching frequency

Select a frequency within the following range:

30,000 Hz f 60,000 Hz

1and note that:

on off

ft t


13 Sept 2010

Step #5 – Calculate the value for the switching inductor

min( )

IN sat OUTon

pk switch

V V VL t

I

Step #6 – Calculate the value for the switching capacitor

( )

( )8pk switch on off

Oripple p p

I t tC

V

1

1

OUT IN sat D

OUT D IN

V V V V

V V V

Step #7 – Determine the regulator efficiency


13 Sept 2010

The 555 Integrated Circuit Timer

• Operates from a wide range of power supplies (see H & H p. 289)

• Timing intervals of several minutes. Frequencies as high as a few MHz

Remember how the comparator functions:

+-

+-

++--

+ -


13 Sept 2010

Summary of Operation:

Three 5 k resistors are configured in series to divide Vcc into thirds. The junctions of these resistors are tied to comparators. This serves as a constant reference voltage that is only dependent upon Vcc.

To force the output of the 555 low, the voltage on the “Threshold”input must exceed 2/3 Vcc. This also turns the discharge transistor “on”. To force the output of the timer high, the voltage on the “Trigger” input must fall below 1/3 Vcc. This also turns the discharge transistor “off”.

The diagram on the previous page shows the 555 connected in free-running (astable) mode.

We can design the 555 to operate at a specific free-running rate by choosing Ra, Rb and C. Let’s examine how this is accomplished.

You might recall that:

defines the charging of an general series RC circuit.

1t

RCV t A e

1. Time to charge 2

03 CCV

2

3 CC

CC

A B

V t V

A V

R R R

21

3

t

RCCC CCV V e

1

3

t

RCe

so 1.09t RC


13 Sept 2010

2. Time to charge 10

3 CCV

11

3

t

RCCC CCV V e

0.405t RC

3. Time to charge 1 2

3 3CC CCV V

1.09 0.405 0.69hight RC RC RC

4. Time to discharge

0.69high A Bt R R C

The next thing we need to do is look at the discharge behavior of the RC circuit. Note that the discharge of an RC circuit is defined as:

t

RCV t Ae

2 1

3 3CC CCV V

1 2

3 3

t

RCCC CCV V e

But here, because of the discharge path.

BR R

0.69low Bt R C


13 Sept 2010

5. Finally, the period of the output signal is:

0.69 0.69

0.69 2

high low

A B B

A B

T t t

R R C R C

R R C

1 1.45

2A B

fT R R C

also


13 Sept 2010


13 Sept 2010

The inductor will be hand-wound using the Ferroxcube 2213PA500-3C8. These are ferrite inductor cores that are snapped around aplastic bobbin on which is wound the coil. Since the wire is wound on a bobbin rather than around the ferrite material itself, it is easy to change bobbins and experiment with the various inductances produced by different numbers of turns and kinds of wire.

Two features of this core:

• This core is a manganese-zinc ferrite substance with medium permeability and low losses.

• It is designed specifically for high flux-density applications such as power supplies.

Winding the Inductor


13 Sept 2010

The inductance of a coil can be determined from the following equation:

where: L = Inductance in HenriesN = number of turnsA = Cross sectional area of coil (m2)r = relative permeability of core material = permeability of air (1.26 x 10-8) H/mℓ = avg. coil length (m)

You can use this information and the data in the Ferroxcube data sheet to determine the number of turns needs for a given inductance. A shortcut is to use the following formula:

910

L

LN

A

where: N = number of turnsL = desired inductance in henriesAL = millihenries/1000 turns (~ 500 for the

3C8 core)

To produce a 275 microhenry inductor, approximately 23 turns are required. The result is not exact, so you will have to experiment with it.

or

2 ANL


13 Sept 2010

Tips for success:

1. Understand how step-down switching regulators work. Refer to H&H (Sect. 6.19) and the reference material posted on the JEDL web site.

2. Understand how the 555 functions and how to select components to make it function predictably as an astable and monostable circuit. The 556 is just two 555’s in one package. There is reference information on using the 555 posted on the web site.

3. Understand how the 723 operates in the application. It is used differently than in the previous project.

4. Read the note entitled “Step-Down Switching Regulator Operation and Design.” This is your primary source of information for this project.

5. Break the circuit into three parts: Error amplifier (723), oscillator and PWM (556), and the output circuitry. Allow one person to focus on one of these so that there is sufficient effort from all team members.

6. Breadboard parts of the circuit just to get familiar with it. In the process you should learn to wind and test the inductor. Play around with it and understand what happens when you change the peripheral components.

7. Implement a Spice simulation of the pass transistor, diode, inductor and output capacitor. The purpose of this is to see how to bias the transistor and to make sure it switches properly. Also this is a check on the inductor

8. Construct the completed circuit and test it. When testing the circuit, always keep a load on the circuit – it doesn’t function well without a load. Try to keep a minimum load of about 200 mA on the circuit when testing.

switching power supply regulators - university ofcallen58/501/switchingpsnotes.pdf · switching...

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