ELG4139 Switching Mode Power Supply

(Project Theme)

The term switch mode power supply is generally used to indicate an item that can be connected to the mains, or other external supply and used to generate the source power. In other words it is a complete power supply.

Unregulated Power Supply

The advantages of the unregulated supply are low cost and simplicity. There are many applications with do not

require precise output voltage.

Linear Power Supply

Linear power supplies are typically only used in specific applications requiring extremely low noise, or in very low power applications where a simple transformer

rectifier solution is adequate and provides the lowest cost. Examples are audio applications (low noise) and low power consumer applications such as alarm panels.

The 50/60Hz mains transformer reduces the voltage to a usable low level, the secondary AC voltage is peak-rectified and a Series Pass Element (SPE) is employed to provide the necessary regulation. The benefits of this solution are low noise, reliability

and low cost. On the downside, these units are large, heavy and inefficient with a limited input voltage range. In order to significantly reduce the size and increase

efficiency, most applications utilize a Switch Mode Power Supply (SMPS). (low cost).

Why Switching in Power Supply?

• The desired output voltage is greater than the input voltage

– Linear regulators cannot provide an output voltage greater than the input voltage

The desired output voltage is opposite polarity than the input voltage

– Linear regulators cannot invert an input voltage

1.5 V Battery Power Supply

3 V Required

Power Supply 12 V Battery

-10 V Required

Switches: Diode

• Conducts in forward direction only.

• Modern power devices can conduct in ~ 1 ms.

• Has voltage drop of (< 1 V) when conducting.

• Dissipates power whilst conducting.

• Ratings up to many 100s A (average), kVs peak reverse

volts.

Switch: Transistor

From data sheet find VCE sat

This is the saturation point

were Vce is at a minimum

and the transistor dissipates

the least power.

Icsat = VCC –VCEsat / Rrelay

Icsat = 500mA B = 100

Find min Value of IB

IBsat = Icsat / B

IB = 500mA/100 > 5mA

Switches: Thyristor

•

Withstands forward and reverse volts until

‘gate’ receives a pulse of current;

then conducts in the forward direction;

• Conducts until current drops to zero and reverses (for

short time to ‘clear’ carriers).

• After ‘recovery time’, again withstands forward voltage.

• Switches on in ~ 5 ms (depends on size) – as forward

volts drop, dissipates power as current rises.

• Therefore dI/dt limited during early conduction.

• Available with many 100s A average, kVs forward and

reverse volts.

Switches: IGBTs

The Insulated Gate Bi-polar Transistor (IGBT):

• Gate controls conduction; switching the

device on and off.

• Far faster than thyrisitor and can operate

at 10s kHz.

• Is a transistor, so will not take reverse voltage (usually a

built-in reverse diode.

• Dissipates significant power during switching.

• Is available at up to 1 kV forward, 100s A average.

Pulse Width Modulation

By controlling mark to space ratio

you can control the power going to

the device.

Minimise transistor losses by having

transistor either on or off.

Switch Mode Power Supplies

The use of switch mode topologies has reduced the size and improved the efficiency of power supplies by increasing the

frequency of operation, reducing the physical size of transformers, inductors and capacitors, and utilizing an ‘on or

off’ switching element to increase efficiency. The compromises in adopting this technique are increased ripple and noise on the

output DC supply and the generation of both conducted and radiated EMI which have to be managed.

The introduction of low voltage semiconductors and the consequent high output current demands have driven the

development of synchronous output rectifier schemes, where the output diodes are replaced by power MOSFETs to reduce

power dissipation in the secondary and achieve high efficiency solutions for these applications.

Switched-Mode DC Power Supplies

• Five configurations:

– Flyback

– Forward

– Push-pull

– Half Bridge

– Full-Bridge

• Operate at high frequencies:

– Can filter out harmonics!

Basic Circuit Configuration

VOUT

VIN

VM

VGATE

L

C

ISW

IL VOUT

VIN

VM

C VGATE

L IL

ISW

VOUT

VIN

VM C

VGATE

L IL

ISW

Buck-Boost VIN < -VOUT < VIN

Boost VIN < VOUT

Buck VIN > VOUT

All topologies consists of the same basic components but are arranged differently

Inductor Technology

• There are a number of inductor technologies to choose from:

• Drum core

• Flat coil

• Toroid

• Bead

• Wire-wound

• Planar

• In addition to inductance and saturation current, the inductor technology will also affect:

• Inductor resistance and impedance

• Size (length, width, height)

• Cost

Output Capacitor Technology

• There are dozens of capacitor technologies to choose from.

• Typically, output capacitors are tantalum or ceramic

• In addition to capacitance and maximum current, the capacitor technology will also affect:

Equivalent series resistance (ESR) and inductance (ESL)

Size (length, width, height)

Cost

Changes in performance versus temperature.

Optimization of Switching Voltage Regulators Step Down Regulator Inductor Selection

ΔIVf

VVV

INswitching

OUTOUTIN

L

• The inductance affects the ripple current of the regulator • Minimizing the ripple current will require a larger (more expensive)

inductor • The ripple current affects the over all ripple voltage

VOUT

VIN

VM

VGATE

L

COUT RL

IIN IL

Optimization of Switching Voltage Regulators Output Capacitor Selection

OUTswitching

RIPPLEC8f

1ESRΔIV

• The selection of the output capacitor affects the voltage ripple

• The impedance of the output capacitor’s equivalent series resistance (ESR) and capacitance both impact the output voltage ripple.

• Low ESR capacitors (ceramic and tantalum) are recommended to minimize the output voltage ripple.

VOUT

VIN

VM

VGATE

L

COUT RL

IIN IL

Optimization of Switching Voltage Regulators Output Resonant Frequency

LC2π

1f resonant

• The inductance and output capacitance determine a L-C resonant frequency

• The acceptable range of L-C values are included in the datasheet to avoid instabilities in the regulation loop.

• For Example:

Parameter Minimum Maximum

Buck Inductance 10µH 100µH

Output Capacitance 10µF ---

VOUT

VIN

VM

VGATE

L

COUT RL

IIN IL

Optimization of Switching Voltage Regulators Recirculation/Catch Diode

• The recirculation diode must have a stand-off voltage higher than the maximum positive voltage seen at VM

• Schottky Diode Minimizes Power Loss • Forward voltage of Schottky diode is less

• A Schottky diode’s reverse recovery time (switching from a forward bias state to a reverse bias state) is very fast so it minimizes the power loss in the freewheeling path.

VOUT

VIN

VM

VGATE

L

COUT RL

IIN IL

- VF

+

Optimization of Switching Voltage Regulators Input Capacitance

2

OUTRMS LOAD

IN LOAD

V 1 ΔII = I 1+

V 3 2I

• To minimize ripple voltage on the battery line, an input capacitor with a low ESR should be used

• During high load currents, the current flows through the inductor continuously

• The input capacitor is exposed to a square wave current with a duty cycle of VOUT/VIN

• The maximum RMS current the input capacitor must withstand:

Summary

• Switching voltage regulators convert a non-useful input voltage to a useful output voltage.

• When in regulation, the power transistor is either completely on or completely off.

• The duty cycle of the power transistor is varied to regulated the output voltage.

• Step down switching voltage regulators are used for high efficiency conversion.

• Step up switching voltage regulators are used because linear regulators cannot perform a step up conversion.

Basic Switch Mode Power Supply An Example Only BUT Not Necessarily to be Implemented!