Introduction to DC-DC Conversion – Cont.

EE174 – SJSUTan Nguyen

SWITCHING MODE POWER SUPPLY (SMPS)

• The switching-mode power supply is a power supply that provides the power supply function through low loss components such as capacitors, inductors, and transformers -- and the use of switches that are in one of two states, on or off.

• It offers high power conversion efficiency and design flexibility.

• It can step down or step up output voltage.• The term switchmode was widely used for this type of

power supply until Motorola, Inc., who used the trademark SWITCHMODE TM for products aimed at the switching-mode power supply market, started to enforce their trademark. Switching-mode power supply or switching power supply are used to avoid infringing on the trademark.

• Typical switching frequencies lie in the range 1 kHz to 1 MHz, depending on the speed of the semiconductor devices.

SWITCHING MODE POWER SUPPLY (SMPS)

• Buck converter: Voltage to voltage converter, step down.

• Boost Converter: Voltage to voltage converter, step up.

• Buck-Boost or FlyBack Converter: Voltage-Voltage, step up and down (negative voltages)

• Cuk Converter: Current-Current converter, step up and down

These converters typically have a full wave rectifier front-end to produce a high DC voltages

Heater: The heater turns on and off every several minutes to keep the room temperature constant. Examples:Vin = 12 Vdc and the load resistor R2 = 0.25 ohms. The objective is to open and close the switch so that the average voltage across R2 is 5 Vdc. The waveform of the voltage across R2 shown below. Vo = Vin x D where D = Ton / (Ton + Toff) : Duty cycle

Simple switching-mode power supply

The switch control signal, which controls the on and off states of the switch, is generated by comparing a signal level control voltage vcontrol with a repetitive waveform.

The switching frequency is the frequency of the sawtooth waveform with a constant peak.

The duty ratio D can be expressed as

st

control

s

on

V

vTt

D ^

Pulse-width Modulator (PWM)

• The buck converter is known as voltage step-down converter, current step-up converter, chopper, direct converter.

• The buck converter simplest and most popular switching regulator.

The Buck Converter

Size:30mm(L)*18mm(W)*14(H) mm

DC-DC Buck Converter Module 4.5-14V to 0.8-9.5V 6A Adjustable Set-Down Regulator

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• converts dc from one level to another

• the average output voltage is controlled by the ON-OFF switch

• PWM switching is employed• lower average output voltage than

the dc input voltage Vd depending on the duty ratio, D

• D=ton/Ts

• Average output:

Applications:• regulated switch mode dc power

supplies• dc motor drives

• LC low-pass filter: to pass the DC component while attenuating the switching components.

• diode is reversed biased during ON period, input provides energy to the load and to the inductor

• energy is transferred to the load from the inductor during switch OFF period

• in the steady-state, average inductor voltage is zero

• in the steady-state, average capacitor current is zero

dds

on

sT

ont

ont

ds

sT

s

DVVTt

dtdtVT

dttvT

V

011

0000

The Buck Converter

• Inductor current iL flows continuously

• Average inductor voltage over a time period must be zero

Assuming a lossless circuit

ratiodutyDTt

VVor

tTVtVVthereforeequalbemustBandAArea

dtvdtvdtv

s

on

d

onsond

T

tL

t

L

T

L

s

on

ons

0

00

00

,,

0

DVV

IIand

IVIV

d

d

dd

1

0

0

00

Buck Converter

Buck Converter

• Assume large C so that Vout has very low ripple

• Since Vout has very low ripple, then assume Iout has very low ripple

• Interchange of energy between inductor and capacitor is referred as flywheel effect.

Flywheel circuit

• the average current through a capacitor operating in periodic steady state is zero

• the average voltage across an inductor operating in periodic steady state is zero

What do we learn from inductor voltage and capacitor current in the average sense?

Buck Converter in Continuous Conduction

Switch closed for DT seconds

Where D = Duty CycleT = Switching period

+ (Vin – Vout) –

Buck Converter in Continuous Conduction

– Vout +

When switch open VL = - Vout, diode is closed (forward biased) so iL continues to flow. This is the assumption of “continuous conduction” in the inductor which is the normal operating condition.

Switch open for (1 − D)T seconds

Buck Converter in Continuous Conduction

Since the average voltage across L is zero

Buck Converter

01 outoutinLavg VDVVDV

outoutoutin VDVVDDV

The input/output again becomes inout DVV

From power balance,

outoutinin IVIV DII in

out

If D is duty cycle average output voltage is

inout DVV

Power Losses in a Buck Converter

There are two types of losses in an SMPS:

• DC conduction losses.• AC switching losses.

Buck Converter Design ExampleFor a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio and “on” time of the switch.

D = V0 /Vd = 5/40 = 0.125 = 12.5%

Ts = 1/fs = 1/4000 = 0.25 ms = 250 μs

Ton = DTs = 31.25 μs

Toff = Ts – ton = 218.75 μs

When the switch is “on”: VL = Vd - V0 = 35 V

When the switch is “off”: VL = -V0 = - 5 V

I0 = IL = V0 / R = 5 A

Id = D I0 = 0.625 A

• The conduction losses of a buck converter primarily result from voltage drops across transistor Q1, diode D1 and inductor L when they conduct current.• A MOSFET is used as the power transistor.

The conduction loss of the MOSFET = IO2

x RDS(ON) x D, where RDS(ON) is the on-resistance of MOSFET Q1. • The conduction power loss of the diode = IO • VD •

(1 – D), where VD is the forward voltage drop of the diode D1.

• The conduction loss of the inductor = IO2 x RDCR, where RDCR is the copper resistance of the inductor winding.

DC conduction losses in Buck converter

Therefore, the conduction loss of the buck converter is approximately: PCON_LOSS = (IO

2 x RDS(ON) x D) + (IO • VD • [1 – D]) + (IO2 x

RDCR)

Power Losses in a Buck Converter

Considering only conduction loss, the converter efficiency is:

Example:For 12V input buck supply 3.3V/10AMAX output buck supply.• Use 27.5% duty cycle provides a 3.3V output voltage.

Vout = Vin x D = 12 x 0.275 = 3.3 V• MOSFET RDS(ON) = 10 mΩ • Diode forward voltage VD = 0.5V (freewheeling diode)• Inductor RDCR = 2 mΩ

Conduction loss at full load:PCON_LOSS = (IO2 x RDS(ON) x D) + (IO x VD x [1 – D]) + (IO2 x RDCR) = (102 x 0.01 x 0.275) + (10 x 0.5 x [1 – 0.275]) + (102 x 0.002) = 0.275W + 3.62W + 0.2W = 4.095WBuck converter efficiency:

Example:For 12V input buck supply 3.3V/10AMAX output buck supply.• Use 27.5% duty cycle provides a 3.3V output voltage.

Vout = Vin x D = 12 x 0.275 = 3.3 V• MOSFET RDS(ON) = 10 mΩ • Diode forward voltage VD = 0.5V (freewheeling diode)• Inductor RDCR = 2 mΩ

Conduction loss at full load:PCON_LOSS = (IO2 x RDS(ON) x D) + (IO x VD x [1 – D]) + (IO2 x RDCR) = (102 x 0.01 x 0.275) + (10 x 0.5 x [1 – 0.275]) + (102 x 0.002) = 0.275W + 3.62W + 0.2W = 4.095WConverter efficiency:

Power Losses in a Buck Converter

AC Switching Losses in Buck Converter1.MOSFET switching losses. A real transistor requires time to be turned on or off. So there are voltage and current overlaps during the turn-on and turn-off transients, which generate AC switching losses.

2.Inductor core loss. A real inductor also has AC loss that is a function of switching frequency. Inductor AC loss is primarily from the magnetic core loss.

3.Other AC related losses. Other AC related losses include the gate driver loss and the dead time (when both top FET Q1 and bottom FET Q2 are off) body diode conduction loss.

Basic Nonisolated DC/DC SMPS TopologiesBUCK COVERTER

Basic dc-dc converters and their dc conversion ratios M(D) = V/Vg.

Linear Regulator:VBAT = 3.7 V nom, BIN_BB = 1.2 VLoad Current = 600 mAPower delivered to load = 600 mA * 1.2 V = 720 mWPower converted to heat = 20 mW * ((3.7/1.2) 1) = 1,500 mWTotal power consumed = 720 mW + 1,500 mW = 2,200 mW

32% goes to work, 68% goes to heating user hand and ear.

Switch-mode regulator: VBAT = 3.7 V nom; BIN_BB = 1.2 VLoad Current = 600 mAConverter efficiency = 90%Power delivered to load = 600 mA * 1.2 V = 720 mWPower converted to heat =720 mW * ((1/0.9) 1)=80 mWTotal power consumed = 720 mW + 80 mW = 800 mW

90% goes to work, 10% goes to heating user hand and ear.

An example of Linear Regulator versus Switch-Mode Regulator

Sample of Linear and Switch-Mode Regulator Output

References:http://en.wikipedia.org/wiki/DC-to-DC_converter

https://www.jaycar.com/images_uploaded/dcdcconv.pdf

Linear Technology - Application Note 140

http://www.smpstech.com/tutorial/t03top.htm#SWITCHINGMODE

Notes from Fang Z. Peng Dept. of Electrical and Computer Engineering MSUhttps://www.google.com/webhp?sourceid=chrome-instant&rlz=1C1OPRB_enUS587US587&ion=1&espv=2&ie=UTF-8#q=picture+of+noise+on+buck+output

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCQQFjABahUKEwj329-J4YvIAhVLy4AKHZiyADY&url=http%3A%2F%2Fusers.ece.utexas.edu%2F~kwasinski%2F_6_EE462L_DC_DC_Buck_PPT.ppt&usg=AFQjCNH1PIzP73b3t11mgGhnUBBg-sVNXg&cad=rja

http://ecee.colorado.edu/ecen4517/materials/Encyc.pdf

https://www.valuetronics.com/Manuals/Lambda_%20linear_versus_switching.pdf