1

WEEK 7

DC SWITCHING POWER SUPPLIES, PART II

2

EXPECTATIONS• Describe the supply isolation characteristics

afforded by transformers.• Draw basic forward, flyback and isolated

Cuk topology schematics.• Determine minimum inductances required

in isolated switching supplies.• Explain the practical tradeoffs between

types of isolated converters.• Describe the operating characteristics of

resonant switching converters.• Draw the sine wave characteristics of a

resonant switch arrangement.• List the three specific modes of load-

resonant converters.• Compare tradeoffs in switching topologies

by considering loading conditions.

Ferromagnetism

• Iron, nickel, cobalt and some of the rare earths (gadolinium, dysprosium) exhibit a unique magnetic behavior which is called ferromagnetism because iron (ferrum in Latin) is the most common and most dramatic example.

• Samarium and neodymium in alloys with cobalt have been used to fabricate very strong rare-earth magnets.

Ferromagnetism • There are many applications of

ferromagnetic materials, such as the electromagnet.

• Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field.

• This tendency to "remember their magnetic history" is called hysteresis.

• The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets.

FerromagnetismHysteresis

Hysteresis

Ferromagnetic Materials

Material TreatmentInitial

RelativePermeability

Maximum Relative

Permeability

Iron, 99.8% pure Annealed 150 5000

Iron, 99.95% pure Annealed in hydrogen 10,000 200,000

78 Permalloy Annealed, quenched 8,000 100,000

Superpermalloy

Annealed in hydrogen, controlled

cooling

100,000 1,000,000

Cobalt, 99% pure Annealed 70 250

Nickel, 99% pure Annealed 110 600

Steel, 0.9% C Quenched 50 100

Steel, 30% Co Quenched ... ...

Alnico 5 Cooled in magnetic field 4 ...

Silmanal Baked ... ...

Iron, fine powder Pressed ... ...

7

Equivalent circuits of a transformer:

(a) ideal transformer(b) transformer with the

magnetizing inductance included

N 1

i

v1

v2

2

N 2

N 1

v1

v2

N 2

i 2 i 1

i 1 i'2

im

L m

(a)

(b)

8

Coupled Inductor

In SPICE, you define a transformer as a coupled inductor, with the following

ratio:

There's also a coupling factor K that comes into play.The magnetizing inductance of a transformer is the inductance you measure across the primary winding with the secondaries open-circuit (floating). It's a function of the core material and geometry, air gap and number of turns.

9

Coupled Inductor

A coupling factor, K, can be used to take care of the leakage inductance

K=1 is often good enough for a simulation; "good" real transformers often have a K very close to 1 (e.g. K = 0.995).

For power converters (e.g. flyback), however, it's good to use leakage inductances.

N1/N2 = K * sqrt(L1/L2)

10

Forward converter

L

i i

iV

D 3 D 1

3

1N

N

2N

D 2

io

voC

11

Flyback Converter

2N

N 1 iV

i i

L m

D 1

vo

io

C

12

Isolated uk Converter

iV

i i

L 1C

2N

N 1

C

vo

io

C 2

11 12

L 2

13

Midpoint Rectifier

v1

v i

vo

14

Push-pull Converter

i

N 1 N 2

D1

D3

D4

D2

L

C

Vi

i i

io

vo

S1

S2

15

Half-bridge Converter

io

L

i i

iV

2N

1N

C 2

C 1 D1

D3 D4

D2

S1

S2

C vo

16

Full-bridge Converter

io

L

2N

1N

C vo

i i

iV

D3

D4

S3

S4

D5 D6

S2

S1 D1

D2

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Voltage-mode resonant switches:(a) L-type half-wave(b) M-type half-wave(c) L-type full-wave(d) M-type full-wave

(a)

(c)

(b)

(d)

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Current-mode resonant switches:(a) L-type half-wave(b) M-type half-wave(c) L-type full-wave(d) M-type full-wave

(a)

(c )

(b )

(d )

19

Waveforms of the inductor current and capacitor voltage in an undamped resonant circuit

iL )t0 (

t0

t0

vC )(

iL

vC

t 0

20

Series-loaded resonant converter

i i

iV vo

io

r r

i r

v inv recv

1

2

C

C

L C

S1

S2

D1

D2

D3

D4

D5

D6

C

21

AC equivalent circuit of the series-loaded resonant

converter

vrecvinv

L r rC ri

R eq

22

Control characteristic of the series-loaded resonant

converter

23

Parallel-loaded resonant converter

i i

iV

r

vinv

1

2

C

C

L

S1

S2

D1

D2

D3

D4

D5

D6

v recrC

i r

vo

io

C

L

24

AC equivalent circuit of the parallel-loaded resonant

converter

vrecvinv

L r

R eq

i

C

rec

r

25

Control characteristic of the parallel-loaded resonant

converter

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Series-parallel resonant converter

i i

iV

r

1

2

C

C

L

S1

S2

D1

D2

vo

io

D3

D4

D5

D6

C

C r2

C r1

ir

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Control characteristic of the series-parallel resonant

converter

Chapter 8 28

Soft-Switching DC-DC Converters

• Is to shape the voltage or the current waveform by creating a resonant condition to:

• Force the voltage across the switching device to drop to zero before turning it ON Zero-Voltage Switching (ZVS)

• Force the current through the switching device to drop to zero before turning it OFF Zero-Current Switching (ZCS)

Hard-Switching and Soft-Switching

Hard-Switching

Zero-Current Switching

Zero-Voltage Switching

Why Soft-Switching?

• Reduce switching losses especially at high switching frequencies

• Increase the power density, since the size and weight of the magnetic components is decreased by increasing the operating frequency

• Reduce the Electromagnetic Interference (EMI)

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ZVS Converter

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Quasi-resonant ZVS buck converter with L-type half-

wave switch

vo

io

iV

i i

L

C

L r

C r

Zero Voltage Switching (ZVS)

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ZVS

ADVANTAGES:

The ZVS enables high frequency operation with high efficiency. • Zero power “Lossless” switching

transitions• Reduced EMI / RFI at transitions• No power loss due to discharging

Goss• No higher peak currents, (ie. ZCS)

same as square wave systems• High efficiency with high voltage

inputs at any frequency• Can incorporate parasitic circuit and

component L & C

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ZCS

Eliminates the voltage and current overlap by forcing the switch current to zero before the switch voltage rises.For high efficiency power conversion, the ZCS topologies are most frequently adopted.

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Quasi-resonant ZCS buck converter with an L-type full-

wave switch

iV

i i

L

C

io

vo

L r

C r

Zero Current Switching (ZCS)

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Quasi-resonant ZCS boost converter with M-type full-

wave switch

L

L r

C r C

i i

iV vo

io

Zero Current Switching (ZCS)

The full-wave ZCS quasi-resonant switch cell

= 0t

i1(t)

I2

v2(t)

0Ts

Vc1

Subinterval: 1 2 3 4

Conductingdevices:

Q1

D2

Q1 D1 D2X

= 0t

i1(t)

I2

v2(t)

0Ts

Vc1

Subinterval: 1 2 3 4

Conductingdevices:

Q1

D2

D1

Q1

D1

D2X

+

v2(t)

–

i1(t) i2(t)

+

v1(t)

–

Lr

Cr

Half-wave ZCS quasi-resonant switch cell

Switch network

+

v1r(t)

–

i2r(t)D1

D2

Q1

+

v2(t)

–

i1(t) i2(t)

+

v1(t)

–

Lr

Cr

Full-wave ZCS quasi-resonant switch cell

Switch network

+

v1r(t)

–

i2r(t)

D1

D2

Q1

Half wave

Full wave

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TABLE 8.1 Comparison of the ZVS and ZCS Resonant-Switch Converters

______________________________________________________________________________

Property ZCS Converters ZVS Converters

____________________________________________________________________________________________________________________

Voltage gain

Buck converter kf 1 - kf

Boost converter 1/(1 - kf) 1/kf

Buck-boost converter kf /(1 - kf) 1/kf - 1

Control constant tON constant tOFF

variable tOFF variable tON

Waveform of voltage across the switch square sinusoidal

Waveform of current through the switch sinusoidal square

Load range 1 < r < ∞ 0 < r < 1

______________________________________________________________________________

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ZCS

ADVANTAGES:

ZCS technology for use in the charging test of a lead-acid battery, to demonstrate the effectiveness of the developed methodology.

These techniques lead to either zero voltage or zero current during switching transition, significantly decreasing the switching losses. This increases the reliability for the battery chargers high quality, small size, light.

The circuit structure is simpler and much cheaper.

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Next WeekUnit 8

Chapter 5AC – AC CONVERTERS

AC-TO-AC POWER CONVERSION