Boost-buck Push-pull Converter for Very Wide Input Range Single Stage Power Conversion

8/10/2019 Boost-buck Push-pull Converter for Very Wide Input Range Single Stage Power Conversion

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BOOST-BUCK PUSH-PULL CONVERTER

FOR VERY WIDE

INPUT RANGE SINGLE STAGE POWE R

CONVERSION

Jonathan J. Albrecht

Jason

Young,

William

A.

Peterson

Martin

Marietta Control Systems

Johnson

City, New York

ABSTRACT

In many avionics systems, the power supply

is

required

to operate over a very wide input voltage range. In these

aircraft systems the power supplies are powered off

a

Permanent Magnet Generator (PMG). The PMG, which

is typically mounted via gear box to the engine

of

the air-

craft, outputs a voltage proportional to the speed

of

the

engine.

It is

usually required to have the avionics start

at very low engine speeds and operate up t the maxi-

mum

engine speeds.

To operate over

this

large input

range, power converter stages are cascaded to reduce the

input range down to manageable levels. The last con-

verter stage then prov ides the filtering, isolation, scaling

and regulation for

the

avionics system. Additionally, air-

craft power sources are provided for backup power

and

typically require power factor correction. Active power

factor correction is also performed with

a

separate cas-

caded pre-regulator converter. Multiple power conver-

sion stages reduce efficiency, increase cost, and increase

size of the power supply system. The power converter

described herein combines the aforementioned functions

into

a

single conversion stage. The converter is

a

push-

pull converter that has both buck and boost regulator

characteristics. At low input voltage the push-pull con-

verter acts

as

a boost, and at high input voltages it acts

as

a buck converter. For power factor correction applica-

tions,

this

push-pull converter, by transitioning between

the boost

and

buck operating modes,

is

able

t

shape the

input current.

OPERATION

To

describe the operation of the new push-pull convert-

er, both modes of operation, boost and buck, will be

as-

cussed separately. A simplified circuit schematic of the

Boost-Buck Push-Pull Converter is shown

in

Figure 1.

The circuit consists

of

a Push-Pull converter with an in-

ductor placed between the input source and the Push-

Pull center tap. The Boost-Buck Inductor is referred to

as an inductor, rather than a transformer, since most of its

characteristics

in

both operational modes are that of

an

inductor. The primary to secondary turns ratios

(N)

of

both magnetics, the Boost-Buck Inductor and the Push-

Pull Transformer, are the same.

INPUT

SOURCE

N

D

BOOST-BUCK

IPl N:l

INDUCTOR

PWM-1

PWM-2

D I

g-f

vo

D2

+

Figure 1.Boost-Buck

Push-pull

Converter

303

0-7803-2482-X/95

4.00

995

EEE


2/6

BUCK OPERATION

The converter will operate in

the

Buck Mode when the

input voltage is greater than NVo. Operating in

this

mode, switches Q1 and Q 2 will be run at

less

than 50%

duty-cycle and

180

out-of-phase

with

each other,

as

shown infigure 2.

This

means that there will be

two

seg-

ments of time

in

each cycle in which both Q1 and Q2 will

be in the off state. This is typical of the Push-Pull con-

verter.

Referring to figure 2, the discussion of the buck mode

operation begins at time

to

when Q1 turns on and Q2 is

off. With

Q1

on, current Ipl flow s through transformer

winding P1 and inductor winding P3. The Boost-Buck

Inductor stores energy due to a VIN-NVO voltage across

it, and the primary current Ip1 begins to ram p up through

winding P3. The current Ipl through winding P1 causes

a secondary current to flow in winding S2 via transform-

er action. This secondary current flows ou t to the load

through diode D2. At time

t l

Q1 turns off,

Ipl

falls to

zero and D2 stops conducting. The energy stored

in

the

boost-buck inductor during the time Q1

was

on now dis-

charges as a current flow through the secondary winding

S3

and out to the load through diode

D3.

The magnitude

of the current is NIpl. At time t2, Q2 switches on while

Q1

remains off. Now primary current Ip2 flows through

windings

P2

and P3. The current through winding

P2

causes a secondary current to flow in winding S1 via

transformer action.

This

secondary current flows out to

the load through diode D

1.

Ip2

begins to ramp up through

the boost-buck inductor winding

P3

and again energy is

PWM-1

PWM-2

BOOST-BUCK

INDUCTOR

VOLTAGE

PRIMARY

CURRENT

IP1 + In

OUTPUT

CURRENT

stored in the inductor. At time tg, 42 turns off,

Ip2

falls

to

zero and D1 stops conducting. The energy stored in

the boost-buck inductor when Q2 was

on

now dis-

charges as a current flow through the secondary winding

S3 and out to the load through diode D3. The magnitude

of this current is NIp2. This cycle is repeated at time t4

at a constant frequency

( l / ~ ) .

The

turn-on times of Q1

and Q2 to and t2, respectively) are always a constant

half-period apart.

From

this

description and the waveforms of Figure 2,

this converter acts as a Buck converter by storing excess

input energy in an inductor during the switch on times

and transfemng

this

stored energy to the output during

the switch

off

times. Additionally, while operating in

this

mode the converter exhibits discontinuous input cur-

rent and continuous output current, just as a buck con-

verter does.

The integral

of

the Boost-Buck inductor voltage over

one time period (T)must be zero. Therefore, from figure

2

The variable

bg

an be expressed in terms of

T

and

to,,

a

(2)

T

to,

- --

on

Using

(1)

and (2) the input

to

output transfer characteris-

tic for Buck mode operation is

2avIN

v, =

(3)

where

a

=

Duty Cycle of one switch

bn/T)

V o = Output Voltage

VIN

=

Input voltage

N =

Turns Ratio (of both transformers)

From figure

2,

the

peak

voltage across each

of

the

switches occurs when both switches are off and the

boost-buck diode, D3,

is

conducting. At

this

point, the

drain-t+source voltage, V&, is VIN

+

NV,.

This

is

less

than the 2vIN vo ltage normally seen across the switches

in a classical push-pull converter.

This

allows the de-

signer to select lower rated switches.

Figure 2. Buck Mode Simplified Converter

Waveforms

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BOOST OPERATION

BOOST-BUCK

INWCTOR

V 1 N - ~ ~ j [ '

The Push-Pull converter will operate in the Boost Mode

when the input voltage is less than NVo. In Boost mode

the duty-cycle

of

both switches is increased to

greater

than

50%.

This means that Q1 and Q2 will be on at the

same time for portions of the switch period (T). The

switch period

(T)

and

the

delay time

T/z)

are kept

constant for both the boost and buck operational modes.

Typical PWM-1 and PWM-2 gate drive signals are

shown in Figure 3.

D2

Figure

4.

Boost Mode

- Flux

Cancellation

Referring to figure 3, the discussion of the boost mode

operation begins at time to when Q1 s turning on and Q2

is already on. During this time, current

Ipl

flowsthrough

transformer winding PI and current

Ip2

flows through

transformer winding P2.

A

redrawn circuit

of

the time

interval is show n inFigure4.With Ip2 flowing into th e

dotted side of the P2 winding , and

Ipl

flowing out of

the dotted side of the

P1

winding, equal but opposite

fluxes are developed in the transformer core. These

fluxes cancel and form an effective short circuit across

the transformer primaries. With this short circuit across

the transform er, a vo ltage equal to

VIN

is placed across

the winding P3

of

the Boost-Buck Inductor. This volt-

age causes the current to ramp up in the inductor and the

inductor begins

to

store energy, as seen in figure

3.

Next,

at time

t l ,

Q2 turns

off

while

Q1

remains

on.

With Q2

off,

Ip2

stops flowing through winding P2 and the flux

cancellation across the transformer primaries ceases.

W T +

* /e:

1 1 1 1

1 1 1 1 1

t tl tz t3 14

b n t o n b n h

BOOST-BUC

INDUCTOR

VOLTAGE

RIMARY

CURRENT

OUTPUT

CURRENT

Figure

3.Boost

Mode Sim plified Converter

Waveforms

Energy is now transferred to the secondary via trans-

former action. The magnitude of this energy is equ al to

the input voltage (Vin) plus

the

energy stored in the

Boost-Buck Inductor. During

this

portion of the switch

period, the voltage polarity on winding P3 reverses at a

magnitude of NVCJ-VIN and current begins to ramp

down. Diode D2 is forward biased and provides current

flow from secondary winding S2 to the output load. At

time t2, Q2 turns back on while

Q l

remains in the on

state. Again, equal but opposite fluxes are developed in

the transformer and both primary windings, P1 and

P2,

form a shor t to ground. The Boost-Buck Inductor stores

input energy, and no energy is transferred to the outputs.

At time t3, Q1 urns off while Q 2 remains on. With Q1

off, Ipl stops flowing through winding PI and the flux

cancellation

across

the transformer primaries ceases.

Energy is now transferred

to the

secondary hrough diode

D1.

Again, the magnitude of

this

energy is equal to the

input voltage (Vin) plus the energy stored in the Boost-

Buck Inductor. During

this

portion

of

the switch period,

the voltage polarity on winding

P3

reverses and current

begins to ramp down. Thiscompletes he switching peri-

od. Thiscycle is repeated at time t4 at a constant frequen-

cy ( l / ~ ) . he turn-on times of Q1 and Q2

to

and t2, re-

spectively) are alway s a constant half-period apart, just

as in the buck mode of operation.

From this description and the waveforms of Figure 3, the

Push-Pull converter in this mode of does act as a Boost

converter by storing excess input energy in an inductor

during the switch on times

b,)

and transferring this

stored energy plus Vin

to

theoutput during the switch

off

times (toff). Additionally, while operating i n

this

mode,

the converter exhibits continuous input current and

dis-

continuous output current, ust like a boost converter. It

should be noted that winding

S3

and diode D3 do not

supply current to the load during boost operation.

Since the integral of the Boost-Buck inductor voltage

over one time period (T) must be zero, the equation for

the inductor voltage is,

305


4/6

2 V O I I = 2t o f f ( wO

VpJ)

4)

5 )

Variable toff can

be

expressed in terms of T and ton as,

T

toff

=

on

Since

a s

the duty-cycle of each switch, it can

be ex-

pressed

as

Using

4),S ) ,

and

(6)

the input to output transfer charac-

teristic for Boost mode operation

is

'IN

v, =

2(1 -

a)N

7)

From figure

3,

the peak voltage across each of the

switches in boost mode occurs when one switch is off

and one is on. At this poin t, vds is just NV,.

BOOST-BUCK TRANSITION

The novel aspect of

this

converter is that it exhibits the

transfer characteristics

of

two different power converter

topologies, boost and buck,

by

changing only the duty

cycle of the power switches

Ql

and

Q2.

Using both the

boost and buck modes, operation over very large input

voltage swings are achievable. Depending on the type

of

control method used, transitioning between the two

modes is smooth and unnoticeable to down stream cir-

cuitry.

Part

of

this

can be seen graphically

by

plotting

both transfer equations (equations

3

and 7) on the same

axis. Duty-Cy cle verses Input Voltage for both modes

is shown

in

figure 5 .

Both

transfer characteristics con-

verge at the

50%

duty cycle point. This allows for a

smooth transfer from one mode to the other.

1.25

1 oo

0.75

2

0.50

c

0.25

0

CONDITIONS:

N

= 1 (tums ratios)

Vout

= 3 5 ~

C

0 2 40

60 80

100 120 140

INPUTVOLTAGE (VDC)

Figure

5.

Boost and Buck Transfer

Characteristics

During

this

transition, the inductor curren t

has

no ripple

because there

is

no voltage across

it.

The input voltage

VIN

s exactly equal

to

NV,. Although

this

condition is

desirable, difficulty arises when using peak current mode

control to control the converter since there

is

no slope.

This

easy transition characteristic

also

lends itself to

power factor correction applications where the converter

must transition between the boost and buck modes, at the

frequency of the input voltage source, to force the input

current to follow the inpu t voltage.

If

a

negative voltage rail is required for a given app lica-

tion, a negative windmg can be added

to

the Boost-Buck

Inductor with an additional diode

as

shown

in

Figure

6.

t

w

PWM-1

1

BOOSTBUCK

INDUCTOR

-

T

+

Figure 6.Boost -Buck Push-Pull ConverterWith Negative Output

+vo

vo

306


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WEINBERG'S CONVERTER

A.H.

Weinberg developed apo wer converter

in

the

1970s

for satellite applications that used a similar concept.

Weinberg's goal was to develop

a

boost converter that

provided continuous output current. Continuous output

current allowed output filters to be smaller thus achiev-

ing weight savings.

Weinberg's converter, shown in Figure

7

as the convert-

er described herein is based on a push-pull converter.

Additionally, Weinberg uses a tapped inductor in the

Push-Pull centertap. The inductor tap ratio and trans-

former turns ratio are equal.

To describe the operation of Weinberg's converter the

condition of

Q1

switching on

with Q off

will be consid-

ered first. When Q1 turns on a voltage equal to Vin -

N(Vo -Vin)

is

place across the

L1

inductor.

This

cause

the primary current to ramp up and the inductor begins

to store energy. The current through the

Q1

switch

causes a current to flow through transformer winding

T/3,

diode

D1,

and out to the output capacitor and load.

When Q1 is switched off, the voltage on the

L1

inductor

reverses and the stored energy is discharged as

a

current

through the

L2

inductor tap through D3, and out to the

load.

This

sequence repeats for the second half of the

cycle when

Q2

turns on. The simplified converter wave-

forms for Weinberg's boost converter are shown in Fig-

ure

8.

Although similar, Weinberg's converter is non-isolated

and runs only

in a

single conversion mode, similar to the

buck mode discussed herein. Weinberg's converter nev-

er has both sw itches on

at

the same time (ie. duty cycles

above

50 ),

herefore does not transition between con-

version modes. With only one conversion mode, the

L1

INDUCTOR

VOLTAGE

PWM-1

.-

-

nr-N(Vo-yln)

RIMARY

CURRENT

Figure 8. Weinberg's C onverter Waveforms

converter can not operate over

a

wide input voltage

range.

EXPERIMENTAL RESULTS

A 150W

converter was breadboarded using

this

design.

The output voltage was regulated to 25 V w hle the input

voltage varied from

10

to 200 V. A

2:1

turns ratio was

used for both magnetics. A switching frequency of

5OoKhz for each switch was selected to reduce the size

of the magnetics.

Figure

9

shows typical converter waveforms

in

buck

mode when the input voltage is at 100 V. The top two

waveforms show

the

duty cycle of the switches to be at

25 . The next waveform shows the input current

through the boost-buck inductor on the primary side

while the next waveform shows

the

current through

the

same inductor on

the

secondary

side. These

two wave-

forms show energy being stored in the primary side dur-

ing switch on time, and that same energy being trans-

ferred to the secondary during the off time.

Figure

10

shows typical converter waveforms at the

boost-buck transition point when the input voltage is at

50

V.

The top two waveforms once again show the duty

cycle of the switches to be at

50%.

The next waveform

shows

th

primary side boost-buck inductor current.

As

was sta ted earlier, the ripple current at thispoint

is

nearly

zero. The final waveform shows the output current.

Figure

11

shows typical converter waveforms in boost

mode where the input voltage is at

25V.

The top two wa-

veforms show the duty cycle

of

the push-pull switches

to be 75%. The next waveform shows the current

through the buck-boost inductor (primary side), and the

P

Figure 7. Weinberg's

Boost

Converter

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f

T

c

c

Figure 9. Converter W aveforms in

Buck Mode

Figure

10.

Converter Waveforms at the

Boost-Buck Transition

, ...

Figure11. Converter Waveforms in

Boost Mode

REFERENCES

1.A.H. Weinberg, A Boost Regulator With A New

Energy-Transfer Principle, Proceedings of

the Spacecraft Power Conversion Electronics

Seminar,

1974

final waveform shows the current through the output rec-

tifying diodes.

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Boost-buck Push-pull Converter for Very Wide Input Range Single Stage Power Conversion

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Transcript of Boost-buck Push-pull Converter for Very Wide Input Range Single Stage Power Conversion