A Unified Model for the ZVS DC-DC

Converters With Active Clamp

byN.Lakshminarasamma, B. Swaminathan,

Prof V. Ramanarayanan, IISC

Department of Electrical EngineeringIndian Institute of Science

Bangalore

Linear Regulator

Best dynamic performance Very good regulation Poor efficiency and bulky

Series Regulator Efficiency = K

(1-K)Vg

KVgR+

-Vg

Switching Regulator

Ideal losses zero Output discontinuous Smoothing filter needed

Switching Voltage Regulator

+

-

TON TOFF

Vg R

Typical Converter

Switches control power flow Reactive elements smoothen power flow Both are non-dissipative elements

LCSVg Vo

Classification of SMPS

SWITCHED MODE POWER

CONVERTERS

HARD SWITCHED POWER

CONVERTERS

SOFT-SWITCHED POWER CONVERTERS

Active Clamp Converters

Resonant Load Converters

Quasi-Resonant Converters

Resonant Transition Converters

Hard Switching Converter

T1 T2

P

t

I

t V

I

V

Soft Switching Converter

V

I t

t

ZVS

ZCS

I

V

I

V

OFF/ON Transient

ON/OFF Transient

Active Clamp ZVS Buck Converter

S2

S1

C R

CR

CC

LR I

D

V

DC

Throw1 Pole

CR

DClamp Capacitor

Clamp Switch

s2

LRS1

CR

+

Interval T1 - Zero-voltage Turn-on

*

R

Vi(t) I t

L

*

RI I

T1 LV

*1

Ns

T II 1

T I

IN - Normalized current

I

V VoS2S1

CRCR

D1

D2

LR

D

VCi(t)

i(0) = -I*

i(T1) = I

Interval T2 - Resonant Commutation

I

V VoS2S1

CRCR

D1

D2

LR

D

VC i(t)

i(0) = I

v(t) v(0) = V v(T2) = 0

R

R R R

C ti(t) I V sin

L L C

R R

tv(t) Vcos

L C

RT2R

Ci( ) I V

L

T2v( ) 0

R RT2 L C2

S S

R R

f fT2

Ts 2 2 f 4f

Interval T3 - Power-on Duration

I

V VoS2S1

CRCR

D1

D2

LR

D

VC i(t)

S1 turned off at end of T3 and CR almost instantly charges to V+VC.

R

R

Ci(t) I V

L T1 T2 T3 DTs T3 T1 T2

DTs Ts Ts

Interval T4 – Assisted Turn-off

I

V VoS2S1

CRCR

D1

D2

LR

D

VC i(t)

i(T4) = I

R C

R R

C V Vi(t) I V t

L L

T4i( ) I

RR

C R

V CT4 L

V V L

S

R

T4 1 f

Ts 1 2 f

CV

V

Interval T5 – Resonant Commutation

I

V VoS2S1

CRCR

D1

D2

LR

D

VC i(t)

v(0) = 0

v(T5) = V

RC

R R R

C ti(t) I V V sin

L L C

CR R

tv(t) V V 1 cos

L C

RT5R

CI( ) I V 1

L

V ( T5 ) = V

R RT5 L C cos1

1S

R

fT5cos

Ts 2 f 1

Interval T6 – Power Freewheeling Duration

At the end of T6 interval current i(t) has reversed and Flows through MOSFET of S2. CR almost instantly discharges to zero. Now S1 may be

switched on with zero voltage across the same.

I

V VoS2S1

CRCR

D1

D2

LR

D

VC i(t)

Theoretical Waveforms

I Active Switch S1

I*

T2T1 T3 T6

I

T4T5

t

I(T5)

kI(T5)

Resonant Inductor LR Current

I Freewheel Diode

t

Pole Voltage Vo

t

Vg

Clamp Ratio and Clamp Voltage

t

Clamp Capacitor CurrentI

kI(T5)

I*

T2T1 T3 T6

I

T4T5

t

I(T5)

kI(T5)

Resonant Inductor LR Current

R

T5

IT5 V Ck 2 1

I ( )T6

RT5C

k 1 LV I( )

T6

S

R

f

2 f

R

k 1 Ts / T6A

k 1 11

T6 2 f

Steady State Equivalent Circuit Model

1: D

(1+k)LR/Ts

Steady State Equivalent Circuit for Active Clamp Buck converter

o R

g g s

V L I(1 k)D

V V T

Ro g

s

L I(1 k)V DV

T

Equivalent Circuit Models of Other Converters

1-D: 1

Rd

1-D: 1 1:D

Rd

1: D1-D: 1

Rd1 Rd2

Equivalent circuits of the active clamped ZVS boost, buck-boost and cuk converters

Spread Sheet Design..\pesc04\work\spreadsheetdesign.xls

Inductor Current I 1.95 2.4 2.8 3.2 3.5

Throw Voltage V 50 50 50 50 50

Resonant Inductor LR 5.00E-06 5.00E-06 5.00E-06 5.00E-06 5.00E-06

Resonant Capacitor CR 1.00E-09 1.00E-09 1.00E-09 1.00E-09 1.00E-09

Resonant Freq r/s wR 1.41E+07 1.41E+07 1.41E+07 1.41E+07 1.41E+07

Resonant Freq Hz fR 2.22E+07 2.22E+07 2.22E+07 2.22E+07 2.22E+07

Switching Freq fS 2.50E+05 2.50E+05 2.50E+05 2.50E+05 2.50E+05

Switching Period Ts 4.00E-06 4.00E-06 4.00E-06 4.00E-06 4.00E-06

Switch Initial Current kI(T5) 1.40 1.87 2.30 2.72 3.03

Clamp Initial Voltage VC 4.61 6.23 7.66 9.08 10.15

Current after T6 kI(T5) 1.40 1.87 2.30 2.72 3.03

Clamp Voltage VC 4.61 6.23 7.66 9.08 10.15

Duty Ratio D 0.259 0.259 0.259 0.259 0.259

S1 ON Time DTs 1.04E-06 1.04E-06 1.04E-06 1.04E-06 1.04E-06

Dbar 1-D 0.741 0.741 0.741 0.741 0.741

Interval 1 T1 3.35E-07 4.27E-07 5.10E-07 5.92E-07 6.53E-07

Interval 2 T2 1.11E-07 1.11E-07 1.11E-07 1.11E-07 1.11E-07

Interval 3 T3 5.90E-07 4.98E-07 4.15E-07 3.33E-07 2.72E-07

Interval 4 T4 6.47E-08 6.29E-08 6.13E-08 5.98E-08 5.88E-08

Interval 5 T5 1.05E-07 1.03E-07 1.02E-07 1.00E-07 9.91E-08

Interval 6 T6 2.79E-06 2.80E-06 2.80E-06 2.80E-06 2.81E-06

Current after T5 I(T5) 1.18 1.61 1.99 2.37 2.66

Current Factor k 1.18 1.16 1.15 1.14 1.14

Pole Voltage Vp 9.65 8.51 7.48 6.46 5.70

Normal Current IN 0.05 0.06 0.07 0.08 0.09

Clamp Ratio VC/V 0.09 0.12 0.15 0.18 0.20

Voltage Ratio V/Vg 0.19 0.17 0.15 0.13 0.11

Table 1 : Spreadsheet Organisation for the Steady-state Performance Solution

Conversion Ratio (In vs V/Vg)

Clamp Ratio ( In vs Vc/Vg)

Conversion Ratio and Clamp Ratio as a function of Normalized Current

0.000.05

0.100.150.20

0.250.30

0.350.40

0.0000 0.0500 0.1000In

Vc/VgD=0.259D=0.332D=0.436

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.00 0.05 0.10In

V/Vg

D=0.259D=0.332D=0.436

Steady State Definitions Of Base Voltages And Currents

Buck Boost Buckboost Cuk

v

Rd

M

IVg

Io

R(1 k)L

Ts

ND (1 k) I

Vo

Ig

R(1 k)L

Ts

R(1 k)L

Ts

N

1

1 D (1 k)I

IL

N

N

D (1 k)I

1 D (1 k)I

N

N

D (1 k)I

1 D (1 k)I

R R(1 k)L (1 k)L;

(1 D)Ts DTs

Ig + IL

Vg+Vo Vg+Vo

Dynamic Model Of Active Clamp Buck Converter

Perturbation of the nonlinear circuit averaged model about a quiescent operating point.

ˆ ˆD+d I+ig gˆV +V

L

+-+

1:D

Rc

C

-+ g g

ˆ ˆD+d V +V

R

S

(1+k)L

T

Small signal ac model of active clamp buck converter

1:D

R

S

(1+k)L

T

Rc

L

CR

gd̂V

D

d̂IgV̂-

-

+

+

+-

Simulated Active Clamp Buck Converter

0

G1

S1

G2

1n

7u

1n

MUR1620CT

5u

IRF250150u

MUR1620CTIRF25050V

110u

12

Output power = 60 watts

Input voltage = 50 volts

Output voltage = 20 volts

Switching frequency = 250 KHz

Resonant Inductor Current Waveform

Clamp Capacitor Current Waveform

Steady State Performance Of Active Clamp Buck Converter

Clam p Ratio

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00 0.05 0.10 0.15

In- Am ps

Vc

/Vg D=0.259

D=0.332D=0.436

Clam p Ratio

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.00 0.05 0.10 0.15In- Am ps

Vc

/Vg D=0.259

D=0.332D=0.436

Conversion Ratio

0.00

0.10

0.20

0.30

0.40

0.50

0.00 0.05 0.10 0.15In- Am ps

V/V

g

D=0.259D=0.332D=0.436

Conversion ratio

0.00

0.10

0.20

0.30

0.40

0.50

0.00 0.05 0.10 0.15IN - Am ps

V/V

g

D=0.259D=0.332D=0.436

SIMULATED RESULTS EXPERIMENTAL RESULTS

Experimental Waveforms Of Active Clamp Buck Converter

Vgs and Vds of S1 showing ZVS; Vgs and Vds of S2 showing ZVS

Experimental Waveforms Of Active Clamp Buck Converter

Pole voltage and Inductor current waveforms; Pole voltage and Clamp capacitor current waveforms

Dynamic Performance Of Active Clamp Buck Converter

Measured output impedance of Hard-switched buck converter and Active clamp

buck converter

Conclusions – Active Clamp Converters

Derived from Hard-Switched Converters by the addition of few Resonant elements following the simple rule.

Circuit equations governing these sub-intervals are identical when expressed in terms of pole current; throw voltage and freewheeling resonant circuit voltage (I, V, and VC).

Steady state and Dynamic equivalent circuits are obtained from this idealized analysis.

The resonant sub-interval introduces lossless damping in the converter dynamics.

Advantages – Active Clamp Converter

High Efficiency - ZVS

Simple Dynamic Model

Wide Variety of Topologies

Thanks