1

Efficiency Improvement in Redundant Power Efficiency Improvement in Redundant Power Systems by Means of Thermal Load SharingSystems by Means of Thermal Load Sharing

Carsten Nesgaard Michael A. E. Andersen

Technical University of Denmark

in collaboration with

2

OutlineOutline

• Load Sharing

• The Power System

• Experimental Verification

• Efficiency

• Reliability

• Causes of power imbalance

• Conclusion

3

Load SharingLoad Sharing

Load sharing is utilized when applications call for:

• Modular structure – increase maintainability

• Simple power system realization

• Short time to market

• Increased reliability – redundancy and fault tolerance

• High-current low-voltage applications

• Distributed networks

4

Powercomponents

PWM control

Load sharecontrol

Currentmeas. OutputInput

R MEAS

+ 9V

- 9V

LS controller

R 3

R 1 R 2

R 4

OP-amp

High side sensing

DC/DC converter

Loadcontrol

DC/DC converter

Loadcontrol

DC/DC converter

Loadcontrol

Load

Load

sha

ring

bus

Load

sha

ring

bus

DC/DC converter

DC/DC converter

DC/DC converter

Load

Loadcontrol

Temp

Loadcontrol

Temp

Loadcontrol

Temp

Powercomponents

PWM control

Load sharecontrol

Currentmeas. OutputInput

2,7V - 20V

R 1

R 2

T Sense

Part of

Load SharingLoad Sharing

5

Converter 1

Converter 2

OutputInput

IOUT /2

IOUT /2

IOUT1 0 0 F 1 0 0 F

4 8 HIR FP 0 6 4 1 0 m

4 7 0 F

R F eedbac k

+ 5 V

MC 3 3 0 7U C 3 9 0 2U C 3 8 4 3

IR 2 1 1 0

P B YR3 0 4 5

R G ate+ 1 6 V

In p u t O u tp u t

1 0 0 F 1 0 0 F

4 8 HIR FP 0 6 4 1 0 m

4 7 0 F

R F eedbac k

MC 3 3 0 7U C 3 9 0 2U C 3 8 4 3

IR 2 1 1 0

P B YR3 0 4 5

R G ate

The Power SystemThe Power System

Buck topology – simplicity of implementation

125 W converters – 5 V output at 25 A

5% output ripple voltage

4 IC’s – lowers overall system reliability

2 freewheeling diodes and 1 MOSFET

L = 48 H, COut = 200 F, RSense = 10 m

6

Experimental VerificationExperimental Verification

Duty cycle differences due to component tolerances, off-set voltages and temperature difference.

The output voltage that results is a combination of each converter’s output voltage.

7

Experimental VerificationExperimental Verification

Current sharing: Thermal load sharing:

0

2

4

6

8

10

12

14

0 5 10 15 20 25 30

Output current

Ind

ivid

ual

co

nve

rter

cu

rren

t

C o nv e rte r 1

C o nv e rte r 2

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30

Output current

Ind

ivid

ua

l co

nv

ert

er

cu

rre

nt

C o nv e rte r 1

C o nv e rte r 2

Current distribution among the two converters as a function of total output current.

8

Experimental VerificationExperimental Verification

0

1

2

3

4

5

6

7

8

9

10

MOSFET switchingcurrent

MOSFET switchingthermal

MOSFET conductioncurrent

MOSFET conductionthermal

Converter 2

Converter 1

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

Capacitor, current Capacitor, thermal

Power losses

0

0,5

1

1,5

2

2,5

3

3,5

4

Sense resistor, current Sense resistor, thermal

Power losses

Power component loss distributions

0

1

2

3

4

5

6

7

8

Diode, current Diode, therm al

Converter 2

Converter 1

0

0,5

1

1,5

2

2,5

3

3,5

4

Inductor, current Inductor, thermal

Power losses

9

Experimental VerificationExperimental Verification

S w itch in g lo ss e s

2

4

6

8

1 0

1 2

1 4

5 1 0 1 5 2 0 2 5O u tp u t C u rre n t

N o m in a l RD S (O N )

N o m in a l RD S (O N ) + 2 .9 m

MOSFET conduction and switching losses.

Both type of losses increase nonlineary with current and temperature.

C o n d u c tio n lo ss e s

5

1 0

1 5

2 0

5 1 0 1 5 2 0 2 5O u tp u t C u rre n t

N o m in a l RD S (O N )

N o m in a l RD S (O N ) + 2 .9 m

Incr

easi

ng te

mpe

ratu

re

Temperature dependance of MOSFET switching losses are described in [3]

10

Experimental VerificationExperimental Verification

Thermal Load Sharing loss distribution

MOSFET

Diode

Sense resistor

Inductor

Capacitor

Loss distribution as a function of combined losses.

Current Sharing loss distribution

MOSFET

Diode

Sense resistor

Inductor

Capacitor

MOSFET loss redistribution

Diode loss redistribution

11

EfficiencyEfficiency

• Initial ‘semi-droop’ method

• Current sharing

• Thermal load sharing

0,4

0,5

0,6

0,7

0,8

0,9

0 5 10 15 20 25 30

Output currentE

ffic

ien

cyS e m i-dro o p s ha ring e ff ic ie nc y

C urre nt s ha ring e ffic ie nc y

The rma l s ha ring e ff ic ie nc y

‘Semi-droop’ at low current levels

The thermal load sharing efficiency

Current sharing technique at heavier loads but at a higher level.Lowest temperature

12

ReliabilityReliability

T S u rfa ce

T Su rfa ce - 1 0 °C

T Su rfa ce - 3 0 °C

1 re s is t o r1 M O S F E T

5 re s is t o rs2 I C 's1 in du c t o r

2 d iod es

4 c ap ac ito rs

1 re s is t o r1 d iod e2 c ap ac ito rs

8 re s is t o rs2 I C 's4 c ap ac ito rs

Temperature distribution for reliability assessment

t-

t

t-

t

t

0

e e f(t) f(t) - 1 R(t)

dtdtdt

t-

t

0

t-

t

0

e - 1 e f(t) Q(t) dtdt

212121System qp pq pp R

qp2 p R 2System

= Accumulated failure rate per unit

R = Survivability

Q = Unavailability

13

ReliabilityReliability

Annual system downtime – current sharing: 10 min. 14 sec.

– thermal load sharing: 6 min. 11 sec.

Change in unavailability (downtime):

Inserting values – an overall reduction of almost 40% can be calculated.

Achieved by simply choosing a different load sharing technique.

1001) - (P1) - (P

P - P2 P - 1) - (PP Q

Converter2Converter1

2

ThermalThermalConverter2Converter2Converter1

14

Causes of power imbalanceCauses of power imbalance

Possible causes of the power imbalance in the two-converter system:

• Lower thermal contact between MOSFET and heat-sink

• RDS(ON) incremental deviation among the two converters

• Unequal switching losses among the two MOSFET’s

• Diode parasitic deviations – causing imbalanced diode losses

15

ConclusionConclusion

• Two parallel-connected buck converters controlled by a dedicated load share IC formed the basis for the experimental verification.

• Theoretical evaluations of the experimental measurements provided the explanation for the efficiency gain.

• Redistribution of the MOSFET transistor losses proved to be the major contributor to the increased efficiency.

• Unequal thermal contact, differences in RDS(ON) and diode parasitic deviations are some of the possible causes.

The concept of thermal load sharing has been presented and analytically proven to enhance system reliability and efficiency.