solving thermal issues of dcr current sensing in voltage ... · 7 volterra semiconductor: ibm power...

1Volterra Semiconductor: IBM Power and Cooling Technology Symposium 2008IBM Power and Cooling Technology Symposium 2008

Solving Thermal Issues of DCR Current Solving Thermal Issues of DCR Current Sensing in Voltage Regulators.Sensing in Voltage Regulators.

AlexandrAlexandr Ikriannikov, Ognjen DjekicIkriannikov, Ognjen Djekic


BackgroundBackground

Popular industry practice is to use DCR of the inductors as a current sense, often using a thermistor in control loop to compensate for the temperature drift of DCR value inside inductors.

This presentation illustrates the issues with current sense and therefore voltage regulator loadline that arise from DCR thermal drift, including an error of the temperature sense.

Pole to compensate inductor zero

Inductor zero

Sensed signal:IL*DCR


Why Current Sense is ImportantWhy Current Sense is ImportantVo

Io

Vo(0)

dVLIMIT

dVERROR

Imax

Rdroop Rdroop(1+σ)

0

Error in current sense leads to output voltage deviation from ideal loadline.When inductor DCR is used for current sense – DCR tolerance affects the loadlinedirectly. Thermal drift of DCR is another contributor to the output voltage error.


Some lowest DCRs with lowest tolerance

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6DCR, [mOhm]

Tole

ranc

e, [%

]

0

0.5

1

1.5

2

2.5

Pow

er D

issi

patio

n, [W

]

PA0512.101NLTPA0511.900NLTPA1212.101NLTIHLM-2525CZ-R10-07IFLP-4040DZ-R34-01HM69S-1110R09LFHM69T-0707R10LFPower Loss per inductor, 40A/phase

Some lowest DCRs with lowest tolerance

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6DCR, [mOhm]

Tole

ranc

e, [%

]

0

0.5

1

1.5

2

2.5

Pow

er D

issi

patio

n, [W

]

PA0512.101NLTPA0511.900NLTPA1212.101NLTIHLM-2525CZ-R10-07IFLP-4040DZ-R34-01HM69S-1110R09LFHM69T-0707R10LFPower Loss per inductor, 40A/phase

DCR Tolerance: Efficiency OR AccuracyDCR Tolerance: Efficiency OR Accuracy

General trend shows that lower DCRsgenerally have higher tolerance in datasheet, and this is not even looking at layout or assembly factors.

If high efficiency is desired: inductors with lowest DCR is a must. This will most likely degrade loadlineaccuracy and current share between phases.

Conduction loss per inductor is shown for Io=160A, four phase solution. DCR thermals not even considered (could be up to 50% more).

TCR=700ppm/C(copper – 3900ppm/C)

10.3x11.2x5.5mm


Test Board for ThermalsTest Board for ThermalsThermocoupleon the top of each inductor

PCB

Thermistor 1

Inductor:Cooper FP4-150

5 mOhm load per phase to imitate VR

power losses

Mock-up of the five phase solution:Typical 150nH inductors with DCR~0.5 mOhm.Each phase has 5 mOhm (+/-1%) thermal source: ~3.1W at 25A/phase. This would correspond to efficiency of ~88% for Vin=12V, Vo=1V for a real VR solution: conservative loss estimate.Each phase (inductor and thermal source) are connected in series to insure the same phase current.Kelvin Sense for each inductor DCR.Thermocouple on the top of each inductor.Three Thermistors near inductors: 1 (left), 3 (middle) and 5 (right).

1 2 3 4 5

Airflow in the wind tunnel

Thermistor 2

Thermistor 3


ThermistorThermistor MeasurementsMeasurements

( ) ( ) ( )32 )ln()ln()ln(1TTT RDRCRBA

T⋅+⋅+⋅+=

0 2 .104 4 .104 6 .104 8 .104 1 .105 1.2 .105 1.4 .105 1.6 .1050

50

100

150

200177.3162052

15.5183203

Tem p1 Rt( )

1.5 10×1 103× Rt0 2 .104 4 .104 6 .104 8 .104 1 .105 1.2 .105 1.4 .105 1.6 .105

0

50

100

150

200177.3162052

15.5183203

Tem p1 Rt( )

1.5 10×1 103× Rt

0 2 .104 4 .104 6 .104 8 .104 1 .105 1.2 .105 1.4 .105 1.6 .1050

50

100

150

200177.3162052

15.5183203

Temp1 Rt( )

1.5 10×1 103× Rt0 2 .104 4 .104 6 .104 8 .104 1 .105 1.2 .105 1.4 .105 1.6 .105

0

50

100

150

200177.3162052

15.5183203

Temp1 Rt( )

1.5 10×1 103× Rt

Thermistors are often used in control loops to compensate for the DCR temperature shift, so thermistor performance is important to evaluate.

Thermistor: NTHS0603N01N1003J. Curve 1 from the datasheet.

Conversion table was used to determine the fit parameters for Steinhart and Hart equation: third degree polynomial.

Established equation matches temperature values in the datasheet table with better than 0.01C accuracy


Temperature Rise: ThermocouplesTemperature Rise: ThermocouplesInductor Temperatures, 600LFM

0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]

0A per phase5A per phase10A per phase15A per phase20A per phase25A per phase

Inductor Temperatures, 400LFM

0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]

0A per phase5A per phase10A per phase15A per phase20A per phase


0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]



0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]

0A per phase5A per phase10A per phase15A per phase20A per phase25A per phase


0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]



0

20

40

60

80

100

120

140

160

1 2 3 4 5Inductor #

T, [C

]


Test board was set in the wind tunnel.Airflow was applied from the right side of the board (Inductor #5).Accurate current was applied to all branches in series. All measurements were done in thermal steady state, after 20 minute delay.Readings from thermocouples attached to the top of inductors are shown above for 600LFM, 400LFM and 200LFM. DCR values and Thermistor values were also monitored directly by Kelvin sense differential pairs.

Airflow Airflow Airflow


Temperature Rise: Actual DCRTemperature Rise: Actual DCRDCR change with current per phase, 600LFM

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0 5 10 15 20 25 30Io, [A]

DC

R, [

mO

hm]

DCR1DCR2DCR3DCR4DCR5Avg. DCR

DCR change with current per phase [%], 600LFM

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30Io, [A]

dDC

R, [

%]

dDCR1dDCR2dDCR3dDCR4dDCR5Avg. dDCR

DCR change with current per phase, 600LFM

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0 5 10 15 20 25 30Io, [A]

DC

R, [

mO

hm]

DCR1DCR2DCR3DCR4DCR5Avg. DCR

DCR change with current per phase [%], 600LFM

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30Io, [A]

dDC

R, [

%]

dDCR1dDCR2dDCR3dDCR4dDCR5Avg. dDCR

The plots show initial DCR tolerances, plus the different thermal drift.Load line of the system will be affected by the average DCR value.Current share will be affected by the difference in individual DCR values.


Temperature Rise: Actual DCRTemperature Rise: Actual DCR

Accurate monitoring of the voltage drop across DCRsillustrates the real current sense changes.

DC

R 1

DC

R 2

DC

R 3

DC

R 4

DC

R 5 0A 5A 10

A 15A 20A

25A0

5

10

15

20

25

30

35

40

45

DC

R c

hang

e, [%

]

Relative DCR change for 600LFM, [%]

0A5A10A15A20A25A

DC

R 1

DC

R 2

DC

R 3

DC

R 4

DC

R 5 0A 5A 10

A 15A 20A

25A0

5

10

15

20

25

30

35

40

45

DC

R c

hang

e, [%

]

Relative DCR change for 600LFM, [%]

0A5A10A15A20A25A

Airflow


Temperature Errors [C], 600LFM

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

0 5 10 15 20 25 30Io/phase, [A]

Tem

pera

ture

Err

or, [

C]

Difference between Thermocouple and DCR reading, inductor 1Difference between Thermocouple and DCR reading, inductor 3Difference between Thermocouple and DCR reading, inductor 5Difference between Thermistor and DCR reading, inductor 1Difference between Thermistor and DCR reading, inductor 3Difference between Thermistor and DCR reading, inductor 5

Temperature Errors [C], 600LFM

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

0 5 10 15 20 25 30Io/phase, [A]

Tem

pera

ture

Err

or, [

C]

Difference between Thermocouple and DCR reading, inductor 1Difference between Thermocouple and DCR reading, inductor 3Difference between Thermocouple and DCR reading, inductor 5Difference between Thermistor and DCR reading, inductor 1Difference between Thermistor and DCR reading, inductor 3Difference between Thermistor and DCR reading, inductor 5

DCR DCR -- ThermistorThermistor Temperature DifferenceTemperature Difference

PCB

1 2 3 4 5

Airflow

DCR measurement => temperature. (benchmark)Thermistormeasurement => temperatureThermocouple measurement => temperature


Average DCR ReadingAverage DCR Reading

DCR Temperatures, 600LFM

90

95

100

105

110

115

120

125

130

135

140

1 1.5 2 2.5 3 3.5 4 4.5 5Inductor #

T, [C

]

DCR Temperatures, 25A per phaseAverageThermistor Readings

DCR Temperatures, 600LFM

90

95

100

105

110

115

120

125

130

135

140

1 1.5 2 2.5 3 3.5 4 4.5 5Inductor #

T, [C

]

DCR Temperatures, 25A per phaseAverageThermistor Readings

Thermistor error in temperature of the local inductor

DCR temperature error from the

average

Thermistor in the middle has a large temperature difference from the local inductor DCR.

But that particular inductor DCR is also much hotter than average DCR temperature.

Put thermistor “in the middle” – two errors are at least in the same direction and will approximately compensate.

However, if airflow direction changes, or one phase is not loaded (or added)….


Temperature Errors from Real Average [C], 600LFM

-26

-24-22

-20

-18

-16-14

-12

-10

-8-6

-4

-2

02

4

0 20 40 60 80 100 120 140

Io, [A]

Tem

pera

ture

Err

or, [

C]

Thermistor 1Thermistor 2Thermistor 3

Temperature Errors from Real Average [C], 600LFM

-26

-24-22

-20

-18

-16-14

-12

-10

-8-6

-4

-2

02

4

0 20 40 60 80 100 120 140

Io, [A]

Tem

pera

ture

Err

or, [

C]


DCR Average DCR Average -- ThermistorThermistor DifferenceDifference

PCB

1 2 3 4 5

Airflow


Vo deviation from ideal due to thermal drift of DCR, 600LFM

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

0 20 40 60 80 100 120 140

Io, [A]

dVo,

[mV]


Vo deviation from ideal due to thermal drift of DCR, 600LFM

-12

-11

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

0 20 40 60 80 100 120 140

Io, [A]

dVo,

[mV]


Load Line ImpactLoad Line Impact

PCB

1 2 3 4 5

Airflow

Based on 1.25mOhm load line.Depending on specification (15mV window?), even compensated thermal drift adds noticeable error to the load line


Adjusting the Thermal CompensationAdjusting the Thermal Compensation

Adjustment of thermal compensation is always needed for particular design.This adjustment will not be valid if something is changed: airflow direction, heatsink shape, one phase not loaded, etc.


DCR ConsiderationsDCR ConsiderationsSmall shift in DCR value causes potentially significant deviation of the load line.

The error will grow in absolute terms as current increases.

The deviation window (mVolts) has to account for the total tolerance, such as offset in voltage and current error amplifiers and tolerance in their gains, etc.

Loosing a big part of tolerance window to DCR static and drift errors puts a serious pressure on the tolerance of the other contributors, generally increasing the silicon price.

In order to keep overall load line accuracy in spec – solutions with DCR sensing often have to choose inductors with acceptable DCR accuracy, not smallest DCR, which is not beneficial for efficiency.

The current sensing across DCR relays on matching the time constants between L-DCR inside inductor and R-C of external filter.

DCR was changing up to 45% in performed experiments.In addition to DCR thermal drift, considered above, there are other problems:

L can change with both current and temperature, as much as 20-30%R and C of external filter also have thermal drift.

The time constant issues can relate only to dynamic performance: they introduce overshot or undershot during the transient, but do not affect loadline in steady state.

Degraded dynamic performance is still a significant concern.


Integrated Power Stage from VolterraIntegrated Power Stage from VolterraVolterra patented and proprietary current sense and current mode control:

Lossless: no external resistors, any inductors can be used - with lowest possible DCR for efficiency boost. Integrated into power stage.Thermally compensated – right on the die. Thermal “measurement” is right where the current sensor is: they are in one IC.Initial trim in manufacturing insures accuracy, arbitrary external components are not involvedExtremely fast.No noise or external filtering issuesNo layout worries about current sense tracesNo Signal/Noise ratio dilemma regarding DCR valueNow applied to Volterra patented Coupled Inductor technology as well.

Inductors in the real end product are the staple construction for the lowest possible DCR found on the market. DCR tolerance or thermal drift are irrelevant. All of the above also means “cheap”.


Load line deviation for proposed solution

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]

Ta=0CTa=25CTa=55CIdeal

Load line deviation for proposed solution

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]


Competitor 1 load line deviation

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]



-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]



-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]



-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]



-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]



-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0 50 100 150Io, [A]

dVo,

[V]


Performance ComparisonPerformance Comparison

Data is shared by the customer (independent and unbiased report).

Samples of several competing solutions (modules) for a particular high volume (VRM10) project were evaluated.

Volterra shows the most consistent performance, the load line is closest to ideal.

Volterra module


CommentsCommentsDCR current sensing noticeably affects the total accuracy of the system with the droop.

DCR toleranceDCR drift

When DCR is lowered for a better efficiency DCR related errors increase. Smaller DCR decreases the current sense signal amplitude, which makes an offset at the input of current error amplifier proportionally larger contributor of errors.Efficiency or accuracy – not both at the same time.

Decreasing DCR makes Signal/Noise ratio worse for a current sense amplifierLoad line may become non-monotonic or irregularOutput voltage ripple may increaseEfficiency may take some hit due to irregular switching.

More advanced current sense and current control techniques, such as offered by patented Volterra technology, can achieve better accuracy of the solution, while being insensitive to the DCR value, DCR tolerance or DCR thermal drift.

This allows to separately optimize system efficiency and accuracy. Such solutions also do not require tuning the thermal compensation for each particular design and particular conditions.

solving thermal issues of dcr current sensing in voltage ... · 7 volterra semiconductor: ibm power...

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