deremurray - apec 2014 - noise susceptibility of delta-vbe temperature sensors

Powervation Confidential | Copyright © 2014 1

APEC March 2014 Authors- Derek Murray, Karl Rinne

Noise Susceptibility of ΔVBE Temperature Sensors in Highly-

Integrated Power Converters


Scope of this Presentation

Some background: • Reasons for temperature sense in power conversion circuits.

• Introduce semiconductor temperature sense techniques.

Describe application + observed error.

Explain the error mechanism (‘charge-pumping’ phenomenon).

Introduce Spice model to simulate the issue.

Present measured Vs modelled data: • Part 1: Show effects of VR load current, VR switching frequency, ΔVBE bias currents, ΔVBE noise

capacitor.

• Part 2: Show that transistor choice has an effect on error magnitude.

Conclusion – Summarise the techniques that analog and application engineers can use to reduce or eliminate the problem.


Reasons for Temperature Sense in Power Conversion Applications

1. Protection against destructive thermal events.

2. Compensation of load current reporting (IOUT) across temperature.


Semiconductor Temperature Sense Methods: Advantages of ΔVBE over Standard Constant-Current


Application Details + Observed Error


Proposed Error Mechanism Hypothesis


Error Mechanism Waveforms (Charge-Pumping)


Spice Model to Simulate ΔVBE Error


Expected Error Behaviour due to Capacitor


Expected Error Behaviour due to IBIAS


Expected Error Behaviour due to Switching Frequency


Reason for Non-Linear ΔVBE Error


Measurement Set-up


Measurement Vs Model: Load and fsw Effect


Measurement Vs Model: Transistor IBIAS Effect


Measurement Vs Model: Capacitor Effect

7 9

20.5

82

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

X7R Type 1 C0G Type 1 C0G Type 2 C0G Type 3

ΔVBE

Temp.

Error

(°C)

Measured Data: ΔVBE Temperature Error for Four 100pF

Capacitors (Vo = 1.0V, fSW = 500kHz, Load = 20A)

X7R Type 1

C0G Type 1

C0G Type 2

C0G Type 3

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

1 10 100 1000

Ext.

Temp.

Error

(°C)

Noise Capacitor (pF)

Comparison of Capacitor Types with Model Data

(Vo = 1.0V, fSW = 500kHz, Load = 20A)

Model Data

X7R Type 1

C0G/NPO Type 3


Test Circuit to Analyse Effect of Transistor

DC Shift ~25mV

Noise pulses: -150mV, 20ns width, at 500kHz


Results: ΔVBE Error for Different Transistors

66 65

75

59 56

64

23 24

43

15

32

0

10

20

30

40

50

60

70

80

90

ΔVBE

Temp

Error

(°C)

ΔVBE Error for Different Transistors

(-150mV, 20ns Noise Pulses @ 500kHz)

Oscilloscope Data, post-processed in

MatLab


Over-riding point: transistor location is critical. Place the sensor in as quiet a location as possible while still obtaining useful thermal information.

Analog designers: Use higher ΔVBE bias currents.

Application engineers: • Use a lower-valued ΔVBE noise capacitor where some filtering is still

achieved, but charge-pumping does not occur.

• Similarly, use a transistor where charge-pumping is less likely to occur.

• Note that these two points are inter-dependent, as they both affect the low-pass filtering behaviour of the circuit. For example, if an MMBT3904 transistor is used, a capacitor of 50-80pF appears optimal. However, with a ‘better’ transistor (BCW33 or BC850), a higher-value capacitor may be more suitable. Further work is required to arrive at a more academic method of choosing transistor/capacitor values. For now, measurement is best!

Conclusion: What can analog designers and application engineers do?


VBE-T characteristics for a range of diode-connected transistors (bias current = 500uA)

200

300

400

500

600

700

800

-40 -20 0 20 40 60 80 100 120

VBE

(mV)

Temperature (°C)

VBE-T Characteristics for a range of diode-connected NPN transistors (Bias current = 500uA)

MMBT3904

MMBTA42

BC850C

BCW33

MMBT4401

BC817

BC848C

PBSS4032NT

ZXTN25012

m = -1.99mV/°C

m = -2.46mV/°C

0°C Offset = 700mV

0°C Offset = 581mV


Improved Modelling Techniques: Co-simulation (Full-Wave + Time-Domain)

deremurray - apec 2014 - noise susceptibility of delta-vbe temperature sensors

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