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DereMurray - APEC 2014 - Noise Susceptibility of delta-Vbe Temperature Sensors
- 1. Powervation Confidential | Copyright © 20141
APEC March 2014
Authors- Derek Murray, Karl Rinne
Noise Susceptibility of ΔVBE
Temperature Sensors in Highly-
Integrated Power Converters
- 2. Powervation Confidential | Copyright © 20142
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.
- 3. Powervation Confidential | Copyright © 20143
Reasons for Temperature Sense
in Power Conversion Applications
1. Protection against destructive
thermal events.
2. Compensation of load current
reporting (IOUT) across
temperature.
- 4. Powervation Confidential | Copyright © 20144
Semiconductor Temperature Sense Methods:
Advantages of ΔVBE over Standard Constant-Current
- 16. Powervation Confidential | Copyright © 201416
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
- 17. Powervation Confidential | Copyright © 201417
Test Circuit to Analyse Effect of Transistor
DC Shift ~25mV
Noise pulses: -150mV,
20ns width, at 500kHz
- 18. Powervation Confidential | Copyright © 201418
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
- 19. Powervation Confidential | Copyright © 201419
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?
- 20. Powervation Confidential | Copyright © 201420
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
