Switched mode power supplies have become ubiquitous in electronics as they provide precise voltages including high power with very high efficiency. The efficiency of these power supplies requires low loss power transistors and the design requires measurement of highly dynamic voltages. Voltage levels can vary from millivolts to hundreds of volts in some applications. In this seminar, the proper use of a digital oscilloscope to accurately measure these voltages will be discussed along with key aspects of instrument performance such as noise and overdrive recovery that affect the accuracy of the measurement.

1. Troubleshooting Switched Mode
Power Supplies

2. SMPS | 2
Agenda
l Switched mode power supply background
l Measurement points
l Voltage and current waveforms
l Maximizing measurement accuracy
l Averaging, high resolution decimation
l Sampling rate
l Analyzing common issues
l Improper inductor size
l EMI
l Load transient behavior

3. Modern Power Supplies:
Inductors, Capacitors and Fast Switches
ı Use ‘Lossless’ Components, In ‘Switching’ Operation
 Inductors store energy, and can deliver the energy at higher or lower
voltage than input
 Capacitors store energy between ‘pumping’ operations of inductors
ı Replace Linear Series Pass And Shunt Regulators
 Linear regulators turn excess voltage into thermal energy
 Efficiencies can be very high – as little as 2% to 3% “wasted” energy
ı Effectively ‘Variable Transformer’ Operation
 Able To Provide Increase/Decrease, Or Both, In Voltage
 Able To Operate Over Wide Ranges Of Input Voltage
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4. Power Supply – Evolution
Instead of “Burning” Excess Voltage, SMPSs Use Inductors and
Capacitors to “Transform” the Voltage.
In a Buck (Down-) Converter, the Inductor “input” is switched between
voltage source and ground
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Currents And Voltages Change Direction / Polarity, At High Speed…
Dynamic Circuits, Where Oscilloscopes Excel At Measurement!

5. Understanding What to Measure
ı Understanding Power Flow and Topology
 The Basic SMPS - Buck Converter Topology – Current Flow
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A diode or transistor may replace
one switch

6. Understanding What to Measure
ı The Basic SMPS - Buck Converter Topology
 The “Switches” are typically implemented as internal, or external, FET’s, or
IGBT’s in high-power applications.
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Shunt resistor

7. Power Flow and Topology
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Vswitch
Iinductor
Vout
Note the slope in
Vswitch
Related to the slope
in inductor current
Proportional to the
internal switch and
current-sense
resistance
Measure V1 and I1 Measure V2 and I2
Use V1 -V2 / I2-I1
To calculate switch resistance

8. SMPS | 8
Maximizing measurement accuracy
l Large dynamic range required for accurately measuring
switching voltage and current
l On state is tens to hundreds (even thousands) of volts
l Off state is often only several mV to a few volts
l Typical 8-bit A/D provides approximately 39 mV on a 10 V scale
l Three possibilities to improve signal to noise
l Use waveform averaging
l High resolution decimation (trade off sample rate and bandwidth for
S/N)
l Overdrive instrument front end
Resolution enhancement (B = bits)
due to averaging

9. SMPS | 9
Noise reduction using averaging
1 mV on 10 V
scale (13.3 bits)
50 averages
Zoom of this
segment

10. SMPS | 10
High Resolution Mode
l Combine consecutive
samples from A/D
converter with
weighting
l Preserves real time
sampling – no smearing
of dynamic signals
l Reduces bandwidth
based on decimated
sampling rate
l Compatible with
segmented memory so
that each cycle can be
analyzed
Combine
samples for
each point

11. SMPS | 11
High Resolution Decimation Mode
Decimate 10
Gs/s to 1 Gs/s
~ 500 MHz BW
4.6 mV on 10 V
scale (11.1 bits)

12. SMPS | 12
Combining averaging and high resolution mode
Decimate 10
Gs/s to 1 Gs/s
50 averages
~ 500 MHz BW
500 uV on 10 V
scale (14.3 bits)

13. SMPS | 13
Slew Rate and Vertical Resolution
N bits
2N levels
Sampling rate = F
Resolution = 1/F

14. SMPS | 14
Slew Rate and Vertical Resolution
N bits
2N levels
Sampling rate = F
Resolution = 1/F

15. SMPS | 15
Slew Rate and Vertical Resolution
l Both vertical and horizontal resolution are critical
l High slew rates
l Measuring short, high amplitude peaks that could damage active
components
l 10 V/ns = 1 V per sample @ 10 Gs/s
l 10 V/ns = 5 V sample @ 2 Gs/s
l Compare to digitizer range
l 39 mV @ 8 bits
l 9.7 mV @ 10 bits
l Measurement resolution can be limited by the sampling rate

16. SMPS | 16
Viewing Multiple Waveforms

17. SMPS | 17
But the Resolution is Reduced by Half…
Full scale waveform
Half scale waveform

18. SMPS | 18
Using Multiple Grids

19. SMPS | 19
Current Measurements
Shunt resistor

20. SMPS | 20
Current Measurements
Current probe
Shunt resistor

21. SMPS | 21SMPS | 4
Inductor Current Waveform
Vg = Vin
V = Vout

22. SMPS | 22SMPS | 4
Inductor Current Waveform
Vg = Vin
V = Vout

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Analyzing the Inductor Current
Ts = 950 ns
D = 0.35
L = 2.2 µH
Vin – Vout = 3.2V
2*∆I = 3.2*950e-9*0.35
(2.2e-6)
= 484 mA
Predicted current ripple:
20 ohm resistive load (90 mA load current)

24. SMPS | 24
Analyzing the Inductor Current
Measured current ripple:
2*∆I = 680 mA
Equivalent Inductance:
L = 950e-9*.35*3.2/0.680
= 1.56 uH
5 ohm resistive load (360 mA load current)

25. SMPS | 25
Using Math Waveforms to Identify Saturation
l Create math waveform = integral(VL/L)
l Ideal current ripple is linear
Measured I(t)
Computed I(t)

26. Output Voltage Ripple
The Basic SMPS – 1.4 MHz Buck Converter – Vout Ripple Spectrum
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Iinductor
Vout
Spikes at multiples of Fswitch

27. Output Voltage Ripple – No Load
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28. Output Voltage Ripple – Small Load
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29. Output Voltage Ripple – Large Load
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30. EMI – Large Load
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Vout
Near field probe

31. Understanding Power Flow and Topology
The Basic SMPS - Buck Converter – Load Transient – Well-Damped Response, Little Overshoot
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ILoad
Vout

32. Load Transient Response
inductor current linearity and output voltage ripple
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Red = Vout
Blue = IL

33. Load Transient Response
ı 1% to 100% load shift with 5 V input
ı 4 µs recovery time
ı Higher Vin-Vout delivers more current to load
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Red = Vout
Blue = IL

34. Load Transient Response
ı 1% to 100% load transient with 3.3 V input
ı 9 µs recovery
ı Smaller Vin – Vout slows down response
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Red = Vout
Blue = IL

35. SMPS | 35
Summary
l Switched mode power supply voltages are dynamic with very
high voltage swings
l Oscilloscope performance is critical for making accurate
measurements
l Both sampling rate (bandwidth) and resolution are important
l Averaging techniques are used to enhance resolution when required
l Trouble shooting techniques
l Analyzing output ripple voltage and EMI
l Observing inductor current
l Using spectrum analysis