Date post: | 21-Nov-2014 |
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Engineering |
Upload: | rohde-schwarz |
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Troubleshooting Switched Mode Power Supplies
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
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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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!
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
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
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
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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
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Noise reduction using averaging
1 mV on 10 V scale (13.3 bits) 50 averages
Zoom of this segment
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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
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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)
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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)
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Slew Rate and Vertical Resolution
N bits 2N levels
Sampling rate = F Resolution = 1/F
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Slew Rate and Vertical Resolution
N bits 2N levels
Sampling rate = F Resolution = 1/F
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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
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Viewing Multiple Waveforms
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But the Resolution is Reduced by Half…
Full scale waveform
Half scale waveform
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Using Multiple Grids
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Current Measurements
Shunt resistor
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Current Measurements
Current probe
Shunt resistor
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Inductor Current Waveform
Vg = Vin V = Vout
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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)
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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)
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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)
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
Output Voltage Ripple – No Load
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Output Voltage Ripple – Small Load
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Output Voltage Ripple – Large Load
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EMI – Large Load
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Vout
Near field probe
Understanding Power Flow and Topology The Basic SMPS - Buck Converter – Load Transient – Well-Damped Response, Little Overshoot
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ILoad Vout
Load Transient Response inductor current linearity and output voltage ripple
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Red = Vout Blue = IL
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
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
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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