troubleshooting switched mode power supplies (presented at eelive!)
DESCRIPTION
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.TRANSCRIPT
Troubleshooting Switched Mode Power Supplies
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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