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