Power Measurements for Switch-Mode Power Supplies SAVE Verona 2011
Agenda Power measurements tools Switch-mode power supplies Automated power measurements Summary Reference information 2
Switch-Mode Power Supply (SMPS) Power Supply Converts Electrical Power From one Form to Another SMPS advantages over a linear regulator power supply Good efficiency Good regulation with changing loads Wide range of AC input voltages Small size Light weight Low heat generation SMPS challenges Greater complexity Ripple voltage Switching frequency & its harmonics High amplitude High frequency energy 3
Switch-Mode Power Supply Verification & Debugging Operation Performance Reliability Regulatory compliance 4
Switch-Mode Power Supply Characterization - Debugging Measurement systems Real time oscilloscope Probes Adapters, Fixtures, Bundles 5
Power Supply Automated Power Analysis Tektronix Oscilloscopes, Analysis Software & Probes DPO/MSO 5K/7K DPOPWR Analysis DPO/MSO/MDO4000 Series DPO4PWR Analysis DPO/MSO3000 Series DPO3PWR Analysis TPS2000 TPS2PWR1 6
Switch-Mode Power Supply Simplified Schematic of the Power Conversion Section AC Input L Output N G Control Circuits 7
Automated Power Measurements Switch-Mode Power Supply 8
Active Component Measurements: Switching Devices P ds T 1 T 2 T 3 T 4 Time Transistor switch circuits dissipate the most energy during transitions Common measurements: Turn Off&On Loss Power Loss Slew Rate Dynamic On Resistance Safe Operating Area V ds I ds T on Loss Conduction Loss T off Loss 9
Switching Loss Basics a (t) v a (t) Energy loss during the transition can be estimated by: E Where: on t 1 t o v a ( t) i a ( t) dt i a (t) OFF t 0 t 1 ON E on is the energy loss in the switch during the transition. v a (t) is the instantaneous voltage across the switch. i a (t) is the instantaneous current through the switch. t 1 is when the transition is complete. t 0 is when the transition begins. The equation for E off is similar 10
Voltage Slew Rate Measurement Turn-Off ΔV/Δt = 263.5kV/s Current Volts Watts 11
Current Slew Rate Measurement Turn-Off ΔI/Δt = -880.2A/s Current Volts Watts 12
Switching Power Using Math Waveform Turn-Off Maximum Power Peaks at 808.4 mw Current Volts Watts 13
Switching Loss Measurements Turn-on loss Energy losses when the switching device changes from its non-conducting state to its conducting state Conduction loss Losses in the switching device when it is in saturation Turn-off loss Energy losses when the switching device changes from its conducting state to its nonconducting state 14
Switching Loss Measurements 3 Conduction Calculation Methods: Waveform, RDS(on) & VCE(sat) 15
Max. Voltage Amps Safe Operating Area (SOA) Switching device operating region Plot of voltage versus current SOA mask is a graphic representation of the switching device's limits on a SOA plot Max. Current Max. Power Switching Device Safe Operating Area Mask Volts 16
Current Axes Safe Operating Area Measurements Switching Device Voltage Versus Current Graph Volts Waveform Current Waveform Switching Device Volts vs. Current Operating Curve Volts Axes 17
Current Axes Safe Operating Area Mask Testing User Defined Mask Black Mask Operating Curve Volts Axes 18
Safe Operating Area Mask Testing Passed/Failed Results Operating Margin Operating Curve 19
Measurement challenges Filtering out the ringing present on the switching signal, which is often mistaken for an ON or OFF transition. Accurately measuring V DSon and V DSoff, which vary from tens of millivolts to hundreds of volts respectively. True turn-on True turn-off Simultaneously measuring V DSon and V DSoff on an 8-bit oscilloscope. Overdrive recovery of the measurement system Successfully and accurately capturing voltage and current data for the switching device under various load scenarios and variations in input line voltages Probes with appropriate dynamic range for the measurement 20
Output Analysis 21
Ripple Measurements on the DC Output Ripple is the periodic AC component On top of the DC voltage output Ripple frequency is related to Line frequency ~120 Hz in countries with 60 Hz power ~100 Hz in countries with 50 Hz power Switching frequency Typically > 100 khz AC component DC component 22
Ripple Measurements on the DC Output 23
Modulation Analysis Probing Probe the Control Signal Voltage of the Switching Device Differential probe Output AC Input Control Current probe 24
Modulation Analysis In a Switch-Mode Power Supply Pulse-width modulation voltages & currents 25
Power-On Modulation Analysis Control Signal Voltage of the Switching Device 26
Power-On Modulation Analysis Switching Device Current, Cursors Reading Positive Pulse Width 27
Power-On Modulation Analysis Switching Device Current, Cursors Reading Current Amplitude 28
Line Power Measurements 29
Input Analysis Probing Power Quality & Harmonics Measurements Current probe Differential probe Output AC Input Control 30
Input/Output (I/O) Analysis Output Ripple A periodic AC component riding on top of the DC voltage output of a power supply. The frequency of the ripple is related to the line/switching frequency of the device. Output Noise Similar to output ripple, however, it is not periodic. Power System Harmonics and Distortion Harmonics are sinusoidal components that have a frequency that is an integral multiple of the fundamental frequency. Distortion may be caused by non-linearities in an active devices, passive components or reflections in the propagation path. Total Harmonic Distortion (THD) is used to quantify Distortion. A pure sine wave will have no harmonics and no distortion and a non-sinusoidal wave will have both distortion and harmonics. Power Quality The measurement of True Power, Apparent Power, Power Factor, Current Harmonics and THD are essential to understand the effect of non-linear loads on the supply. 31
Input/Output Analysis Measurement Challenges Successfully and accurately separating line ripple from switching ripple. Isolating the noise in the system from the ripple and isolating the noise source. Accurately measuring the voltage and current. Measurement system with enough bandwidth to look at all harmonics of interest. Calculating the various power quality components. 32
Power Quality Measurements Definitions Power Factor for Pure Sine Waves A measure of the efficiency with which AC power is delivered Dimensionless number between 1 & 0 Power factor = cos(phase angle between volts & current) Power factor = true power/apparent power Power factor = 1 Maximum efficiency True power = apparent power Reactive power = 0 Voltage & current waveforms are in phase Power factor = 0 Diagram from Wikipedia Reactive power = apparent power True power = 0 Stored energy in the power supply returns to the AC line on each cycle 33
Power Quality Measurements Definitions Pure Sine Wave Example Peak RMS RMS =.707 * Peak RMS = Root Mean Square Phase Crest Factor = Peak/Average Power Crest Factor = Peak/RMS = 1.41 34
Reactive Power Imaginary Axis Power Quality Measurements Definitions Power Types in Pure Sine Wave Circuits True power (Watts, W) Transfers energy to the load Only true power is dissipated in a pure resistive load Real axis Apparent Power True Power Real Axis Reactive power (Volt-Amperes reactive, VAR) No net energy transferred to the load Power is stored in power supply inductive & capacitive components Power flows back & forth between the AC line & the power supply Imaginary axis Apparent power (Volt-Amperes, VA) Vector sum of true & reactive power 35
Power Quality Measurements Distorted AC Input Voltage & Current Sine Waves Current Volts 36
Power Quality Measurements AC Input Voltage & Current 37
Harmonics Measurements Current Harmonics Power distributed among line frequency harmonics Harmonics can feed back into the line, causing overheating Indicator of power quality A requirement in many standards Total Harmonic Distortion (THD) The cumulative value of distortion contained in the harmonics of the fundamental line frequency 38
Harmonics Measurements Graph AC Input 39
Harmonics Measurements Graph AC Input 40
Harmonics Measurements Table AC Input 41
Harmonics Pre-Compliance Testing EN61000-3-2 & MIL-STD-1399 Pass/Fail 42
Pulse Period Trend Analysis White line shows as amplitude the value variations of the different pulse periode Characterisation of the regulation function behavior Automated analysis of the displayed waveform sections
Magnetic Characteristics Measurements Measure the voltage across a magnetic component of the SMPS Measure the Current through a magnetic component Run magnetic property measurement prior set up of physical parameter of transformer 44
Measurement challenges Connecting probes to the magnetic device in the SMPS Accurately measuring and capturing voltage and current waveforms Accurately capturing high dv/dt and di/dt Analyzing data and generating plots and reports 45
Inductance Basics Inductance is defined as: L Vdt I Where: L is the inductance (Henry). V is the voltage across the inductor. I is the current though the inductor. dt is the rate of change in a signal; the slew rate. 46
Inductance Measurements As simple as probing the voltage across, and the current through the magnetic component 47 Tektronix DPO7000 Oscilloscope with DPOPWR
Magnetic Power Loss Basics Magnetic Power Loss = Core Loss + Copper Loss Core Loss includes hysteresis loss and eddy current loss Copper Loss is due to resistance of the copper winding wire Important to know different power loss components to identify root cause Measure Total Magnetic Loss Derive Core Loss from vendor s data sheet Solve for Copper Loss Multiple-winding inductor: TotalPowerLoss PowerLoss L1 PowerLoss L2 PowerLoss L3 48
Hysteresis Curve and Magnetic Properties Measurements Just measure voltage and magnetizing current Software calculates the Maximum Magnetic Flux Density, Remanence Flux Density, Permeability, Coercive Force 49
Summary Switch-mode power supply verification & debug Reliable product operation Regulatory compliance Faster time to market products with Automated power measurements Quick, repeatable measurement analysis Comprehensive set of power measurement tools DPO & MSO Series real time Oscilloscopes Power Analysis modules Probes Calibration tools 50
References: Power Measurement Information www.tek.com/power Primer & application notes Power measurement poster Manuals 51 Power Measurements for Switch-Mode Power Supplies
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