Measurement and Analysis for Switchmode Power Design

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Buck Peters
5 years ago
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1 Measurement and Analysis for Switchmode Power Design

2 Switched Mode Power Supply Measurements AC Input Power measurements Safe operating area Harmonics and compliance Efficiency Switching Transistor Losses Measurement challenges Transformer B-H curve Dynamic Control Loop Step load and start up behavior Output Ripple

3 AC Input Line Power and Harmonics AC In + + DC Out PWM Controller Feedback

4 Line Voltage Line Current Line Power RMS line voltage, RMS line current, real power, apparent power, power factor and crest factor

5 A

8 Line Harmonic Analysis Line harmonics can be measured against compliance standards like EN

9 Power supply efficiency measurement

10 Safe Operating Area Mask Testing

12 Switched-Mode Power Supply AC In + + DC Out DC AC PWM Controller Feedback The measurements we will talk about here are useful for any inverter based power conversion device

13 Energy Loss Loss displayed in Joules

14 Power Loss Loss displayed in Watts Power = Energy / Time

15 Conduction Loss Measurement Challenge Although the peak to peak waveform may be hundreds of volts, during the conduction stage the voltage is close to zero. Measuring the conduction loss or dynamic on resistance is a challenge due to the limited dynamic range of the oscilloscope

Current Saturation Voltage Conduction Region Conduction Region Parameter Gate ParameterGate Power Conduction Conduction Generally it is not possible to measure total

16 Current Saturation Voltage Conduction Region Conduction Region Parameter Gate ParameterGate Power Conduction Conduction Generally it is not possible to measure total cycle loss in the same waveform capture. Measurement challenge: Why will this setup produce an incorrect switching loss measurement for the complete switching cycle?

17 The Challenge of Power Measurement Upper V GS measurement required between point A and B +175 Volts A B Output? Volts

18 A Minus B Method Both A & B must be on screen. This determines the maximum sensitivity the oscilloscope can be set at. Limited channel accuracy matching severely limits the ability to Reject (Subtract) the signal that is Common to both A & B. Line Voltage 1 W Shunt Load Circuit Scope remains safely grounded Channel A - Channel B is not adequate when: The Common Mode Voltage >> Voltage Being Measured Because oscilloscope amplifiers and passive probes are not precisely matched for higher frequency gain (or attenuation), CMRR above a few khz will be very low.

19 Ground Referenced Need for Differential Ground referenced measurements upset by alternate ground path low amplitude signals Currents in the ground distribution system result in Ground is not ground syndrome Common problem when using coaxial shunts Noise in the system can be >> signal of interest Moving the ground ground closer to the device will result in an alternate ground path and large circulating currents in the probe ground lead V+ V+ + I I v - I

20 Floating the Scope Floating the scope can result in a shock hazard, damage to the DUT, damage to the scope and poor measurement accuracy. If the reference voltage connected to the probe ground lead is high enough, conductive surfaces of the scope, including the ground leads of other probes can become shock hazards If an insulation fault were to occur in the primary wiring of the scope, it could raise the front panel of the scope to line potential. If the reference voltage is large, there is a possibility of damaging components in the scope power supply or power line filter from the dielectric breakdown. This type of damage may not show up immediately, but appear later even after the scope is returned to grounded operation. The capacitance between the scope case and earth ground can result in damage to the device under test when the probe ground lead loads the circuit with the full capacitance between the scope case and ground. The scope ground is a discharge path for radiated EMI, and with the scope ground severed, radiated EMI from the scope may intefere with the measurement. Measurement corruption can occur when the ground lead inductance acts on the capacitance to ground to create a series LC resonant circuit which resonates with the common mode signal. Because this capacitance is much larger than the scope input C, the resonant frequency is much lower. Thus, when measured with a floating scope, the waveform of a typical upper gate drive signal often has a significant ring which is really not present in the true signal. This measurement distortion is large enough to make many routine measurements, such as upper gate drive, virtually impossible.

21 Using Differential Amplifiers CMRR 100,000:1 Overdrive recovery <100 ns Differential Amplifier Line voltage at 400 V full scale (with 8-bit resolution) Line voltage waveform crest at 2 V/div (with 8-bit resolution) Differential Amplifier connected to oscilloscope

200 mv/div To measure a device s saturation voltage to 100 mv accuracy when the off voltage is 400 V requires 250 ppm

22 Transition Losses and Conduction Loss Due to the wide dynamic range of voltage, different vertical sensitivities are needed to accurately resolve conduction loss and turn-on/turn-off loss Voltage waveform at 50 V/Div Voltage waveform at 200 mv/div To measure a device s saturation voltage to 100 mv accuracy when the off voltage is 400 V requires 250 ppm measurement capability The signal conditioning must have linear performance far better than required by an 8 bit digital oscilloscope

23 Solution 1: Overdriving the Signal Differential probe response is very slow to stabilize, and never reaches the correct saturation voltage level Differential amplifier response rapidly stabilizes and reaches the correct saturation voltage level Differential Amplifier Differential Probe

24 Using Differential Amplifier for Saturation Measurements CMRR 100,000:1 Overdrive recovery 400 V to 100 mv <100 ns Precision Offset Generator 0.5% DA1855A Differential Amplifier Differential Amplifier connected to oscilloscope

27 Using High Definition Oscilloscopes 12-Bit Capture 8-Bit Capture

28 Using High Definition Scope with High Accuracy Probes 12-Bit Capture, 1% accuracy Probe 12-Bit Capture, Standard Probe

probes with high accuracy and high CMRR.

29 Example Hardware Configuration Voltage and current probes to match the accuracy of HDO scopes High voltage differential probes with high accuracy and high CMRR. Current probes offer high accuracy and low noise. CP030A and CP031A HVD3102 and HVD3106

30 Rds On Resistance Measurement Overdrive recovery of differential amplifier and high resolution oscilloscope combination

31 Eliminating Sources of Error DC Offsets, Deskew Before making detailed device loss measurements, fine adjust to eliminate DC offset errors and scope probe propagation delay differences

32 Two Ways to Fine Adjust Current Probe DC Offset During Off-state, utilize Math integral function and adjust for zero slope Utilize Power Analyzer s automatic calculation of Off-State Losses and fine adjust to zero

remove propagation delay differences between voltage and

33 Deskewing Voltage and Current Probes Use a deskew calibration source, with V and I coincident edges, to remove propagation delay differences between voltage and current probes Line up the knee of the curve to deskew for power measurement

measurement error Device turn-off transition loss, V x I, is

34 Sources of Error Skew Between Voltage and Current Probes Timing skew between voltage and current probes results in measurement error Device turn-off transition loss, V x I, is properly measured at 7.88 uj of energy versus uj without proper deskew

35 Switched-Mode Power Supply AC In + + DC Out PWM Controller Feedback The transformer provides isolation between the power supply input and output

36 Power Analyzer BH Curve

37 Power Analyzer BH Curve

39 BH Curve Definition

40 Voltage Current B= V(t)dt H = ni l B-H Curve shows the hysteresis loop for the magnetic material in inductors and transformers Coil Characteristics Input: # of windings Cross sectional area Magnetic path length Cursor are used to measure magnetic field strength, H, and magnetic flux density, B H is calculated from the current, # windings and magnetic path length B is calculated as the integral of the voltage across the coil Parameter math is utilized for calculation of the magnetic permeability of the material B and H constants are individually entered and the resulting parameter is calculated as B/H

41 Control Loop Measurements AC In + + DC Out PWM Controller Isolated Feedback

Cycle 1 Period Cycle 2 Period Cycle 3 Period Cycle 4 Period Cycle 5 Period Cycle 6 Period Cycle 7 Period Cycle 8 Period Cycle 9 Period Voltage 2.001 ns 2.004 ns 1.991 ns 2.001 ns 1.999 ns 1.

43 Cycle 1 Period Cycle 2 Period Cycle 3 Period Cycle 4 Period Cycle 5 Period Cycle 6 Period Cycle 7 Period Cycle 8 Period Cycle 9 Period Voltage ns ns ns ns ns ns ns ns ns Time Period ns ns ns ns ns ns ns ns ns Time Parameter Track can be used to determine power supply modulation

44 Pulse width begins to decrease Load disconnected Track function plots changing pulse width Settling time

49 Control Loop Measurements AC In + + DC Out PWM Controller Isolated Feedback

50 Power supply ripple measurement

51 Radiated Immunity Testing

Radiated Immunity Testing - Real Time Functional Performance Evaluation Deviation detection of a device under test (DUT) during exposure to a disturbance Functional state of the DUT is output through

52 Radiated Immunity Testing - Real Time Functional Performance Evaluation Deviation detection of a device under test (DUT) during exposure to a disturbance Functional state of the DUT is output through non-conductive fiber optic cables Mechanical mode tuner Devices under test are exposed to electric fields high enough to effect operation of nonshielded equipment. Transmit and receive antennas generate a controlled electric field RF-hardened fiber optic transmitters

test criteria such as signal high level, signal low level, frequency, duty

53 Outside the reverberant chamber, oscilloscope masks test for acceptance criteria Optical receiver and O/E converter 16 channels performing mask test criteria such as signal high level, signal low level, frequency, duty cycle, and other criteria fit within tolerance limits described in the test plan

54 High Voltage Fiber Optically-isolated Probe? Amplifier/Modulating Transmitter A frequency modulating optical transmitter is used for signal and data transmission across a fiber optic cable. Attenuating Tip Accessories Available in a variety of voltage ranges, e.g., +/-1V, +/-5V, +/-20V and +/-40V with a simplified pin socket termination Fiber Optic Cable A standard 1m length cable is provided, but longer ones may be purchased for use. De-modulating Receiver The optical signal is received and de-modulated to an electrical output to the oscilloscope with correct voltage scaling.

55 High Voltage Active Single-ended (Fiber Optic) Probes Parameter Bandwidth Voltage Range (SE) Voltage Range (CM) Loading Attenuation CMRR Value 60 MHz 2 to 80V Virtually Unlimited 1-10MΩ 34-22pF Z IN =50kΩ@100 khz 2x to 80x >140 db A new topology specifically for measuring small signals floating on a HV DC bus

Switched Mode Power Supply Measurements

Power Analysis 1 Switched Mode Power Supply Measurements AC Input Power measurements Safe operating area Harmonics and compliance Efficiency Switching Transistor Losses Measurement challenges Transformer