LeCroy PMA1 User manual

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Copyright © June 1999, LeCroy. All rights reserved. Information in this publication supersedes all earlier versions. Specifications
subject to change.

LeCroy and ProBus are registered trademarks of LeCroy Corporation.

Analog Persistence and PowerMeasure are trademarks of LeCroy Corporation.

PMA1-UG-E Rev A 0699

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The Tools and the Work They Do........................................................ 1-1

Equipment Required ............................................................................ 1-2

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Matching the Time Delay in Your Measurements System ................ 2-1

Checking Channel-to-Channel Time Skew............................................. 2-2

Initial Setup........................................................................................... 2-2

Deskew Setup (Voltage Channel).......................................................... 2-3

Deskew Setup (Current Channel).......................................................... 2-4

Channel-to-Channel Propagation Delay (Skew) Matching ....................... 2-5

Clearing the Deskew Setup .................................................................. 2-7

Setup and Configuring for Power Device Analysis........................... 2-8

Configure for Device Measurements...................................................... 2-9

Preliminary Trigger Setup..................................................................... 2-9

Initial Setup (Optional Event Trigger)................................................... 2-10

Initial Setup (Main Trigger) ................................................................. 2-11

Setup for Power Device Analysis...................................................... 2-12

Power Device Analysis Setup...................................................................2-12

Power Device Analysis Setup (Deskew Value Check)..............................2-15

Power Device Analysis Measurements ............................................ 2-16

Instantaneous Power Measurement – Steady State................................. 2-17

Instantaneous Power Measurement – Event Triggered............................2-18

Safe Operating Area Measurement – Steady State..................................2-19

Safe Operating Area Measurement – Event Triggered.............................2-20

Saturation Voltage and Dynamic On-Resistance Measurement...............2-21

Fine DC Level Adjustment........................................................................2-22

Effects of Probe Compensation on Saturation Voltage Measurements..... 2-23

Clearing the Power Device Analysis Setup........................................... 2-25

Measuring Device dv/dt ..................................................................... 2-26

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How Modulation Analysis Works........................................................ 3-1

The Modulation Analysis Display........................................................ 3-3

Configuring for Modulation Analysis.................................................. 3-4

Setup for Modulation Analysis Measurements.................................. 3-5

Modulated Signal and Trigger Setup................................................. 3-5

Trigger Setup – The Event Trigger.................................................... 3-6

Initial Setup – The Modulated Signal................................................. 3-7

Finishing the Setup and Making Modulation Measurements ........... 3-8

Activating the Modulation Analysis Menu.......................................... 3-8

Modulation Analysis Controls............................................................ 3-9

Optimizing the Display .................................................................... 3-10

Clearing the Modulation Analysis Setup.......................................... 3-11

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Line Power Analysis Overview............................................................ 4-1

Configuring for Line Power Analysis.................................................. 4-2

Initial Setup – The Line Voltage and Current Signals............................... 4-3

Finishing the Setup and Making Line Power Measurements ........... 4-4

Activating the Line Power Analysis Menu............................................... 4-4

Selecting the Voltage and Current Channels.......................................... 4-5

Line Power Measurement..................................................................... 4-6

Line Harmonics Measurement .............................................................. 4-7

The Measure Harmonics Displays......................................................... 4-8

Clearing the Line Power Analysis Setup............................................ 4-9

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PMA1 Menu Overview.......................................................................... 5-1

Units .................................................................................................. 5-1

Scaling ............................................................................................... 5-2

Current Input Setup Menus................................................................... 5-3

Voltage Input Setup Menus .................................................................. 5-5

Clearing the Channel Assignments and DC Offsets ......................... 5-6

Chapter 1 - The PowerMeasure PMA1 Software

The Tools and the Work They Do

The PowerMeasure Analysis (PMA1) software package  when combined with a
LeCroy digital storage oscilloscope (DSO), and current and differential
measurement tools  provides a complete set of hardware and software tools
for the design and analysis of power conversion circuits. PMA1 consists of three
major measurement areas:

Power Device Analysis: This section covers measurements made on power

switching devices used in power conversion products such as power
supplies, electronic motor drives (adjustable-speed drives), or high­efficiency lighting circuits. Measurements covered include device
instantaneous power, safe operating area (SOA), saturation voltage, and
dynamic on-resistance. With PowerMeasure Analysis, these device
measurements can be made either on a test stand or while the devices
are operating in a power conversion circuit.

Modulation Analysis: This section covers the acquisition and analysis of

information contained in a power conversion circuit’s modulated control
signal. It facilitates the analysis of modulation changes in pulse width
(PWM), duty cycle, frequency, or period as the circuit responds to change
in line and/or load, as well as during start-up and shut-down.

Line Power Analysis: This section covers the measurement of line voltage and

current applied to an offline power conversion device. Real power,
apparent power, power factor, and line harmonics are measured.
Analysis of line harmonic content is covered to assist the design and
evaluation engineer in designing for pre-compliance to EN61000-3-2
requirements.

1-1

Equipment Required

DSO: PowerMeasure Analysis software operates on any LeCroy LC, 9300C, or

LT (

Waverunner

acquisition of many cycles, a DSO with a minimum of 1 mpoint per channel
should be used. Steady state analysis can be accomplished with shorter record
length. Two acquisition channels are adequate for most measurements, but a
four-channel DSO is required if analysis of multiple devices or complex triggering
is desired.

Voltage Probes: Since most voltage signals associated with power conversion
circuits are not ground related, differential measurement capability is required. To
carry out all the measurements covered in the Power Device Analysis section,
the CMRR, CMR, Overdrive Recovery, and compensation flatness performance
of the LeCroy DA1855A differential amplifier and its associated DXC series
passive differential probes is required. The measurement requirement in the
Modulation and Line Power Analysis sections can be made with medium­performance differential probes.

Current Probes: The Power Device Analysis section requires precision wide­bandwidth current probes with DC measurement capability. The LeCroy AP015
DC to 50 MHz current probe is recommended.

) Series DSO. For an alysis of phenomenon requiring the

Deskew Signal Source: A source of time-coincident voltage and current signals
is required to allow the propagation delay differences in the voltage and current
signals to be matched. The LeCroy DCS015 Deskew Calibration Source is
provided for this purpose.

Note: The time delay of a DA1855/ DXC100 (A or non-A versions) connected to

the DSO with a 1.2 m 50 coaxial cable is the same as the AP015 Current Probe’s
delay, and no deskew adjustment is required.

1-2

Chapter 2 – Using Power Device Analysis

Matching the Time Delay in Your Measurements System

Because the signals associated with power devices are relatively fast, it is important to
determine whether the time delay for the current and voltage signal paths are the
same. Signal delay characteristics of the voltage and current probes as well as the
distance the signals must travel from the probe tips to the input of the DSO can cause
time-coincident points on the voltage and current signals to be sampled by the DSO at
different times. A small time difference can cause significant errors to occur in the
measurements.

Skew Error

Skew Error Corrected

Voltage

Current

Instantaneous

Power

Figure 2.1: Significant error occurs in an instantaneous power measurement when the current signal takes

longer than the voltage signal to get to the DSO, because the current’s signal path is longer. This error can be
corrected by matching the delay of the voltage and current signal paths using the deskew function.

It is advisable to use the deskew function to check and match, if necessary, the time
delay of the current and voltage channels. This is very important if these signals are
going to be used to make Instantaneous Power, Safe Operating Area, or Dynamic On­Resistance measurements.

2-1

Checking Channel-to-Channel Time Skew

Select which DSO channels are to be used as the
voltage and current inputs, and connect the
voltage and current probes to those channels.
Connect the DCS015 Deskew Calibration Source
to the DSO’s calibrator output or one of the

unused channels.
Connect the voltage and current probes to

the DCS015 as shown. Be sure to observe
polarity on the current and voltage signals.

MINUS
VOLTAGE
PROBE

PLUS
VOLTAGE
PROBE

INITIAL SETUP

LeCroy

DCS015

CURRENT

CURRENT
PROBE

Set the time/div, trigger delay, trigger level, voltage, and current channel coupling to
obtain the display as shown above. It is important to trigger on the negative slope of
the voltage waveform. Position both traces around the center screen, and set the
trigger level and delay as shown. Note: The choice of which channel to use for current
and voltage is arbitrary. For consistency, all examples use Channel 2 for voltage and
Channel 3 for current.

2-2

Deskew Setup (Voltage Channel)

/

Once the DCS015’s voltage and current waveforms are properly displayed, use the
PMA1 software to match the time delay in the voltage and current channels.

Press the MEASURE TOOLS (LT Series) or CURSORS/MEASURE (LC Series)
button on the DSO front panel to bring up the

Power Measurement

selection.

MEASURE

menu, which includes the

MEASURE

TOOLS

CURSORS

MEASURE

Follow the menu sequence given below:

Change the horizontal time/division to 10 nsec/div.

The display should appear similar to this.

2-3

Deskew Setup (Current Channel)

After the voltage channel is properly set up, follow the menu sequence given below to
set up the current channel:

Display should appear similar to this.

The channel selected as the

Current Input

channel will be assigned Ampere units

even if a voltage or nonProBus-compatible current probe is used.

2-4

Channel-to-Channel Propagation Delay (Skew) Matching

After the voltage and current channels are properly set up, follow the menu sequence
given below:

Skew Error

The display should appear similar to this.

The signal time skew will be shown as delay difference between the voltage and

current waveform. The original current waveform is replace by a “resampled” math
waveform that can be delayed up to ± 2 msec. In this example the
delay difference (time skew) is a little more than one 10 nsec per division. The amount
of delay will depend on which voltage and current probes are used, as well as the
length of the probes and the coaxial cable used to connect the DA1855A to the DSO.

When the DA1855A Differential Amplifier (connected with a 1.2 m coaxial cable),
DXC100A Differential Passive Probe Pair, and AP015 Current Probe are used,
the time delay is matched and no deskew adjustment is needed.

2-5

Channel-to-Channel Propagation Delay (Skew) Matching - continued

Alignment point

Adjust the deskew value until the current waveform coincides with the voltage

waveform. If the current waveform’s fall time is slower than that of the voltage, align
the beginning of the wavefroms (alignment point).

Turn the until the alignment points on the current and voltage waveforms
coincide.

Pressing the will return the

Deskew Value

to zero.

The time delays of these two channels are now matched, and they can be used to
make accurate measurements that require precise time alignment of the current and
voltage waveforms. This deskew value is only valid for this particular setup. Changing
probes or bandwidth for either the current or voltage channel requires the channel’s
time-delay difference be checked and possibly corrected.

This process can be used to characterize and correct the delay difference between
more than one current and one voltage channel. For instance, if the user plans to use
one voltage channel and alternate measurements between two current channels, the
relationship between the voltage channel and each of the current channels can be
characterized. The amount of deskew required for each combination should be
recorded for later use.

Proceed directly to the power device measurement section by pressing the RETURN
(See page 2-8.) If the Power Measurement Application is not going to be used
immediately, it should be turned off by following the instructions on the next page.

2-6

Clearing the Deskew Setup

If no immediate use of the PMA1 software is planned, the channel assignments and
other alterations made during the deskew process should be cleared.

Press the RETURN until the on-screen menu is cleared. Then press the

MEASURE/TOOLS to bring up the

MEASURE

menu.

Press

RETURN

Press

MEASURE

TOOLS

Selecting
and

Application OFF

Current Input

in the Power Measure menu changes the

assignments to

NONE

. The

Deskew Value

The assignment of Ampere units to the channel selected as the
channel will be removed.

Voltage Input

remains unchange d.

Current Input

2-7

Setup and Configuring for Power Device Analysis

g

The Power Device Analysis portion of PMA1 lets the user make difficult device
measurements on the devices while they operate in circuit. The exact setup for each
measurement will differ depending on what device type is to be analyzed and where it
is located in the circuit. It will make the setup easier if the measuremen ts are planned
and set up in advance. If possible, obtain representative waveforms of the voltage,
current, and trigger inputs before evoking PMA1 software.

The following diagram shows a typical setup used to analyze the power FET in an off­line switching power supply. A differential amplifier is used to acquire the voltage
across the device, and a current probe is used to acquire the current through it.

Current

Probe

Load

Change

trigger point

CONTROL
CIRCUIT

I

D

+

V

DS

-

Figure 2.2: Typical connections to a circuit under test required to make instantaneous power, safe operatin

area, saturation voltage, and dynamic on-resistance measurements.

The circuit shown is an off-line flyback power supply. Examples in this section are
based on similar connections to a circuit of this type. Measurements also can be made
on devices such as power transistors, snubber diodes, or similar devices in other
topologies.

2-8

Configure for Device Measurements

Plan the measurement in advance and do a preliminary setup before evoking the
PMA1 Power Measurement menus. This will minimize the numbe r of time s it is
necessary to leave the menu structure.

Select which DSO channels are to be used as voltage and current inputs. Follow the
procedure contained in the

Matching the Time Delay in Your Measurement System

section to correct any delay difference between the current and voltage.
Connect the voltage and current probes to the appropriate points in the circuit under

test. Figure 2.2 can be used as a guide. If measurements are to be made on the
device as a function of an event such as load change or start up, select a signal to be
used as a trigger for this event. The example in Figure 2.2 uses the DSO’s Channel 4

to acquire a trigger signal indicating when the load changes from maximum to
minimum. The DSO’s EXT trigger input also could be used.

Preliminary Trigger Setup

Before entering the Power Measurement software menu, it is important to determine
the source and setup of the triggers. Identify the signal on which the main
measurement will be triggered, as well as the signal on which the acqu isition of an
extended measurement record is to be triggered. The main trigger can be the device’s
voltage or current signal while the event trigger is usually associated with load change
or turn-on and turn-off. Establish the event trigger first (if required), then set up the
main trigger. Choose the desired trigger signal and establish a stable display.

Turn on “EVENT” trigger point

Load Change “EVENT” trigger point

CONTROL
CIRCUIT

Figure 2.3: Typical connections to circuit under test for MAIN and EVENT triggers.

2-9

VDS “MAIN” trigger point

INITIAL SETUP (Optional Event Trigger)

Determine the event around which a change in the operation requires recording an
extended record of information. Examples of such an event include the change from
maximum to minimum load, and start up. Presetting the trigger of such an event will
make the final power measurement setup easier.

In this example, Channel 4 is used to acquire a signal that indicates the power

supply’s load changes from maximum to minimum. Set up the trigger so the
acquisition of a record can be initiated from this event. The Load Change “EVENT”
trigger shown in Figure 2.3 was used in this example. The event you want to trigger on
may be different.

2-10

INITIAL SETUP (Main trigger)

The following example uses the power transistor’s drain-to-source voltage as a trigger
source. This is the same signal that will be used to measure the device’s
instantaneous power loss or safe operating area performance.

V/div and
time/div
settings will
depend on the
particular
circuit under
test.

Set the time/div, trigger delay, trigger level, and voltage channel coupling to obtain a
display similar to that shown above. It is usually desirable to trigger on the negative
slope of the voltage waveform.

2-11

Setup for Power Device Analysis

/

After the measurement is planned, the voltage and current channels are identified and
deskewed, and preliminary triggering is established, use the PMA1 software to finish
the setup and make the measurements.

Power Device Analysis Setup

Press the MEASURE TOOLS (LT Series) or CURSORS/MEASURE (LC Series)
button on the DSO front panel to bring up the

Power Measurement

selection.

MEASURE

menu, which includes the

MEASURE

TOOLS

CURSORS

MEASURE

Follow the menu sequence given below:

2-12

Power Device Analysis Setup - continued

In the

Setup List

menu, select the channel that is connected to the voltage
measurement point. In the example, it is Channel 2. Adjust the volts/div and time/div to
obtain a stable display of at least one cycle of the voltage signal across the device.

The display should appear similar to this.

It is important to set the V/div to allow the largest expected voltage excursion to
remain on screen during the all conditions of the test. If the voltage signal goes off
screen during the test, erroneous results will be obtained.

It is good practice to balance the DC offset of the voltage probe at this step. Press the

Autozero

to autobalance the DA1855A amplifier.

2-13

Power Device Analysis Setup - continued

In the

Setup List

menu, select

Current Input

. Select the channel that is connected to
the current measurement point. In the example, it is Channel 3. Adjust the A/div and
time/div to obtain a stable display of at least one cycle of the current signal through the
device.

The display should appear similar to this.

The channel selected as the

Current Input

channel will be assigned Ampere units

even if a voltage or nonProBus-compatible current probe is used.
It is important to set the A/div to allow the largest expected current excursion to remain

on screen during the all conditions of the test. If the current signal goes off screen
during the test, erroneous results will be obtained.

It is good practice to degauss the current probe at this step. Remove the current probe
from the circuit under test, close the probe and press the

Degauss

. Return the

current probe to the circuit under test.

2-14

Power Device Analysis Setup (Deskew Value Check)

After the voltage and current channels are assigned, the
value of time delay correction required between the two
can be checked by selecting

Setup List

menu. If the deskew value was set using the

process on page 2-5, that valu e will be displayed in the

Deskew Value

menu readout.

The deskew value can be changed in this menu by
turning the . Pressing the will return the value to
zero.

NOTE: The deskew value should not be changed in this
menu unless the correct value is known. Voltage and
current signals that occur in the circuit under test may not
be properly phase-related, and using them for deskew
purposes can cause major errors to occur in the
measurements. Time-coincident voltage and current
signals such as those provide by the DCS015 Deskew
Calibration Source should be used.

Time Deskew

from th e

2-15

Power Device Analysis Measurements

Note: The previous sections should be completed before making these measurements.

After the channel-to-channel propagation delay is matched and the previous
measurement setup is completed, measurements on the device under test can
proceed.

Press the RETURN to bring up the following menu:

Select
loss in the power FET.

Select

Operating Area.
Select

saturation voltage and its dynamic on-resistance.

The

Level

voltage and current probes without removing the voltage
or current probe from the circuit. Pressing either
returns the respective offset to zero.

Instant Pwr

Safe Op Area

Dynamic Res.

Fine Adjust V DC Level

knobs corrects small DC offset errors in the

to measure the instantaneous power

to measure the power FET’s Safe

to measure the power FET’s

and

Fine Adjust I DC

NOTE: The

Measurement

Dynamic Res.

assigned in the
return to the
them.

Setup

menu selections

will not be available unless both voltage and current channels were

Setup

menu. Press the

menu to assign the voltage and current channels or to change

Instantaneous Pwr, Safe Op Area,

Setup

in the Power Device menu to

2-16

and

Instantaneous Power Measurement – Steady State

Note: The previous sections should be completed before making these measurements.

Selecting

Instant Pwr.

in the

Measurement

menu brings up the following display. The
deskewed current waveform (A) is displayed in the first grid, and the voltage waveform
(2) is displayed in the second.

The instantaneous power waveform (B) is displayed in the third grid. Use the DSO’s

ZOOM + MATH position and zoom controls to obtain the desired display.
The fourth grid (C) displays a copy of the instantaneous power waveform (B) and is

used to expand the waveform B so individual cycles of a multiple cycle record can be
viewed.

Fine Adjust V DC Level

and

Fine Adjust I DC Level

are provided to allow the user to
compensate for any residual DC offset that the voltage and current probes may
exhibit. Because the magnitude of residual DC offset error is usually unknown, use of
these adjustments requires user knowledge of the waveforms.

Turn the to adjust the DC Offset Level to the desired level. Pressing the will
return the offset to zero.

For a detailed explanation on how to adjust the DC Offset Level, see page 2-22.

2-17

Instantaneous Power Measurement – Event Triggered

Note: The previous sections should be completed before making these measurements.

To analyze the instantaneous power of the power device during transitions such as

turn-on and load change, trigger the acquisition on an “EVENT” trigger.

The above is a 10 msec window of a power FET’s drain-source voltage, drain current
(deskewed), and instantaneous power dissipation that occurs during the circuit’s
transition from maximum to minimum load. The ZOOM of the instantaneous power
waveform (B) is used to examine the instantaneous power dissipation during one
cycle in trace (C). In this case, the acquisition of the 10 msec record was triggered by
the “EVENT” trigger previously set up on Channel 4.

2-18

Safe Operating Area Measurement – Steady State

Note: The previous sections should be completed before making these measurements.

Selecting

Safe Op Area

in the

Measurement

menu brings up the following display.
The voltage waveform (2) is displayed in the first grid and the deskewed current
waveform (A) is displayed in the second grid. In the XY plot, voltage points are plotted
on the horizontal axis, while current is plotted on the vertical. The delay difference
between the voltage and current samples have been removed by the deskew function.

Fine Adjust V DC Level

and

Fine Adjust I DC Level

are provided to allow the user to
compensate for any residual DC offset that the voltage and current probes may
exhibit. Because the magnitude of residual DC offset error is usually unknown, use of
these adjustments requires user knowledge of the waveforms.

Turn the to adjust the DC Offset Level to the desired level. Pressing the
will return the offset to zero.

For a detailed explanation on how to adjust the DC Offset Level, see page 2-22.

2-19

Safe Operating Area Measurement – Event Triggered

Note: The previous sections should be completed before making these measurements.

To analyze the safe operating area performance of the power device during a

transition such as turn on or load transition, trigger the acquisition on an “EVENT”
trigger.

The above is a 10 msec window of a Power FET’s drain-source voltage, drain current
(deskewed), and safe operating area measurement that occurs during the circuits
transition from maximum to minimum load. In this case, the acquisition of the 10 msec
record was triggered by the “EVENT” trigger previously set up on Channel 4.

2-20

Saturation Voltage and Dynamic On-resistance Measurement

Note: The previous sections should be completed before making these measurements.

Selecting

Dynamic Res.

in the

Measurement

menu brings up the following display.
The deskewed current waveform (A) is displayed in the first grid and the voltage
waveform (2) is displayed in the second grid. The delay difference between the
voltage and current samples have been removed by the deskew function. By changing
the DSO’s V/div setting and adjusting the trigger level, the voltage waveform displayed

is the device saturation voltage. [Note: A DA1855(A) differential amplifier and
DXC100A is required to make this measurement.] The voltage waveform is divided by
the deskewed current waveform, and the resulting resistance waveform is displayed in
the third grid (B). Trace (C) is a ZOOM of the resistance trace (B) to be used to
examine detail.

Because the device saturation voltage waveform (B) is off screen during the non­saturation portion of the waveform, the math-generated resistance waveform should
be ignored during this time.

Fine Adjust V DC Level

and

Fine Adjust I DC Level

are provided to allow the user to
compensate for any residual DC offset that the voltage and current probes may
exhibit. Because the magnitude of residual DC offset error is usually unknown, use of
these adjustments requires user knowledge of the waveforms.

Turn the to adjust the DC Offset Level to the desired level. Pressing the will
return the offset to zero.

For a detailed explanation on how to adjust the DC Offset Level, see page 2-22.

2-21

Fine DC Level Adjustment

Measurement errors caused by channel-to-channel time-delay differences can be

corrected using PMA1’s deskew capability. Another major cause of errors is DC offset
in the measurement equipment. Minor DC offset errors in the current or voltage
channel can cause major errors in power calculations. Because it is not possible to
design amplifiers and current probes with zero DC offset, PMA1 provides fine DC
offset adjustments to correct these errors in both the current and voltage channels.

To use these adjustments it is necessary to know where zero is on a current or
voltage waveform. The following illustrates how to compensate for residual DC offset
in a current probe when the zero current point on the waveform is known.

Zero Current Point

To set the

Fine Adjust I DC Level:

DC Offset Error

Zero Current

Indicator

Turn the until the zero current point
aligns with the zero current indicator.

Pressing the will return the offset to its
pre-adjusted state.

Adjustment of the

Fine Adjust DC V Level

operates in the same fashion. Placing the
voltage plus and minus differential probes on the same point in the circuit will provide
a zero voltage reference point for the adjustment.

NOTE: These adjustments remain in effect until

Application Off

is pushed (see
page 2-25). Failure to turn this function off after using PMA1 can cause errors in
subsequent measurements.

2-22

Effects of Probe Compensation on Saturation Voltage Measurements

To measure a switching device’s saturation voltage while the device is operating in
circuit requires the combination of several capabilities in the measurement system.
First, because the measurements are not ground referenced, differential voltage
measurement is needed. The amplifier also must be able to quickly recover from
overdrive, and the amplifier as well as the probes must have low high-frequency
aberrations.

An example:

To measure the saturation

V

OFF

voltage of a device to
100 mV accuracy when the
off voltage is 400 V requires

V

DS

Saturation
Voltage

250 ppm meaurement
capability.

It is obvious that the DSO
input or an input preamplifier

0 V

such as the DA1855A needs
to recover and settle to a
value greater than 250 ppm
before the measurement can
be made. The DA1855A is
designed with this capability.

From this example, the probe’s high-frequency performance required for this
measurement is clear.

Less obvious is how the probe’s LF compensation adjustment can have a large effect
on the accuracy of device saturation voltage measurement. Most DSO users are
familiar with the requirement of adjusting passive probes for low-frequency
compensation. Under normal usage, the entire waveform is on-screen when a passive
voltage probe’s low-frequency compensation is adjusted. A low-frequency
compensation adjustment made with the entire waveform on-screen is usually
adequate for most measure men ts

However, when a signal’s amplitude is greatly magnified as it is during a saturation
voltage measurement, a small error in the LF compensation flatness can cause a
major error in the saturation volta ge measurement. The following figures illu strate how
this seemingly minor adjustment can make the saturation voltage’s DC level appear to
be incorrect.

2-23

Effects of Probe Compensation on Saturation Voltage Measurements ­continued

B

A

Figure A: A voltage probe appears to be properly
compensated on a 400 V squarewave when viewed
at 100 V/div.

0 V

Figure B: When viewed at 500 mV/div, the same
400 V squarewave shows the probe compenstion is
slightly peaked.

0 V

C

D

Figure C: When the horizontal time per division is
decreased to a value normally used to view 20 to
150 kHz swictchmode power conversion circuits, the
slightly peaked LF compensation appears as a DC
level shift.

0 V

Figure D: Viewing a power FET’s saturation voltage
with the slightly peaked LF compensation makes the
voltage appear to go negative. In this example, the
repetition rate of the power supply is 60 kHz.

0 V

2-24

Clearing the Power Device Analysis Setup

After using PMA1’s Power Device Analysis section, it is important to clear the channel
assignments and other alterations that were made while making power device
measurements. Press the RETURN until the onscreen menu is cleared. Then
press the MEASURE/TOOLS to bring up the

MEASURE

menu.

Press

RETURN

Press

MEASURE

TOOLS

Selecting
and
made while using the
Any

Application OFF

Current Input
Deskew Value

in the Power Measure menu changes the

assignments to

NONE

Fine Adjust V DC Level

and removes any DC offset adjustment

and

Fine Adjust I DC Level

Voltage Input

set while deskewing channel-to-channel delay remains

unchanged. The assignment of Ampere units to the channel selected as the

Input

channel will be removed.

features.

Current

2-25

Measuring Device dv/dt

The speed of a power device’s dv/dt during turn-on and turn-off can be measured
using PMA1’s derivative math function.

Note: This measurement does not require the use of the PMA1 menus.

In the example above, a power device’s Drain to Source Voltage (VDS) signal (1) is set
up to be displayed in the first grid.

Math Channel A is used as a ZOOM of the Drain to Source voltage (A), and the trace
is ZOOMed to 50 ns/div showing the voltage change as the device turns from on to
off.

In the third trace (B), math Channel B is used to average trace A to remove noise from
the VDS signal before its derivative is taken.

In the fourth trace (C), math Channel C uses the derivative math function to display
the waveforms dv/dt.

Cursors can be used to find the signal’s dv/dt at any point.

2-26

Chapter 3 – Using Modulation Analysis

How Modulation Analysis Works

Switchmode power conversion circuits use some method of transferring energy from
an unregulated source to a regulated output(s) on a cycle-by-cycle basis. Output
regulation is achieved by modulating the amount of energy transferred in each cycle.
The most common modulation method u sed is Pulse Width Modulation (PWM).

The Modulation Analysis section of PMA1 is intended to provide the user with tools to

view the information contained in the control circuit’s modulated signals. The most
common method of controlling the energy-per-cycle transfer in power conversion
circuits is through the use of PWM. Other methods (such as frequency modulation)
also are used, but no matter which method is used it is difficult to view and analyze the
modulation.

Figure 3.1:
(A) Gate drive pulse width at minimum load

(B)

Gate drive pulse width at full load

Figure 3.2: Analog Persistence display
shows pulse widths at minimum and
maximum load, as well as other pulse widths
that occur during load transition.

When operating in steady state, a power supply’s pulse width will be narrow during
periods of low load and wider when the load is higher. This difference is easy to see
on a DSO in the XY display mode. What happens to the pulse width during a change
in load or some other EVENT is much harder to see. The use of Analog Persistence
mode yields more information about the supply’s step response, but does not display
the change-in-width information as a function of elapsed time.

3-1

Modulation Analysis provides the user with a method of seeing the information

6

contained in the modulated signal. It does this by taking the time (width) information in
the modulated signal that is normally displayed on the horizontal axis along with
elapsed time and displays it on the vertical axis.

P

P

P

P

2

P

P

4

P

P

7

Gate drive pulses

(voltage vs time)

Gate drive pulses

(width vs time)

W

Figure 3.3: Example of how Modulation Analysis measures the width of individual pulses and displays their
value on the vertical axis.

W

P1

W

P2

W

P3

P4

W

W

P6

W

P5

P7

As the number of pulses increase per division, the display of their individual widths

forms a “waveform” that represents the change in pulse width as a function of elapsed
time. This “waveform” can be used to gain valuable information about the power
supply’s response to various events, such as load change (step response) or its soft
start performance.

3-2

The Modulation Analysis Display

A 20 msec record of the power FET’s gate
drive pulses is captured.

Two zoom

traces

(C & D) are

provided to

allow

individual

pulses in the

captured
record to be
examined in

detail.

W

N

W

N

The width of every
pulse is measured
and its value is
displayed on the
vertical axis.

Capture of a record is “EVENT” triggered as
the load changes from maximum to
minimum.

Figure 3.4: Modulation Analysis displays a PWM circuit’s step response as the output load changes from
maximum to minimum. In this example, over 1000 gate drive pulses are recorded in the fi rst gri d [2], and a
record of their individual widths is displayed in the second grid [B: Jwidth (2)].

3-3

Configuring for Modulation Analysis

l

The Modulation Analysis portion of PMA1 lets the user capturie and analyze

information contained in the power conversion circuit’s modulation. The exact setup
for this measurement may be different depending on the specific circuit topology and
where in the circuit under test the modulation signal is to be acquired.

The following diagram shows a typical setup used to acquire the modulated signal at
the power FET’s gate in an off-line switching power supply. The LeCroy DA1855A
Differential Amplif ier is u s ed to acquire the device’s gate drive signa l.

Load

Change

trigger point

CONTROL
CIRCUIT

I

D

+

V

GS

-

Figure 3.5: Typical connections to a circuit under test needed to acquire the power device’s gate drive signa

from which the circuit’s Pulse Width Modulation can be obtained.

The circuit shown is an off-line flyback power supply. Examples in this section are
based on connections to a circuit of this type. Other signals in the circuit can be used
to measure the modulation, but the gate drive signal is usually a good place to acquire
a relatively noise-free signal.

3-4

Setup for Modulation Analysis Measurements

Plan the measurement in advance and do a preliminary setup before evoking the
PMA1 Power Measurement menus. This will minimize the numbe r of time s it is
necessary to leave the menu structure.

Modulated Signal and Trigger Setup

Modulation analysis measurements usually are made to find the circuit’s response to
some event. Identify which signal in the circuit is to be used as a source of modulation
information (modulated signal) and a signal (EVENT) that can be used to trigger the
acquisition of the record of the modulated signal. Connect the differential probes to the
appropriate points in the circuit under test to acquire the modulated signal. An
“EVENT” such as turn-on, turn-off, line trigger, or load change can be used to trigger
the modulated signal record acquisition. In Figure 3.3, the power transistor’s gate drive
signal is used as the source of modulation information, and a load change on the
output is used as the “EVENT” Trigger.

IMPORTANT – If possible, establish a stable display of the modulated signal and
determine the source of the EVENT trigger before entering the PowerMeasure
software menu.

Turn on “EVENT” trigger point

Line voltage “EVENT” trigger

CONTROL
CIRCUIT

V

GS

Figure 3.4: Typical connections to the circuit under test for acquiring a source of the feedback modulation and
various EVENT trigger sources.

Load change “EVENT” trigger point

Modulated signal source

3-5

Trigger Setup – The Event Trigger

Determine the event around which the acquisition of an extended signal modulation
record will be required. Triggering the acquisition of the modulated signal on these

“EVENTS” can test the circuit’s response to events such as line voltage change, turn­on, turn-off, and load change. Typical trigger points are illustrated in Figure 3.4.

In the example used here, the acquisition is triggered as the power supply’s 5 V
supply load changes from maximum to minimum. Presetting the trigger of such an
event will make the final modulation measurement setup easier.

Set the time/div, trigger delay, and trigger level for the “EVENT” trigger channel to
obtain a display similar to that shown above. If the modulated signal is to be acquired
as the result of a one-time event such as turn-on, test the “EVENT” trigger for
satisfactory operation in SINGLE trigger mode.

In this example, Channel 3 is used to acquire the load change signal, and the DSO is
set up to trigger from this channel. Other channels or the DSO’s EXT trigger input also
could be used for this purpose.

3-6

Initial Setup – The Modulated Signal

Set up a stable display of the signal that will be used as the source of the modulation

information. Ensure that a clean signal can be acquired that will allow the signal’s
width (or other characteristic) to be readily measured. The following example uses the
power transistor’s gate-to-source voltage as a modulation signal source.

V/div and
time/div
settings will
depend on the
particular
circuit.

Set the time/div, trigger delay, trigger level, and voltage channel coupling to obtain a
display similar to that shown above.

3-7

Finishing the Setup and Making Modulation Measurements

/

After the measurement is planned, the modulated signal channel is identified, the
probes are connected to the proper point in the circuit under test, and preliminary
triggering is established, use the PMA1 software to finish the setup and measure the

signal’s modulation.
Change the trigger source to the “EVENT” trigger previously set up (Channel 3 in the

example). If the event is repetitive, the DSO’s NORMAL trigger can be used. For
events that occur only once, such as start-up, SINGLE trigger should be used.

Activating the Modulation Analysis Menu

Press the MEASURE TOOLS (LT Series) or CURSORS/MEASURE (LC Series)
button on the DSO front panel to bring up the

Power Measurement

selection.

MEASURE

menu, which includes the

MEASURE

TOOLS

CURSORS

MEASURE

Follow the menu sequence given below:

Press the

Voltage Input

to select the previously set up modulated signal

voltage channel.
In the

Type

menu, select the form of modulation to be analyzed.

Width

this example and is the selection that is used for PWM modulation analysis.

3-8

is selected for

Modulation Analysis Controls

Change the horizontal time/div to a value that will allow the capture of a modulated
signal record sufficiently long to cover the time of interest and use the pre-selected

“EVENT” trigger to acquire a record.

Level

Press the

POS

in the

level

menu to measure the width betw een a posi ti v e-

going edge and the ne xt falling edge.
Press the
Turn the

FIND LEVEL

level

knob to set the voltage level on the modulated signal at which the

to find the 50% level of the modulated signal’s width.

width is to be measured. Set this to a level on the modulated signal where both the
rising and falling edges are free from noise. When measuring the modulation of the
gate drive signal, it is best to avoid placing the level around the pedestal.

Press the

hysteresis

to select the number of divisions the modulated signal

must change before a slope change is recognized.
Press the

FIND RANGE

to find the range of the modulated signal’s width.

3-9

Optimizing the Display

Use the DSO’s ZOOM + MATH position and zoom controls to optimize the display.

width/div

ZOOM B changes

the pulse width per

division in the

display.

POSITION B

indicates a zero

pulse width.

ZOOM and POSITION

C and D examines

individual gate drive

pulses in the modulated

signal record.

The modulated signal (2) is displayed in the first grid (the gate drive voltage waveform
in this example). The waveform that results from measuring the width of each pulse
(B) is displayed in the second grid, and zoom traces (C) and (D) of the modulated
signal (2) are displayed in the third and fourth grids. These grids are used to expand
the waveform B so individual cycles of a multiple-cycle record can be viewed.

3-10

Clearing the Modulation Analysis Setup

After using the Modulation Analysis section of PMA1, it is important to clear the
channel assignments and other alterations that were made while making
measurements. Press the RETURN until the on-screen menu is cleared. Then
press the MEASURE/TOOLS to bri ng up the

MEASURE

menu.

Press

RETURN

Selecting

Application OFF

assignment to

NONE

Press

MEASURE

TOOLS

in the Power Measure menu changes the

Voltage Input

.

3-11

Chapter 4 – Using Line Power Analysis

Line Power Analysis Overview

The Line Power Analysis section of PMA1 provides the user with tools to measure 50
and 60 Hz line voltage (V
(Watts), and Power Factor (cos 0), as well as evaluate harmonic currents injected into
the power line. Harmonic measurements are made in relationship to the requirements
of standard EN60001-3-2.

EN61000-3-2

The user is encouraged to refer to the latest version of EN61000-3-2 for full definitions
and limits set forth by the standard. The following is provided for convenience.

Classification of Equipment:

For purposes of harmonic current limitation, EN61000-3-2 classifies equipment as
follows:

), line current I

RMS

), Apparent Power (VA), Real Power

RMS

Class A: Balanced three-phase equipment and all other equipment, except

that stated in one of the following classes.

Class B: Portable tools.
Class C: Lighting equipment, including dimming devices.
Class D: Equipment not in Classes B, C, or motor driven and having an

input current with a “special wave shape” as defined in the
following figure:

i .
i

pk

π

/3

1

π

/3

π

/3

CL

0.35

ω

0

0

π/2

π

t

Equipment is deemed to be Class D if the input current wave
shape of each half-period is within the figure’s envelope for at
least 95% of the time. The input current’s peak value defines
the envelope’s centerline, CL.

4-1

Configuring for Line Power Analysis

SETUP: To make line power analysis measurements, the equipment should be set up

as follows. In the examples below, Channel 2 is used for voltage and Channel 3 is
used for current. Any channel can be used for voltage or current. In the case of 3φ
systems, multiple voltage and/or current channels can be set up before analysis is
started.

Current Probe

H

Differential Voltage

Line

Power

Source

L

Probes

Equipment

Under

Test

EQUIPMENT UNDER TEST: The equipment being tested for power consumption and
line harmonics.

LINE POWER SOURCE: The power source should be low distortion. EN61000-3-2
specifies maximum crest factor and harmonic distortion for the power source while it is
connected to the EUT. The test can be run with the available power line, but the
distortion in the source will directly affect the quality of the measurements.

4-2

Initial Setup – The Line Voltage and Current Signals

Set up a stable display of the line voltage and current signals similar to those shown.

V/div setting
will depend on
the particular
line voltage
being tested.

Set the time/div, trigger delay, trigger level, and voltage channel coupling to obtain a
display similar to that shown above.

Polarity of the current waveform must match that of the voltage waveform.

4-3

Finishing the Setup and Making Line Power Measurements

/

After the measurement is planned and the voltage and current channels are
connected and preliminarily set up, use the PMA1 software to finish the setup and
make the line power measurements.

Activating the Line Power Analysis Menu

Press the MEASURE TOOLS (LT Series) or CURSORS/MEASURE (LC Series)
button on the DSO front panel to bring up the

Power Measurement

selection.

MEASURE

menu, which includes the

MEASURE

TOOLS

CURSORS

MEASURE

Follow the menu sequence given below:

In the

Setup List

menu, select

Voltage Input

4-4

.

Selecting the Voltage and Current Channels

Press the Voltage Input to select the previously set up line voltage signal
channel.

In the

Setup List

menu, select

Current Input

.

Press the Current Input to select the previously set up line current signal
channel. The channel selected as the

Current Input

channel will be assigned Ampere
units even if a voltage or nonProBus-compatible current probe is used. Remove and
degauss the current probe. Polarity of the current probe should match the voltage
waveform.

4-5

Line Power Measurement

After the Voltage Input and Current Input selections have been made, press the
RETURN on the DSO front panel to bring up the following display:

This display screen shows the line voltage and current waveform, as well as the power
and energy waveforms.

Parameters displayed are:

Line Voltage {
Line Current {
Real Power {
Apparent Power {

crms
crms

rpwr

(2)}
(3)}

(2,3)}

apwr

(2,3)}

Power Factor {pf (2,3)} (readout is in milli-units)

Menu selections are provided for the user to select the class of the equipment under
test (EN61000 A, B, C, or D classification) and the line frequency at which it is
operating (50 or 60 Hz).

4-6

Line Harmonics Measurement

Before evoking the Measure Harmonics menu selection:

Press the

Class

to select the proper EN61000-3-2

equipment class.
Press the

Frequency

(50 Hz or 60 Hz) upon which the equipment under test is
operating.

Press the

Units

the harmonic displays.
If changes are needed in the voltage or current input

setups, pressing the
previous menu where the selections can be changed.

After the above selections are made, press t he
the following display:

to select the line frequency

to select Amps or dBuA units for

Setup Inputs

Measure Harmonics

will return to the

to bring up

Press the

Measure Power

to return to the Measure Power Display (changes in

class and line frequency can be made there).
Press the
Turn the

Show Table

to change to the tabular display.

cursor Position

to move the cursor along the harmonics (trace should

be stopped before using the cursor).

4-7

The Measure Harmonics Displays

Line Current
waveform

Harmonics that
exceed limits
are shown
extending
beyond the
template.

Frequency cursor readout (trace
should be STOPPED)

EN61000-3-2
equipment
class and line
frequency
selections are
displayed.

Toggle
between
display formats
by pressing this
button.

4-8

Harmonics
above the limits
are shown as
FAILED.

Clearing the Line Power Analysis Setup

After using the Modulation Analysis section of PMA1, it is important to clear the
channel assignments and other alterations that were made while making line power
measurements. Press the RETURN until the on-screen men u is clear ed. Then
press the MEASURE/TOOLS to bring up the

MEASURE

menu.

Press

RETURN

Selecting
and
channel selected as the

Application OFF

Current Input

assignments to

Press

MEASURE

TOOLS

in the Power Measure menu changes the

NONE

Current Input

. The assignment of Ampere units to the

channel will also be removed.

Voltage Input

4-9

Chapter 5 – Using NonProBus Probes

PMA1 Menu Overview

PMA1 menus give the user maximum flexibility by providing the correct units and
scaling for power measurements. When LeCroy probes equipped with the ProBus
interface are used, correct use of units and scaling is automatic. When nonProBus
current or voltage probes are used, PMA1 software provides methods of entering the
correct units and scaling for a variety of current and voltage probes.

Units

When a channel is selected as a

Current Input

in the PMA1 setup menus, its units
are automatically changed to Amperes. After the assignment is made, data acquired
through the channel is treated as current in any math function. Therefore, multiplying a
current channel waveform by a voltage waveform results in watts, dividing a voltage
waveform results in resistance, etc. This allows the proper units to be displayed even
when a shunt resistor and a voltage probe are used to measure current.

Figure 5.1: Ampere units are applied when Channel 2 is assigned as a current channel.

5-1

Scaling

When a channel is selected as a

Current Input

or

Voltage Input

in the PMA1 setup

menus, its scale can be set to take into account the non-ProBus probe’s overall
effective gain. This includes gain as well as attenuation factors. For non-normalizing
current and voltage probes, the attenuation or gain can be set between ÷10,000
attenuation to X1000 gain in a 1-2-5 sequence. For current or voltage probes with
normalizing amplifiers, special factors can be applied so that the amplifier’s readout
can be entered directly into the menu system.

Figure 5.2: Effective gain of a voltage or current probe can be set from ÷10,000 attenuation
to X1000 gain.

5-2

Current Input Setup Menus

Adj. Current

menu with no amplifier normalization factor.

When an non-ProBus cu rrent probe is us ed that has no
normalization factor, press the to select
the Ampl. mV/div menu.

Determine the probe’s effective gain factor and press the
to enter that value in the

Effective Gain

If the current probe’s DC offset cannot be adjusted to
zero on the probe, use the
to correct the level. Press the to reset the offset
adjustment to its pre-adjusted value.

NONE

menu.

Fine Adjust I DC Level

in

Adj. Current

menu when the current probe has an amplifier with a 10 mV/div
normalization factor. This menu is useful with the AM503 amplifier and its family of
current probes.

When a nonP roBus current pr obe is used that has a
10 mV/div normalization factor, press the to select

10

in the Ampl. mV/div menu. When this selection is

made, the

Amp/div

menu will appear.

Set the probe amplifier to the desired amp/div setting and
turn the until the proper Amp/div factor appears in
the

Amp/div

window.

If the current probe has DC offset that cannot be adjusted
to zero on the probe, use the

Fine Adjust I DC Level

to correct the level. Press the to reset the offset
adjustment to its pre-adjusted value.

5-3

Current Input Setup Menus – continued

Adj. Current

menu when the current probe has an amplifier with a 50 mV/div
normalization factor. This menu is useful when the DA1855 Differential Amplifier is
used to measure the voltage across a resistor shunt.

When a nonP roBus different ial voltage amplifier is used
that has a 50 mV/div normalization factor , press the
to select 50 in the Ampl. mV/div menu. When this
selection is made, the

Amp/div

menu will appear.

Set the differential amplifier to the desired effective gain
setting and turn the until the proper Amp/div factor
appears in the

Amp/div

window.

If the differential amplifier has a DC offset that cannot be
adjusted to zero, use the

Fine Adjust I DC Level

to
correct the level. Press the to reset the offset
adjustment to its pre-adjusted value.

5-4

Voltage Input Setup Menus

Voltage Input

menu when the nonProBus voltage probe is used on the voltage input
channel. This menu is useful when a non-A LeCroy DA1855 or Preamble Instruments
1855 Differential Amplifier is used to measure the voltage. It also can be used for
other voltage probes and amplifiers.

If the differential amplifier has a DC offset that cannot be
adjusted to zero, use the

Fine Adjust V DC Level

to correct the level. Press the to reset the offset
adjustment to its pre-adjusted value.

Set the differential amplifier to the desired effective gain
setting and turn the until the proper effective gain
factor appears in the

Effective Gain

window.

5-5

Clearing the Channel Assignments and DC Offsets

After using any section of PMA1, it is important to clear the channel assignments and
other alterations that were made while making measurements. Press the RETURN
until the on-screen menu is cleared. Then press the MEASURE/TOOLS to bring
up the

MEASURE

menu.

Press

RETURN

Selecting
and
made while using the
Any
unchanged. The assignment of Ampere units to the channel selected as the

Input

Application OFF

Current Input

assignments to

Fine Adjust V DC Level

Deskew Value

channel will be removed.

set while deskewing channel-to-channel delay remains

Press

MEASURE

TOOLS

in the Power Measure menu changes the

NONE

and removes any DC offset adjustments

and

Fine Adjust I DC Level

Voltage Input

features.

Current

5-6