Power Analysis Demo Guide
LAB 5: FREQUENCY RESPONSE ANALYSIS
Control Loop Response (Bode Plot) and Power Supply Rejection Ratio (PSRR)
NOTE: Some minor details in the screen shots in this document may differ from the instrument you are using.
NOTE: Always use the latest instrument software for demonstrations. Go to www.tek.com/software and search for “5 Series MSO”
software. Follow the installation instructions on the web page.
1 5-PWR Demo Guide
5 Series/Series 6 MSO Power Analysis Lab
POWER TRAINING GUIDE
2 Control Loop Kit Demo Guide
INTRODUCTION
The stability of a control loop determines the closed loop
performance of most key aspects of the power supply.
An unstable power supply can oscillate, resulting in very large
ripple, poor step load response, and excessive noise and EMI.
A power supply does not have to be fully oscillating to be
inadequate. The control loop stability controls PSRR, dynamic
response and output impedance. Poor stability means that all
these responses will also be poor.
Because the performance is not simply pass or fail, and
changes with part tolerances over the life of the power supply, it
is extremely important to understand the initial performance.
The Bode plot is the generally accepted method for assessing
the stability of a loop. A Frequency Response Analyzer, or FRA,
is used to measure the Control Loop Response, popularly
known as Bode plot, and the Power Supply Rejection Ratio
(PSRR); two key metrics of power supply performance.
The Bode plot is ideally measured with a FRA but now, due to
the addition of specialized software, can also be obtained using
oscilloscopes, along with a built-in or external function
generator.
The control loop response computes and plots gain as 20 log
(Vout / Vin). Phase margin is measured at the gain crossover
frequency, which occurs at the frequency where the gain plot
crosses 0 db. The corresponding point on the phase plot gives
the phase margin. The Gain Margin is calculated using the
Phase Margin.
In this app note the Picotest VRTS1.5 demo board (adjustable
regulator board) is used to demonstrate this test.
An injection transformer is used for connecting the FRA to the circuit
being tested for control loop stability measurements. The goal of the
transformer is to inject a signal into the control loop being
measured, without impacting the performance of the loop. In order
to accomplish this to a reasonable degree, the transformer is
isolated and, therefore, is capable of floating on a high voltage, such
as a Power Factor Corrector (PFC), which is often close to 400VDC.
The usable bandwidth of an injection transformer is significantly
greater than the 3dB frequency limits. Because the transformer
bandwidth itself is outside of the measurement, it leads many to
incorrectly believe that the transformer is a non-critical element.
The bandwidth of the transformer is strongly related to the
terminating impedance. Most network analyzers provide a 50Ohm
oscillator source impedance. Assuming this impedance, the optimal
termination resistor is 5 Ohms. This significantly attenuates the
injection signal, which is beneficial, as a common error in Bode
measurements is using a signal which is too large, and therefore not
a small signal measurement. An added benefit of this low value is
that it can always be left in circuit, simplifying the connection to the
network analyzer or scope without appreciably impacting the output
voltage of the circuit being tested.
Picotest supports the oscilloscope stability testing with three
injection transformer options for different circuit applications as
discussed below.
5 Series/Series 6 MSO Power Analysis Lab
POWER TRAINING GUIDE
3 Control Loop Kit Demo Guide
Control Loop Analysis
This guide provides step-by-step instructions and introduces the
capabilities of the 5/6Series MSO FRA measurements.
The first step is to set up the test so that the injection signal from
the built-in signal generator can be used to inject an AC signal
into the control loop. An injection resistor (5-10 ohms) in the
feedback path is required. An injection transformer is used to
inject the signal across the resistor.
Shown below is the Bode plot measurement test setup.
What You Will Need
❑
MSO 54 / MSO 64, 5 series / 6 series MSO Scope
❑
5-PWR /6-PWR Power Measurement and Analysis SW
❑
5 -AFG / 6-AFG Arbitrary / Function Generator
❑
Two TPP0502 500 MHz, 2X TekVPI® passive voltage probe
❑
Picotest Injection transformer
(https://www.picotest.com/tektronix.html) to inject signal from
the AWG into your control loop through your injection
resistor
− Picotest J2100A 1Hz - 5MHz, supports PFC regulators
5MHz high enough for most supplies, low distortion, 5
Ohm termination for minimum impact to loop
− Picotest J2101A 10Hz – 45MHz, 10Hz supports off-line
power supplies, 45MHz high enough for state-of-the art
regulators
− Picotest J2110A Active Injector DC-45MHz
❑
Sample DUT Board: Picotest makes two good training test
boards - VRTS1.5 (left) and the VRTS3 (right)
These demonstration boards are self‐contained and
designed to support a wide range of typical distributed
power system measurements including Bode plots and
PSRR. Input power is provided via a USB port.
− 1 BNC-BNC Cable
− 2 sets of banana plug to mini-grabber cables
− DC Power supply (Keithley 2230-3-1 or 8Vmin)
Objectives
- Obtain a basic understanding of the FRA and PSRR
measurements in a power supply.
- Learn how to make FRA measurements with a 5 /6 Series
MSO oscilloscope.
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