Control Loop Analysis

Control Loop Analysis

Power supply design engineers need to ensure the stability of the closed loop response system of their Power supply designs to ensure the designs have sufficient margins to handle different load conditions before the feedback loop reaches the unstable state.

Designers need to change the connection of their setup from one test equipment to other and this is innovatively addressed by the Tektronix MSO5/6 series oscilloscope with built-in AFG and FRA suite in the 5/6-PWR Automated Power measurement and Analysis software. The Control Loop Analysis Kit combines powerful software and hardware that enables the most accurate characterization and optimizes your Power Supply designs.

Characterize:

  • Stability of the closed loop circuit with the measurement of Control Loop Response (BODE plot) with Gain Margin and Phase Margin measurements.
  • The performance of AC-DC and DC-DC converters with Power Supply Rejection Ration (PSRR) measurement.

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FRA-KIT

Eliminate the need for multiple test set-ups

bode_gain_phase_graph

Typical BODE plot plots phase and Gain values over frequency and calculate PM and GM.

rejectionratio_vs_frequency

Typical Rejection ratio Vs Frequency plot of a DC-DC converter.

Power design teams need to ensure their closed loop circuitry operates in stable region in varying load conditions. To measure the Frequency response of a power system a known signal must be injected into the feedback loop. For this measurement the Arbitrary/Function Generator (AFG) option in the 5/6 Series is used to generate sinewaves over a specified range of frequencies. One can also use an external generator, Tektronix AFG31000 series function generators are supported. The stimulus from the AFG is injected using a small (5-10Ω) injection resistor in its feedback loop so that it does not load the design.

An injection transformer with a flat response over a wide bandwidth is connected across the injection resistor and isolates the ground signal source from the power supply. The Picotest J2101A Injection Transformer has a range of 10Hz-45MHz which aligns well with the function generator option for the 5/6 Series MSO. Low capacitance, low attenuation passive probes such as the TPP0502 are recommended for the LDO voltage measurements. This enables measurements with vertical sensitivity of 500 µV/div on the 6 Series MSO, and 1 mV/div on the 5 Series MSO.

Get more visibility with your software

2-blade-for-software

To simplify the complexity of multiple test set-ups, Tektronix has enabled Control Loop and PSRR measurements by creating software that can run on the same oscilloscope used for automated Power measurements. This simplified solution enables design teams to design, verify, validate and test their designs effectively and faster thereby enabling ease of use and faster time to market.

The FRA capability in the Oscilloscope Power analysis software ensures the test is performed automatically which reduces human error dramatically, ensures repeatability and finally helps designers with one oscilloscope-based solution to complete majority of their measurement needs.

What’s in the FRA-KIT system?

Tek-Control-Loop-Analysis_SBG-new
  1. 5/6 Series MSO Oscilloscope - High resolution (12 bit) scope with built in Function Generator.
  2. 5/6-PWR software – Automated, accurate and repeatable power measurements for Control Loop response(BODE plot) -calculating gain and phase margin measurements, PSRR and Impedance measurement.
  3. Two TPP0502 – high-bandwidth, general-purpose probes from Tektronix that offer breakthrough specifications previously unrealized in this product class. With the recommended TPP0502 probes which are ideal for FRA measurements. With their low input capacitance these probes reduce the loading on the circuit which can hamper the measurement accuracy as we are dealing with very low voltage levels. We can expect better measurement accuracy as the recommended probes do not alter the signals, which otherwise may impact the results dramatically.
  4. Picotest has created an FRA bundle that enables the Tektronix 5/6 Series MSO to perform the Control Loop and Power Supply Rejection Ratio measurements. It includes the J2120A line injector for PSRR and the J2100A injection transformer for Bode plots and stability measurements. This bundle by Picotest, also includes the VRTS1.5 test/training board.

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