Step-Down | PartQuest™ Explore

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Mike Donnelly
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TDFS Impedance Stability of Transmission Line Fed LED Driver - Switching

Mike Donnelly
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10 years

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Original member of the PartQuest Explore development team. Modeling and Applications Specialist

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This example demonstrates the use of the TDFS method (Time Domain Frequency Sweep) to measure impedance stability ratio. The system is a switching DC/DC step-down power converter used for LED lighting, supplied by a battery through a long power cable.The TDFS impedance stability measurement model applies a 0.2A sinusoidal stimulus current at the point where the cable connects to the converter. The voltage at that point, as well as the stimulus current splits (toward the source and the load) are measured, and the associated source and load impedances vs. frequency are computed.The ratio of the source to load impedance is the impedance stability ratio (Tm). Similar to the open-loop frequency response requirement for closed-loop stability, Tm must not have magnitude = 1.0 at phase = 180 degrees, at any frequency.For this system, with a cable length of 400 meters and 8 AWG wire, the results show that the impedance ratio magnitude (green waveform) reaches unity (0 dB) at close to 2 kHz, where the phase (light blue waveform) is approximately 165 degress. This implies a "phase margin" of only 15 degrees, For a longer cable, this margin is further reduced. As shown in the companion example, "Transmission Line Fed LED Driver - Switching", for the cable length = 800 meters the system is unstable.This instability is the result of the "source" impedance (which includes the cable) variation over frequency, interacting with the varying load impedance. Note that at low frequency, the load effectively has a negative impedance, due to the constant power nature of the DC to DC converter.

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Copy of TDFS Impedance Stability of Transmission Line Fed LED Driver - Switching - on Sun, 03/21/2021 - 11:01

077zz
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This example demonstrates the use of the TDFS method (Time Domain Frequency Sweep) to measure impedance stability ratio. The system is a switching DC/DC step-down power converter used for LED lighting, supplied by a battery through a long power cable.

The TDFS impedance stability measurement model applies a 0.2A sinusoidal stimulus current at the point where the cable connects to the converter. The voltage at that point, as well as the stimulus current splits (toward the source and the load) are measured, and the associated source and load impedances vs. frequency are computed.

The ratio of the source to load impedance is the impedance stability ratio (Tm). Similar to the open-loop frequency response requirement for closed-loop stability, Tm must not have magnitude = 1.0 at phase = 180 degrees, at any frequency.

For this system, with a cable length of 400 meters and 8 AWG wire, the results show that the impedance ratio magnitude (green waveform) reaches unity (0 dB) at close to 2 kHz, where the phase (light blue waveform) is approximately 165 degress. This implies a "phase margin" of only 15 degrees, For a longer cable, this margin is further reduced. As shown in the companion example, "Transmission Line Fed LED Driver - Switching", for the cable length = 800 meters the system is unstable.

This instability is the result of the "source" impedance (which includes the cable) variation over frequency, interacting with the varying load impedance. Note that at low frequency, the load effectively has a negative impedance, due to the constant power nature of the DC to DC converter.

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Copy of TDFS Loop Stability for Step-Down DC to DC (Buck) Converter - Switching - on Tue, 10/20/2020 - 13:58

anita.w.zhou
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This example demonstrates the TDFS (Time Domain Frequency Sweep) Loop Stability Instrument Model. It is used to compute the open-loop transfer function of an operating (closed-loop) switching power converter. There is no need for state-average or continuous equivalent models for the modulator section of the design, as normally needed for frequency-domain (or "AC") analysis. Rather, the actual circuit component models can be used directly, because the open-loop transfer function is computed from time-domain simulation results.

In this case, the converter is operating at 200kHz switching frequency, and is converting the 12V DC input to a regulated 5V output, while supplying a 5A current to the 1 Ohm load resistor. The TDFS measurement instrument indicates that the open-loop gain crossover frequency is at 26 kHz, and the phase margin is just under 60 degrees. This verifies that the opamp-based lead-lag compensator is providing adequate stability margin under these operating conditions.

Note that the TDFS instrument model characterizes the open loop transfer function by injecting a small sinusoidal stimulus signal in series with the loop, and then measures the complex ratio of the return signal to the injected signal, is described in:

D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries.

The companion example, "Step-Down (Buck) DC to DC Converter - Switching", shows the line and load transient response of this converter design. Another companion example, "Step-Down (Buck) DC to DC Converter - Continuous", uses a state-average model of the switching (or modulator) section of the converter, so it supports traditional "AC" or frequency-domain analysis.

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Copy of Transient Stability Testing of Transmission Line Fed LED Driver - on Tue, 03/24/2020 - 11:15

darkson00
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Copy of Transient Stability Testing of Transmission Line Fed LED Driver - on Thu, 03/12/2020 - 07:50

aruninis
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Copy of Transient Stability Testing of Transmission Line Fed LED Driver - on Wed, 03/04/2020 - 09:55

danirech
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Este es un ejemplo complementario del diseño: "Estabilidad de impedancia TDFS del controlador LED alimentado por línea de transmisión - Conmutación". En esta versión, se agrega un interruptor para encender dos de los LED después de 2 ms, para inyectar una carga transitoria en el sistema. Este transitorio expondrá la gravedad de la respuesta de llamada amortiguada en la entrada del convertidor o la inestabilidad del sistema si la relación fuente / impedancia de carga es inadecuada.

La configuración inicial para este diseño utiliza una longitud de cable de 400 metros con 8 AWG = 2.1 mOhm / metro conductores y un condensador de entrada del convertidor = 22uF. Esto es consistente con el diseño complementario. Puede intentar usar un cable de mayor longitud (por ejemplo, 800 metros, 5 AWG = 1 mOhm / metro) haciendo una copia de este diseño y volviendo a ejecutar la simulación. Verá que el circuito se vuelve inestable en esa longitud más larga. También puede probar valores más grandes de capacitancia de entrada para mitigar el problema de inestabilidad.

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TDFS Loop Stability for Step-Down DC to DC (Buck) Converter - Switching

TakayukiAtago
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TakayukiAtago

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Description

This example demonstrates the TDFS (Time Domain Frequency Sweep) Loop Stability Instrument Model. It is used to compute the open-loop transfer function of an operating (closed-loop) switching power converter. There is no need for state-average or continuous equivalent models for the modulator section of the design, as normally needed for frequency-domain (or "AC") analysis. Rather, the actual circuit component models can be used directly, because the open-loop transfer function is computed from time-domain simulation results.In this case, the converter is operating at 200kHz switching frequency, and is converting the 12V DC input to a regulated 5V output, while supplying a 5A current to the 1 Ohm load resistor. The TDFS measurement instrument indicates that the open-loop gain crossover frequency is at 26 kHz, and the phase margin is just under 60 degrees. This verifies that the opamp-based lead-lag compensator is providing adequate stability margin under these operating conditions.Note that the TDFS instrument model characterizes the open loop transfer function by injecting a small sinusoidal stimulus signal in series with the loop, and then measures the complex ratio of the return signal to the injected signal, is described in:D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries.The companion example, "Step-Down (Buck) DC to DC Converter - Switching", shows the line and load transient response of this converter design. Another companion example, "Step-Down (Buck) DC to DC Converter - Continuous", uses a state-average model of the switching (or modulator) section of the converter, so it supports traditional "AC" or frequency-domain analysis.

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Nonlinear R-L-C

Norm
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PWM Modulator v8

Norm
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Mike Donnelly
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Mike Donnelly
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077zz
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anita.w.zhou
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darkson00
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aruninis
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danirech
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TakayukiAtago
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Norm
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