Constant Power Loads | PartQuest™ Explore

Application Area

PascalV
Designer125146

Member for

7 years 6 months

4

designs

1

groups

Add a bio to your profile to share information about yourself with other SystemVision users.

Description

About text formats

Transmission Line Fed LED Driver - Switching

KwangabaSafari
Designer123756

Member for

7 years 6 months

5

designs

1

groups

Add a bio to your profile to share information about yourself with other SystemVision users.

Description

This example illustrates the challenging problem of operating a DC/DC converter at the end of a long cable or transmission line. In this case, a set of three LEDs are co-located with a Step-Down (Buck) switching converter, Together the LEDs consume 25W at 6VDC, and they are supplied by an 18V battery at the other end of an 800 meter cable. The simulation results indicate that the circuit operates properly with only one LED turned-on, but becomes unstable with the other two LEDs are switched on at 1 ms.The transmission line model is specified to use 16 coupled RLC lumped segments, each representing 50 meters of cable length. The conductor DC resistance is set to 1 mOhm/meter, corresponding to 5 AWG wire. This should result in a relatively small voltage drop at the expected 1.5A current needed to support the load. But the effect of the transmission line is more than the DC resistance. The effective impedance and propagation delay, combined with the effective negative impedance of the power converter (i.e. constant power load), result in an unstable power system interaction.You can try using a reduced cable length (e.g. 400 meters, 8 AWG = 2.1 mOhm/meter, or 100 meters, 14 AWG = 8.3 mOhm/meter) by making a copy of this design and re-running the simulation. You will see that the circuit performance is greatly improved with a shorter cable.The concept of Impedance Stability Ratio, and the ability to measure it using SystemVision's TDFS (Time Domain Frequency Sweep) method, will be demonstrated in a related design example and discussed in Part 3 of the TDFS Blog Series, both coming soon!

About text formats

Automotive Electrical Power Distribution System

franck
Designer17016

franck

Member for

8 years 10 months

18

designs

1

groups

Add a bio to your profile to share information about yourself with other SystemVision users.

Description

About text formats

Automotive Electrical Power Distribution System

Darrell
Designer10

Darrell

Member for

11 years

624

designs

10

groups

Big fan of VHDL-AMS

Description

About text formats

TDFS Input Impedance of Step-Down (Buck) DC to DC Converter - Switching

Mike Donnelly
Designer19

Mike Donnelly

Member for

11 years

1,697

designs

10

groups

Member of the PartQuest Explore Development Team. Focused on modeling and simulation of analog, mixed-signal and multi-discipline systems covering a broad range of applications, including power electronics, controls and mechatronic systems.

Description

This example demonstrates the use of the TDFS method (Time Domain Frequency Sweep) to measure the input impedance of a switching DC/DC power converter. The converter circuit is identical to the design shown in the companion example: “Step-Down (Buck) DC to DC Converter - Switching”.In this example, the TDFS impedance measurement model applies a 12V DC bias to the line input, in addition to a sinusoidal stimulus with 3V peak amplitude, and a frequency range from 3 kHz to 30 kHz. The input current is measured and the impedance vs. frequency is computed. The results show that the impedance magnitude (blue waveform) is approximately 5.0 Ohms at low frequency, This is approximately the reflected value of the 1 Ohm load resistor, multiplied by the effective DC/DC conversion ratio squared: 1 Ohm * (12.0/5.0)**2 = 5.76 OhmThe phase measurement (red waveform) shows -175 degrees at low frequency, indicating that this is effectively a "negative resistance". That is, the input current decreases when the line voltage increases. This behavior can be observed directly in the time domain waveforms, where the line voltage (brown waveform) and the input current (green waveform) are almost completely out of phase at 3 kHz. This negative impedance, or constant power load characteristic, can be destabilizing in power distribution systems. This will be demonstrated in a related design, and in Part 3 of the TDFS Blog Series, coming soon!

About text formats