Buck Converter

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Mike Donnelly
Designer19

Mike Donnelly

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11 years

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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.

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Managing PWM Waveforms - Buck Converter

Norm
Designer43361

Norm

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8 years 7 months

328

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Description

To demonstrate data reduction in a PWM design I've copied and modified Mike Donnelly's Buck DC to DC Converter at https://www.systemvision.com/design/buck-dc-dc-converter-switchingI've added a simplified softstart to simulate startup. Sofstart is implemented by ramping up internal voltages over a 5ms period after a 0.5ms delay for a total startup time of 5.5ms. Mike's load transient is timed to activate after steady state. A line transient can be simulated by adding a pulsed voltage source in series with v_line.The total time to simulate startup, steady state and the load transient is set to 10.5ms of which only the final 4ms involve the response to the transient disturbance. Downloaded .csv files of the output voltage (v_out_buck), inductor current (l1inductor/i), or any other signal cover over 84,000 time points.For a 40us switching cycle, a 1us resolution samples each switching cycle 40 times and requires only 4,000 time points to cover the steady state and load transient response. A voltage source is included in the design to mark off this sampling resolution with a sawtooth signal that starts at 6.5ms. The sawtooth appears at the net named x_smpl. Downloading x_smpl along with v_out_buck to Excel enables edits that reduce the original 84,000 time points to 4,000.To demonstrate the data reduction technique for large .csv downloads,t he design includes a dedicated table voltage source which has been preloaded with a set of 4,000 samples from an earlier simulation. The voltage signal vout_reduced displays those 4,000 samples. The table voltage source can be edited to insert to store reduced data records of any transient response generated by this buck converter.Copies of the table voltage source can be placed into this or any other design in order to display the transient response or apply it to another simulation. This mechanism can be used to test designs that are driven by the buck converter output for their ability to withstand the transient disturbance. Other simulations can be used to compare the simulated response to measured data or to analyze the simulated transient response.Watch for an accompanying blog that will explain how the table voltage source works.

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TestOsc

RobertDavis
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RobertDavis

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9 years 6 months

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Rev0 - NCV881930 Virtual Evaluation Board

RobertDavis
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RobertDavis

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9 years 6 months

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TDFS Loop Stability for Buck DC to DC Converter - Switching

瀧澤登
Designer123146

瀧澤登

Member for

7 years 7 months

365

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3

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Description

This design demonstrates the use of the TDFS (Time Domain Frequency Sweep) simulation method, to measure the open-loop frequency response of an operating closed-loop system containing switching elements.The stability of the "Buck DC to DC Converter - Switching" design is assessed. This is a switching circuit, it does not use a state-average model for the modulator, so the standard AC Analysis method cannot be used. Rather, the frequency response is generated from time-domain simulation results. The TDFS approach can also be used for systems that contain sampling or digital control aspects.This particular example is directly comparable to the design titled "TDFS Loop Stability for Buck DC to DC Converter - State Average". In that design, both the TDFS and AC Analysis methods are used to measure the open loop transfer function of an equivalent non-switching circuit.Note that the approach used to characterize the loop stability, by injecting a small sinusoidal stimulus signal in series with the loop and then measuring the complex ratio of the ground referenced return signal to the injected signal, is described in:D. Venable, “Testing Power Sources for Stability”, Venable technical paper #1, Venable Industries

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SatishBA
Designer47381

SatishBA

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8 years 6 months

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PWM Loop gain

Norm
Designer43361

Norm

Member for

8 years 7 months

328

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Description

To demonstrate data reduction in a PWM design I've copied and modified Mike Donnelly's Buck DC to DC Converter at https://www.systemvision.com/design/buck-dc-dc-converter-switchingI've added a simplified softstart to simulate startup. Sofstart is implemented by ramping up internal voltages over a 5ms period after a 0.5ms delay for a total startup time of 5.5ms. Mike's load transient is timed to activate after steady state. A line transient can be simulated by adding a pulsed voltage source in series with v_line.The total time to simulate startup, steady state and the load transient is set to 10.5ms of which only the final 4ms involve the response to the transient disturbance. Downloaded .csv files of the output voltage (v_out_buck), inductor current (l1inductor/i), or any other signal cover over 84,000 time points.For a 40us switching cycle, a 1us resolution samples each switching cycle 40 times and requires only 4,000 time points to cover the steady state and load transient response. A voltage source is included in the design to mark off this sampling resolution with a sawtooth signal that starts at 6.5ms. The sawtooth appears at the net named x_smpl. Downloading x_smpl along with v_out_buck to Excel enables edits that reduce the original 84,000 time points to 4,000.To demonstrate the data reduction technique for large .csv downloads,t he design includes a dedicated table voltage source which has been preloaded with a set of 4,000 samples from an earlier simulation. The voltage signal vout_reduced displays those 4,000 samples. The table voltage source can be edited to insert to store reduced data records of any transient response generated by this buck converter.Copies of the table voltage source can be placed into this or any other design in order to display the transient response or apply it to another simulation. This mechanism can be used to test designs that are driven by the buck converter output for their ability to withstand the transient disturbance. Other simulations can be used to compare the simulated response to measured data or to analyze the simulated transient response.Watch for an accompanying blog that will explain how the table voltage source works.

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

Galina
Designer21

Galina

Member for

11 years

104

designs

3

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Member of the PartQuest Explore team.

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!

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Buck Converter

KarlWoolley
Designer131621

KarlWoolley

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7 years 5 months

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Transmission Line Fed LED Driver - Switching

KwangabaSafari
Designer123756

KwangabaSafari

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7 years 7 months

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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!

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Mike DonnellyDesigner19

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RobertDavisDesigner256

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RobertDavisDesigner256

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SatishBADesigner47381

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Mike Donnelly
Designer19

Norm
Designer43361

RobertDavis
Designer256

RobertDavis
Designer256

瀧澤登
Designer123146

SatishBA
Designer47381

Norm
Designer43361

Galina
Designer21

KarlWoolley
Designer131621

KwangabaSafari
Designer123756