LED | PartQuest™ Explore

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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LED Dimmer Circuit using 555 Timer

Mike Donnelly
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Mike Donnelly

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

Description

This LED Dimmer Circuit uses a 555 Timer to control the PWM duty cycle of the current drive. Rather than apply proportional but continuous current to the LED for dimming, which can cause color shifts, modulating the duty cycle allows the LED to operate at its nominal current during the “ON” portion of the cycle. Because the frequency response of human vision is limited, using a PWM frequency of 250 Hz avoids the perception of flicker for the observer.

The LED model has an internal monitor for the "perceived" light output (blue waveform), which is a low-pass filtered version of the instantaneous light output. The filter pole frequency is set to 15 Hz to represent the bandwidth of the human eye. The value of the dimmer setting (green waveform) is increased from 10% to 90% at time 100msec. The LED current pulses (red waveform) are shown before and after the duty-cycle transition.

Part of this design is based on a dimmer schematic found on-line:

<a href="http://www.555-timer-circuits.com/led-dimmer.html">http://www.555-timer-circuits.com/led-dimmer.html</a>

That original circuit is actually incorrect, one of the diodes connected to pin 7 on the 555-Timer needs to be reversed. There was a comment from a reader who said he built the circuit as shown and it didn’t work. This is a good example of the value of simulating circuits before building hardware!

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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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077zz

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Description

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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tunable example 2

bedrosian1134
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Description

This LED lighting example demonstrates the value of simulating both the electrical and thermal* aspects of power dissipating circuits together, simultaneously.

In this application example, a Vishay NTCS0603 Thermistor provides feedback of the enclosure temperature. This feedback is used to control PWM dimming of the LEDs, thereby limiting the internal temperature when operating at high external ambient temperature conditions.

This is a "Live" design, the user can change key parameter values and then run new simulations to see the results. These parameters include "r_mirror", the resistance of the current mirror that controls the capacitor charging rate of the 555 timer, and thereby set the PWM frequency. The user can also change "r_offset" that controls the temperature level at which the dimming operation begins. Finally, the user can set "r_iLED_set", to control the ON-state operating current of the LEDs.

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* To reduce the time needed to simulate the transition and settling at 6 different temperature levels, all thermal time constants were reduced by approximately 1000x. The actual thermal response time constant of the NTCS0603 is approximately 3 seconds (depends on mounting), not 3 msec! Also, the enclosure thermal capacitance value would more likely be 3 (J/degC) instead of 3 (mJ/degC), giving a thermal time constant for the enclosure of 10 (degC/Watt) * 3 (J/degC) = 30 seconds. This time scaling does not affect the static relationship between the outside temperature and PWM dimming.

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Copy of Joule Thief Transformer Physical Design For LED Lighting - on Thu, 02/25/2021 - 10:30

mikeydalby
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Copy of simple circuit with LED and transistor - on Thu, 02/25/2021 - 10:15

amrishahid
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Copy of simple circuit with LED and transistor - on Thu, 02/25/2021 - 10:12

amrishahid
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TEST TUNABLE Analog LED Driver with Thermal Protection using FloTHERM Netlist

Galina
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Galina

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Description

This LED spotlight example demonstrates the value of simulating both the electrical and thermal aspects of power dissipating circuits together, simultaneously.

In this design, a Vishay NTCLE100 Thermistor is used in a detection circuit to monitor the enclosure temperature. It is used for thermal shut-down protection, to keep the enclosure temperature well below the "Tg" (glass transition temperature) of the spotlight's Nylon 6 polymer lens. This is particularly helpful when operating at higher external ambient temperatures.

The "Thermals" (thermal dynamics) model was automatically generated from a full 3D-CFD analysis of the spotlight board layout and enclosure, using FloTHERM. The model is in the IEEE Standard VHDL-AMS format, so it can be directly imported into the SystemVision "1D" circuit and system simulation context. The ability to include an accurate model of the thermal environment is key to having "thermally-aware" circuit function design and board layout processes.

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Mike DonnellyDesigner19

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Mike Donnelly
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Mike Donnelly
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Mike Donnelly
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Mike Donnelly
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077zz
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bedrosian1134
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mikeydalby
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amrishahid
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amrishahid
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Galina
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