LED driver for high power machine vision flash
Machine vision systems use very short flashes of bright light to produce high-speed images for a variety of data processing applications.For example, fast-moving conveyor belts are used for rapid labeling and defect detection through machine vision systems.Infrared and laser LED flashlights are commonly used for short range and motion detection machine vision.The security system emits a high-speed, imperceptible LED flash to detect movement, capture and store security images.
ne challenge with all of these systems is the generation of very high currents and short (microsecond) LED camera flash waveforms that may be spread out over longer periods of time, e.g. 100 ms to more than 1 s.Generating short LED flash square waves with long intervals is not easy.The challenge is further increased when the driving current of the LED (or LED string) rises above 1 A and the LED turn-on time is reduced to A few microseconds.Many LED drivers with high-speed PWM capabilities may not be able to effectively handle longer turn-off times and short periods of high current without reducing the square wave quality required to properly process high-speed images.
Proprietary LED flash
Fortunately, the LT3932 high-speed LED driver provides machine vision camera flash for LED strings up to 2 A with off times of up to 1 second, 1 hour, 1 day or more.The LT3932's special camera flash feature allows it to maintain output capacitance and control loop charge status even during long shutdown periods.After sampling the state of the output and control loop capacitors, the LT3932 continues to trickle charge these components during long shutdowns to compensate for the usual leakage current, which other LED drivers do not consider.
The LT3932's proprietary flash technology can be extended and the drivers can be connected in parallel to provide higher LED flash current.The desired flash shape and integrity remain the same.Figure 1 shows how easy it is to connect two drivers in parallel to support A 3A camera flash, and designs up to 4A are possible.
LED flash requirements for machine vision systems are much higher than standard PWM dimming drivers can achieve.That said, most high-end LED drivers are designed to provide PWM dimming brightness control at a PWM frequency of at least 100 Hz.This is because, at lower frequencies, even if the LED waveform is square and repeatable, the human eye sees an annoying flicker or stroboscopic flicker.At 100 Hz, the theoretical maximum turn-off time is about 10 ms.During the 10 ms turn-off time, if designed correctly, the LED driver will lose very little output capacitance charge, causing it to start the control loop in approximately the same state, thus ending the last PWM on-pulse.The quick response and ramp up of the inductive current and the next LED PWM conduction pulse can be fast and repeatable, and the startup time is extremely short.Longer turn-off times (frequencies below 100 Hz) may result in a loss of charge in the output capacitance due to leakage, making it impossible to respond quickly when the LED is turned on again.
Parallel LED drivers to provide higher current
The LED driver acts as a current source, regulating the current sent through the leds.The current flows only in a single direction, so multiple LED drivers can be connected in parallel and the current can be aggregated through the load.The current source does not need to guard against current flowing backwards through a converter, nor does it need to worry about output mismatches.On the other hand, voltage regulators themselves are not good at equalizing current.If they are both trying to adjust the output voltage to a certain point, and their feedback networks are slightly different, the regulator may absorb the reverse current.
The LED driver keeps its output current constant regardless of whether other drivers provide additional current and aggregate it on the output load.This makes it very simple to parallel LED drivers.For example, an LED flash system with two parallel LT3932 LED drivers shown in Figure 1 can drive four leds at an efficiency of 3 A, with 10μs short pulses breaking apart over A longer period as defined by machine vision systems.During the PWM on-time, each LT3932 converter generates half of the total series current;During the PWM off time, the converter turns off and saves its output state.The turn-off time can be short or long without affecting the repeatability of flash waveform.
Figure 1. Parallel LT3932 1.5A LED driver generates 3 A machine vision LED pulses with long turn-off time relative to standard PWM dimming frequencies.
Figure 2. The 3a camera flash waveform shown in Figure 1 in parallel with the LED driver looks the same regardless of the PWM off time.The waveform shows that (a) 10μs pulses after 10 ms and (b) 10μs pulses after 1 s are identical.The LT3932 LED flash waveform looks the same after a day or more of PWM off time.
During long shutdowns, the parallel camera flash application is almost as simple as a single converter.The converter observes the shared output voltage at the end of the last PWM on-pulse, charging the output capacitor to that state and maintaining it, even during long off periods.Each converter disconnects its PWM MOSFET from the shared load and supplies current to its output capacitor to compensate for the energy leakage so that the capacitor is charged close to the final voltage state and maintained.Any leakage of these capacitors during long turn-off times can be compensated by a small amount of maintenance current.When the next PWM conduction pulse starts, the PWM MOSFET of each converter turns on, and the output capacitance starts in roughly the same state as the last pulse, whether after 10 ms or an entire day.
Figures 2 (a) and 2 (b) show the LT3932 parallel LED driver driving 4 leds at 3 A with machine vision camera pulse of 10μs.Whether it is a 10 ms PWM off time (100 Hz) or a 1 s PWM off time (1 Hz), the LED pulses are steep and fast, which is ideal for machine vision systems.
Higher current is also possible
Parallel LED drivers are not limited to two converters.Three or more converters can also be connected in parallel to produce higher current waveforms with steep edges.The system has no master or slave devices, so all converters provide the same amount of current and share the load equally.It is recommended that all parallel LED driver converters share the same synchronous clock and remain in phase.This ensures that the output capacitance ripple of all converters has roughly the same phase, so the ripple current does not flow in the opposite direction or between different converters.It is important to keep the PWM pulse waveform out of phase with the 2 MHz synchronous clock.This ensures that the LED flash waveform remains square and free of jitter, resulting in optimal image processing results.
The LT3932 demonstration circuit (DC2286A) is designed to drive 1 A LED current through one or two leds (acting as step-down LED drivers).As shown in Figure 1, it is easy to change and parallel to achieve higher current, higher voltage, or parallel operation.Figure 4 shows how two such circuits can be easily connected together to drive four leds with 10μs, 3 A pulses from A 24 V input.For testing purposes, the pulse generator can be used to provide a synchronous clock signal, as shown in Figure 4.In mass production machine vision system, clock chip can be used to generate synchronous clock signal and PWM pulse.For higher current pulses, more demonstration circuit DC2286A converters can be added using the same parallel scheme.
Figure 3. Example of machine vision on an industrial conveyor belt.The detection system moves at many different speeds, but the flash technology must be fast and sharp.
Figure 4. Two DC2286A LT3932 demonstration circuits can be easily connected in parallel to create the 3 A to 4 A machine vision LED flash application shown in Figure 1.
conclusion
Machine vision systems can use parallel LED drivers to create fast, square, high-current waveforms needed for automatic image processing.The LT3932 LED driver's proprietary camera flash technology can be extended to higher currents by paralleling converters.With parallel LT3932 converters, 3 A and higher microsecond pulses can be achieved even with longer turn-off time.The LED camera flash waveform remains square and jitter free regardless of the length of time between LED flashes
Thermal solutions for LED is very important as the power is gradually higher and higher, Sinda Thermal can provide varieties LED heatsinks and coolers which inlcuding aluminum extruded heatsink, high performance heatsink, copper heatsink, skived fin heatsink, and heat pipe heatsink. please contact us if you have any questions about thermal solution.
website: www.sindathermal.com
contact:castio_ou@sindathermal.com
Wechat: +8618813908426
