This technical note is intended to provide an introduction to the pulse width modulation technique (PWM) which can be used to drive Violumas LEDs. ed.
This technical note is aimed to provide product designers with an introduction to pulse width modulation technique, the factors which need to be understood to use PWM with UV LED systems, and how such a system can be implemented.
This resource is intended for engineers using Violumas LED products. Product designers are solely responsible for selecting the appropriate Violumas products and ensuring that the applicable standards and safety requirements are met. Violumas cannot be held responsible for any damage caused by following these guidelines.
Pulse-width modulation (PWM) refers to switching a device ON and OFF at a high frequency for dimming LEDs. It is a very well-known technique which has been used with visible LEDs to attain an average irradiance level. The fundamentals which govern PWM for UV LEDs remain the same as the visible LEDs. However, the perception of PWM in the case of visible LEDs is governed by the flicker levels, which is not necessarily concerning for UV LEDs, since such LEDs are not meant to be seen directly, and human eye exposure should be always avoided.
PWM provides more control over the output level and is ideally suited for applications needing high efficiency and a controlled temperature. Pulsed operation of UV LEDs is already used in many industries such as curing [1]. When compared to a constant current control, PWM provides a wider dimming range.
The time taken for a PWM signal to complete a full ON-OFF cycle is called a time period. The inverse of the time period is the frequency.
A duty cycle is the fraction of one period when a system is switched to high (ON level). It is typically expressed as a ratio or percentage.
For example, if a PWM signal is in an ON-state half the time and in the OFF state for the other half, the duty cycle of this PWM signal would be 50%. In order to increase the duty cycle of the signal the ON time needs to be increased for the same time period.
True PWM dimming for constant current LED sources implies that the current is switched from high to low directly such that the LED power varies at every cycle. If the time period is the same and the pulses are narrow, implying that the ON time in a particular cycle is short, the average current will be lower as compared to a wider pulse within the same time period. 
In some LED power supplies, PWM can be used to achieve constant current dimming. In this case, an RC filter is embedded into the PSU.
While the signal applied to the power supply is a PWM signal, the actual signal to the LEDs is not a PWM signal but a constant current signal, for which the current is determined based on the additive PWM signal. For example, in a Meanwell Power supply, PWM is one way to achieve constant current dimming.
It should also be noted that switching the LEDs ON/OFF implies a lower energy dosage depending on the duty cycle, which might impact the performance of certain applications such as curing.
When deciding to use PWM, it is essential to ensure that the LED driver is compatible and whether it would be a true PWM dimming or a constant current dimming using PWM.
For example, RECOM dc-dc driver, RCD-48, offers true PWM dimming as shown in the figure. Note that this driver requires an external PWM signal to be generated and applied to the PWM pin.
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