Using Real PWM Dimming Constant Voltage Driver for Strips

Overview

Nowadays more and more strips or signage need dimming function to save energy further. And there are mainly two methods to dim, one is constant current mode dimming and the other is PWM mode dimming. This article compares the two ways and tells why PWM dimming is better than constant current.

 

Definitions

PWM (Pulse Width Modulation) mode.

The output voltage and current is switched at a high frequency between 0 and the rated value, which means the LED is either off or running at its rated current. The PWM duty cycle determines the LED brightness. See the following diagram for an example of an LED that is dimmed to approximately 25% using PWM. The 25% level is a result of the current flowing for 25% of the time, then being turned off for the remaining 75% of the time.

Figure 1 PWM mode LED waveform (from Lutron application note #360)

 

Constant Current Mode.

The driver reduces the output voltage and go into the constant current model. Then the driver output current is continuous reduced to dim the LED light.

Figure 2 Constant current mode LED waveform (from Lutron application note #360)

 

Comparison

Color temperature consistency.

Most of the LED’s light feature changes with the operation current so engineers may have to choose PWM mode for the applications which are sensitive to color temperature.

Accuracy.

When designing a 10% dimmable light, most of the constant current mode driver accuracy is normally +/-3%, which means the real output at the 10% level can vary from 7%-13% (Upowertek driver does 9-11%). While PWM mode can do 1% min dimming level and also the variation range is from 0.95-1.05% which is not noticeable for human eyes.

Current balance for each strings.

When the driver goes into the constant current mode, the current regulator of each LED string will not be able to regulate the current, so if there is multiple strings paralleled, we don’t know how much current there is in the strips.

Figure 3 shows the constant current dimming solution. The LEDs with linear regulators are driven by constant current drivers. Suppose we have to use a 250W driver to power six strips or signage modules with 700mA linear regulator each. The LED voltage is around 51V and 3V is the linear regulator voltage drop to make sure it works.

Figure 3. Constant current solution with 100% output

So it works well when the light is 100%.

Now let’s see what happens when we dim it down to for example 2.4A. To achieve lower output current, the LED driver has to reduce the output voltage to around 52V, then the linear current regulator will not work and they are not able to regulate the current to 700mA at all, so the current will be imbalanced and we don’t know how much current there is for each string.

Figure 4. Constant current solution with 60% output

And most of the current solutions in the market can only dim to 10% as minimum which is actually not low enough for most of the strip and signage applications.

Figure 5 shows Upowertek’s dimmable constant voltage driver working with the linear regulators. With this PWM dimming input, each string always works in 700mA current and difference between dimming levels is only PWM duty cycle. The minimum dimming level can reach down to 2%. And that is able to meet most of the design targets.

Figure 5. Upowertek constant voltage solution with 60% output

Comparison with Meanwell’s HLG/ELG solutions, PWM dimming does not have any problems of “dead-travel” of dimmers. Please check the reference.

https://www.meanwell.com/newsInfo.aspx?c=5&i=610

Mean Well amplitude current adjusting LED driver HLG-100 connected to LED strip at different load (length) conditions: (a) full load(length) resulting in the best dimming performance; (b) 70% load(length) implying non-perfect dimming experience due to small dead travel; (c) 30% load(length) implying worst dimming result due to huge dead travel.

 

 

Conclusion

PWM mode constant voltage LED driver has great advantages over constant current mode and the cost does not add much. It is more suitable for applications where end users have high and restrict requirements for the quality of the light. And Upowertek constant voltage drivers cover from 50-400W output power range and please contact with us by [email protected] to get the best quotation and service from Upowertek.

 

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