Often encounter such a problem. "Do you want to switch the power?" I said LED driver is not switching power supply! He said no drive is switching power supply. Is the kind of constant voltage switching power supply on the market? Yep! It is estimated that manufacturers also have some constant pressure power needs to be developed. Asked the question is the boss can understand, but there are a lot of competent technology to ask this way.

So it is necessary to explain the switching power supply. For the current mainstream technology, the switching power supply is through the PWM signal control power switch during the turn-on and off, and then closed-loop control loop feedback on the PWM signal duty cycle (PWM) or output frequency (PFM) to adjust to To achieve the purpose of controlling the output current or voltage. Regardless of the topology of the power supply are. Where signal sampling is a major part of this closed-loop control loop. The mode of signal sampling is voltage sampling and current sampling. Only the voltage sampling control loop switching power supply we call the constant voltage switching power supply, only the constant current control loop switching power supply we call the constant current switching power supply, the more modern power supply at the same time with voltage and current sampling loop Of the power supply. So the LED driver is switching power supply. If as a power engineer, will do LED driver, but will not do switching power supply. Which is called engineer?

LED driver is mainly based on constant current, then the LED driver is actually a constant current switching power supply, early this is the case. But because there is no voltage control loop regulation, in the light load state LED drive power output voltage is not controlled, the output voltage is very free and very high. So often burn the LED. After the case of burning LED, the LED power control loop has been improved, that is, increase the voltage sampling feedback. So that when the load current consumption is less than the set constant current current when the power supply in constant voltage mode. So the current mainstream LED drive constant voltage and constant current are available. Insert a digression. I remember in 2011 CNLEDWl forum was mentioned constant voltage cross-flow drive power of a statement, was everyone attack, saying that there is constant voltage constant current power supply such a statement. Can only say that these are too receptive to the new students, and not now can not have it? There is such a self-styled thinking but also attack others.

Do not run so far away. Both constant current loop and constant voltage loop they do not conflict? How did they coexist? In fact they are in a logical or dynamic relationship, can not work at the same time. Ie either working in constant current mode or working in constant voltage mode. When the current consumption of the load circuit is greater than or equal to the set constant current value in the constant current mode, the output constant current value. When the current consumption of the load is less than the constant current of the power supply, the constant sampling failure of the sampled signal can not be detected. At this time the voltage will rise, when the voltage rises to the constant voltage set value, the constant voltage sampling signal to the control loop to a control signal into the constant pressure control mode. The power supply operates at this constant voltage point.

But there will still be problems, and some designers in order to compatible with a wide range of LED operating voltage work, which will set the control point is too high, resulting in a low voltage load is the voltage exceeds the LED breakdown voltage and damage the LED. At the same time because of the power control loop gain and power supply power range. This will inevitably lead to power instability. This forces the PSR power supply IC to add an over voltage protection (OVP) function. The advantage of this is to turn off the power output when the power is out of the design power range or voltage range to prevent power supply from being overloaded or damaged. But will be periodically output to detect whether the load is back to normal or replace. So that we often said that the phenomenon of hiccups, which is the case of LED load reversal and good protection. But the post-level increase in the dimming control has a great impact. Because if the PWM dimming circuit in the latter stage, when the PWM signal is in the shutdown phase of the power is bound to enter the constant voltage or even impact OVP, resulting in power to stop output. This is what I think the IC to join the OVP function of the drawbacks.

Finally, explain the difference between my previously mentioned duty cycle loop control and output frequency control loop. PWM loop control is in the power supply operating frequency remains unchanged, according to the feedback loop provided by the control loop to adjust the duty cycle. PFM loop control is in the power supply conduction time to maintain the same time, according to the control loop to provide the feedback signal to adjust the operating frequency.