Often encounter such a problem. "Can you make the switch?" I said LED drive is not the switching power supply! He said not to drive the switch power supply. Is the kind of constant voltage switching power supply on the market do? On right! It is estimated that manufacturers also need to develop some constant voltage power supply. The problem is that the boss can understand, but there are a number of competent technical projects.
Therefore, it is necessary to explain the switching power supply. The current mainstream technology for switching power supply is based on and off during the PWM signal to control the power switch, the control closed-loop feedback on the duty cycle of the PWM signal (PWM) or output frequency (PFM) of regulation to control the output voltage or current objective. Regardless of the topology of the power supply are. Signal sampling is a major part of the closed-loop control loop. There are two kinds of sampling modes: voltage sampling and current sampling. Only voltage sampling switch power supply control loop is called a constant voltage switching power supply, only constant current control loop of switching power supply that switching power supply is a constant current power supply, the more modern at the same time with the power supply voltage and current sampling circuit. So the LED drive is also the switching power supply. If as a power engineer, will do LED drive, but will not do the switching power supply. What is an engineer?
LED driver is mainly based on constant current, then the LED driver is actually a constant current mode switching power supply, and so is the early. However, because there is no regulation of the voltage control loop, in the light load state, the output voltage of the LED power supply is not controlled, the output voltage is very high. So often burn LED. After the burning of LED, the control circuit of LED power supply is improved, which means that the voltage sampling feedback. Thus, when the load current is less than the constant current, the power supply operates in constant voltage mode. Therefore, the current mainstream LED drive constant voltage and constant current have. Insert a digression. I remember in 2011 CNLEDWl forum mentioned constant pressure cross flow power supply is a saying, was attacked, saying that there is constant voltage and constant current power supply. Can only say that these can not accept the new kind of thing, not previously can not have it? This complacent thinking also needs to attack others.
Don't run so far. Both constant current and constant voltage loop loop they don't conflict? How do they coexist? In fact, they exist in a logical or relationship, it can not work at the same time. They also work in the current mode or in constant voltage mode. When the current of the load circuit is greater than or equal to the constant current value, the constant current mode is output. When the current consumed by the load is less than the constant current of the power supply, it is impossible to detect the constant current failure of the calibrated sampling signal. At this time the voltage will rise, when the voltage rises to the constant voltage setting value, the constant voltage sampling signal is provided to the control loop of a control signal, into the constant voltage control mode. Power supply works at this constant voltage point.
But this will still have problems, some designers in order to LED compatible with a large range of working voltage, thus the control points set too high, resulting in low voltage with load voltage exceeds the breakdown voltage is LED and LED damage. At the same time, because of the gain of power control loop and the limitation of power supply range. This is bound to lead to unstable power supply. This forces the manufacturer to do PSR power IC with an over voltage protection (OVP) function. The advantage of this is to turn off the power supply when the power supply is beyond the design power range or voltage range, and to prevent the power supply from being damaged when the load is abnormal or damaged. However, the output will be periodically to detect whether the load is restored or replaced. In this way, we often say that the hiccup phenomenon, which has a good protective effect on the situation of LED load connection. But it has a great influence on the dimmer control. Because if in the latter stage to increase the PWM dimming circuit, when the PWM signal is in the closing phase of the power supply is bound to enter the constant voltage or even impact the OVP, resulting in the power to stop the output. This is what I think IC to join the drawbacks of OVP function.
Finally explain the difference between the duty cycle control circuit and the output frequency control loop that I mentioned earlier. PWM loop control is to adjust the duty cycle according to the feedback signal provided by the control loop when the power supply frequency is kept constant. The PFM loop control is to adjust the operating frequency according to the feedback signal provided by the control loop when the working time of the power supply is kept unchanged.