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Detailed explanation of commonly used LED drive power



General LED driver

There are many types of LED power supplies, and quality and price of different power supplies vary greatly, which is also one of important factors affecting quality and price of product. LED drive power can generally be divided into three categories: one is a switching DC power supply, another is a linear IC power supply, and third is a step-down power supply with resistance and capacitance.


Switch DC source

Transformers are used to convert high voltage to low voltage, while rectification and filtering are performed to obtain stable low voltage DC current. Switching DC power supply is divided into isolated power supply and non-isolated power supply. Isolation refers to isolation of high and low output voltage, which is very safe, so shell insulation requirements are not high. The safety of non-insulated ones is somewhat worse, but cost is relatively low. Traditional energy-saving lamps use a non-isolated power source and are protected by insulating plastic shells.

The safety of switching power supply is relatively high (generally, output voltage is low), and performance is stable. The disadvantage is complexity of circuit and high price. Switching power supply technology is mature and performance is stable, and it is currently main power supply for LED lighting.


IC line source

Single IC or multiple ICs are used for voltage distribution, there are several types of electronic components, very high power factor and energy efficiency, no electrolytic capacitor required, long life and low cost. The disadvantage is that high voltage output is not isolated, but there is a stroboscope, and case must be protected from electric shock by insulation.

Those on market claim no (removal) of electrolytic capacitors and long life using linear IC power supplies. The IC drive power supply has advantages of high reliability, high efficiency and low cost, and will become an ideal LED drive power supply in future.


Resistance and capacitance step-down power supply

A capacitor is used to provide control current by charging and discharging it. The circuit is simple and inexpensive, but its performance is poor and its stability is poor. The LED burns out easily when mains voltage fluctuates. , At same time, output high voltage is non-isolated, and an insulating protective shell is required. Low power factor, short life, generally only suitable for economical low power products (within 5W).

Large power products have large output current, and capacitor cannot provide large current, otherwise it is easy to burn out. In addition, country has power factor requirements for high-power lamps, that is, power factor above 7W must be greater than 0.7, but capacitance-resistive buck power supply is far from achieving (usually between 0.2-0.3), so high power products should not be used. RC buck power supply. On market, almost all low-cost, low-demand products use step-down RC power supplies, and some high-end, cheap budget products also use step-down RC power supplies.


The difference between a switching power supply for LEDs and a drive power supply

In short, LED driver is also a type of switching power supply, but has some features that are also common to this type of switching power supply, so it is commonly classified as an LED driver.

These features:

  1. Its output voltage is a multiple of 3.2, which means that output voltage is in form of 3.2V, 6.4V, 9.6V, 12.8V, but usually does not exceed 25.6V Because after this number is exceeded, when LED is turned on, there will be a possibility of last LED turned on instantly burning out due to poor consistency of product. And this voltage is not constant, it changes with load to achieve purpose of constant current.

  2. Its output current is constant, and ideal circuit is that no matter how characteristic curve of LED changes, control power supply current remains same. However, limited by accuracy of components, there will still be a small amount of change, and this change is also an important parameter in judging whether drive circuit is excellent. The function of LED conductance and voltage is a non-linear "three" stage" relationship, so it's important to keep current constant.

  3. It all starts with a soft start. Since LED stability is very poor and activity of internal PN junction changes instantly when it is turned on, LED driver is usually designed as soft start to avoid this defect.

  4. Its circuit requirements are simplest, because in many cases circuit needs to be installed in a small space to match convenience of LED lighting, so circuit must be as simple as possible, which can also save costs Reduce energy consumption.

  5. In general, no insulation is required because many products have same structure as conventional lighting, and safety is similar to that of lighting. But first element is "additional reading element". Please don't misunderstand it when you get it, because some drivers still need to be isolated. This feature only applies to our current popular circuits and is not necessarily suitable for future circuit development. needs. .

Summarizing, its characteristics are soft start, constant current, step voltage and simple circuit.

One more moment:

Many people emphasize direct current one-sidedly, but keep silent about voltage, which is wrong. Because concept of DC has nothing to do with voltage. For example, if power supply is only DC onoutput 30V, then when circuit is opened, its voltage will be 30V. The working components burn out sooner more accurately circuit can react.

Because any circuit requires a response time, and working device in circuit is a semiconductor. The PN junction can only respond after power supply gives a fetch signal and PN junction of LED starts working directly, so its "response" is faster than "many PN junctions interact" in circuit, and it will be burned. in advance! Of course, such a drive is used in special cases, but such an LED drive does not allow output end to be opened. To be precise, "it is not allowed to connect LED after opening output terminal."