Working principle of LED light source

The main parameters of LED light source are VF and IF, and other related parameters are color/color temperature/wavelength/brightness/lighting
angle/efficiency/power consumption, etc. LED is a PN junction diode. It can only conduct current when sufficient forward voltage is applied
. VF forward voltage is to establish a normal working state for LED light emission, and IF forward current is
to promote LED light emission. The brightness of light emission is proportional to the current flowing through. White light LED VF nominal voltage: 3.4V
± 0.2V. When LED light sources are mass-produced, the VF of each batch of LEDs has a certain degree of discreteness. In order to
meet the consistency required by customers, LEDs must be sold in different VF grades when leaving the factory;
when customers order, the LED light sources required for the same batch of lamps must be selected with the same grade VF or adjacent grades, otherwise it will affect
the brightness of LED lamps produced in the same batch; LED IF working current is selected according to application needs, and different
current grades cannot be mixed.

LED light source driving circuit
The driving circuit of LED light source is to convert 12V DC voltage into a stable constant current source. The design of the circuit
is based on the principle of simplifying and saving costs. From the many LED driver chips that can meet the requirements of this circuit design, we should
choose a chip with high integration, good performance, simple application circuit, and low price
. Therefore, choosing a driver chip with fewer peripheral devices in the driving circuit is the primary consideration for production cost.
PT4115 only needs four parts when used in 1-6W white light LED light source driving solutions (Figure 2).

Figure 2 PT4115LED light source driving circuit

Cin is the input filter capacitor, Rs sets the current IF flowing through the LED, Rs=0.1/ILED; L is the freewheeling inductor,
D5 is the freewheeling diode. Since the adapter has provided a 12V DC voltage,
the bridge rectifier D1-D4 used for AC voltage input rectification in the original figure can be omitted. Although there are fewer parts, the requirements for parts are higher. When designing,
the requirements in Table 1 should be selected to make the circuit enter a good working state. The switching frequency of PT4115 adopts
the frequency jitter technology to effectively reduce EMI and omit the anti-EMI circuit.

Table 1 Selection of key components
The constant current accuracy of LED light source driver chips is crucial for LED lamp manufacturers.
At present, the constant current accuracy of LED light source driver ICs will have a certain degree of discreteness during mass production.
When LED lamp manufacturers are debugging in mass production, the same power supply, the same LED light source load, and the same constant current
source power board will have a certain tolerance in the output current of the constant current source power board due to the individual differences in the constant current accuracy of different driver ICs of the same model,
which will make the luminous brightness of the same LED light source load different. This
will increase the time for online debugging of the constant current source power board during mass production and affect productivity. Therefore, constant current
source power board manufacturers should choose driver ICs with high constant current accuracy. The constant current accuracy should be at least less than ±3%. If
±1% is more ideal, but its price will be higher than ±3% products.

Several key technologies
1) Selection of inductors
Taking the design case of 3X1W high-brightness white LED light source as an example, three 1W LED light sources are connected in series, and their
working current can be designed to be 300-350mA. The inductance of L should be 68uH-100uH, Q value greater than 50,
and saturation current greater than 800mA.
The optimal design operating frequency of PT4115 is below 1MHz. A large inductance will affect its working frequency
. The inductance of this solution is designed to be above 68uH, so that the system operating frequency can be controlled below 1MHz. When
the inductance is small, the operating frequency tends to be higher. Due to the response speed limitation of the internal current detection circuit of PT4115,
the normal detection of the internal current is affected, and the on/off control of the internal switch cannot be better realized. In addition
, since high frequency will bring greater switching losses, the chip will operate at a higher junction temperature, the electrical stress will increase,
which is not conducive to stable operation. If the inductance is too small, the SW terminal of PT4115 may burn out and there will be no output.

The smaller the DCR of the inductor, the higher the efficiency. It is recommended to use EPC13 MnZn 4000 core. If
the saturation current of the inductor is too small and the current of the D5 Schottky diode is too small,
the freewheeling current of the entire circuit will be insufficient, and the LED light source will produce visible flashes. By
appropriately increasing the saturation current of the inductor and the current of the Schottky diode, the freewheeling current of the entire circuit can be increased to eliminate the flashes caused by it.
2) Selection of Adapter
The adapter provides a stable AC power supply -> AC-DC step-down conversion for
the LED desk lamp in this solution. Its real-time load output capability will affect the performance of the LED desk lamp in this solution.
The adapter used for the 3X1W white light LED desk lamp in this solution should have an output current greater than 1A when loaded and a voltage stable at DC12V. Some adapters with poor load capacity will drop to 7V or even 6.5V
when connected to the LED light source load of this solution. For LED driver ICs with an operating voltage starting from 8V, they will enter the undervoltage protection state and stop working. Once the driver IC stops working, the voltage rises back to 12V, and the LED driver IC enters the working state again. This cycle repeats, making the LED desk lamp flash visible to the human eye. At this time, only by replacing the adapter with a good load capacity can the LED desk lamp work normally. At the same time, the LED driver IC with an operating voltage range starting from 6V should be selected , which can also reduce the requirements for adapter selection to reduce production supporting costs. 3) Conduction and radiation of EMI The overall design of this LED desk lamp solution should consider the conduction and radiation of EMI. The key to the conduction and radiation of EMI is the power converter, so it is necessary to select an adapter that can pass EMI and even CE and UL to match, so that the LED desk lamp produced can be exported to the European, American and Japanese markets. The constant current source power board uses a DC/DC switching device as the driver IC, and the switching frequency will radiate when working. Therefore, it is built into the metal shell of the desk lamp base to effectively reduce radiation. The magnetic circuit shielding in the metal base should be considered when designing the mechanical structure.