1、 Overview



LED bulb has excellent energy efficiency and long service life. It is considered by the market as a powerful substitute for incandescent lamp and compact fluorescent lamp. However, at present, the main obstacles affecting large-scale adoption come from the relatively expensive procurement cost and the dimming compatibility problem like compact fluorescent lamps. For most consumers, the current purchase of LED bulbs is a frustrating experience. Once you buy it home, you will find that most LED bulbs cannot be used for parallel dimming of multiple lamps, and the connection of multiple lamps with a single dimmer is used in many occasions, such as hotels, shopping malls, kitchens and so on. At present, many customers use dimmable chips ssl2101 / ssl2102 / ssl2103 as thyristor dimming LED driving circuit. Considering that the dimming compatibility of multiple lamps is different from that of single lamp, it is necessary to find out the factors affecting the dimming compatibility of multiple lamps in parallel and provide the simplest and easy solution.



This paper selects ssl2103 120VAC GU10 as the research object, trying to solve the above problems.



Schematic diagram of parallel adjustment connection of 1 and 10 lamps

 

Dimming circuit design of LED multi lamp parallel connection


 

2. Schematic diagram of ssl2103 120VAC GU10

Dimming circuit design of LED multi lamp parallel connection


  The circuit board uses a large buffer capacitor and WB is not used_ DRV pin, so there is no weak discharge current (for 2101, a large resistance is generally connected to WB, so the weak discharge current is small). The above working mode is called low ripple mode. Many customers adopt this scheme.



When a single GU10 circuit is not connected to a dimmer, it is connected to a 120V 60Hz AC input. The waveforms of input current and voltage are shown in the figure below. The output current is 345.3ma.

Dimming circuit design of LED multi lamp parallel connection


3. Principle and idea of multi lamp parallel dimming



For the thyristor dimmer, when one dimmer is connected to multiple LED lamps, the most important thing is to damp the AC signal, prevent the oscillation of AC current and appropriate discharge current.

The following figure shows the voltage and current waveforms output by the dimmer when the thyristor dimmer is connected. We subdivide it into three stages.

Dimming circuit design of LED multi lamp parallel connection


Stage 1: front cut thyristor dimmer on



      In this stage, for the dimmer, the impedance characteristic of the load is very complex. In fact, it is not a simple resistance characteristic, which also includes capacitance. At this stage, it is very important to select the appropriate resistance value to damp the AC signal. At the same time, in order to prevent AC current oscillation, an appropriate x capacitance value must be selected.



Stage 2: the front cut thyristor dimmer is turned off and the weak catharsis method is turned on



       In this stage, the dimmer keeps turning off. The DC resistance of the load, that is, the weak discharge resistance and the upper resistance of the partial pressure of PWMLIMIT after the rectifier, is very important. Both resistors have a weak discharge effect on the dimmer. When multiple lamps are connected in parallel, we hope that the dimmer will remain off at this stage, so as to avoid false triggering. Improper discharge current (such as several milliamps, but lower than the maintenance current of the dimmer) will lead to false triggering or re triggering of the dimmer. This is very important for the compatibility of multi lamp parallel dimmers.



Stage 3: forced drainage and conduction



      At this stage, the forced discharge remains on so that the dimmer can be reset correctly.

2、 Test and verification process



Observe the compatibility between the dimmer and the LED drive circuit by adjusting the values of input damping resistance, input x capacitance, parallel resistance at the output end of the rectifier bridge (weak discharge resistance), partial voltage resistance on the pwmlimit pin and other components.



1. Adjust the input resistance

Dimming circuit design of LED multi lamp parallel connection


Parallel 4.7 N F, W B at input_ When D R V is floating and the resistance on pwmlimit partial voltage is 1m, by adjusting the value of R1 + R4, it is found that only one dimmer will flicker when the resistance value of R1 + R4 is 100 Ω (there are 11 dimmers in total), which is basically the same as that under 150 Ω and 100 Ω. However, reducing the resistance value will lead to incompatibility of multiple dimmers.



See Table 1 for details.



Summary: when the value of R1 + R4 is 100 Ω, 10 dimmers work well (11 dimmers in total). If the resistance value is further reduced, the effect is not ideal, and more flickering phenomena will appear.



        Table 1

Dimming circuit design of LED multi lamp parallel connection


       2. Adjust input x capacitance



When the input resistance is 100 Ω, wb-drv floats, and the resistance on pwmlimit partial voltage is 1m, adjust the input capacitance and observe the dimming under different capacitance values. When the capacitance value increases by 10nf from the initial value of 4.7nf, it is found that the current waveform will become very poor and the dimming effect is not ideal. After that, I tried to remove the input capacitance and found that the effect was better; When the input capacitance is 2.2nf, adjust the dimmer without flashing. When the input capacitance increases, the peak and peak of the input current will increase. See Table 2 for details.



      Table 2

Dimming circuit design of LED multi lamp parallel connection


 

Summary: the dimming effect is better when the input x capacitance is 2.2nf and 4.7nf. Increasing the capacitance will cause multiple dimmers to be incompatible. The larger the input x capacitance, the greater the maximum peak to peak value of the input current.



      Table 3 maximum peak value of input current under different x capacitance values

Dimming circuit design of LED multi lamp parallel connection


3. Parallel resistance at output end of rectifier bridge



When the input capacitance is 4.7nf, the input resistance is 100 Ω, the wb-drv floats, and the resistance on the pwmlimit partial voltage is 1m, the output end of the rectifier bridge is connected in parallel to generate a weak discharge current (there will always be a weak discharge resistance for the scheme of ssl2101). Observe the performance of the dimmer with different weak discharge resistance. See Table 4 for details.

Dimming circuit design of LED multi lamp parallel connection


Table 4

Dimming circuit design of LED multi lamp parallel connection

Summary: in low ripple mode, weak discharge resistance has a great influence on multi lamp dimming. Its resistance value is less than 1m, which is easier to flash.



4. Change the partial voltage resistance on brightness



Considering that the partial voltage resistance behind the rectifier bridge will also produce a similar weak discharge effect. Therefore, an attempt was made to change the partial voltage resistance on the pwmlimit and brightness pins. When the input capacitance is 4.7nf and the input resistance is 100 Ω, see Table 5 for details.

 

Dimming circuit design of LED multi lamp parallel connection






Summary: the partial voltage resistance on the brightness and pwmlimit pins has a weak discharge effect, and the R7 resistance is maintained at about 1m, which has a good dimming effect. 5. Additional tests



The input capacitance is 2.2nf, WB_ DRV floats and the resistance on pwmlimit partial voltage is 1m. Adjust the input resistance (R1 + R4). It can be compatible with all dimmers under 100 Ω resistance and most dimmers under 90 Ω and 80 Ω resistance. The effect of further reducing resistance is not ideal. Therefore, when the input capacitance is 2.2nf, selecting a resistance of 80 Ω can reduce the loss and avoid low efficiency. In this way, dimming compatibility and efficiency can be considered.

 

Dimming circuit design of LED multi lamp parallel connection


 3、 Summary


When the ssl210x series is used to build the commercial thyristor dimming circuit board, in the low ripple mode (large buffer capacitance and small weak discharge current), the factors affecting the dimming compatibility of multiple lamps are: input damping resistance, input x capacitance, weak discharge resistance, partial voltage resistance on pwmlimit and brightness pins.



At 120VAC, we recommend an input damping resistance of 100 ohms and an X capacitance of 2.2n? 4.7n, weak discharge resistance > 1m, and pwmlimit partial voltage resistance > 1m.

4、 Appendix

It is known from the above that in order to have better compatibility of the dimmer, the input x capacitance must be small, but this will affect EMI. In order to deal with EMI test, we provide 2 circuits for your reference.



4.1 LC + PI filter

Dimming circuit design of LED multi lamp parallel connection


An LC filter is used at the AC end (the capacitance C is consistent with the X capacitance used in the above test), and a PI filter is used after the rectifier bridge.



4.2 two stage differential mode inductance + common mode inductance

Dimming circuit design of LED multi lamp parallel connection


The two-stage structure is mainly used in high-power boards, generally greater than 10W.  

Responsible editor; zl

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