Soft start, I believe that hardware engineers will not be unfamiliar with this term. Feel free to open a data sheet of the switching power supply chip, you can see the description of soft-start (soft start). With the increase of chip integration, the soft-start circuit is also integrated into the power chip. This reduces the engineers' work and also causes some engineers to understand the soft start and pay insufficient attention. So what is the role of the soft start circuit?
A large-capacity capacitor usually exists in a power supply circuit, and a large surge current is required to apply a voltage to a capacitor, which may cause a decrease in input power. The soft-start circuit is used to reduce the inrush current when the power is turned on, so that the output voltage rises slowly and reduces the influence on the input power. Let us take a look at the power supply design, the soft start circuit is added, how to ensure the burner is stable.
The P800 is a 4-channel, multi-function online programmer from Zhou Ligong Zhiyuan Electronics. Each channel can output independent power supplies that are adjustable from 1.25V to 7V. Inside the programmer, the power supply of each channel uses the same power supply VDD, and the power supply of each channel is independent of each other through the switching circuit shown in the figure below.
A simple analysis of the above circuit: When the control signal EN_VDDx is high, Q2 is saturated, Q1 is pulled low, Q1 is turned on quickly, and the power supply VDD is output to the VDD_OUT of the corresponding channel and supplied to the target board to be burned. This seemingly simple circuit has a very unstable phenomenon in the multi-channel asynchronous online burning test. What is going on?
We used the P800 to perform asynchronous burning test on four ARM core boards. It was found that when one channel was inserted and powered on, other channels would fail to burn. Since the signal lines of the four channels are independent of each other, only the power supply VDD is shared, so we suspect that the ARM board initialization may interfere with VDD and affect other channels.
To verify this conjecture, we used the oscilloscope ZDS2022 to observe the change in VDD during VDD_OUTx power-up and captured the waveform below.
As can be seen from the waveform diagram, during the rise of VDD_OUTx, VDD drops instantaneously from 3.12V to 2.14V, and then slowly rises back to 3.12V, with a maximum drop of 980mV. Since the power supply of the other three channels is also provided by VDD, it is inevitable that these three channels will fail to be programmed online.
Why does VDD_OUTx power-up cause VDD to drop? Observing the waveform diagram, we can also find that VDD_OUTx rises from 0V to 2V and only takes 3μs. According to the capacitor charging formula: I=C&TImes;dU / dt, the decoupling capacitor of VDD_OUTx is 4.7μF, and the surge current is estimated to be 3A! As mentioned earlier, excessive inrush current eventually causes a reduction in input power.
In order to limit the inrush current, the soft start can be introduced into the switching circuit. By using the characteristic of Q1's on-resistance RDS(on) as a function of VGS, VDD_OUTx is slowly ramped up to VDD by delaying the speed at which Q1 is turned on. The soft start circuit introduced is shown as C1 and R4 in the figure below.
When the collector of Q2 goes low, C1 discharges through R4, and the gate voltage of Q1 decreases slowly, thus controlling Q1 to turn on slowly, so that VDD_OUTx does not mutate. Observe the VDD change during VDD_OUTx power-on with the oscilloscope ZDS2022, and obtain the following waveform.
Compared with the waveform before adding soft start, it can be seen that the rise time of VDD_OUTx is extended to 400μs, and the VDD drop problem is also significantly improved. After repeated tests over a long period of time, there was no more burning failure.
It is such an inconspicuous soft-start circuit that greatly improves the stability of programming. Some small details in life can always bring unexpected surprises, as well as work.
Besides Dmx Cable, we also privide ower and Signal Cable for led lighting connection, such as 444T Cable, 442t cable, 2 female end, 44I cable, and so on. In order to catch less cable wiring in a low voltage led lighting project we combine power and signal wire in one cable, which share same GND wire. The function is same as 2 wire but save time to connect the led lighting cables and save cost in extension cable in a big project.
Photo show of Power And Signal Cable:
Power And Signal Cable
Signal Cable,Dmx Lighting Cable,Led Wiring,444T Cable
Shenzhen Iseeled Technology Co., Ltd. , https://www.iseeledlight.com