Power over Ethernet (PoE) is a technology that allows data signals to be transmitted while providing DC power to devices such as IP phones, wireless access points, and network cameras over existing Ethernet Cat.5 cabling without any modifications (power transmission via network cables). The TT9930 is a DC/DC controller used in PoE systems. It meets the operator’s requirements for low-cost PoE cameras.
During the debugging process, the TT9930 prototype experienced noise and could not work when connected to a camera. What could be the cause of this issue? In this article, we will detail the solutions and hope to help everyone.
Below are images of the test prototype:
TT9930 prototype image
【Application】 Video Surveillance / IP Phone / Wireless AP
【Specifications】 12V2A
【Control IC】 TT9930
About TT9930: High-Performance DC/DC Controller
Product Description
TT9930 is a DC/DC controller used in PoE systems (Power over Ethernet). It adopts primary-side control, external MOS design, suitable for flyback topology, providing precise constant voltage control loop with high system efficiency.TT9930 is available in SOP-8L package.
Features
● Low startup current (approximately 7μA)
● Under-voltage lockout function
● Primary-side control mode
● Light load frequency reduction to lower power consumption
● VCC over-voltage protection
● Output over-voltage protection function
● Output short-circuit protection
● Internal over-temperature protection
● Soft-start function
● Peak current control mode
● Built-in LEB circuit
● Overload protection
Basic Applications
● Video Surveillance
● IP Phone
● Wireless AP
Typical Applications
Pin Arrangement
Pin Description
【Problem Description】
During the debugging process, the prototype experienced noise and could not work when connected to a camera.
【Solution Approach】
The noise may be caused by the following reasons:
1. Magnetic component vibration: Magnetic components in switch-mode power supplies, such as transformers or inductors, may vibrate or resonate during operation. This may be due to looseness, improper soldering, or mechanical damage during design or manufacturing, causing noise.
2. Switching noise from the switch: In switch-mode power supplies, the switch (such as MOSFET) generates noise during the switching process due to rapid current and voltage changes. This switching noise may propagate through the circuit and convert into audible noise.
3. Ground interference: In switch-mode power supplies, the connection and layout of the ground play an important role. Incorrect ground design may lead to interference in the ground loop, resulting in noise issues.
4. Loop issues in the switch-mode power supply may cause fluctuations or instability in output voltage or current, introducing noise or interference signals to the output of the switch-mode power supply.
Connecting the camera is equivalent to adding a load, which may have the following reasons:
1. Insufficient input voltage: The switch-mode power supply requires a sufficient input voltage to operate normally. If the input voltage is too low, it may not provide enough energy to start the circuit. Please ensure that the input power supply voltage meets the design requirements of the switch-mode power supply.
2. Abnormal control signals: The primary-side control circuit of the switch-mode power supply may be affected by abnormal control signals, preventing it from starting. Please check the connections and logic of the control signals to ensure they are correctly transmitted to the control circuit.
3. Overcurrent or short-circuit protection triggered: The switch-mode power supply may have overcurrent or short-circuit protection functions, which automatically disconnect the output to protect the circuit and load when the output load is abnormal. If it cannot start under low voltage conditions, it may be because the protection circuit detects an anomaly, causing the output to be cut off. Please check if there are any abnormalities in the output load.
4. Switch failure: The switch (such as MOSFET) in the switch-mode power supply may be damaged or fail, preventing the circuit from functioning normally. Please check if the switch is working properly and ensure it is correctly driven and controlled.
【Key Points for Debugging】
First, check the noise issue. According to the customer’s description, the noise suddenly appeared during debugging, indicating that the original circuit and transformer were functioning correctly. It is likely that a component failure caused the noise. Check if there are any issues with the waveform.
The switching frequency is good, around 180KHz, and the waveform is also very regular, indicating that the loop is generally fine. This prototype uses primary-side control, indicating that the transformer is not a major issue. The TT9930 has a loop compensation pin (COMP), check if the noise is caused by the COMP pin.
Upon investigation, it was found that the C4 compensation capacitor was damaged, and replacing it resolved the noise issue in the prototype.
Next, check the issue of not being able to start with a load. Using a DC source to simulate the voltage output from the switch, it was found that cameras could work normally above 48V, while at 38V, the cameras could not start. It was suspected that insufficient input voltage led to inadequate energy for the circuit to work. Below are the waveform images of the circuit working normally:
CH4 green line is the voltage after the rectifier bridge, CH2 sky blue is the voltage after the protocol chip XS2100S and the input capacitor, and CH1 dark blue is the output voltage. Upon investigation, it was found that it was not insufficient input voltage causing the prototype to not work. At 38V input, it was found that there was no current passing through the input capacitor, and the input capacitor needed to work after passing through the XS2100S, and it was found that the XS2100S had not started.
【Final Result】
Finally, it was attempted to directly power the input capacitor without going through the protocol chip XS2100S, and it was found that the circuit could work normally, and the camera could start normally, indicating that there was an issue with the XS2100S circuit. After consulting the XS2100S datasheet, it was found that the input power was too high at low voltage, causing the protocol chip to automatically protect itself, preventing the subsequent circuit from starting. Finally, by reducing the OCP, the camera was able to work normally.
About Threeda Microelectronics
Threeda is a high-tech enterprise focused on ADC, DAC, PoE, and DC/DC chip-level solutions. We are currently promoting the full range of Qicheng Micro products, hoping to carry forward the 15 years of accumulation and persistence in the power supply industry. Threeda also provides chip-level customization services.
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