SONY GT3 POWER SYPPLY REPAIR
The power supply of the Sony LED TV model KLV-32S550 was fluctuating. The feedback reference, which should be found at pin 2 of the CXD9841P, was missing. This reference is generated by the TL431, and the voltage divider resistors were found to be faulty.
Using a TL431 as a reference and regulator in conjunction with the feedback pin of a power supply IC like the CXD9841P can provide precise voltage regulation. Here’s how to integrate a TL431 to regulate the feedback:
Understanding the TL431
The TL431 is an adjustable precision shunt regulator. It operates like an adjustable Zener diode with a reference voltage of 2.495V typically, and it can be adjusted from 2.495V up to 36V using an external voltage divider.
Circuit Setup
- **Determine Desired Output Voltage**:
– Decide the output voltage (VOUT) you need for your application.
- **TL431 Configuration**:
– The TL431 is typically used with a voltage divider network to set the desired reference voltage.
– The basic setup involves connecting the anode to ground, the cathode to the power supply IC’s feedback pin through a resistor, and the reference pin (REF) to the midpoint of the voltage divider.
- **Design the Voltage Divider**:
– Use two resistors (R1 and R2) to set the reference voltage at the REF pin.
– The reference voltage (VREF) of the TL431 is 2.495V.
Calculating Resistor Values
The output voltage \( V_{\text{OUT}} \) can be calculated using the following formula:
\[ V_{\text{OUT}} = V_{\text{REF}} \left(1 + \frac{R1}{R2}\right) \]
Rearranging to solve for \( R1 \):
\[ R1 = R2 \left(\frac{V_{\text{OUT}}}{V_{\text{REF}}} – 1\right) \]
Example Calculation
Suppose you want an output voltage \( V_{\text{OUT}} \) of 5V:
- **Choose \( R2 \) (e.g., 1kΩ)**:
- **Calculate \( R1 \)**:
\[ R1 = 1kΩ \left(\frac{5V}{2.495V} – 1\right) \approx 1kΩ \left(2.004 – 1\right) \approx 1kΩ \times 1.004 = 1.004kΩ \]
You can use a standard resistor value close to 1.004kΩ, such as 1kΩ.
Implementation Steps
- **Connect TL431**:
– Anode to GND.
– Cathode through a resistor (R3) to the feedback pin of the CXD9841P.
– REF pin connected to the junction of R1 and R2.
- **Design the Voltage Divider**:
– Connect R1 between VOUT and REF.
– Connect R2 between REF and GND.
- **Set Up the Feedback**:
– Connect the feedback pin of the CXD9841P to the cathode of the TL431.
Example Circuit
Here’s an example of the connection:
“`plaintext
VOUT —- R1 —-+—- REF (TL431)
|
R2
|
GND
FB (CXD9841P) — R3 –+– Cathode (TL431)
|
VOUT
“`
Fine-Tuning and Testing
- **Fine-Tune Resistor Values**:
– Adjust R1 and R2 values to fine-tune the output voltage to the desired level.
- **Testing**:
– Power up the circuit and measure the output voltage.
– Ensure the feedback regulation works correctly and maintains the desired output voltage.