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

 

1. **Determine Desired Output Voltage**:

   – Decide the output voltage (VOUT) you need for your application.

 

2. **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.

 

3. **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:

 

1. **Choose \( R2 \) (e.g., 1kΩ)**:
2. **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

 

1. **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.

 

2. **Design the Voltage Divider**:

   – Connect R1 between VOUT and REF.

   – Connect R2 between REF and GND.

 

3. **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

 

1. **Fine-Tune Resistor Values**:

   – Adjust R1 and R2 values to fine-tune the output voltage to the desired level.

 

2. **Testing**:

   – Power up the circuit and measure the output voltage.

   – Ensure the feedback regulation works correctly and maintains the desired output voltage.

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