LED TV Basics: How The PWM Controlled Power Supply Works

Hello friends, M. A. Mustafa here. This post is a preview from my upcoming 'LED TV Basics and Servicing' PDF book. I hope this chapter on PWM-controlled power supplies helps you master LED TV basics and repair.

The power supply circuit is the heart of modern LED TVs. As you know, different brands of TVs use varied power supply circuit designs, but even with different designs, their core operating technology is almost the same. While older TVs used "voltage mode power supplies," modern TVs widely use current mode PWM switch-mode power supplies (SMPS). For this reason, it's not possible to provide detailed explanations for all types of circuits in one book, which could be confusing.

To repair all models of LED TVs on your own, without relying on YouTube videos or other people, it is crucial to have a clear understanding of how the power supply works. For your convenience, I have selected a common schematic circuit diagram of a PWM-controlled LED TV's power supply. In this chapter, we will provide a step-by-step, detailed explanation of what each component does. To avoid repeatedly flipping pages to refer to the diagram, you can print a copy and keep it with you as you read.

In this chapter, we will discuss the current mode PWM SMPS widely used in today’s LED TVs. As its technology is nearly the same across all types of TVs, it will be easy for you to understand. Here, a tested and complete SMPS schematic circuit diagram is used to provide a detailed explanation of its operation, potential problems, and simple solutions. By understanding this, you will be able to repair any modern LED TV power supply circuit.

The schematic circuit diagram above may seem quite complex at first glance. To overcome this complexity and make the subject incredibly simple, you need to understand what each component does individually. To learn this, you must read the following sections with patience.

SMPS Component - F1

F1 is a fuse. It is typically made from an alloy with a low melting point. In the event of a short circuit or overload in the SMPS section, it melts quickly, cutting off the power flow and providing protection to the rest of the circuit.

SMPS Component - L1

L1 is called a Common Mode Choke or EMI Filter Transformer. It is an extremely important part of the power supply circuit.

Function of the Common Mode Choke:

Its main job is to filter out high-frequency noise or interference (EMI) that can come from two directions:

External Noise:

This choke blocks harmonic frequency noise generated by external sources like factory motors, welding machines, or lightning, which enters the power supply circuit through the AC line.

Internal Noise:

It also prevents the high-frequency noise generated by the SMPS circuit from spreading out through the AC line and causing noise on the screens of other TVs.

The choke is designed to block common-mode noise (noise that flows simultaneously on both lines) while allowing normal AC current to pass through without interference. This makes the circuit more stable and reliable.

Fig 1: A complete schematic circuit diagram of a PWM-controlled SMPS for an LED TV, explaining the functions of each component in this guide.

SMPS Component - D1

D1 is a Bridge Rectifier, which is internally composed of four silicon diodes. Its main job is to convert the incoming AC voltage into a full-wave DC voltage. This DC voltage is not smooth but pulsating and must be smoothed out by a capacitor. AC is input at its Pin-2 and Pin-4, while negative voltage is output from Pin-3 and positive voltage from Pin-1. This rectified voltage serves as the operating voltage for the SMPS circuit. Depending on the line voltage, this operating voltage will be between 130V DC and 300V DC.

SMPS Component - C1

C1 is a high-voltage electrolytic capacitor. Its value usually ranges from 100µF to 220µF, depending on the TV’s size and load, and its voltage rating is typically 400V. The job of this capacitor is to smooth the pulsating DC voltage received from the bridge rectifier (D1). Because it filters the voltage pulses, it is called a filter capacitor. The positive voltage is supplied from this capacitor's positive pole, which serves as the SMPS circuit's operating voltage or main positive supply.

SMPS Components - R3 & C2

R3 is a Startup Resistor with a value of approximately 82KΩ. In the circuit diagram, you'll see that it connects the main positive supply to the PWM IC's VDD pin. Its primary function is to provide the initial operating voltage to turn on the IC.

On the other hand, the C2 capacitor is connected from the VDD pin to the ground. It filters and smooths this operating voltage. The PWM IC's internal Zener diode typically holds this voltage steady at 5V.

SMPS Components - D3, R2, & Auxiliary Winding

In this section, the Auxiliary Winding (Coil B), along with the D3 Diode and R2 Resistor, forms a Feedback Supply Circuit. If you look closely at the circuit diagram, you'll see that it functions as a half-wave rectifier circuit.

The Function of the Circuit:

The main job of this circuit is to provide a continuous and stable operating voltage to the PWM IC. The startup resistor (R3), which we discussed earlier, only provides the initial voltage to start the IC. Once the circuit is running, the PWM IC gets its necessary voltage from this auxiliary winding. It holds the IC's VDD pin at a specific voltage (such as 5V or 15V), allowing it to work efficiently.

Functions of the Components:

• D3 Diode: This diode converts the pulsating AC voltage from the transformer's auxiliary winding into a DC voltage.
• R2 Resistor: This resistor filters the voltage and controls the current, ensuring that the correct and stable voltage reaches the PWM IC's VDD pin. In this way, the auxiliary winding ensures the long-term performance of the circuit.

SMPS Component - T1

The T1 SMPS Transformer is a Ferrite Core Transformer. The main reason for using a ferrite core is its ability to operate at much higher frequencies than a conventional iron core. As a result, the transformer is smaller and much more efficient.

This process ensures that one part of the circuit is electrically isolated from the other, which is crucial for human safety.

SMPS Component - Q1 & R1

Q1 is an N-channel MOSFET. In an SMPS circuit, it works as a continuous electronic switch, which is why it's called a switching MOSFET. It is essential to know how its three pins—Source, Drain, and Gate—function.

• Source Pin: When the MOSFET is turned on, electrons flow from the Source pin toward the Drain. This Source pin is connected to the ground through a 0.22Ω resistor, R1, which is used for current sensing.
• Drain Pin: To perform switching, the MOSFET's Drain pin requires a positive voltage supply. This voltage is provided to the Drain pin through the SMPS transformer's primary coil (A). When the MOSFET turns on, current flows through this primary coil. When it turns off, a voltage spike is generated in the coil. This process transfers power to the transformer's secondary coil.
• Gate Pin: The Gate pin is the MOSFET's control pin. It works like a switch, directly receiving the clock signal from the PWM IC. When a Logic-1 signal arrives at this pin, the MOSFET turns on and allows current to flow from the Drain to the Source. When a Logic-0 signal arrives, the MOSFET turns off, and the current flow stops. This continuous on-off process is how the MOSFET performs the switching function of the SMPS.

SMPS Component - U1 (PWM IC)

PWM IC Pin Functions

• GND Pin: This pin provides the negative voltage supply for the IC’s internal circuits. It is connected to the DC ground of the live line section.
• FB (Feedback) Pin: The FB or voltage feedback pin is connected to the optocoupler’s collector to keep the output voltage stable. The PWM Duty Cycle is determined by the voltage level of this pin.
• RT (Resistor-Transistor) Pin: This pin is used to determine the frequency of the IC's internal oscillator. It is connected to the ground via a resistor (R5). The value of this resistor sets the frequency range of the oscillator.
• CS (Current Sense) Pin: This is the current sense pin. It is connected to the SMPS regulator output MOSFET's Source terminal and monitors the current flow.
• VDD Pin: VDD is the IC's main power supply pin. This pin provides the positive voltage supply to the DP2263 IC's internal circuit, allowing it to function.
• Gate Pin: The Gate pin is connected to the external MOSFET’s gate terminal. The PWM signal pulse is output from this pin, which turns the MOSFET on and off.

The number of this PWM IC is OB2263CP. For detailed information about the IC, its internal circuit, and more, the OB2263CP IC Datasheet PDF is available here.

SMPS Components - U2, TL431, C4, R6, R7 (Isolated Feedback Circuit)

The Main Function of the Feedback Circuit:

The main purpose of this circuit is to keep the output voltage (+12V OUT) stable at a specific level. If the output voltage increases for any reason, this circuit sends a signal to the PWM IC (via the FB pin), telling the IC to decrease its switching duty cycle. Conversely, if the voltage decreases, the duty cycle is increased. The entire process is completed with electrical isolation between the primary and secondary circuits via the optocoupler, which is essential for safety.

Component Descriptions:

• R7 & R6 (Voltage Divider): These two resistors together form a voltage divider. They take a small sample of the output voltage (+12V OUT) and send it to the TL431's REF pin. The ratio of the values of R6 and R7 determines the final output voltage.
• TL431 (U2): This is a precision voltage reference and error amplifier. It turns on when the voltage at its REF pin exceeds a fixed internal reference voltage (2.5V). The TL431 compares the voltage received from R6 and R7 with its internal 2.5V reference. Based on this comparison, it controls the optocoupler's LED.
• PC817 Optocoupler (U3): This is the core part of the isolation device. Based on the TL431's signal, its internal LED lights up and turns off. The light from the LED activates the phototransistor on the primary side, which then sends an electrical signal to the PWM IC's FB pin. In this way, information is transferred between the primary and secondary circuits without a direct electrical connection.
• C4: This is a compensation capacitor. Its main job is to stabilize the feedback loop and prevent unexpected fluctuations or oscillations in the output voltage.

The entire process is as follows: R6/R7 > TL431 > Optocoupler > PWM IC. This circuit ensures that the output voltage remains precisely stable at all times.

SMPS Components - D4, C5, & Coil C

This part of the circuit, consisting of D4, C5, and Coil C, is the output section of the SMPS. The main job of this section is to convert the high-frequency AC voltage from the transformer into a DC voltage and send it to the various parts of the TV.

Functions of the Components:

• 'C' Secondary Coil: This is the transformer's output coil. Due to the high-frequency switching in the primary coil, a high-voltage AC wave of the same frequency is generated here. The voltage level is determined by the turns ratio of the transformer's coils.
• D4 Diode: This is a rectifier diode. Its main job is to convert the AC voltage from the secondary coil into a unidirectional, pulsating DC wave. It allows current to flow in only one direction, which blocks the negative portion of the AC wave. The diodes used in this type of circuit are Fast-Recovery or Ultrafast-Recovery diodes. Since the "reverse recovery time" of a normal diode is too slow for it to work at high frequencies, these special types of diodes can turn on and off very quickly, which is essential for an SMPS circuit.
• C5 Capacitor: This is a filter capacitor. Its job is to smooth the pulsating DC voltage from the diode and convert it into a stable DC output voltage. Much like a reservoir, it holds charge from the voltage pulses and releases that charge when the voltage drops, keeping the voltage level almost constant.
• 
  

These three components work together to convert high-frequency AC into a specific and stable DC voltage, which is indispensable for the TV circuit.

Now that you understand the theory behind PWM power supply circuits, watch this video for a practical demonstration of common problems and their real-world solutions.