Why AC Convert to DC?
Most household appliances operate on a 5V or 3.3V DC voltage. Without converting AC to DC, these appliances cannot function.
Why Isn’t DC Transmitted Directly from Power Plants?
Power plants are often located in remote mountainous or coastal regions. AC voltage is advantageous for transmission from these areas to urban areas. High-voltage AC transmission with low current reduces transmission losses. The high-voltage AC is converted to 220V AC at power stations before being distributed to homes.
Common Methods for Converting AC to DC
There are generally two methods for converting AC to DC:
- Transformer Conversion
- Switching Conversion
How Conversion Works and Its Principles
- Transformer Conversion:
Step-Down Transformer:
- The high-frequency transformer (since the frequency of AC high voltage is 50–60 Hz) converts high-voltage AC to low-voltage AC.
Rectification:
- The reduced AC voltage is converted to DC through rectification.
Filtering:
- The rectified DC has a severe ripple, which is smoothed using capacitor filtering to reduce the ripple.
2. Switching Conversion:
Bridge Rectification:
- AC is converted to DC using a bridge rectifier.
Smoothing:
- The voltage is smoothed using capacitors.
Chopping:
- DC voltage is chopped using switching elements.
Step-Down Transformer:
- The chopped high-frequency AC is stepped down using a transformer.
Rectification:
- The output is rectified again using diodes.
Filtering:
- The final DC voltage is smoothed using capacitors.
Advantages and Disadvantages of Each Conversion Method
| Method | Transformer Method | Switching Method |
| Diagram | Simple circuit diagram | Complex circuit diagram |
| Advantages | 1. Simple circuit 2. Less noise, good EMI 3. Inexpensive | 1. High efficiency 2. Less noise, good EMI 3. Inexpensive |
| Disadvantages | 1. Large size, heavy 2. Serious heat generation 3. Poor efficiency | 1. Complex circuit 2. Many high-voltage components 3. Switching noise, poor EMI |
For AC-DC power supplies, common topologies include:
- Forward Converter
- Flyback Converter
- Half-Bridge Converter
- Full-Bridge Converter
Complete AC-DC Circuit Framework
A complete AC-DC circuit typically includes the following modules:
1. Input Filter Circuit: An AC-to-DC power supply typically requires the addition of an input filter circuit. It removes high-frequency noise and interference from the input AC to ensure stable DC output and reduce interference to subsequent circuits.
2. Rectifier Bridge: Converts the AC input into DC. A rectifier bridge is usually composed of four diodes, which can convert the positive and negative half-cycles of the AC into forward and reverse DC, respectively.
3. Filter Circuit: This circuit is used to smooth the output DC and remove ripples. It generally consists of capacitors or inductors forming a low-pass filter, which reduces output ripples by storing or dissipating charge.
4. Voltage Regulator Circuit: Ensures stable output voltage. The regulator circuit usually consists of regulators that adjust the output voltage through feedback.
5. Output Protection Circuit: It protects the load and power circuits. Common output protection circuits include overcurrent protection, overvoltage protection, and short-circuit protection.
6. Control Circuit: It manages the operation of the entire power supply. It typically includes a switch-mode power supply controller, feedback control, and protection circuits.
Example Circuit Analysis
The following is an example analysis using the “HFC0500” power chip:
| Pin | Pin Name | Description |
| 1 | TIMER | This pin combines soft start, frequency jitter, and fixed-time over-load protection (OLP). It also retains power and X-cap discharge functions. |
| 2 | FB | This pin combines soft start, frequency jitter, and fixed-time overload protection (OLP). It also retains power and X-cap discharge functions. |
| 3 | CS | Current sense pin. It detects the primary current and operation status of the switch, providing slope compensation and current limit protection. |
| 4 | GND | Ground pin for the chip. |
| 5 | DRV | Drive signal output pin. |
| 6 | VCC | Power supply pin. |
| 7 | HV | High voltage start-up pin, including power-on and X-cap discharge functions. |
The fuse in the diagram provides protection against abnormal currents. After the fuse, a common-mode choke filters out common-mode interference from the signal. CX1 is an X capacitor, which is placed across the input lines to eliminate differential-mode interference. (Additionally, Y capacitors are placed between the input lines and ground to eliminate common-mode interference.)
1. The rectifier bridge then converts the AC to DC. Interference is filtered out using capacitors. The capacitor, resistor, and diode between C1 and T1 form an RCD snubber circuit, which primarily protects the switching transistor below.
2. Pin 5 of the HFC0500 chip is the drive signal output pin. It controls the switching of the transistor on the right, chopping the DC.
3. The chopped DC is then stepped down by the high-frequency transformer T1 and converted into a square wave.
4. The square wave is then half-wave rectified by the rectifying diode D6 on the right.
5. Finally, the voltage is smoothed using capacitors C10 and C11, producing the desired DC voltage.
6. This figure shows the feedback and voltage regulation sections of the circuit. Feedback control adjusts the output voltage, with the optocoupler providing isolation, feedback signals, and switching functions.
Key Points in PCB Design
1. Power Loop Minimization: When designing the power supply PCB, it is crucial to minimize the power loops. Smaller loops have stronger anti-interference capabilities and generate less interference to the external environment. The three key loops are:
- Input Loop: (C1-T1-Q1-R11/R12/R13-C1)
- Auxiliary Winding Loop: (T1-D4-R4-C3-T1). This loop reduces leakage inductance voltage, decreasing electromagnetic interference and transformer losses. Additionally, it can improve the transformer’s autotransformer ratio and energy transfer efficiency, enhancing its electrical performance.
- Output Loop: (T1-D6-C10-T1).
2. GND Handling: The ground (GND) of the input loop and the control circuit should be separated, and they should only connect at C1.
3. Q1 Heatsink Connection: Connect the Q1 heatsink to the main GND plane to improve anti-interference capability. Due to the strong interference and noise at high-frequency switching points, you can isolate this area by partially hollowing out the circuit board around it.
4. Feedback Line Sensitivity: The feedback line is quite sensitive and easily susceptible to interference. Therefore, the power line and the feedback line should be separated, and the feedback line should be kept as short as possible.
5. Optocoupler Isolation: Hollow out the board frame around the optocoupler to isolate both sides of the optocoupler, ensuring signal integrity and reducing interference.
