To prevent fires caused by high-powered appliances, many universities have issued "power rationing orders," setting maximum power consumption and total power limits for dormitories. Students' washing machines, air conditioners, and hair dryers often exceed these limits and become unusable. As a result, a "dormitory-specific" power converter has become a hot seller. Electricity meters only limit the wattage of resistive appliances with a heating element ("tungsten filament"). Other appliances, such as computers and desk lamps, which lack this heating element, are not subject to wattage restrictions. The "power converter" exploits this design flaw in electricity meters, disguising itself as a resistive appliance like a computer.
Do not use high-power appliances for extended periods under load, as negligence may lead to safety hazards. Unplug appliances when not in use. Minimize the use of high-power appliances, considering the fragile school power supply system, to reduce unnecessary safety risks! 1. The total power of all connected appliances must not exceed their rated power. 2. Do not bundle the wires during use to prevent abnormal overheating. 3. Do not block the ventilation holes during use.
This article briefly describes the power conversion and transmission process in converter circuits. It addresses the problem of voltage and current spikes generated by MOSFET switching devices during turn-on and turn-off, which in turn cause electromagnetic interference. By comparing the structure and parameters of traditional planar MOSFETs and superjunction MOSFETs, the article seeks to find a solution using superjunction MOSFETs that produces better performance.
I. Working principle of power converter circuit
A power converter circuit is a circuit that converts power supply voltage or current into the desired voltage, current, and frequency. Its working principle mainly involves controlling the on/off state of a switching transistor to change the power supply voltage, thereby achieving the required output voltage, current, and frequency. Its main workflow includes:
1. Input power transformer
The input power of a power converter circuit is usually AC power, which needs to be transformed by a transformer to convert it into the required voltage and current.
2. Rectification and Filtering
The voltage output from the input power transformer contains both AC and DC components. To ensure the stability of the output voltage and the normal operation of the circuit, rectification and filtering are required to remove the AC components and retain only the DC components.
3. Power switching circuit
The power switching circuit is the core component of the power converter circuit. Its function is to control the conduction and cutoff of the switching transistor, thereby controlling the change of the power supply voltage and realizing the adjustment of the output voltage.
4. Output filter circuit
The output filter circuit is used to filter out noise signals in the output voltage, ensuring the stability and quality of the output voltage.
II. Application Scenarios of Power Converter Circuits
1. Power adapter
Power adapters are one of the most common applications of power converter circuits. Their main function is to convert AC power into the required DC voltage and current to supply various electronic devices.
2. Motor driver
Power converter circuits are also commonly used in motor drivers to control the motor speed and direction by controlling the power supply voltage and frequency.
3. Solar panel controller
A solar panel controller is a device that converts the electrical energy output from a solar panel into the required voltage and current. It includes a power converter circuit that regulates the output electrical energy of the solar panel by controlling the power supply voltage and frequency.
This article briefly describes the power conversion and transmission process in converter circuits. It addresses the problem of voltage and current spikes generated by MOSFET switching devices during turn-on and turn-off, which leads to electromagnetic interference. By comparing the structure and parameters of traditional planar MOSFETs and superjunction MOSFETs, this article seeks to identify the reasons why using superjunction MOSFETs generates worse electromagnetic interference, and analyzes and improves the process.
With the continuous development of switching power supply technology, power MOSFETs, as one of the core electronic components of switching power supplies, suffer from switching losses, which are a major source of energy loss. Based on the fundamental principles of energy saving and loss reduction, power MOSFET technology is evolving towards improving switching speed and reducing on-resistance. COOL MOSFETs are a new type of superjunction power MOSFET with lower on-resistance, faster switching speed, and higher power conversion efficiency. However, the ultra-fast switching performance of superjunction MOSFETs also brings unnecessary side effects, such as higher voltage and current spikes and poorer electromagnetic interference.
At the instant the switching transistor is turned on, since the voltage across the capacitor cannot change abruptly, the voltage across the stray capacitor Cp is initially negative at the top and positive at the bottom, generating a discharge current. As the switching transistor gradually turns on, the power supply voltage Vin charges the stray capacitor Cp, and its voltage becomes positive at the top and negative at the bottom, forming a current spike flowing through the switching transistor and Vin. At the same time, the Cds capacitor discharges through the switching transistor, also forming a current spike, but this spike current does not flow through Vin, only forming a loop inside the switching transistor. In addition, if the converter operates in CCM mode, because the voltage across the primary inductor Lp decreases, the diode D begins to withstand reverse bias voltage and is turned off, causing a reverse recovery current. This current is coupled to the primary side through the transformer, also forming a current spike flowing through the switching transistor and Vin.
During the turn-on phase of the switching transistor, diode D is cut off, the voltage across capacitor Cp is Vin, and the current through the primary inductor Lp increases exponentially, approximately linearly.
At the instant the switching transistor is turned off, the primary current id charges Coss. When the voltage across Coss exceeds the sum of Vin and nVo (the voltage reflected back from the secondary coil of the transformer to the primary coil when diode D is turned on), diode D is forward biased and turned on under the voltage generated by the freewheeling current of the primary inductor Lp. Lk and Coss resonate, generating high-frequency oscillating voltage and current.
During the turn-off phase of the switching transistor, diode D is forward-biased and turns on, releasing the energy previously stored in Lp to the load. At this time, the secondary coil voltage is clamped to the output voltage Vo, and coupled back to the primary side through a transformer with a turns ratio of n, causing the capacitor Cp voltage to be charged to nVo (positive at the bottom and negative at the top). The voltage across the primary inductor Lp is also clamped to nVo. When the freewheeling discharge of Lp ends, D is reverse-biased and turns off, and Lp, Coss, and Cp resonate, causing the voltage across Cp to decrease.
The working principle of a power converter circuit mainly includes the following key steps and components:
Power electronic devices: These are the core components of a power converter, including diodes, thyristors, MOSFETs, and IGBTs. These devices control the actual conversion of electrical energy through rapid switching. For example, in a chopper, the current in the inductor can be controlled by fast-switching devices to regulate the output voltage.
Control Circuit: The control circuit typically consists of a microprocessor, sensors, and feedback circuitry. It continuously adjusts the operating state of the power electronic devices based on actual output demands to ensure output voltage stability. For example, when the output voltage is lower than a set value, the control circuit increases the duty cycle to raise the output voltage.<sup>1</sup> Output Circuit: The output circuit receives the converted electrical energy and supplies it to the load. The output terminal may contain more filtering components to ensure output stability.<sup>1</sup>
Why didn't the power station directly transmit DC power in the first place?
Because our electricity is usually generated in remote mountainous or coastal areas, AC voltage is more advantageous when transmitting electricity from these areas to urban areas. Transmitting AC voltage using a high-voltage, low-current method can reduce transmission losses. High-voltage electricity is converted to 220VAC in stages by the power station before being transmitted to homes.
What are some common ways to convert AC to DC?
There are generally two ways to convert AC to DC:
1. Transformer conversion
2. Switching mode conversion
How is the conversion performed, and what are the underlying principles?
① Transformer conversion:
1. Transformer conversion first uses a low-frequency transformer (because the frequency of AC high voltage is 50-60HZ) to convert AC high voltage into AC low voltage.
2. Then, the stepped-down AC voltage is converted to DC voltage through rectification.
3. However, since the DC ripple after conversion is too severe, we need to find a way to reduce the ripple. At this time, we can use capacitor filtering to smooth the voltage.
A diode-based rectifier circuit, when used to measure AC signal voltage or convert AC signals to DC signals, exhibits suboptimal linearity and accuracy. This circuit utilizes an absolute value circuit composed of an operational amplifier (OP amplifier) because it converts the input signal to its average value using an equalization capacitor, thus achieving high accuracy even with a small input voltage. While sinusoidal voltages can be represented by their average value, measuring pulse waveforms using the average value makes it difficult to obtain the optimal value.
OP amplifier A1 is an AC amplifier with a gain of 10. The required gain depends on the magnitude of the input signal, and the gain A can be calculated using the formula A = 1 + (R3/R2). The low-frequency FL of A1 is approximately 1.6 Hz (FL = 1/2πC1.R2 ≈ 1.6). C1 and C3 are also related to the low frequency. Both have a capacitance of 10uF, but because they are connected in series, the total capacitance is 5uF, and its FL is approximately 0.5 Hz. OP amplifiers A2 and A3 are standard absolute value circuits. A2 is a negative output half-wave rectifier circuit, where the output and input signals are added together for full-wave rectification. The ratio of the adding resistors R6 and R7 is crucial; their ratio is 2:1, so 150K and 75K resistors from the E series are selected. C4 is the equalization capacitor; its capacitance must be determined based on the frequency range of the input signal. If the capacitance is too small, rectified ripple will occur. On the other hand, the output response also depends on C4; if rapid measurement is required, its capacitance cannot be too large. The output is the average value of the full-wave rectification, which for a sine wave is equal to 2/π (0.636EI) of the peak voltage, and this should be noted.
