The rapid development of electronic information technology has propelled the field of power supply technology forward at a rapid pace, while also bringing unprecedented opportunities and challenges to power supply engineers. From small household appliances to large-scale instruments and equipment used in the power industry, all require power supplies to provide energy, which necessitates a large number of engineers with expertise in power supply to complete the design and development.
In the vast cosmos of the electronic world, power supplies, like an ever-burning star, provide the lifeblood for countless electronic devices. From simple flashlight batteries to complex server power systems, each power supply embodies the wisdom and hard work of engineers. So, how exactly are these amazing power supplies "forged"? Let's unveil their mysteries together.
Power engineers are personnel who design and develop power supplies for switches, communications, and other equipment.
So, how does a seasoned power supply engineer work?
There are ten main points:
1. Receive the power supply design requirements! Assess costs and determine feasible solutions.
2. Based on the customer's quotation, provide the approximate component and production costs, and determine the feasible circuit.
3. Develop a schematic diagram! Determine the selected power transistors, transformers, and the most stable, simplest, and convenient-to-produce schematic scheme.
4. Design the PCB based on the schematic diagram and the sample or enclosure requirements given by the customer.
5. According to the schematic diagram, assemble the appropriate components and adjust the electrical parameters to ensure the machine can operate normally under the minimum requirements.
VI. Apply load test, reaching 80 kW, and check the output waveform, voltage requirements, electromagnetic performance, power transistor temperature, voltage stability, and conversion efficiency. During this process, adjust the parameters of the electronic components appropriately.
7. Enhanced testing! This includes tests for overload, short circuit, low voltage, overvoltage, high temperature, and shock resistance.
8. Based on the sample, determine the accurate parameters of the schematic diagram, establish the orientation diagram and material diagram, and send them to the production department, warehouse manager, and merchandiser to carry out small-batch production of the sample.
9. The sample undergoes rigorous testing. Once all performance parameters are satisfactory, the salesperson sends it to the customer for evaluation. If it passes evaluation, mass production can begin.
10. We will track and improve the project in future production to deliver goods to customers in the shortest time and with the best quality.
None of the ten points above mentioned how the engineers derived their parameters.
In fact, all the parameters engineers use come from experience and debugging. When designing power supplies, 95% of the parameter calculations are unnecessary. For example, for a 1000W inverter, you can use two 55W inverters, because each can reach 500W, or you can use four 40W inverters, because each 40W can reach 350W. Transformers—why would you need to calculate them? But experience is required.
Of course, you can also refer to the completed products to determine the quantity! We also need to verify the number of coils. Circuit parameters for electronic components are limited to a few: resistors, capacitors, diodes, transistors, inductors, thermistors, varistors, MOSFETs, transformers, fuses, integrated circuits, etc.
The combination of various components forms our basic circuit: amplifier, filter, isolation, signal source, voltage regulator, comparator, current amplifier, voltage amplifier, and other resident circuits. Of course, you also need to add some independent circuits that you come up with yourself!
Therefore, the power supply board can be viewed as a unified whole, and then commonly used circuits can be combined to create the necessary components. This approach yields a good product. For new products, it's crucial to use circuits that you are familiar with and confident in. Otherwise, the designed product will encounter numerous production problems.
In simple terms, a power supply is a device that provides suitable voltage, current, waveform, and frequency to electronic devices. For example, a direct current (DC) power supply can be understood as a power source with a zero frequency and a straight waveform. An alternating current (AC) power supply can be understood as a power source that alternately changes voltage (positive and negative values are switched between two electrodes) and has frequency and waveform.
Regardless of the type of power source, its value at any point in time can be solved using differential and integral mathematics, and there is a unique solution. For example, a square wave is composed of sine waves of infinite magnitude. Therefore, a square wave can be decomposed into odd harmonics and new harmonics. We usually take the third harmonic as the value to meet the requirements, and the fundamental wave accounts for the largest part of the power.
Since modern power supplies primarily use transformers with high-frequency magnetic cores, field-effect transistors (FETs) have become the main power devices. As we all know, FETs operate in a switching state, so when used as power transistors, the power supply output is a pulsed square wave. Therefore, power supplies using FETs as power sources carry a significant amount of harmonics and fundamental frequencies.
For power supplies using MOSFETs as switching power transistors, it's important to understand that 90% of the losses generated by MOSFETs occur during the turn-on and turn-off time, due to a significant instantaneous resistance during both periods. Therefore, solving switching power supply problems primarily involves minimizing turn-on and turn-off losses. Harmonics can be addressed using filters.
Another point we need to understand is that field-effect transistors are very sensitive to instantaneous voltage changes, so the power supply for them must have a stable voltage!
Finally, it's important to understand that its gate resistance is very high, so a very small voltage is enough to turn it on, and it basically doesn't require any current. Therefore, a field-effect transistor (FET) is a voltage-controlled device.
Precautions for using field-effect transistors:
1. The power supply voltage must be stable.
2. Control the power-on and power-off losses.
3. Appropriately reduce the gate resistance to prevent false turn-on.
4. A low-pass discharge circuit and a fast-charging circuit are required because the MOSFET has a capacitor at its gate, and the capacitor needs to charge and discharge quickly! Therefore, a totem pole circuit is needed.
How to design a good power supply:
First: Determine your power supply wattage and input voltage. Based on the power supply wattage, select the switching current of the switching transistor, the transformer size, and the input voltage to determine the voltage rating of the switching transistor and the number of input turns of the transformer.
Second: Regardless of the voltage or power supply, any MOSFET used must have a low-pass discharge and fast-charge circuit.
Third: The transformer must have an absorption circuit to absorb harmonics.
Fourth: The gate resistance of the MOSFET should be grounded and pulled low using a resistor.
Fifth: Select an appropriate switching frequency to ensure the lowest static loss while maximizing the MOSFET conversion efficiency.
Sixth: When the input voltage exceeds 75V, the resonant circuit should be considered as a negative auxiliary function circuit.
Seventh: When the power is too high, the integration of PFC circuitry into the design should be considered.
Eighth: The drive signal must be stable and have a voltage of 5V or higher.
Whether it's switching power supplies or inverter power supplies, high-frequency design has become the main technical approach and mainstream! For many people who have never designed power supplies or whose power supply design skills haven't reached the level required for project development, the thought of design immediately brings to mind all sorts of calculation models! This idea is correct, but the approach is wrong!
