There are two types of electricity in the world
👉One type is alternating current (AC).
Its direction changes from positive to negative.
👉One type is direct current (DC).
Its direction never changes.
The electricity in our household sockets is alternating current (AC).
The electricity generated by the battery is direct current (DC).
Sometimes, we need to convert alternating current (AC) to direct current (DC), such as in a mobile phone charger.
Sometimes, we need to convert direct current (DC) to alternating current (AC), such as in the uninterruptible power supply (UPS) of data centers.
Sometimes, we need to increase or decrease the voltage of direct current (DC), and mobile phones contain many boost circuits.
Sometimes, we need to change the frequency and amplitude of alternating current, such as in inverter air conditioners and inverter refrigerators.
The art of these alternating currents, both perpendicular and perpendicular, high and low, is power electronics.
On the stage of power electronics, our power device dream team is absolutely dominant, including but not limited to: diodes, MOSFETs, IGBTs, HEMTs, IGCTs, GTRs…
You may or may not be familiar with their names, but they all serve the same purpose: to either conduct or cut off the current.
Yes, it's just an electronic switch, not much different from the wall switches in our homes.
Some power devices have two terminals, such as diodes.
Packaged diode; circuit symbol of a diode
It is a one-way lane
Current flows smoothly from the anode to the cathode without obstruction; however, the flow from the cathode to the anode is blocked.
Because a diode only has two terminals, it cannot control its own switching on and off; it must accept current when it comes in and cannot retain current when it leaves.
Some power devices have three terminals; they are controllable power devices, and can loudly declare "I don't want it" no matter how large the current.
Among the top-performing controllable power devices, the IGBT is undoubtedly the most powerful.
An IGBT is a super electronic switch that can withstand extremely high voltages.
The AC voltage in our home sockets is 220V, while the paper-thin IGBT chip can withstand a voltage of up to 6500V. The maximum current drawn by all household appliances when fully turned on generally does not exceed 30A, but a fingernail-sized IGBT chip can carry approximately 200A of current!
The image below shows four IGBT chips and four diode chips mounted on a substrate.
However, exposed chips like these cannot be used directly. We need to encapsulate the chip in a casing, fill the casing with insulating material, and then connect the chip's electrodes to external terminals to form a usable IGBT product.
Some IGBTs contain only one chip, while others may contain a dozen or even twenty. This results in a wide variety of IGBT single transistors and modules. The maximum current of a single-transistor IGBT is around 100A, while the maximum rated current of an IGBT module can reach 3600A!
Single-tube module
In circuit diagrams, IGBTs are generally represented as shown in the image below. G represents the gate, which is used to receive commands. C represents the collector, and E represents the emitter. The collector and emitter are used to conduct current. Normally, the IGBT is off. Once the gate receives a turn-on command, current will continuously flow between the collector and emitter.
It's like a light switch on your wall. Press it once, and the switch closes and the light turns on; press it again, and the light turns off.
Of course, operating the IGBT is no longer done by hand, but by electronic pulses.
When a high-level signal arrives, the device is turned on; when a low-level signal arrives, the device is turned off.
A manual switch might operate once or twice per second, while our electronic switch can operate tens of thousands, even hundreds of thousands of times per second!
This is why we need electronic switches, or power devices.
When several power devices are combined together, they can be called a "topology". Each topology has its own unique features.
A topology consisting of 4 or 6 diodes can convert alternating current (AC) into direct current (DC).
Single-phase rectifier bridge Three-phase rectifier bridge
A topology consisting of 4 or 6 IGBTs or MOSFETs can convert direct current into alternating current;
Full-bridge inverter circuit Three-phase bridge inverter circuit
A topology consisting of IGBTs, diodes, and other passive components can increase or decrease DC voltage.
Boost circuit Buck circuit
The topology seems a bit complicated?
No problem, we can design some logos for them.
Rectifier circuit, Inverter circuit, Boost/Buck circuit
No matter how complex or large an electrical device is, it is composed of these basic topologies and components.
Often, electrical energy undergoes more than one transformation within the same device, such as in electric vehicle charging stations.
Mobile phone chargers typically output 5V, but do you know how high the battery voltage of an electric vehicle can be? It can reach up to 750V!
Batteries can only be charged with direct current (DC), while three-phase industrial power grids use 380V alternating current (AC) (different from the 220V AC of household power grids). So how do charging stations convert 380V AC into 750V DC?
First, the 380V AC power goes through a rectifier circuit and is converted into approximately 500V DC power. At this point, the DC power is not high enough and cannot be adjusted, so it cannot be directly used to charge the battery.
The rectified DC power enters an inverter circuit, which is a topology composed of IGBTs or MOSFETs, as we introduced earlier. Under the control of the gate signal, the IGBTs or MOSFETs switch at high speed, converting the DC power into extremely high-frequency AC power. This AC power is a square wave, and by adjusting the proportion of the high-level signal, the equivalent voltage can be freely adjusted.
The high-frequency alternating current then enters a rectifier circuit, ultimately becoming an output voltage of 250V~750V to charge the car.
The topology of the entire power conversion is shown in the figure below:
It looks a bit complicated, but it becomes simple with symbols: it's just a rectifier circuit + an inverter circuit + a rectifier circuit, like this:
See, power electronic systems are that simple. Just like building blocks, you can put all the functions together. Now you can build your own system.
Within a small charging station, electricity undergoes a complex transformation process: AC-DC-AC-DC. In fact, in the electrical devices around us, from small items like mobile phones and computers to large ones like electric buses and high-speed trains; from readily accessible refrigerators and televisions to unseen factory assembly lines, electrical energy undergoes countless transformations. This art of electrical energy transformation is what we call power electronics. From a ray of sunlight or a gentle breeze, to tens of thousands of volts on a high-voltage transmission line, and finally to the 5V output of a mobile phone charger, this is the romantic journey of electrical energy.
