How power electronics work
Modern power conversion systems are based on a continuous fast switching rather than the traditional use of transformers. Each conversion system consists of several components, with three core components :
- a controller (giving a signal, to tell what to do, in the form of varying voltages)
- a transistor (a type of switch that will turn on or off based on the voltage applied. Today MOSFETs of other FETs are used as well as IGBTs. (MOSFET stands for metal-oxide-semiconductor field-effect transistor and IGBT for insulated-gate bipolar transistor)
- a gate driver (amplifying the signal from the controller to the transistor)
Additional components such as resistors, capacitors, transformers and inductors are also required depending on the type of conversion and power.
Power conversion is achieved by a controlled repeated switching of the transistor which allows to transform the incoming electricity into the desired type. How this exactly is achieved has all to do with sinusoidal wave forms, frequencies, pulses, modulators, Fourrier transformation and other state of the art engineering techniques.
The key issue with that switching on and off is that energy is lost in the process.
This has to do with the way transistors function.
How transistors work
For a transistor to be switched on, a high enough input (gate) voltage is required. This happens by charging the transistors’ gate capacitor (think of a capacitor as a battery) to the right voltage. Similarly, to switch the transistor off, the gate capacitor must be discharged.
This charging and discharging of the transistor takes time.
During that time electricity is conducted through the transistor, however it is “lost” in the switching. (energy is never “lost”, in this case it is rather converted in heat)
As you can see from the graphs, if the capacitor is (dis)charged 2x faster, the wasted energy is in this case 4x smaller.
By reducing charging and discharging time, less energy is wasted and more efficient energy transfer is achieved.
Why is conversion efficiency important?
Because energy is converted multiple times between production and consumption. Improving efficiency allows to save a lot of energy and it allows for smaller and lighter conversion systems. The picture illusatrates this. Up to 35% of energy is lost in this example.
Why is that ? That's because conversion loss is electrical energy transformed in heat.
Heat needs then to be dissipated and this requires additional voluminous components. For example active cooling device (cooling fan (big and consuming energy) or a heat sink piece of metal attached to the switch to help it cool down (think of it as a radiator that diffuses heat).
From the above one can thus understand the critical importance of fast (dis)charging of a transistor.
Not only does it reduce losses it also allows for faster switching cycles, the total time required to perform one switch-on switch-off cycle (a duty cycle).
And now you’re about to discover why switching speed is important in power electronics.
Switching Speed
Let’s say you want to transfer 10L of water from one place to another within one unit of time. You can either use a 10L bucket and transfer it once, of you could use just a 1L bucket and transfer it 10x in the same amount of time (i.e transfer 10x as fast).
Now, imagine the bucket is proportional to the size of the components required in a conversion system. If you can switch 10x faster, components can become 10x as small …. it’s magic! (no, just physics)
If you’ve read the above, you understand the critical role of power switches like MOSFETs.
That’s the reason why everybody in the power electronics world is so enthusiastic about WBG (wide band gap) materials such as GaN and SiC that enable switching speeds 10x higher than with silicon.
This is truly great …. Provided you have the right gate driver. Remember one of the 3 components in an electric conversion system, the amplifier?
You guessed it, it’s useless to have a fast switch if ….it cannot be driven fast enough !
The Gate Driver
As mentioned above, a gate driver is an electronic component that uses the low power input of a controller and amplifies it to control (drive) a transistor.
Gate drive (or equivalent) circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A driver without sufficient drive to switch rapidly may be destroyed by excess heating
Another way to see gate drivers is as the interface between low voltage and low current and high voltage and high currents. That’s sounds tricky and dangerous and you’re right. That’s where isolation comes into the picture
