The E-bike has 3 main parts. The battery, the controller and the motor:
The controller is basically used to send the battery's power to the motor in a controlled way. It also acts as the "brain" for the e-bike similar to a car's ECU.
Inside the controller are little transistors and some other parts. Those transistors are like little pumps that send power to the motor and warm up as they get used.
Image of a typical transistor source: www.adafruit.com
At some power level they get too hot and can easily stop working. To keep them from overheating you can do a few basic things:
Reduced the power (nobody wants to do that!)
Blow air or run liquid over the controller or add cooling fins etc.
Use more efficient transistors.
Add more transistors!
In this quick video we discuss :
CrossCurrent, OceanCurrent, CrossCurrent AIR
Due to the compact size, it can fit many geometries of bikes. The power has to be better managed due to the heat build up. It is more economical and is the most commonly used controller for mass produced e-bikes.
CrossCurrent S, CrossCurrent AIR, OceanCurrent.
The 9 Transistor controller is physically larger than the 6 transistor and can have some difficulty to fit inside the frame without hanging the controller on the outside of the frame.
It is more expensive, but it is easier to manage the controller's temperatures if you run them at the power level of the 6 transistor controller.
HyperFat and other high power bikes.
The classic 12 transistor controller is longer and has a lot of trouble to physically fit on the space provided on the frame.
It is also more expensive and most bikes may not need such big controllers. To really take advantage of the controller, you need a battery that can deliver enough power.
Most standard e-bike battery packs just cannot deliver enough power without hurting the packs. Smaller motors will overheat with the power that a 12 Transistor can deliver.
This controller takes up more space and needs a bigger battery, so eventually you run out of room on the bike and need to make some design considerations on the frame. The bike gets heavier and then needs more battery etc. If we continue down this road, you end up with a heavy electric bicycle or a lightweight motorcycle - both hard to pedal.
A 12 Transistor controller is used on the HyperFat which has a relatively big and physically short battery. The motor is also beefed up to handle the power.