The block diagram for the Basic PWM Controller is shown in the figure below:
The PWM Controller has two DC supply regulators, i.e. 5V and an isolated 12V power supply. The Accelerator voltage is fed to the PWM Micro Processor. The resulting PWM signal is connected to the Opto-coupler 1 input to provide isolation from the EV high voltage battery. The Opto-coupler 1 output is supplied to the MOSFET gate driver circuit. The PWM pulses are shaped and amplified here to drive the ten MOSFETs.
The Power Supply circuit is shown below:
The PWM Micro Processor circuit is depicted in the next picture:
The Opto-Coupler1 and MOSFET driver circuits are shown below:
A safety relay is provided to ensure that when the Accelerator voltage is zero, the MOSFET gate voltage is also zero.
The PWM Controller is based on the ATmega8 processor.
The program is written in Basic Language. The listing is given below.
The program was created and compiled with BASCOM AVR and the programmer used is the USB-ISP programmer, both available from www.mcselec.com
This no-clutch conversion requires large current when the motor RPM is low. (low EMF). Due to the PWM technique used the peak currents through the MOSFETs will be proportional to the PWM ratio. Therefore the software program fixes the PWM ratio for the first three Accelerator positions to 1/4. This prevents the peak current to exceed 4 times the required current. T1 and T2 is in the ratio of 1 to 4 as shown below.
This PWM ratio is too high for smooth take-off and therefore the software program will switch this PWM signal on and off at a ratio of T3/T4 as shown in the picture. This ratio is varied from 1/4 to 1/2 to 3/4 and then normal PWM control for the rest of the Accelerator positions. (There are 10 Accelerator positions.)
The PWM frequency is 8 kHz and the T3-T4 frequency is 50 Hz.