There are three parameters which may damage the MOSFETs, namely:
- High Temperature
- High Current
- High Voltage Spikes.
The Optional Functions Block Diagram is shown in the picture below:
The following optional electronics may be added to the Basic controller electronics in order to prevent damage.
High Voltage Spikes:
A Voltage Spike Detector circuit is used to cut off the PWM signals to the MOSFET gates. These spikes originate from unwanted stray inductance in the controller layout and they are proportional to the instantaneous current that flows through the motor and MOSFETs. The spike detector output is latched when triggered. The PWM signals are then prevented to switch the MOSFETs. The spike detector can only be reset by the PWM Micro Processor when the Accelerator has been returned to zero, using a gate circuit. The trigger level can be set by the input resistor ratio. The comparator is set for 6V. A 1k ohm and a 30k ohm resistor (2 x 15k ohm in series) will set the trigger point for 186V. (The MOSFET safe Drain Source Voltage is 200V). The spike detector circuit diagram is shown below:
Due to the clutchless design, the initial currents through the motor may be extremely high. This occurs at low RPM with low EMF. An over current latch circuit may be used to prevent damage to the MOSFETs. The circuit below uses a DIY current sensor (described under Current Sensor, see menu). The sensor output is amplified and fed to a comparator. It is recommended that the circuit is calibrated for 400A. The current detector can only be reset by the PWM Micro Processor when the Accelerator is returend to zero, using a gate circuit. See circuit below:
The heatsink temperature of the MOSFETs is measured with the aid of a temperature sensor IC. The output of this sensor is fed to the PWM Micro Processor which will stop the PWM signals going to the MOSFET gates until the temperature returns to normal. The software can be set for a safe temperature.
PWM Micro Processor Circuit for the Optional Functions:
The PWM Micro Processor circuit for the optional functions is shown below. The temperature sensor output, spike-detector-latch-reset-signal and current-sensor-latch-reset-signal connections are indicated in this circuit diagram. The diagram also shows a safety circuit for the MOSFET gate inputs. The safety circuit will disconnect the power to the output stage of the gate drivers when the Accelerator returns to zero. This prevents faulty conditions at the MOSFET gates which may switch on the MOSFETs permanently.
The spike detector latch output and the current sensor latch output are connected to the PWM Gates Circuit as indicated in the PWM Gates circuit below: