The DC Chopper
    Components
  PWM
  MOSFET
  Diode
    Circuitry Safety Features
  Snubber
  Zener
    Benefits
    Cost
   
    Developers:  Jason Allan
  Eric Helander
 
    Supervisor:  Dr.A.K.S Bhat
 
 
 
 
 
 
 
 
 
 
 
 
Product
 
Our product is a motor control unit consisting of a DC chopper circuit that incorporates a pulse wave modulator, a MOSFET and a freewheeling diode. It has small circuitry, is essentially lossless and infinitely variable. All the components are very simple and easily accessible.
 

The DC Chopper
 
A DC chopper uses three main components to create variable speed capability on a 12 volt battery-driven motor. A DC chopper circuit is pictured below.
 
  The MOSFET allows current from the battery to pass through it, but when it allows current to pass through it is governed by the pulse wave modulator (PWM). The PWM creates pulses, and the high section of these pulses turns on the MOSFET. The longer the MOSFET is turned on, the faster the motor spins. Thus, by varying the high section, commonly referred to as the duty cycle, it is possible to vary the speed of the motor. The duty cycle is controlled by a potentiometer which acts as the throttle. Due to the properties of the motor and the characteristics of inductance, a freewheeling diode allows the motor to continue to operate even when it is not drawing any current from the motor. This is illustrated in the timing diagram below.
 
 
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Components
 
The Pulse Wave Modulator
 


The purpose of the pulse wave modulator (PWM) is to provide a gating signal to the MOSFET to turn it on and off. The PWM creates a square pulse whose duty cycle (time in high state divided by its period) is controlled by a potentiometer which the user varies with the throttle.

We constructed this sysmtem by using two 555 timer chips. One chip is assembled using the astable configuration to set the frequency of the pulses. The output of the astable 555 is fed into the trigger of the second 555, which is monostably configured. A potentiometer connected to the monostable 555 is varied to set the duty cycle of the output waveform. The current output of the PWM was not enough to properly turn on the MOSFET, so we added a high current FET driver (a UC2710 chip) to amplify the current from the PWM.

The 555 timer configuration was selected over more complicated PWM chips due to greater reliability and versatility. The cost of the 555 timer is also relatively inexpensive, another important consideration. The configuration of the 555 timers is shown below.

 
 
MOSFET
 
The MOSFET in our circuit acts as a switch. It allows current to flow through it for certain periods of time. These periods are controlled by the PWM current waveforms that flow to the gate of the MOSFET. The MOSFET conducts for the high portion of the gating signal, and does not conduct for the low portion of the gating signal. The higher the duty cycle of these input waves, the longer the MOSFET acts as a closed switch, connecting the source to the motor. We attached a large heatshink to the MOSFET to prevent overheating and breakdown due to large currents.
 
Free Wheeling Diode
 
The free wheeling diode has a unique function in the circuit. It ensures that the output voltage during each "off" time, allotted by the MOSFET, is equal to 0 V. It achieves this by acting as a sink for the motors internal inductance. That is, when the MOSFET stops conducting, the current stored in the motor's inductance discharges itself through the motor and the diode. The observed effect is that the motor continues operation despite drawing no current from the battery.
 
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Circuitry Safety Features
 
The MOSFET is exposed to high currents and a lot of stress, which raises the concern of safety, and preserving the life of the MOSFET. The MOSFET in our design is protected two ways. First, it has a snubber circuit to protect against from the switching stresses of high voltages and currents and to lower the power loss. Second, it has zener diodes to limit the voltage levels between the gate and the source on the MOSFET. The function of these circuits is discussed below.
 
The Snubber Circuit
 
Much of the power lost when using transistors is due to switching. Snubber circuits reduce power losses in transistors during switching and protect them from the switching stresses of high voltages and currents. Switching contributes to a large amount of the power lost when using transistors. Therefore, to conserve energy in our circuit it was a good idea to implement a snubber circuit into our design. The figure below displays the snubber configuration that we used and a simplified version of the rest of the DC chopper.
 
 
Consider the above figure without the snubber circuit addition. When the transistor is operating in the on state, the diode D1 is off, and the MOSFET is carrying the load current. As the transistor turns off, the diode remains reverse biased until the transistor voltage Vm, is equal to the source voltage Vs and the load voltage Vl decreases to 0 V. When the transistor voltage reaches Vs, Id increases to Il and the diode begins to conduct while the MOSFET current decreases to zero. During this operation there is a region where the MOSFET has a current going through it and a voltage across it. Therefore there is a power loss spike as shown in the figure below. A similar effect occurs when the MOSFET is turned on again. There is a build up of voltage and a flow of current simultaneously. These power loss spikes are undesirable and a snubber circuit helps lessen their impacts.
 
 
One purpose of the snubber circuit is to alter the voltage and current waveforms produced during switching to an advantage. With our snubber circuit invoked there is another path for the load current during turn-off and the capacitor in it reduces the rate of change of the MOSFET voltage. It delays the voltage transition from low to high. The capacitor charges to the MOSFET off-state voltage Vm, and remains charged until the MOSFET turns on again. The figure below displays the resulting waveforms and the power loss spike.
 
 


Because the voltage and current levels exist at varying degrees of magnitude, the resulting power loss can be configured so it is a lot lower than normal operation. The size of the capacitor determines the rate at which the voltage changes across the MOSFET at turn-off. There is also a resistance in the snubber circuit. The resistance is chosen such that the capacitor is discharged before the next time the MOSFET turns on. Care is required when selecting the values of capacitance and resistance are to be implanted in a circuit. A large capacitor reduces the power loss in the transistor, but in turn it increases the amount of power lost in the snubber resistor. If designed properly a snubber circuit can reduce the total switching losses, but the most important result is that it reduces the losses in the MOSFET itself which in turn reduces its cooling requirements. This is very important because MOSFET's are quite susceptible to temperature variations and are much harder to cool than resistors. Therefore, the snubber makes the entire design much more reliable.

For our circuit we chose a capacitor value of 0.02 uF. Our PWM operates at approximately 22 kHz. The resistance required for the appropriate discharge of the capacitor is given as follows:

R = ton / (5 * C)

For this calculation duty cycles ranging from 10% to 90% will be considered. The respective on and off times are:

ton1 = 0.01 * 1 / (22 *10^3) = 4.545 * 10^-7 s
ton2 = 0.9 * 1 / (22 * 10^3) = 4.091 * 10^-5 s

Therefore the resistance value parameters are found to be:

R(ton1) = 4.545 O
R(ton2) = 409.1 O

In our circuit we have two 630 O, 0.5 W resistors in parallel. The only function of the two in parallel is to equivalently achieve a 315 O, 1 W resistor. This resistance value easily fits within the defined parameters found above.

Finally, a snubber circuit can also assist in the safe operating ranges of a MOSFET by helping to reduce its voltage and current stresses. As discussed above, the junction temperature must be kept within certain limits. Some of the limits are called FBSOA (forward bias safe operating area) and RBSOA (reverse bias safe operating area) A snubber circuit can alter the trajectory of voltage and current waveforms to make sure they stay well within the safe operating areas. The figure below shows a general illustration of the effects of a snubber circuit.

 
 
In summary, snubber circuits are very effective additions to MOSFET's. They allow the MOSFET to operate longer and harder by lowering its operating temperatures and helping it stay within safe operational areas. They also can be configured to save power losses due to MOSFET switching. In this project, we were dealing with high current and fast switching so a snubber circuit was a very helpful addition to our DC chopper design.
 
The Zener Diodes
 
MOSFETS have safe operating ranges for voltage levels that should never be breached. Zener diodes function to limit the voltage levels between the gate and the source on the MOSFET, providing another layer of safety for our circuit. The zener diode configuration shown in the figure below illustrates this simple but very helpful safety addition.
 
 
From our datasheets for our MOSFET, we found that for continuous operation, the gate to source voltage should not exceed 20 Vdc. For repetitive operation, the gate to source voltage should not exceed 40 Vpk. We will be operating in repetitively because we are driving the MOSFET with pulses. Therefore our operating range lies somewhere within 20 to 40 V. By using two zener diodes, connected as shown above, we ensure that the voltage from gate to source will not exceed 24 V. This safety measure keeps these voltage levels in check, avoiding damage to the MOSFET.
 
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Benefits
 
There are many benefits of the DC chopper circuit modification are numerous. The most obvious benefit is the ability to have absolute control over the acceleration of the scooter. This has many uses. The operator can now maintain a constant velocity that isn't the scooters full speed. The scooter was slated to reach speeds up to 13 mph. But with this modification it has the ability to moderate the speed and ride at at any desired speed. It makes riding in general much easier, as the scooter no longer rapidly accelerates with little warning, and now you can easily navigate tight spaces or just match the speed of a person walking beside you.

Another less obvious benefit is that there can be a much smaller current draw at start up. Rather than the all-or-nothing approach as designed by the manufacturer, the user can now control the acceleration rate which in turn controls the amount of current drawn from the battery. Because the amount of large current draws is reduced, the life of all the electrical components is increased. An additional advantage of drawing less current is that it can actually increase the time that a battery can operate under one charge. It is analogous to driving a car with the gas pedal all the way down to reach a certain speed, coasting, then flooring it again. Gasoline consumption would be astronomical and it would be hard on the engine, compared to someone driving a car with the gas pedal mediating their speed.

 
 


The above figure above illustrates the difference in current draw between the manufacturer's switch throttle and the modified DC chopper circuit. In the above scenario the scooter is started from rest and the user rides some flat distance at a constant rate. All of a sudden the user must slow down to navigate a tight space. The red current spikes represent the user periodically turning on the motor to keep their speed up, as would be necessary with the scooter's original circuitry. These large current draws are taxing on the scooters battery and decrease its operating time. Finally the user goes up to speed and again and rides on flat ground until the battery runs out of power, represented by the vertical red line plummeting to zero as the scooter stops.

With our DC chopper system the acceleration can be controlled at a constant current while maneuvering through the tight area. This avoids current spikes and helps the battery operate longer. At start-up and when the user accelerated up to cruising speed, the rate of increase was carefully controlled to reduce the amount of current drawn.

The combination of greater user control over speed, extended battery use and extended life of components clearly demonstrate the advantages of the DC chopper system over the manufacturer's switch operation.

The design criteria of ease of installation and reasonable cost without reducing the top speed available were all met, resulting in a practical, desirable product.

 
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Cost
 
Component Price
IRF 540 MOSFET 1.93
Diode

1.56

ICM7556 2.11
UC2710 FET Driver 9.32
2 Prototype Circuit Boards 14.67
2 12V Zener diodes 2.67
Various resistors and capacitors N/A
Total

32.26