Pulse Width Modulator for 12 and 24 Volt applications
This circuit was featured in an article in Home Power Magazine #75
(C) G. Forrest Cook 1999
A pulse width modulator (PWM) is a device that may be used as an efficient light dimmer or DC motor speed controller. The circuit described here is for a general purpose device that can control DC devices which draw up to a few amps of current. The circuit may be used in either 12 or 24 Volt systems with only a few minor wiring changes. This device has been used to control the brightness of an automotive tail lamp and as a motor speed control for small DC fans of the type used in computer power supplies.
A PWM circuit works by making a square wave with a variable on-to-off ratio, the average on time may be varied from 0 to 100 percent. In this manner, a variable amount of power is transferred to the load. The main advantage of a PWM circuit over a resistive power controller is the efficiency, at a 50% level, the PWM will use about 50% of full power, almost all of which is transferred to the load, a resistive controller at 50% load power would consume about 71% of full power, 50% of the power goes to the load and the other 21% is wasted heating the series resistor. Load efficiency is almost always a critical factor in solar powered and other alternative energy systems.
One additional advantage of pulse width modulation is that the pulses reach the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily. Finally, in a PWM circuit, common small potentiometers may be used to control a wide variety of loads whereas large and expensive high power variable resistors are needed for resistive controllers.
The main Disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference (RFI). RFI may be minimized by locating the controller near the load, using short leads, and in some cases, using additional filtering on the power supply leads. This circuit has some RFI bypassing and produced minimal interference with an AM radio that was located under a foot away. If additional filtering is needed, a car radio line choke may be placed in series with the DC power input, be sure not to exceed the current rating of the choke. The majority of the RFI will come from the high current path involving the power source, the load, and the switching FET, Q1.
PWM Frequency: 400 Hz
Current Rating: 3 A with an IRF521 FET, >10A with an IRFZ34N FET and heat sink
PWM circuit current: 1.5 ma @ 12V with no LED and no load
Operating Voltage: 12V or 24V depending on the configuration
The PWM circuit requires a steadily running oscillator to operate. U1a and U1d form a square/triangle waveform generator with a frequency of around 400 Hz.
U1c is used to generate a 6 Volt reference current which is used as a virtual ground for the oscillator, this is necessary to allow the oscillator to run off of a single supply instead of a +/- voltage dual supply.
U1b is wired in a comparator configuration and is the part of the circuit that generates the variable pulse width. U1 pin 6 receives a variable voltage from the R6, VR1, R7 voltage ladder. This is compared to the triangle waveform from U1-14. When the waveform is above the pin 6 voltage, U1 produces a high output. Conversely, when the waveform is below the pin 6 voltage, U1 produces a low output. By varying the pin 6 voltage, the on/off points are moved up and down the triangle wave, producing a variable pulse width. Resistors R6 and R7 are used to set the end points of the VR1 control, the values shown allow the control to have a full on and a full off setting within the travel of the potentiometer. These part values may be varied to change the behavior of the potentiometer.
Finally, Q1 is the power switch, it receives the modulated pulse width voltage on the gate terminal and switches the load current on and off through the Source-Drain current path. When Q1 is on, it provides a ground path for the load, when Q1 is off, the load's ground is floating. Care should be taken to insure that the load terminals are not grounded or a short will occur.
The load will have the supply voltage on the positive side at all times. LED1 gives a variable brightness response to the pulse width. Capacitor C3 smooths out the switching waveform and removes some RFI, Diode D1 is a flywheel diode that shorts out the reverse voltage kick from inductive motor loads.
In the 24 Volt mode, regulator U2 converts the 24 Volt supply to 12 Volts for running the pwm circuit, Q1 switches the 24 Volt load to ground just like it does for the 12 Volt load. See the schematic for instructions on wiring the circuit for 12 Volts or 24 Volts.
When running loads of 1 amp or less, no heat sink is needed on Q1, if you plan to switch more current, a heat sink with thermal greas is necessary. Q1 may be replaced with a higher current device, suitable upgrades include the IRFZ34N, IRFZ44N, or IRFZ48N. All of the current handling devices, switch S1, fuse F1, and the wiring between the FET, power supply, and load should be rated to handle the maximum load current.
The prototype for this circuit was constructed on an IC proto board with parts and wires stuck into the holes of the proto board. One version of the finished circuit was used to make a variable speed DC fan, the fan was mounted on top of a small metal box and the PWM circuit was contained inside of the box.
A simple circuit board (see picture) was built using a free circuit board CAD program, PCB (1) that runs on the Linux operating system. The circuit board image was printed on a PostScript laser printer onto a mask transfer product called Techniks Press-n-Peel blue film (2). The printed on film is then ironed on to a cleaned piece of single sided copper clad board. The board is etched with Ferric Chloride solution. Holes are drilled with a fine gauge drill bit, parts are soldered in, and the board is wired to the power and load. This technique is great for producing working boards in a short time but is not suitable for making large numbers of boards. A board pattern is shown in Fig 3, this may be photo-copied onto a piece of press-n-peel blue film, or used in a photographic etching process.
Alternately, the dead-bug construction method may be used, this involves taking a piece of blank copper PC board, glueing a wire-wrap IC socket sideways to the board with 5 minute epoxy, then soldering all of the parts to the wire wrap pins. Grounded pins can be soldered directly to the copper board.
No alignment is required with this circuit.
This circuit will work as a DC lamp dimmer, small motor controller, and even as a small heater controller. It would make a great speed control for a solar powered electric train. The circuit has been tried with a 5 Amp electric motor using and IRFZ34N FET and worked ok, D1 may need to be replaced with a faster and higher current diode with some motors. The circuit should work in applications such as a bicycle motor drive system, if you experiment with this, be sure to include an easily accessible emergency power disconnect switch in case the FET shorts out and leaves the circuit full-on.
Wire the circuit for 12 Volts or 24 Volts as per the schematic, connect the battery to the input terminals, and connect the load to the output terminals, be sure not to ground either output terminal or anything connected to the output terminals such as a motor case. Turn the potentiometer knob back and forth, the load should show variable speed or light.
U1: LM324N quad op-amp
U2: 78L12 12 volt regulator
Q1: IRF521 N channel MOSFET
D1: 1N4004 silicon diode
LED1 Red LED
C1: 0.01uF ceramic disc capacitor, 25V
C2-C5: 0.1uF ceramic disk capacitor, 50V
R1-R4: 100K 1/4W resistor
R5: 47K 1/4W resistor
R6-R7: 3.3K 1/4W resistor
R8: 2.7K 1/4W resistor
R9: 470 ohm 1/4W resistor
VR1: 10K linear potentiometer
F1: 3 Amp, 28V DC fast blow fuse
S1: toggle switch, 5 Amps
Jameco 1-800-831-4242 http://www.jameco.com/
Digi-Key 1-800-DIGIKEY http://www.digikey.com/
(1) Free PCB Software for Linux
(2) Techniks, Inc P.O. Box 463 Ringoes, NJ 08551 908-788-8249 Press-N-Peel
Printed Circuit Artwork
PWM PCB artwork in GIF format and in PostScript format
PWM parts silk screen in GIF format and in PostScript format
Silkscreen clarifications: the unlabeled circular pattern near LED1 is a wire jumper and the part labeled FB1 is really R9.
The connections for the 8 pin header on the printed circuit board are as follows:
Header layout as oriented
in PCB artwork:
7 5 3 1
8 6 4 2
1: VR1 clockwise
2: VR1 counter clockwise
3: 12V Supply + and Load +
4: VR1 wiper
5: 12V Supply + and Load +
6: 12V Supply -
7: Load -
8: 12V Supply -
See the variations on this circuit for more ideas.
Electric Vehicle Controller Operation
Understanding power, voltage and current in an EV motive system
By Mark E. Hazen
This article is intended to help EV owners understand motor controller operation in terms of power, voltage and current. Basic calculations reveal extreme differences between the average battery current, as viewed on the in-cab ammeter, and the motor current. This understanding is then applied to motor/controller operation under different load conditions to see how a controller might fail.
Basic Premise: PB = PM
To begin, our basic premise is that the power supplied by the battery bank (PB) is, for the most part, equal to the power used by the motor (PM), with the realization that a relatively small amount of power is wasted as heat in the controller. This premise helps simplify the discussion.
In reality, the power used by the motor is equal to the supplied battery power minus losses in the controller and cabling. The losses in the controller and cabling are very small, compared to what is being used by the motor, so we will ignore controller and cabling losses, along with other miner losses, in our calculations.
So, we are saying that supplied battery power (PB) equals motor power (PM): PB = PM
Transformation Ratio: ton/T
Power is measured in Watts (W) where,
Power = Current X Voltage = Amps X Volts (100 A X 150 V = 15,000 W = 15 kW)
Figure 1 shows the battery side calculations, on the left side, that reveal the relationship between the motor current and the battery Current. The average battery current (IB) is low compared to the motor current (IM) and the ratio of the battery current to the motor current is equal to the ‘on’ time (ton) to total cycle time (T) ratio (ton/T).
The ‘on’ time (ton) is the time during which the switches in the controller are turned on allowing current to flow. The total cycle time (T) includes the ‘off’ time when the controller switches are turned off.
The power delivered by the battery bank is the product of a high battery bank voltage and a battery current that is a fraction of the motor current (IM) where IB = Iaverage = IM(ton/T).
PB = IM(ton/T) VB
(ton/T) can be referred to as a ‘transformation ratio’ that determines IB from a given value of IM, or can be used to determine a value of IM based on a known value of IB.
On the right side of Figure 1, we see that the average motor voltage is affected by transformation ratio ton/T where VM = Vaverage = VB(ton/T).
The power used by the motor is the product of the motor current and the average motor voltage as determined by ton/T.
PM = IMVB(ton/T)
Now, let’s work with these relationships in an actual example.
In this example, your foot on the accelerator pedal has positioned the throttle potentiometer to create a pulse width whose ratio of ton/T is 0.1.
If ton = 10us and T = 100us, then transformation ratio ton/T = 0.1
If VB = 150 V and average battery current IB = 100 A, then
PB = 100 A X 150 V = 15 kW
VM = 0.1VB = 0.1 X 150V = 15 V = average motor voltage
Because PB = PM, PM must equal 15 kW = IMVM = IM X 15 V
IM = 15 kW/15 V = 1000 A
IB/IM = 100 A/1000 A = 0.1 = transformation ratio ton/T
Note that the motor current is 10 times higher than the average battery current in this example when ton/T = 0.1. However, the average motor voltage is only 15 V, so 15 V X 1000 A = 15 kW.
Power from the battery (PB) = Power used by the motor (PM)
Power = Current (I) X Voltage (V)
Power In = Power Out
Battery Side Calculations Motor Side Calculations
PB = IBVB PM = IMVM
Where, IB = Iaverage = IM(ton/T) Where, VM = Vaverage = VB(ton/T)
So, PB = IM(ton/T) VB So, PM = IMVB(ton/T)
Note: The battery current is low because of the transformation ratio (ton/T), but VB is the full voltage of the battery bank. Note: The motor current is high, but the motor voltage is low because of the transformation ratio (ton/T). The motor voltage reaches maximum when ton/T = 1 where VM = VB.
Figure 1: Battery Side and Motor Side Calculations
Figure 2 is an overall system view showing the currents and voltages used in our calculations.
Figure 2: A System View of Voltages and Currents
Figure 3 is a graph that shows what the motor current would be at different values of ton/T. In this graph, the average battery current is held constant for all values of ton/T so that you can see the relative values of motor current and battery current for all throttle positions. You can see that when ton/T is low, the motor and controller switch pulse currents are very high.
Figure 3: Motor Current as a Function of ton/T at Constant Average Battery Current
Now that we understand that the controller transforms current and voltage according to the transformation ratio ton/T, we can apply this to different conditions to further understand controller and motor operation.
Motor Acts as Generator Creating a Counter Voltage
When the throttle is first applied to the controller and the controller begins to feed current and voltage to the motor, the motor starts rotating and accelerating. Initially, the motor offers very little opposition to the current flow because the motor’s opposition to current is based on its RPMs. The faster the motor turns, the more opposition the motor develops to the current. This opposition that is developed by the motor is called counter electromotive force (CEMF), which is simply an opposing voltage that is being generated by the motor as it turns. In other words, the motor is also acting as a generator, generating a voltage that is in opposition to the voltage applied to the motor.
If the motor is free to accelerate rapidly, the generator action (CEMF) will increase quickly and will work to reduce the current being demanded by the motor. On the other hand, if the motor is caused to labor under mechanical load for an extended period of time, the current through the motor will be high for that period of time.
Motor Loading and Consequences
So, what can cause this high-current condition over an extended period of time? If you start out in first or second gear, the high gear ratio allows the motor to increase its RPMs quickly, so the motor and controller experience moderately high currents for only a relatively short period of time. On the other hand, if you start out in third, forth or fifth gear, or even direct drive, the motor will take much longer to accelerate and to generate an opposing voltage to limit current. During the labored acceleration period, both the average battery current and the motor currents are very high, much higher than when starting out in first gear.
As the throttle is pressed, the ratio of ton/T increases toward 1, which is 100% duty cycle or full ON. At any given ton/T ratio, the actual battery current and the motor current will be determined by the mechanical load on the motor, where the mechanical load is determined by the gear ratio and the mass of the vehicle, and a few other things.
When you start out in first gear, the motor has the leverage of the gear ratio to work against the load. Therefore, the motor doesn’t demand as much current, and power, as it would if starting out in a higher gear.
So, if you start out in a high gear or direct drive, the motor will demand very high starting and running currents because of the heavy motor loading and because the motor cannot accelerate quickly. You are using much more power and energy when forcing the motor to operate under heavy load. What is more, if you go to full throttle right away from start, the motor receives full voltage when its opposition to current (CEMF) is very low. This causes a worst-case condition in which the motor current is extremely high, stressing all system components: motor, batteries and controller.
Heavy loading is hard on the motor because the very high sustained currents heat up the windings and the brushes. If the winding insulation breaks down because of high temperatures, the motor is toast. The brushes and commutator also wear out more quickly.
Heavy loading is also hard on the controller because, especially at duty cycles lower than 50%, where ton/T is less than 0.5, the motor currents can be very high, threatening the MOSFET, or IGBT, switches inside the controller. (Refer again to Figure 3.) Electronic devices used in controllers have a continuous current rating, specified usually at two temperatures (25oC and 125oC), and a peak pulse current rating. If either of these ratings is exceeded, the switches can and will fail.
The current limiting circuit inside the controller cannot protect the electronic switches from all scenarios. At low duty cycles where ton/T is less than 0.5, the high average battery current is transformed into even higher motor current pulses that may not be totally captured in time by the current limiting circuit. Slight time delays in current limiting activation open windows of opportunity for unbridled current surges to destroy the switches.
Even assuming that the current limiting circuit is ideal, there are cases in which the current rating of the controller switches is exceeded. Where a motor is connected directly to the vehicles differential, average battery current may already be 400 A at 10% throttle (ton/T = 0.1), which means that the motor current and the peak pulse current through the controller switches starts at 4000 A. This may, or may not, be within the capability of the switches at a temperature of 25oC, but, to make things worse, as the switches heat up, their current handling abilities decrease.
How about at full throttle? If the switches are turned full ON, the battery current and the motor currents must be equal. This is only true if the motor current is less than the current limit of the controller. If the current limit is 500 A and the motor is demanding more, the current limit circuit will kick in and actually reduce the duty cycle to an amount less than 1, where ton/T is less than 1. This forced duty cycle reduction keeps the average battery current at 500 A while the motor current is higher during ton. While you are reading 500 A on your ammeter, the switches in the controller are experiencing much higher pulse motor currents.
As an example, under direct drive conditions the throttle may be applied full ON to cause the vehicle to accelerate rapidly under this heavy mechanical load. In so doing, the controller goes into current limiting right away, which means that the controller is not really full ON. The current limiting circuit reduces the ton/T ratio well below 1 to keep the average motor current and battery current at the limit. If the limit is 500 A and ton/T is held back to initially 0.2 (20%), the motor current and peak pulse current through the switches will be 2500 A. This high pulse current may be a threat to the switches, especially as temperature rises.
As the direct-drive motor increases its RPMs, the ratio of ton/T increases toward 1 and the motor and peak controller currents decrease while the average battery current is held at 500 A. Also, the average continuous current through the switches in the controller stays at 500 A for an extended period of time. This period of acceleration causes rapid heating of the switches in the controller, which may cause them to fail if the heat sinking cannot take the heat away quickly enough.
Many good controllers are destroyed because some EV owners don’t understand this. A controller that is designed for reliable everyday transportation use can easily be destroyed when employed in heavy-load and racing applications.
EV’ers that race their vehicles experience this with every run. That’s why those who race buy 1,000 A and 2,000 A controllers that can survive the heavy loading of racing.
Racers know that there is a big difference in the price of a racing controller and a street controller. Novices get into trouble when they buy a street controller to save money and use it for racing, or abuse it by using high gears or direct drive all of the time.
The bottom line is, if you want to go direct drive, start out in high gears or want to race, buy a higher-priced controller that is designed for heavy loading applications.
Summary of Key Points
• Motor current is a lot higher than average battery current (ammeter current) when the controller switches’ duty cycle is less than 50%, where ton/T is less than 0.5.
• Starting out in a high gear or direct drive or full throttle represents heavy loading, which may exceed the ratings of the controller switches.
• As the temperature of the controller switches increases, the continuous and peak current handling capability of the switches inside the controller decreases.
• Street controllers are designed to be low-cost and reliable for everyday transportation applications.
• Racing controllers are much more costly than street controllers because they are made with more costly heavy-duty components to withstand the high currents caused by heavy mechanical loading.
• Don’t use a low-cost street controller for heavy-loading applications such as high-gear starts, direct drive or racing.
• Symmetrical Layout Enhances Power Controller
• Nov 1, 2007 12:00 PM
By Mark E. Hazen, Engineer and Technical Writer, evhelp.com, Ocala, Fla.
Configuring paralleled power MOSFETs in a circular layout sets the stage for the balancing of currents and heat dissipation in the power stage of an electric vehicle's motor controller.
• NEWS & FEATURES FROM AUTO ELECTRONICS
• Committed to improving hybrid electric cars
• New Motors for Hybrid Vehicles
• Battery Firms Battle for Hybrid Hegemony
• Innovative Bipolar Plates for Fuel Cells
See More Headlines
• TOP ARTICLES
• Exploring Current Transformer Applications
• Ultracapacitor Technology Powers Electronic Circuits
• Buck-Converter Design Demystified
• Sensorless Motor Control Simplifies Washer Drives
• PET RESOURCES
• Buyer's Guide
• Engineering Jobs
• Power Electronics Events
• Rent Our Lists
• Spotlight on Digital Power
• The conversion of a gas-powered pickup truck to an all-electric power train requires the introduction of several electronic subsystems to support battery-powered operation. I became familiar with these requirements late last year when I modified my 1998 Chevy S10 pickup truck to operate as an electric vehicle (EV).
• A low-voltage charger was needed for the 12-V system battery, while a high-voltage three-stage charger was needed for the lead-acid battery bank (16 series-connected 6-V golf-cart batteries) that provided the truck's motive power. The truck's new electric motor also demanded a heavy-duty power controller to deliver power to the motor, an industrial-grade series-wound dc motor from Advance DC Motors.
• To complete the conversion and get the EV running, I initially used an off-the-shelf breadbox industrial power controller. This power controller contains a pulse-width-modulated (PWM) controller, gate driver and power MOSFETs, as well as protection functions such as adjustable current limiting, low-voltage cutoff and overtemperature protection. With a voltage range of 96 Vdc to 144 Vdc, and a maximum load current rating of 500 A, this off-the-shelf controller was certainly adequate for the EV application.
• However, I decided to create a novel design that would be even more robust and efficient, offering greater electrical and thermal margin than the purchased power controller. The resulting power controller, which I have dubbed Hazen's Power Wheel, employs a circular design that permits a symmetrical configuration of all the power MOSFETs.
• The idea behind the circular and symmetrical concept is to distribute electrical and thermal currents evenly to help ensure that all MOSFETs are treated equally. The Power Wheel design does not “force” all the MOSFETs to operate equally; rather, it requires highly controlled semiconductor manufacturing conditions and/or somewhat sophisticated electronic controls to do that.
• Instead, the physical design sets the stage for operational fairness for all the MOSFETs, which means equal and symmetrical gate drive, power current flow paths, and heat distribution and dissipation. Although the Power Wheel targets the EV motor-drive application, the same concept may be applied in other high-power applications where multiple switches are paralleled.
• Power Controller Design
• Like the off-the shelf power controller, the Power Wheel design includes the PWM circuit, MOSFET gate driver, MOSFET power stage and protection circuitry. However, in this article, the focus will be on the power stage, which consists of 15 MOSFETs that are physically configured in a circular symmetrical layout (Fig. 1).
• The MOSFET gate driver is actually a single-gate “super driver,” consisting of a MOSFET half bridge, which provides ample and equal drive to all MOSFETs. The switching frequency of the power controller is a fixed 4 kHz. As shown in Fig. 2, the control circuit includes a trimmer-adjustable current-limit circuit that prevents the motor current from exceeding a maximum level in the range of 325 A to 1350 A. The control circuit also includes a watchdog circuit that shuts the controller down if the control resistor, which is connected mechanically to the gas pedal, becomes open or disconnected.
• Fig. 2 includes a side view of the power stage. From this view it can be seen how the 15 MOSFETs that drive the motor are sandwiched between two large aluminum discs that connect to the drain and source of each transistor. The 15 MOSFETs are actually mounted around the rim of the drain disc as illustrated in Fig. 1.
• The MOSFETs chosen for this design were International Rectifier's IRFP90N20s, each rated at 200 V and 94 A (90 A is the package limit). Together, these MOSFETs deliver overall ratings of 200 V and 1350 A.
• Each MOSFET in Fig. 1 is mounted directly to the drain disc with 4-40 hardware and thermal compound for good heat conductivity. A copper bus bar (-M(FEED)) collects the total motor-drive current at the center of the disc. The only physical contact between the bus bar and disc is at the center. The disc in Fig. 1 is illustrated as semitransparent to show the bus-bar connection behind the disc.
Nov 1, 2007 12:00 PM
By Mark E. Hazen, Engineer and Technical Writer, evhelp.com, Ocala, Fla.
NEWS & FEATURES FROM AUTO ELECTRONICS
Committed to improving hybrid electric cars
New Motors for Hybrid Vehicles
Battery Firms Battle for Hybrid Hegemony
Innovative Bipolar Plates for Fuel Cells
See More Headlines
Exploring Current Transformer Applications
Ultracapacitor Technology Powers Electronic Circuits
Buck-Converter Design Demystified
Sensorless Motor Control Simplifies Washer Drives
Power Electronics Events
Rent Our Lists
Spotlight on Digital Power
Fig. 3 shows the source disc in place with a representative MOSFET sandwiched between the two discs. The discs are separated with nylon spacers and nylon bolts. There is a small gap between the top surfaces of the MOSFETs and the source disc. The power cable that comes from the negative supply terminal of the motive battery bank connects to the -VSS bus bar, which delivers the current to the center of the source disc for unbiased distribution through the disc to the source leads of all MOSFETs. Again, the bus bar only contacts the disc at its center.
Also note the gate-drive distribution disc shown in Fig. 2 and in the top center of Fig. 3. This small disc evenly distributes gate drive to all MOSFETs via an interconnecting lead and small gate resistor for each.
As a side note, the source and drain leads of each MOSFET are pinned to the edges of the discs using brass washers and screws. Also, the electrical portion of this design does not use any electrical means of load balancing among MOSFETs. The physical symmetry of the design and the quality of the MOSFETs have eliminated the need for that.
The two juxtaposed aluminum discs are both visible in Fig. 4, which shows the Power Wheel installed in the pickup truck. As evidenced by this photo, the Power Wheel has all the characteristics of an early prototype or proof of concept.
Under the hood, the performance of this controller is very strong. From the beginning, I included in the physical design a 4-in. center-mounted fan to force air over the drain disc (Fig. 4). As it turns out, the drain disc becomes barely warm in normal operation. Nevertheless, the fan will remain to provide for additional thermal margin.
To assess the performance of the Power Wheel, temperature readings were taken around the aluminum drain disc at each MOSFET location (Table). Readings were taken with a handheld noncontact infrared digital thermometer. To obtain these readings, the vehicle was driven 8.5 miles in a city environment. The ambient temperature was 31°C.
The cooling fan and white disc cover, which can be seen in the under-hood photo (Fig. 4), were removed prior to the test. Also, as can be seen in the photo, the controller is mounted vertically and the bottom MOSFETs are within 15 cm to 20 cm of the electric motor, which during this test had an outer-case temperature of 52°C. The closeness of the electric motor causes the ambient air temperature to be higher near the bottom of the power stage.
Therefore, it is expected that the lower and back drain disc areas would be warmer because of their juxtaposition to the electric motor and vehicle firewall. It is also expected that, when the cooling fan is used, temperatures would be more even around the drain disc because the temperature of the flowing air would be more constant, forcing all MOSFET sites to increase temperature starting from the same ambient point.
What is interesting here is that all the temperatures are quite low (barely warm), which indicates that the cooling cover and fan are actually not needed. The reason that all the temperatures are low is that the MOSFETs are highly efficient with a low RDSON of 0.023Ω and the switching losses are also low at the 4-kHz switching frequency. The lack of hot spots around the drain disc indicates that the MOSFETs match fairly closely and that heat is dispersed quickly in the aluminum drain disc.
In addition, the controller has adjustable current limiting, which was set to limit the total drain current to 325 A maximum or 21.7 A maximum per MOSFET. The actual average road current is around 150 A, so the average current for each MOSFET is only 10 A, which is only 2 W to 3 W per MOSFET (P = I2R = 102 × 0.023 = 2.3 W plus switching loss).
Needless to say, this controller is nowhere close to being at risk — even on the hottest summer day. More details regarding the conversion of the Chevy S10 are presented at www.evhelp.com
Click here for the enhanced PDF version of this article