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A high-current motor speed control for 12V & 24V systems This pulse width-modulated 20A speed control can be used for controlling 12V DC motors in cars. Examples are pumps for fuel injection, water/air intercoolers & water injection on modified performance cars. It could also be used for headlight dimming in the daytime & for running 12V motors & pumps in 24V vehicles. Design by RICK WALTERS These days, car manufacturers are coming to realise that running pumps full bore all the time is wasteful of the battery/electrical system and also causes premature wear of the fuel pump. A prime example of this is the pump use to pressurise the fuel rail in fuel injection cars. The pump runs continuously, regardless of the fuel demand, and the excess fuel is bled off to the fuel tank to keep the pressure constant. In the future, most cars will have fuel pumps which are variable speed controlled according to fuel demand. In the meantime, you can do it now with this design, using the car’s map sensor output as a measure of fuel demand. However, the exact method for doing this is beyond the scope of this article. The circuit can control 12V loads up to 20 amps and it uses just two Mosfets to do it. Other possible applications for this PWM circuit are for control of 12V and 24V motors in model locomotives and cars and in control applications in manufacturing. The circuit has excel­lent line and speed regulation and uses just one low-cost IC as well as the two Mosfets. Note: this circuit is not suitable for operating 12V audio equipment in 24V vehicles since its output is pulsed at around 2kHz. As presented, the circuit incorporates a “soft start” fea­ture which is desirable to reduce inrush currents, particularly if the device is used to control 12V incandescent lamps. However, for some pump applications the soft start may not be wanted and so we’ll tell you how to disable it. We are presenting this project as a standalone PC board. If you want to put it in a case it is a simple matter to install it in a suitable plastic box but that will be up to you. The PC board has all components on it except for a diode (D2) and a capacitor which must be wired across the motor being driven. If the circuit is used to control incandescent lamps, the diode and capacitor are not required. Circuit description This small PC board will provide speed control of 12V or 24V motors drawing up to 20A. Not shown on this prototype board is the input protection diode D1 26  Silicon Chip The heart of the circuit shown in Fig.1 is a TL494 pulse width modulation (PWM) controller. It varies the output voltage fed to the motor by rapidly turning Mosfets Q3 & Q4 on and off. Because the Mosfets are Fig.1: the heart of the circuit is a TL494 pulse width modulation (PWM) controller. It varies the output voltage fed to the motor by rapidly turning Mosfets Q3 & Q4 on and off. Note that diode D2 is essential to the circuit operation. being switched fully on or fully off, they dissipate very little power, even when handling cur­rents as high as 20 amps total. This means that they do not get very hot and no heatsink or very small heatsinks (depending on the output current) are required. Note that the TL494 is normally used in switchmode power supply applications but it is suitable for virtually any PWM application. Its block diagram is shown in Fig.2. The chip con­tains the following functions: •  An oscillator, the frequency of which is determined by a capacitor at pin 5 and a resistor at pin 6. •  A stable +5V reference at pin 14. •  A “dead time” comparator with one input driven from the oscillator. •  Two comparators (pins 1, 2, 15 & 16) with their outputs ORed together via diodes (pin 3). •  A PWM comparator with one input from the oscillator and the other from the ORed output of the two comparators. •  A flipflop driven by the dead time and PWM comparators. •  Two 200mA transistors with uncommitted emitters (pins 9 & 10) and collectors (pins 8 & 11), with their bases driven by the outputs of the flipflop. In simple terms, the TL494 operates as follows. Its oscil­lator is set to run at 2kHz and it produces a pulse train at its outputs at this frequency. The width of the pulses is varied (ie, pulse width modulated) and the ratio of the “on” time to the “off” time controls the amount of power fed to the load which in this case is the motor. A fraction of the output voltage is fed to one input of one of the comparators, while the other input is connected to a reference voltage. If the output voltage rises slightly, the comparator input will sense this change and will alter the output onoff ratio and consequently the output vol­tage. This keeps the voltage at the comparator input equal to the reference voltage. This is done by reducing the driving pulse on time, reduc­ing the time the switching device is turned on, thereby bringing the output voltage back to the required level. The converse applies for falling output volt­ages. Now if we refer to the circuit of Fig.1 again, we see that the TL494 is fed via a 7812 12V regulator. This is not strictly essential for the TL494 since it can operate with a supply rang­ing from +7V to +40V. However, it is important that the gate drive to Mosfets Q3 & Q4 does not exceed their specifications and so this condition is met with REG1. In this circuit, the output duty cycle must be able to be controlled over a wide range, from virtually zero up to the maximum of around 90% and so the two internal transistors (C1 pin 8 and C2 pin 11) have their collectors connected to the +12V supply and are used as emitter followers to pull the bases of Q1 & Q2 to +12V. The 2.2kΩ resistor at pins 9 & 10 is the common emitter load and it pulls the bases to ground. Thus, the emitters of Q1 & Q2, together with the gates of Q3 & Q4, swing from 0V to +12V and so the gate drive signal is limited to this voltage. Q1 & Q2 are included for another reason and that is to rapidly charge and discharge the gate capacitances of the Mosfets each time they turn on and off. This improves the switching action of the Mosfets; ie, it speeds up the turn-on and turn-off times and thereby reduces the power dissipation in the Mosfets. Soft start A soft start circuit is incorporated to June 1997  27 Fig.2: functional block diagram of the TL494. This chip is in­tended mainly for switchmode power supplies but we have adapted it to control motors and resistive loads. reduce surge cur­rent into the motor at turn on. When power is first applied, the REF output, pin 14, rapidly charges its associated 10µF capaci­tor, C1. This pulls the INH(hibit), pin 4, high as the 10µF capacitor (C2) between pins 14 and 4 is initially discharged. While pin 4 pin is high there is no output from pins 9 & 10. As cap­ac­itor C2 charges through the 100kΩ resistor the voltage on pin 4 will gradually fall and the output pulse width will increase, giving a smooth rise in the output voltage. In order to control the output voltage precisely, the TL494 monitors both sides of the motor; ie, the input voltage before the 12V regulator (MOTOR +) and the voltage at the Mosfet Drains (MOTOR -). The MOTOR+ voltage is fed via the 20kΩ and 2.2kΩ voltage divider resistors to comparator 1, pin 1. The MOTOR- voltage is attenuated by the 18kΩ and 4.7kΩ resistors and fed through a 47kΩ resistor to pin 2. The voltage tapped off the +5V reference by the speed control, VR1, is also fed through a 47kΩ resistor to pin 2. When the speed control wiper is at minimum setting (ie, 0V), the voltage at the junction of the 18kΩ and 47kΩ resistors will be forced to be twice 28  Silicon Chip that on pin 1 of IC1 (nominally 1.4V for +14V input), as the voltage drop across each 47kΩ resistor will be 1.4V. The voltage at the MOTOR- terminal will be about +14V and so the motor will not run. As VR1 is advanced, the voltage at the MOTOR- terminal will decrease, thereby applying a larger voltage to the motor so it can run. Normally, the reference voltage on pin 1 of IC1 is fixed and referred to the 5V reference at pin 14. In our case this would not be desirable as the output voltage sensed and regulated by IC1 is between the MOTOR- output and ground (across the 4.7kΩ resistor). This means that as we vary the supply voltage, the voltage between MOTOR- and ground will be held constant but the voltage across the motor will vary in a direct relation to the voltage change. By connecting the 20kΩ resistor between the input rail and pin 1 of the TL494 we compensate for this. Protection Reverse polarity protection is provided by diode D1. It is rated at 3A average but has a one-off surge rating of 200A and will blow the fuse if the leads to the battery are reversed. Two essential components to the circuit are not mounted on the PC board but are wired directly across the motor itself: D2 and C3. Diode D2 is the most important as it prevents the genera­tion of excessive voltage spikes, each time the Mosfets turn off. D2 must be a fast recovery diode because of the very fast switch­ing of the Mosfets. The importance of diode D2 and the associated 0.22µF ca­pacitor C3 is demonstrated in the oscilloscope wave­forms of Figs.3, 4, 5 & 6. The waveform in Fig.3 shows the circuit driving a resistive load which could be a heater element or an incandes­ cent lamp. Notice that the waveform is a clean pulse with a duty cycle of about 74%. This gives a voltage of about 8.8V across the load. Now have a look at Fig.4. This shows the circuit set for the same output when driving a motor instead of a resistive load. The scope’s vertical sensitivity has been changed to 20V/ div instead of 5V/div. Notice the enormous spike voltage amounting to almost 80V peak-to-peak, each time the Mosfets turn off. This spike voltage is enough to blow the Mosfets because their Drain-Source voltage rating (VDS) is only 60V. Fig.3: this scope capture shows the waveform across a resistive load which could be a heater element or an incandescent lamp. Notice that the waveform is a clean pulse with a duty cycle of about 74%. This gives a voltage of about 8.8V across the load. Fig.4: this waveform shows the circuit set for the same output as for Fig.3 but driving a motor instead of a resistive load. The scope’s vertical sensitivity has been changed to 20V/div instead of 5V/div. Notice the enormous spike voltage (amounting to almost 80V p-p) each time the Mosfets turn off. This spike voltage is enough to blow the Mosfets because their Drain-Source voltage rating (VDS) is only 60V. Fig.5: this waveform was produced with the same circuit conditions as for Fig.4 but with D2 connected across the motor to clip the voltage spikes. We now see the motor’s back-EMF during the Mosfet off period, showing a value about half of that applied by the control circuit. Fig.6: this scope waveform shows the effect when both diode D2 and the 0.22µF ca­pacitor are fitted to the circuit. Note that the capacitor has a filtering effect which acts to remove most of the hash generated by the motor’s commutator. Fig.5 shows the same circuit conditions but with diode D2 connected across the motor to clip the voltage spikes. We now see the motor’s backEMF during the Mosfet “off” period, showing a value about half of that applied by the control circuit. Finally, Fig.6 shows the effect when both the diode and 0.22µF capacitor are fitted to the circuit. The capacitor has a filtering ef­fect, removing most of the hash generated by the motor’s commuta­tor. The reason that diode D2 and the 0.22µF capacitor C3 are fitted directly across the motor instead of being mounted on the PC board is that this method stops the motor leads from radiating commutator hash which could otherwise interfere with sensitive circuitry elsewhere in the car. The current rating of diode D2 must suit the rating of the motor. It’s not much use connecting a 5A diode across a motor that pulls 20A; it will just blow the diode and then blow the Mosfets. Finally, also not mounted on the PC board is the in-line input fuse F1. This must also match the rating of the motor. PC board assembly The PC board for this design is coded 11106971 and measures 68 x 50mm. It is fairly easy to assemble as it only has a few components on it. Begin by checking the copper pattern against the PC artwork (Fig.8) and repair any defects such as undrilled holes, shorts or open tracks. The component overlay is shown in Fig.7. June 1997  29 Fig.7: the component overlay for the PC board. Fit and solder the resistors, using a cut pigtail from one of them for the one link. This done, fit the IC, REG1 and trimpot VR1, followed by the transistors, capacitors and the Mosfets. If you intend to operate the controller from a 12V battery and don’t intend to draw more than 6A you can use one Mosfet. Provided a small heatsink is fitted you can probably draw up to 10A with one Mosfet. For higher currents, two Mosfets must be used, as shown on the circuit of Fig.1. If you want the full 20A load current, both Mosfets should be fitted with small heatsinks. Testing If you are careful with the assembly, it should work first up. Turn VR1 fully clockwise (minimum speed) and solder a resis­tor of around 100Ω 5W across the motor terminals. If you have a variable power supply, feed 14V to the DC input and ground. If you don’t have a power supply you will have to connect the con­troller directly to a +12V battery. With the negative meter lead connected to the 0V line, you should be able to measure about +12V on pin 16 and +5V on pin 14 of IC1. The voltage on pin 1 of IC1 should be around +1.4V with 14V input and +1.2V with 12V input. If these values are OK proceed with the following tests. If you now connect the meter leads across the 100Ω resistor it should read zero volts. Rotate trimpot VR1 slowly anticlockwise and the voltage should increase up to about 12V when fully rotated. Because IC1 has an internal “dead time” of 10%, the output devices can 30  Silicon Chip Fig.8: actual size artwork for the PC board. only be turned on for 90% of the time and the output voltage will never be the same as the input. For 14V input, the maximum output will be about 12.5V. Be careful not to burn yourself as the 100Ω resistor will become hot at the maximum setting of VR1. Using the speed controller As noted above, the rating of the in-line fuse will depend on the load you plan to drive. Obviously a 20A PARTS LIST 1 PC board, code 11106971, 68 x 50mm 1 5kΩ PC trimpot (VR1) Semiconductors 1 TL494CN switching regulator (IC1) 1 7812 regulator (REG1) 1 BC639 NPN transistor (Q1) 1 BC640 PNP transistor (Q2) 1 or 2 BUK456-60A/B/H N-channel Mosfets (Q3,Q4) Capacitors 2 100µF 50VW PC electrolytic 2 10µF 16VW PC electrolytic (C1,C2) 1 0.22µF 100VW MKT polycarbonate (C3) 2 0.1µF MKT polycarbonate 1 .068µF MKT polycarbonate Resistors (0.25W, 1%) 1 1MΩ 1 10kΩ 1 100kΩ 1 4.7kΩ 2 47kΩ 2 2.2kΩ 1 20kΩ 2 4.7Ω 1 18kΩ 1 100Ω 5W (testing) fuse will not protect a 1A motor. If you don’t want the soft-start facility, it can be disa­bled by omitting capacitor C2. We recommend that the soft-start facility be included for incandescent loads. However, for motor loads, a better approach would be to connect a 1kΩ 1W resistor across the output terminals and then place a switch in series with the motor or whatever load you wish to drive. You then set up the drive voltage you require with trimpot VR1 and use the in-line switch to connect and disconnect the motor. If resistive or incandescent loads are to be driven, D2 and C3 are not necessary but they must be included when driving any motor, regardless of its current rating. D2 must be rated to handle a current at least equal to that drawn by the motor. A suitable cheap diode is the MUR1515 which is rated at 150V 15A and should cover most applications. If you want to run a 20A motor, then use two MUR1515s in parallel. Make sure that they are connected in the right direction across the motor; ie, anodes to the positive supply line. If connected the other way around, you will blow the fuse and perhaps the Mosfets too. C2 should be an MKT poly­ carbonate capacitor with a rating of at least 100VW. The type of FET used depends on the current drawn by the controlled device. The BUK456-60s specified are readily avail­ able and have an “on” resistance of .028Ω. If you want high currents and 24V operation, the MTP60N06 is a more suitable device. It has an “on” resistSC ance of .01Ω.