Totem Pole Mosfet Driver For Mac
Hello Everyone, So I am reading more and more into driving a motor with a FET from the arduino and have a few scenarios and questions. First here is what I am using for a system. 24V motor, 500W, 27A rated Estimated power = 500W/24V = 21A at max TTL MOSFET, 2000pF gate cap, 22mOhm RDSon Arduino controller, PWM out (500Hz?), 5V out Original option.
Drive the FET with with arduino directly. (Attached - ugly drawing) So basically I would just put a 100ohm resistor in series with the gate and have a 4.7K pull down. This results in a small voltage drop (4.9V still on gate) and would have the gate pulled down when off. I may have to do two FETs to reduce heat though since the following: Watts = I^2.R = 21^2.0.022 = 9.702W So that is 4.851W per FET. The problem I could see is that makes the gate capacitance 4000pF.
Could the arduino still switch at a fast frequency to control both of these with a PWM? Can the arduino provide enough current to switch fast enough? See any issues? Could I run this with only one FET? New option - totem pole. So I looked for alternatives in case the power wasn't there for the arduino to provide.
Totem Pole (Attached) First off, I haven't done a totem pole before so I have some questions: I know that you need a current limiting resistor to drive the BJTs. I know you need an out resistor for the FET gate. Do you need a pull down resistor? If so, do you put it on the output side of the totem pole or at the collector of the PNP? Does the gate resistor count as the ground resistor because the PNP is on when the PWM is off? So a PWM will turn on the NPN when high and the PNP when low so the gate will see the 5V for the on duration and be grounded during the off.
Does that sound right? Any suggestions for using a totem pole to drive a FET?
You'll need something like a MIC4422 to drive those MOSFET gates fast enough for reasonable PWM - at those power levels the switching losses will dominate and are proportional to switching time. MOSFET drivers like the MIC4422 can provide enough current to drive the gates properly for power switching (several amps if necessary). You'll need a nice beefy schottky diode to go across the motor too - or maybe a snubber network. Realistically this isn't a project to take on without access to a current-limited 24V supply and an oscilloscope - its all too easy to blow up power MOSFETs (I do mean blow-up - eye protection is wise). You also need to think about the stall-currents that could flow - these might be 10 to 20 times the full load current. Current monitoring is pretty much required to prevent over-current damage (MOSFET power dissipation proportional to current squared remember). Hello Mark, Thank you for the reply.
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I understand that fast switching requires the ability to be able to charge and discharge the gate capacitance sufficiently all the way for operation and heat issues. I have some half bridges that I can probably drive this with if need be but I guess I am wondering if something like this really requires this? We are talking about 500 Hz that the PWM is capable of for the Arduino. You are talking about 5ms switching.
If I use the setup that has a 50 ohm resistor on the gate with a 10K pull down, I will be having time constants at 110 microseconds. That is almost 50 time constants per cycle assuming max current. Now with that said, I won't be supplying the max current of 100 milliamps since the channel doesn't allow it.
So assuming that you can only draw 40mA per channel that gives about 20 time constants per cycle which seems pretty reasonable to me. Anything wrong with my theory? If need be, I could just drive an H-bridge with the microcontroller to switch the FETs sufficently correct?
With that being said, my setup doesn't currently work with the 50 ohm on the gate and the 10K pull down. I am still probing my circuit for the issue. I am using a 24V battery for the main voltage source and a 12V battery for the controller. The 12V is regulated to 5V and provides the power to the arduino. I have an oscope and all the necessary tools. I observe the PWM is correct and at 5V to drive the TTL logic MOSFET.
Another question though on the same note. Any ideas on what may be causing the microcontroller to reset by switching a relay? I am driving a relay with a BJT that is driven by the arduino. It is current limited to 20mA by the base resistor.
I am able to switch it fine but when the relay opens back up it resets the arduino. The relay has a diode in parallal with the coil to take care of any voltages so I don't think it is that surge that does it. I don't get those figures - where does 5ms come from? 500Hz is 1ms between switches, if you take 10us to switch then 1% of your time is in switching, with 24V and 27A that means you are dissipating VA/4 or so watts for that 10us on average assuming linear current ramp IIRC - thats 162W on average for those 10us slots, so 1.62W on average across the entire waveform (switching loss).
So long as the device is able to handle 160W pulses of 10us duration (there are graphs for this on most datasheets), this will be OK. However if you want to switch faster for less noise and higher efficiency you'll want to switch faster than 10us really. 10us switching of 4000pF / 10V is several mA, so you'll probably be able to go to 2 or 3 us switching without a driver chip. However you have to consider another issue with high power control - the output voltage swings couple back to the gate through the gate-drain capacitance, and there is a large inductance connected to the drain (fast voltage swings). This means than unless you drive the gate with a nice low impedance you could get the gate going negative enough volts to fry the microcontroller (then the MOSFETs may well blow). People often add zener diodes between gate and source to handle this - but a more robust solution I think is to drive the gate with a suitable driver (a few ohms output impedance).
And once you remember that high stall currents can flow you'll start to worry about whether the devices are beefy enough to survive a high-current excursion for long enough for the microcontroller to react and shutdown (measure the DC resistance of the motor at rest, it'll be in the milliohms.) Basically be conservative, assume the worst, detect overcurrent, build with care and test at low power first. The 5ms is suppose to come from frequency to time which should actually be 2 ms for a 500Hz PWM signal. I see what yours saying though. I am still not super clear on the gates of mosfets as you can see.
Can you explain the switching loss a little more in depth (i don't see where this comes from)? The only power calculations I did were for the load power dissipation needed to drive the motor. 27A rated at 22mOhm when the FET is on give me 16.038 Watts for power. I plan on running two FETs in parallel like mentioned before to give me a max of 8 Watts of power for normal use per FET. The other thing is I am using 5V TTL MOSFET so I wouldn't switch at the 10V so the current would be higher.
So this gives me three viable options that I believe I have components for and I am capable of. 1.) MOSFET driver. I don't have the driver chips so I would have to wait and buy one. I have never used one before but have read up on them when I was planning my original H-bridge design for forward and reverse. I found a lot of information on using these on where he has built multiple projects. It is pretty stable and can easily be driven by the arduino.
So that option is there is needed but not preferred since I can't implement it without buying the hardware. I would probably go back to regular logic MOSFETs as well rather than the TTL for those. I have some of them but this would be a redesign of my current circuit. 2.) PNP BJT driver. I could use some PNP to connect the regulated 5V to the gates of the FETs. I can drive the BJT with the arduino and that seems like it would be pretty simple.
I am only going to be able to get about 100mA out of the BJT most likely though. I also have to make sure the voltage drop of the PNP doesn't interfere with the TTL logic or I will have to add more circuitry and implement the 12V signal to drive the TTL. This I have parts for and it is cheap and simple. The question comes down to is how much current for nice smooth switching. 3.) SN754410 H-bridge. I have this chip as well.
I can use simple TTL logic from the arduino to toggle one of the quad outputs (or two) and connect the gates to the H-bridge. This is able to supply 1A continuous if needed for a lot more power. I can connect either the 5V or 12V gate to this and be able to control either. This maybe a good option but I am not sure if there is a restriction on using an H-bridge to drive a FET. Any ideas on this?
I will look into the zener if need be for more protection. I guess I can explain a little bit better how ideally this project works. I attached a drawing of my current setup. The drawing is crowded but here are the basics. There is a key switch that connects the 12V to the regulator that is smoothed with caps. 12V is supplied to the 24V relay coil that is hooked to another switch to make or break that for an estop.
There is a third switch that turns on the controller. The controller handles the following: Reads pot for amount of throttle (alters PWM to the FETs). Read that the relay has voltage Turn the relay on by activating the BJT Turn on the FETs but PWM controlling the gates So the way the software works is that when there is a throttle given by the pot, the controller reads to make sure the relay has voltage. If it does then the relay is activated while the FETs are still off (switches 0 volts).
Totem Pole Output
The controller then PWMs the FETs to conduct the main path to the motor. When the drive cycle is over, there is timeout period that will wait to make sure the user is done driving the motor. Once that is complete the FETs are verified to be 0 and the relay opens. This enables to arcing in the relay for safe operation (except for Estop).
Totem Pole Mosfet Driver For Mac
I have fuses on both the 24V line (30A) and the 12V line (3A) (I didn't include). I am hoping that with this architecture and control I can keep the current pulses down and a safe way to operate the motor. I have a bunch more indicators that I left out as well to determine what stay the software is in.
Currently everything works except the FETs. The FETs currently won't turn on the LED when I PWM the gates.
That means that the conduction path is not complete. So hints why I have so many questions about FET.
I have already broke a few too (measured diode voltage drops and resistance of the pins to verify) so I know I am not perfect and implementing them yet. So any suggestion are always appreciated. Sorry the long and lengthy post but I just want to make sure I understand what is happening so I can control and modify it better.
The MC34152/MC33152 are dual noninverting high speed drivers specifically designed for applications that require low current digital signals to drive large capacitive loads with high slew rates. These devices feature low input current making them CMOS/LSTTL logic compatible, input hysteresis for fast output switching that is independent of input transition time, and two high current totem pole outputs ideally suited for driving power MOSFETs. Also included is an undervoltage lockout with hysteresis to prevent system erratic operation at low supply voltages. Typical applications include switching power supplies, dc-to-dc converters, capacitor charge pump voltage doublers/inverters, and motor controllers. This device is available in dual-in-line and surface mount packages.
Features. Two Independent Channels with 1.5 A Totem Pole Outputs. Output Rise and Fall Times of 15 ns with 1000 pF Load. CMOS/LSTTL Compatible Inputs with Hysteresis. Undervoltage Lockout with Hysteresis. Low Standby Current. Efficient High Frequency Operation.
Enhanced System Performance with Common Switching Regulator Control ICs. Pb-Free Packages are Available.