Slow, Mixed and Question: The amount of ripple voltage at a power supply’s filter output is inversely proportional to the v… Decay Modes. Why Do We Need To Complicate Things?
If you are driving inductive loads, whereas it is a brushed or brushless DC motor, stepper motor, solenoid or a relay, you must have experienced a little bit of a problem in the form of an unwanted current flowing in the unwanted direction. If you did not take this fact of the laws of physics into account, chances are you have had the only once enjoyable experiencing of smoking your transistor. Lets take a quick look at what is happening with our inductor. It is a known law of physics that inductors will not tolerate abrupt changes in current either when they are being charged or when they discharge. This is in essence because as you apply a voltage and a current starts to flow through the conducting element, a magnetic field is generated.
The magnetic field at the same time generates a current that fights the incoming current, making the incoming current needing to fight its way into the inductor. Case A of our picture shows a current happily flowing into our inductor. I say happily because in essence nothing stops this flow. As soon as the FET is energized, the current starts to flow until the inductor saturates. But what happens when the FET is disabled?
We need to provide a way for this current to find a safe path which not encompasses the destruction of our transistor switch. And the solution often comes in the form of what is called a free wheeling diode. It is only when the FET is OFF, that the inductor operating as a source makes the voltage across the diode positive, hence making it conduct. But why do we need to bother about this when dealing with H Bridges? The previous example shows a simple single FET driver. Are H Bridges subjected to the same problems? In essence the problem still exist because inductive loads will still try to conduct through a disabled FET when said switch gets disabled.
So an H Bridge would suffer the same fate as the single transistor driver if an alternate path is not provided. On an H Bridge you only enable as much as two FETs at any given time. If the inductive load was say a DC motor, then the motor would spin in one direction, say clockwise. Unfortunatelly, all is good only if we never disable those FET’s. Because as soon as you do, then the current will try to keep on flowing on the same direction, which should result in flames right?
What if we add freewheeling diodes to save the day? Four of them should do, right? As it turns out, we do not need them! The FETs are considerably much more efficient than diodes anyway, so we can decrease the amount of power loss in the form of heat.
The first thing we must understand is that Shoot Through must be avoided at all times. So, if we are going to use some or all of the unused FETs on the system, it is imperative that we do not turn them ON while the previously active FETs are still ON. If AH is ON, we can not let it be ON, while AL is ON, and so forth. In Fast Decay Mode we use the opposing FETs to offer an alternate path for the current to flow through. Notice that it looks identical to the polarization in which we allowed for the current to flow in the opposing direction. However, it is very important we remember current does not tolerate abrupt changes. Hence, long before the current can flow in the opposite direction, it must decay to zero.
Then, it can start flowing as we saw before. Fast Decay Mode is called as such because this is the fastest the current will die to zero. Since the inductance voltage can only be as large as the voltage source, the new voltage we are applying to the inductor is larger and as such will fight considerably hard for the already existing current to die out. Remember Shoot Through is forbidden, so we must disable one FET before we enable the other one. It must be a break before make kind of deal.
The speed at which the FETs are turned ON and OFF is in the nanoseconds range. Often, anywhere in the vecinity of 100 to 200 nano seconds. You may be asking, how about the current during these 200 nano seconds? Can it damage my already disabled FET? A very important distinction to be made is between asynchronous and synchronouse current recirculation.
This is because you are not controlling the occurrence of the alternate path creation. It will happen, but you do not control when this happens. In Slow Decay mode we use the FETs on the same H Bridge segment. Per example you can use either both high side FETs or both low side FETs. The typical convention is to use the low side FETs. The idea behind this method is that current is allowed to decrease through zero as the inductor recirculates the current through a resistive path.
Notice that as both low side FETs are enabled, the current is basically dissipating a voltage across the two FET’s RDSon. It is called slow decay because although current eventually decays to zero, it takes longer than fast decay mode. How slow the decay mode is depends on the motor inductance and the FETs RDSon. The smaller the RDSon, the longer it will take for the current to decay to zero. When it comes to DC motors, however, there is a very interesting effect while using this decay mode. While on fast decay mode the DC motor rotor coasts down in speed, with slow decay mode the rotor stops very quickly.
This is because as can be seen by the above picture, you are shorting the DC motor terminals. This in essence implies the BACK EMF voltage source inside the motor to be shorted. And if there is no BACK EMF, there can be no speed. One is directly tied to the other. They imply a direct relationship with how fast the current decays through the winding. However, when it comes to how fast the DC motor speed decays, it is the total opposite.
Kind of annoying, but I guess one thing leads to the other. So be careful not to think that fast decay will stop the motor very fast, because it is actually the opposite. Mixed Decay Mode There is one third of current decay which we call Mixed Decay Mode because it is actually a mixture of Slow and Fast Decay modes. By definition, you inccur in Mixed Decay if through the entire current decay cycle a percentage of it is slow and the remaining percentage is fast. Why do we need this feature?
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In DC motors it is pretty much unused, but when it comes to steppers, specially while microstepping, Mixed Decay is a must! This may be a little bit advanced, but what happens is that as you are trying to synthesize a sine wave across your stepper motor winding, drawing said wave shape will become harder at some points in time. So a solution would be to use fast decay. This will definitely solve the problem, but another problem occurs. Fast decay mode has the side effect of high current ripple. Remember the current is swinging considerably more than on slow decay mode.
So what we need is an in between. Some decay rate that is not too fast or too slow. Mixed Decay mode gives us that. Some devices will allow you to control the exact rate of the mixed decay mode.
The DRV8811 is a good example. Other devices, such as the DRV8824 and DRV8825, just give you a fixed rate of mixed decay mode. For most applications, this is enough. I am simulating fast decay, slow decay and mixed decay modes and I have a doubt. It wouldn’d be faster in fast decay mode if you first turn the AH FET off, then you turn the BL off, and then you turn AL on and finally BL on?
I don’t think the H Bridge will ever be in slow decay. And before I go with my analysis, let me point out that the break and make connection of switching the FETs is in nano seconds, not micro seconds or milli seconds. This has to be VERY fast! Otherwise, the current in the winding would die out through the body diodes.
You are right about the order in which things happen. First thing you do is disable the high side FET. This may take about 100 ns, which is what is called the fall time. While the FET is turning OFF, the respective low side body diode will turn ON. In other words, for a few nano seconds, you will have asynchronous slow decay.
However, this really does not last too long as you will disable the low side FET shortly after. So the duration of slow decay is pretty much negligible and there is really not much you can do about it. Seems like a good topic to add to this blog. I would like to know if there is any maths analysis for explaining the fast decay. I mean some kind of calculation that allows to determine the decay time as a function of coil paramaters and power supply voltage, etc.
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I am not a mathematician, so will try to explain it as best as I know, although there might be a few elements missing. It is pretty lenghty, though, so I think I will make it into one of my next posts. In this case, the shoot-through is avoided through the usage of special circuitry that ensures each H Bridge half is never fully enabled at the same time. That is, the High Side FET cannot be enabled at the same time as its Low Side FET. To do this, a delay called dead time is induced when you are switching from 1010 to 0101. On discrete implementations, the controlling circuitry will need to take this timing constraing under consideration. I used to use Allegro A3977 which is nice IC but like the TI DRV8811 it has 2.
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Also want to point out that the DRV8818 came out a few weeks ago and it has lower RDSon than the DRV8811, so it is easier to get to the 2. I don’t think TI was trying to do a pin to pin compatible version of the A3986, although I have heard they will do their own external FET power stage in the upcoming year or so. What I do know, however, is that for 8A steppers they offer the DRV8412 or DRV8432 which can do up to 7A peak or 12. PCB as your heat sink or you add a hefty heat sink on top of the device. First off, the IR2104 already has shoot through protection, so there should be no way to enable both same side FETs at the same time. IRF540 FET’s body diodes will take the current. In actuality it can go up to 110A if pulsed!
I must add, however, that using the body diodes is not the most efficient thing to do. This is piece of cake as all you need to do is invert the H Bridge. I must also point that only using fast decay is ill fated if wanting to microstep, and at 8A I am thinking you will need hearing aid after some time operating the stepper. The problem is that fast decay will give you horrendous current ripple which in turn translates to sub harmonic components beating the heck out of your ear canal.
To make matters worst, high current ripple also means lower average current which in turns mean lower torque. However, as you microstep and draw your sinewave, slow decay can only work for so long. The solution is to use fast decay on quadrants 2 and 4. This is why there is mixed decay, or a combination of fast and slow.
Since you are doing your own controller, you should be able to find the ration of fast and slow which gives you the prettiest sine wave shape. This almost looks like a post in itself! Actually you saying that’s its possible without adding dead time because the IR2104 already has shoot through protection ! Thanks for the article, very clear, interesting and amazing current pictures! Thanks for pure and comprehensible information.
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I am using DRV8825 to control a stepper motor. If you have used this IC, please mail me a schematic. 65V to decay mode pin on the IC. What is the reason for that ?
Yes, I have used the DRV8825 in many different projects, including my CNC Plasma Cutter and my 3D Printer. There is no need to use external FETs with these devices. If you do, however, the FETs should take precedence when they are enabled. The diodes will only work during dead time. This is why you are in essence wasting diodes. The FETs are as protected as they can be.
On any DRV88xx device, you will need to violate Abs Max to destroy the H Bridge. The FETs are protected against over current and over temperature. When they connect the DECAY at half supply, this is like placing the device on mixed decay mode. On DRV8825 there is no need to do this as the device will enter mixed decay mode when the pin is left floating. You can save on the two resistors. Why have nFAULT and nHOME pins been left unconnected ? 2 voltage to appear on the pin and hence activate the mixed decay-mode.
FAULT is an open drain output you can use to determine if there are problems such as Over Current or TSD. On the AE-CNC25, there is no need to use the limited signal pool with a fault signal per axis so I chose to ignore the faulting capability. Home is kind of an useless signal. It tells you when the internal look up table is pointing to the first step. I can assure you on a CNC machine there is absolutely no use for this feature.
The only time I ever used this signal was when exploring highly intricate algorithms in which I was trying to measure phase angles and such. 3K resistors on nSLEEP and nRESET because on the CNC machine environment I do not want to enter SLEEP mode or RESET the device’s core. If I need to, I’ll recycle power. As of today, though, I haven’t needed to.
I also like adding pull up resistors on nENABLE so the device is enabled by nature. This way if I am running an experiment, I do not need to send the enable signal. Do note that just because I placed pull up resistors does not mean I need to populate them. You can always choose to build the board without them.
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If you leave them off and want to add them at a later time, well that doesn’t work as well. For the most part, I use them, though. The CNC Control machine will then send the respective control signals to those signals that do need to change. In other words, there is no need to tie every single line to a microcontroller output.
Yes, the DECAY pin has internal pull up and pull down so when the pin is left disconnected it sits at 1. 67V telling the system to enter mixed decay mode. 18 do have an analog input which will give you tunable mixed decay mode. Avayan, I did not get this part of your reply. Do you recommend using Pull-ups ?
This is also highly dependent on the application. For example, if it is a medical or automotive application, the glitch may be lethal. If it is a toy, chances are it will not make much difference. I would not be afraid if on a CNC machine there are a few glitches here and there on the stepper motor drivers when you are powering up the machine. First thing I do is home the system anyway, plus I doubt that a few nano seconds of enablement are going to make that much of a difference. Thanks for the kudos on my CNC projects. I am doing a low power robot project using a bipolar stepper motor for movement.
I build the stepper motor driver myself with H-bridge L293E, controlled by PWM signal generated from microprocessor. However, when I observed the voltage on both side of the winding, there was a voltage drop opposite to the supply voltage. CH1 was one side of the winding and CH2 was the other side of the same winding. I suspect that this was happened due to the current circulation. Good job in designing your own stepper driver!
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Excuse the question, but what is exactly your concern? The lack of 0V when the output is supposed to be LO and the loss in voltage when the output is supposed to be HI? If you look at the datasheet’s page 5, on the table with VOH and VOL, you will see that this is expected. This driver uses bipolar transistors, so you have to take into consideration the loss caused by the VCE. If your input voltage was 24V, the output deformation would be less visible as 1.
5V is much smaller when compared to 24V, than when compared to 5V. There are several phases work modes mentioned in the most drivers’ datasheets: 1-2, W1-2, 2W1-2, 4W1-2. I should say your incompetence is not as competent as you may think? Years ago, I made it a point not to mix them up with the microstepping nomenclature. I think this may be like trying to explain Intel Pentium technology by looking at an ARM microcprocessor datasheet.
What I have noticed is that Japanese manufacturers, like Toshiba, Rohm, Shindengen, love the one-twos nomenclature. By looking at their datasheet, it seems that 2 Phase excitation is the same as FULL STEP. Somehow, 1-2 Phase excitation becomes HALF STEP. 4W1-2 is the same as 16 degrees of microstepping. Like I said, I just don’t mix degrees of microstepping with the one-twos as I don’t even know where the one-twos came from. Microstepping is so easy to deduce.