A reader posted a good series of questions for the AE-DualDC3A driver board so I thought of answering them on a post in order to offer some information on these topics. I will continue with the 3D Printer mania soon, I promise! So the questions are:
1. Since there are two DRV8800 devices on a DualDC3A module, can I drive them in parallel for increases current capability?
2. Is the board setup for sharing both device’s PHASE and ENABLE Signals or do I need to add jumper wires?
3. Due to board size, how much power can we expect to dissipate without requiring extra heat sinking?
These are all great questions and the truth is they can in essence be answered with “Yes”. Well, at least the first two… But let me get deeper into details.
The DRV8800 can be driven in parallel. I actually wrote an article about how to do this for the Servo Magazine a few years ago. Sorry, I can’t possibly remember the month and year in which the article came out, but don’t bother too much with it as there is one important item I forgot to add in this article. I was too fresh on paralleling H Bridges back then.
The real problem with paralleling two or more DRV8800’s (the most I have done is four units!) is what happens when they get disabled. Although you are enabling/disabling them all at once by applying a PWM at the ENABLE pin, they can still get disabled when they trip due to the current set point being met or an OCP event taking place. When this happens, which device shuts down first? We cannot know but I can tell you this: Once that happens, the device that shuts down first will take all the recirculating current until the other device switches to the same state. This may not be a problem for the FETs, which can take twice the current for a few nano seconds, but the body diodes can be severely affected. Think smoke here! Hence, for this to work you would need to add Schottky diodes at the outputs. TI has an application note depicting this implementation, HERE. I will let the application note do the talking on how to select diodes and what to expect.
Needless to say, the board is not setup to do parallel driving without some modifications. These modifications are not that hard to accomplish, though, and here they are in no particular order:
1. The PHASEA pin needs to have a jumper to the PHASEB pin.
2. The ENABLEA pin needs to have a jumper to the ENABLEB pin.
3. The MODEA pin needs to have a jumper to the MODEB pin.
4. OUTA+ output needs to have a jumper to the OUTB+ output.
5. OUTA- output needs to have a jumper to the OUTB- output.
6. Schottky diodes needs to be added as depicted on the application note in order to protect the internal body diodes.
To be honest, I do not think paralleling DRV8800’s is the way to go anymore. It may have been 4 years ago when there were not too many options to pursue, but if I wanted higher current to drive a DC motor I would chose a DRV8840 or DRV8842 which can do up to 5A. I have a module, the AE-MegaBridge, that is designed around these devices, so that is what I would use. To parallel DRV8800’s would results in an implementation larger than a single DRV8840 and the hassles would be too many. DRV8840/42 also adds current regulation which allows you to control torque, in case that is something you are looking to do, although it is kind of rare on DC motor applications.
But say you still want to pursue the DualDC3A in parallel mode with the Schottky diodes, how much power could we drive out of the device? Unfortunately, not a lot, not continuously at least. Brushed DC motor drivers are really not meant to be driving humongous currents for prolonged periods of time. Basically, when an H Bridge is chosen you are assuming the motor will run at a rather small current for the most part, with the caveat that you may require current bursts during startup, stalling and load changes. In other words, if a device is rated at 3A, you should not run it continuously at 3A, or if you need 3A continuous current, then you should be looking at a 5A power stage.
With that being said, a device like the DRV8800, which happens to be at up to 3A, simply cannot be expected to run at 3A continuously unless you add liquid nitrogen. The RDSon is too high for this. And the DualDC3A board does not help as its size is too small. The idea behind this module was to power two small DC motors requiring any current less than 1A continuous, and up to 3A peak for a few hundred milliseconds if at all.
If you parallel both devices, however, you are in essence halving the RDSon which allows you to run cooler. Say now you should be able to run continuously at 2A and reach peaks of anywhere in between 5A and 6A. I haven’t done the experiments to be able to tell you for how long something like this would work without adding heat sinking, but I know that at 1A, a single DRV8800 device should run pretty much forever, so the same should apply for two devices in parallel passing the 2A. At these currents, both devices would be subjected to about 1.55W (1A*1A*1.55Ohms), and since the device’s package is rated up to 4W I think we are in good shape. Do note that due to the board size, each device will be heating each other which is definitely not a good thing, but I think we would be well within a safe range.
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