Mosfet driving and bridge
Erik Friesen
Posts: 1,071
In this design, I am using the mcp14700 (u2 - 4) - http://ww1.microchip.com/downloads/en/DeviceDoc/22201a.pdf and the Infineon ipg20n (u6 - 8)- http://www.infineon.com/dgdl/IPG20N06S2L-50_DS_10.pdf?folderId=db3a304412b407950112b4322c6d574b&fileId=db3a30432313ff5e0123a3bf936226b1
R8-13 are 15 ohms. Is this a mistake? Should these be straight through?
Here are the symptoms. Mosfets and drivers are running warm to the touch, not hot. However, when the motor (brushless)gets under load, or gets into a debug type jerk and start due to testing, it is blowing both the driver, and the mosfet, although not every time. Failure is a demonstrated by a high current draw from the 5v reg, which is usually from the 14700 driver. Loads are less than 1.5 amp at this point.
R8-13 are 15 ohms. Is this a mistake? Should these be straight through?
Here are the symptoms. Mosfets and drivers are running warm to the touch, not hot. However, when the motor (brushless)gets under load, or gets into a debug type jerk and start due to testing, it is blowing both the driver, and the mosfet, although not every time. Failure is a demonstrated by a high current draw from the 5v reg, which is usually from the 14700 driver. Loads are less than 1.5 amp at this point.
Comments
-Phil
that you must consider the gate load capacitance to determine the resistor value. Possibly to
decrease losses by fine tuning the gate drive voltage for a given frequency?
May not be the problem, but it's a learning experience for me,.
How are you generating your PWM? PICs that are used with BLDC motors have dead-band built into their PWM peripherals.
Have you incorporated all those precautions mentioned in the Microchip data sheet?
Which data sheet do you refer to?
@capt quirk
The data sheet for the mcp14700 gives no recommendation and does not show resistors in their sample schematic, thus the uncertainty.
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en542235
This is a four layer board, so ground and power come from internal layers.
based on your hardware and your software, and is used to fine tune the
gate voltage in order to reduce switching losses that may increase heat.
So, I am curious what your gate drive voltage is, is it regulated, the pwm frequency.
I believe, that with this information and what you supplied, I could calculate
your switching losses that may be contributing to the heat.
You can get a feeling of this by looking at the gate voltage (post-resistor) on an oscilloscope. Are the leading and trailing edges of the PWM vertical, or do they rise and fall more slowly? If the latter, either your resistor is too large, your gate driver is inadequate, or both.
-Phil
Are you sure the ringing is still there when you remove the scope probes? Yeah, rhetorical question, I know; but how are you probing the gate, and where are you connecting the ground lead? In situations like this, it's best not to use the ground lead with the alligator clip, but to attach a short wire to the probe's ground collar and solder it directly to the MOSFET's source pin.
-Phil
I suppose it could be ringing because of the probe, but wouldn't that show up in other places? The whole probe ground thing is murky in my mind, as my scope has them all tied together internally, as well as to the ground plug on the plugin. (fluke pm3382) Really to do it right a person ought to have a differential probe set, which I can't afford right now.
-Phil
not hot. However, when the motor (brushless)gets under load, or gets into a debug type jerk and start
due to testing, it is blowing both the driver, and the mosfet, although not every time. Failure is a
demonstrated by a high current draw from the 5v reg, which is usually from the 14700 driver.
Loads are less than 1.5 amp at this point.[/QUOTE]
I am curious if your problem occurs before you start sampling the zero-cross? and if your load is a propeller.
Also, I have been reading about motor misfiring or not being in sync from start up to the zero cross range,
and programming for that possibility. I have also read about "over-current" protection during start-up, but
I think that may protect the motor coils?
I have started to suspect a transient problem, or at least enough to fry the driver chips, which in turn then fries the mosfets. I also suspect the IPG20 mosfet is way overrated in the datasheet. Or, the 15a avalanche rating is not enough for my motor.
Its a learning process by force.
was unable to deal with the spike. That very low esr ceramic caps are preferred over electrolytic, and that an x7r temp rated ceramic capacitors
are recommended.
Bill M.
How did you create your PDFs? Neither one opened in my PDF viewer but caused errors, and all I got was blank pages.
-Phil
The supply can give 4 amps or so, but peak pulse amps are much higher, I estimate closer to 10, based on mclv testing.
I don't know if I have it figured out yet, but one major source I have identified is fairly severe ground bounce, enough to reset the uP. 100 ohms resistance on the gates has made a huge difference. I think the ground bounce was also affecting the driver outputs. These mosfets switching speed is specked at 5ns rise time, 15ns fall, compared to the mclv board at 250 rise, 140 fall.
I also changed the .1uf bypass caps on the driver for 1 uf, now the switching and driving waveforms look the same as the mclv board.
Yet another problem, I have forward and reverse constants, I had accidentally added a 0 to the end of one, thus one direction was causing issues.
http://www.hobbypartz.com/86ma64-2215-740kv.html Yes I know, its not rated to 24v.
Are you suggesting that I need to get a ceramic that is specked as low esr? There is only about 4 0805 size ceramic caps listed as low esr at mouser, and they are all y5v.
The mclv is using 3.3uf tantalum for the bootcap, which I doubt is lower esr than a ceramic.