Electronics > Projects, Designs, and Technical Stuff
Circuit for MosFets in parallel for extra current capacity.(Solved)
<< < (8/10) > >>
Niklas:
I used to have the same hobby, flying 10 cell (NiMH) F5F electric gliders, but have not been active for at least 10 years. Brushless motors were new back then and the LiPos were just introduced.
There might be an easier way to implement the safety feature, but it depends on the speed controller's safety. Usually there is a microcontroller that does the control signal processing and commutation calculations. Upon power on, the software waits for the control signal to be "motor off" before enabling the speed controller function. It is also common that it will use the motor windings to beep when this is done.
There is also another software function that performs a watchdog timeout function. The control signals from the receiver is sent out at approx 20-50 times per second, depending on the manufacturer. The signal is a 5V pulse, positive logic, with pulse width varying between 1.2 and 1.8 milliseconds, depending on the demanded speed or servo angle. If that signal is not received, or the pulsewidth is out of range, then the software should disable the speed controller.
A small switch, inserted on the control signal between the receiver and the speed controller, and a pulldown resistor (10k, signal-GND on the ESC side of the switch) could be an alternative. The control signal wire is either yellow, orange or white and the GND wire is usually black or brown. If you use an extension cable, then you don't need to modify the speed controller itself.
spec:

--- Quote from: Niklas on January 01, 2019, 01:42:26 pm ---I used to have the same hobby, flying 10 cell (NiMH) F5F electric gliders, but have not been active for at least 10 years. Brushless motors were new back then and the LiPos were just introduced.
There might be an easier way to implement the safety feature, but it depends on the speed controller's safety. Usually there is a microcontroller that does the control signal processing and commutation calculations. Upon power on, the software waits for the control signal to be "motor off" before enabling the speed controller function. It is also common that it will use the motor windings to beep when this is done.
There is also another software function that performs a watchdog timeout function. The control signals from the receiver is sent out at approx 20-50 times per second, depending on the manufacturer. The signal is a 5V pulse, positive logic, with pulse width varying between 1.2 and 1.8 milliseconds, depending on the demanded speed or servo angle. If that signal is not received, or the pulsewidth is out of range, then the software should disable the speed controller.
A small switch, inserted on the control signal between the receiver and the speed controller, and a pulldown resistor (10k, signal-GND on the ESC side of the switch) could be an alternative. The control signal wire is either yellow, orange or white and the GND wire is usually black or brown. If you use an extension cable, then you don't need to modify the speed controller itself.
--- End quote ---
Brilliant :-+
David Hess:

--- Quote from: spec on January 01, 2019, 08:29:40 am ---The first story concerns a friend's high-end MOSFET audio power amplifier. He said that it sounded very good but there was something about the sound that was not right somehow, and he couldn't quite put his finger on it. I had a listen and on his outfit it sounded superb... at first. But, after a while, I too detected an odd characteristic to the sound, so I took his amp home to investigate.

To short a long story, I found that the output power MOSFETs were bursting into oscillation at random points of the output voltage waveform. The amplitude of the oscillations varied and, for a while, they would be absent. There were no gate stoppers and relatively long traces between the MOSFETs and the driver circuit. So, obviously, I fitted gate stoppers. This greatly reduced the tendency for the MOSFETs to burst into oscillation and lowered the frequency too. But it was not a complete cure.

The MOSFETs were decoupled, but only with high-value aluminum electrolytic capacitors, so I added polypropylene decoupling capacitors. The result was that there was no sign of the parasitic oscillations and the owner was over the moon that his amp had regained it's original pure sound.
--- End quote ---

My stories involving MOSFET linear output stages without gate resistors usually involve the MOSFETs breaking into a roughly 10 to 100MHz howl and self destructing and the better the low loss high frequency decoupling is, the more quickly they self destruct.  Testing shows no problems until the output cable is attached.

For switching stages time spent in the linear region is short so oscillation is very rarely a problem but larger values may be used to control ringing in the gate drive signal.  But an ESC is usually driving a relatively long lead (transmission line) to a motor which can lead to problems.

The safe rule of thumb is to include a lossy element on at least one lead of any transistor.  Strong decoupling and a capacitive load are especially problematical.

spec:
Ha Ha I love these stories. It is a consolation to know that others have problems too.

Before, I got the hang of decoupling, stopping, etc, I went though a phase where everything I touched oscillated, unless I tried to make an oscillator that is.

We had a name for intermittent circuits- 'Thursday Circuits/Equipments'. Thursdays were often the day for customer demonstrations/handover and, guess what, that would be the day that the gremlins would show themselves.

By the way, I started a new thread on this general topic: https://www.eevblog.com/forum/chat/thursday-circuits-gremlins-repairs/
spec:

--- Quote from: David Hess on January 01, 2019, 02:22:50 pm ---The safe rule of thumb is to include a lossy element on at least one lead of any transistor.  Strong decoupling and a capacitive load are especially problematical.
--- End quote ---
Good idea, presumably a lossy feritte bead would be one way.

Quite right about strong decoupling being critical, but you often need strong decoupling to get a good frequency response, supply line rejection, etc. Also poor decoupling can cause parasitic oscillations/ringing too.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod