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Electronics => Beginners => Topic started by: drm on January 17, 2015, 03:30:35 pm

Title: Ground plane continuity.
Post by: drm on January 17, 2015, 03:30:35 pm
hi dear ppl.

Attached is a pic of a PCB with broken gnd plane.

Surroundings traces run a 5Amp 20khz pwm, to a DC motor.

Should I modify my pcb, or the gap in between does not matters?

Title: Re: Ground plane continuity.
Post by: T3sl4co1l on January 17, 2015, 06:20:07 pm
Is this a two layer board?

Also, there's this PRNTSCR button... ;)

Tim
Title: Re: Ground plane continuity.
Post by: ampdoctor on January 17, 2015, 09:29:20 pm
Im not sure I fully understand your question. Are you just asking if it's ok to get rid of the dead copper? If that's the case, I can't really think of a good reason not to remove the islands. Shouldn't matter one way or another. If you have other concerns, then it might be better to supply the entire layout so people can get a better idea of what's going on.
Title: Re: Ground plane continuity.
Post by: Phoenix on January 17, 2015, 10:06:57 pm
My concern is why you're running a ground plane near a 5A 20kHz switching node at all?

Ground planes are great and all, but sometimes you dont want one.
Title: Re: Ground plane continuity.
Post by: drm on January 18, 2015, 09:05:28 am
GND planed was added just in case to decrease an uggly noise
that can be seen on my tv screen, when this pwm controller is driving
the load (driller motor).

The circuit is built on the protoboard at this time , now i want to take it to PCB.


Title: Re: Ground plane continuity.
Post by: Yansi on January 18, 2015, 09:47:38 am
There is no way that 5Amp motor controller should make such terrible noise that affects even your TV screen. This is simply telling you, that your design is badly screwed. Not many people have actually any knowledge how to design power electronics.

So at first, get rid of those silly copper pools. They are useless.

Second, the power circuit should look like this and not else. The decoupling cap shoud be electrolytic, designed a chosen based on current ripple. If you cannot use big enough electrolytic anywhere near, use there 1-2.2uF beefy MKP (polypropylene) foil cap, 100 to 250V rated an then place the electrolytic somewhere near.

Third: Make sure the mosfet is not switching too fast, otherwise it will produce a lot of unnecessary noise and EMI. At 20kHz, you can sure leave it switching slower, that means to use higher value gate resistor, about 100 ohms. The switching loss therefore increases, but at 5 amps current, it is no issue, the power loss increase will be small. Advantages are reduced EMI, which is what you want.

Proper frequency. If you regulate the motor at 20kHz, it is okay. Some people actually think, they can use any frequency, which they want. Not true. The frequency of PWM should by f >> Us / (0.4 * La * In), where Us is the supply voltage, La is the inductance of the rotor winding and In is the nominal current (rated) of the motor. (Some math and thought going on, I don't want to dig into that now) Usualy the minimum frequency is in the range of a few kilohertz, but for practical reasons, it s used higher, so you can't hear the PWM sound from the motor. And also there is no reason to raise the frequency too high.

The circuit should look like this. For example there is an 555 timer as a controller. It is also useful to put a RC or better an LC filter between the fat power supply and the control circuit, so the noise and ripple from the power stage do not enter the controller circuitry so easily.

The loop consisting of diode, decoupling cap and mosfet shoud have the minimum possible length of trace. Lets say 2" / 50mm max. Any length extra wil degrade the performance and adds EMI, because HF ringing can occur in the circuit.
Title: Re: Ground plane continuity.
Post by: drm on January 18, 2015, 10:15:06 am
So guess EMI disturbance is caused by gate drive signal, with less than 1useg risetime, as u pointed out.

For 50useg period, its a hard switched mosfet.


Upper ringing can be seen on the mosfet drain.


Will try to make it turn on slower...

Thanks.
Title: Re: Ground plane continuity.
Post by: Yansi on January 18, 2015, 10:50:30 am
Firstly, make proper layout. Fiddling with switching times won't help until you have a properly layed out board.