Have noticed when putting a 20pf scope probe on the gate to a sfp9540 mosfet, that some oscillation is damped out when full turn on is reached. There is 22ohm gate resistor, and is being driven by an inverting TC4426 mosfet driver. Is it worth putting a 20pf cap on the gate to clean up the waveform? Would the other side of the cap go to gnd?

Follow up question, there is a bit of oscillation encountered at turn off too. Any reason to go after that, and how? A 47k pulldown resistor is used on the MCP4426 inverting input.

 

Just leave the scope probe attached

Just kidding. It would be helpful if you posted a schematic, so we can see what you're working on.

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It is a continuing experiment with a solar battery charge controller. Playing with a buck converter configuration at the moment. Please note the irfr5305 has been replaced with the sfp9540, as the irfr5305 seemed a little too touchy for my bumbling around.

P.S. Schematic doesn't show pull down resistor on TC4426 input, but its there.

Attached Thumbnails

 

 

I'm wondering if you've formed a closed-loop margnially stable oscillator between your MOSFET and the driver power connection. If so, then bypassing capacitors and careful layout might fix the problem.

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The loop is not closed yet, just straight pwm at whatever volts, amps the circuit allows. The lamp load is 500-600ma in full sun. Its a really crummy, cloudy day out, and the circuit is not even stable enough to test under load yet.

Was hoping to work on the feedback loop today.

 

Sorry, talking about the wrong oscillating loop. Will stick on a 20pf bypass to gate and take another look at the waveform. The driver and mosfet are on vero board, could that be the problem?

 

My guess it's caused by coupling from your transistor back to it's driver. When the transistor switches, it draws current through the resistance and inductance of the wires and voltage source resistence, which couples back to the driver. Use seperate connections for your switch and driver back to your voltage source. Try larger guage wire, and bypass caps. You many also increase the value of your 150UF smoothing cap. Your voltage source resistance might bite you, and require a redesign.

But if using a small cap on the gate works, then go with it.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
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Hi there,


Oscillation problems like this often occur because of the small inductance in series with
the source lead. The inductance works with the miller capacitance to cause turn on
turn off, turn on, turn off, etc., until the device finally turns on or off compeletely.
Any extra lead length there only makes the problem worse.
The cure is to shorten the source lead as much as possible. When that doesnt work,
making the gate resistor a higher value has almost the same effect as more
capacitance, but both cause slower switching times so the device has to be checked
for higher switching losses.
Also to check is the bypass cap for the driver chip. If the driver chip doesnt have any
now would be a good time to add one cap across the power supply pins. If the
chip cant get the required power fast enough it wont be able to drive the MOSFET
properly.

I almost forgot to mention:
Sometimes lowering the drive resistance helps instead of increasing it.

Hey, i noticed you didnt show the power supply voltage there?

 

Oscillations also appear to happen when the scope lead isn't properly grounded.

I lost count of the supposedly experienced engineers that I had to teach how to take high speed measurements.

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1) Yes I was taking the driver supply from the pfet source. Re-routed as seperate supply line. Driver gnd is direct wire to supply gnd.

2) Mosfet driver is bypassed with 1.5uf tantalum cap.

3) Oscilloscope probe is grounded to supply gnd.

4) Solar panel voltage is around 20.4V in full sun.

5) Source and Drain to pfet is about 3" (each leg) of 16 gauge wire.

6) Gate wire and trace about equal length for about 1" total.

Happy to show my ignorance here, so included the waveform pics. First picture is taken right at the mosfet gate. Adding the 20pf cap did nothing really. Second pic is at the pfet drain. I was confused when looking at the output side of the pfet drain, and sort of transposed that to the gate. It may actually have nothing to do with the gate, but more of a function of the circuit?

Just learning about buck converter, so if someone could comment on cause and effects of the two waveforms, would be much appreciated.

Thanks to everyone for the help thus far.

Trace Notes: Vert 5V/div Horiz 5us/div

Attached Thumbnails

   

 

Hi,

Oh ok, that oscillation isnt always a problem. As long as the ringing peak doesnt
go too high. That's caused mostly by the resonance between the diode capacitance
and the output inductance.

Here are the two waveforms annotated a bit...you might have to zoom in to
see the annotations though.

Attached Thumbnails

 

 

I'm happy that you showed the waveforms. They don't look like scope artifacts, so that's cleared up.

I haven't done the calculation, but you should confirm that 33µH is correct. To me it seems pretty small for a 30 kHz switcher. It also could be saturating. You should also confirm that it's rated for the current you're passing through it. Do you have a part number or markings?

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de KI6RWX

 

Keep in mind that solar panel is a current source clamped by forward biased diodes.

You can make it look like a short period voltage source with a big enough cap. Don't know what current you are pulling during coil charge but 150 uF may not be large enough.

Does ringing go away if powered from a real voltage source power supply?

 

MrAl:
Thanks for the annotations, really helps.

Meneary:
The inductor is a high current model Bourns.The inductor was mostly chosen for the highest inductance at that amp capability.

The lamp load is only 500-600ma, which will be replaced by a 12 deep cycle battery only when things get ironed out. The solar panel is rated at 17V, 4.4A under ideal conditions.

Bumped the switcher back to 100khz and the gate waveform now has a flat top....yeah! Had been vacillating back and forth on just what frequency to use. The drain waveform spends much less time oscillating, although the peaks look a little higher.

I would post the waveforms, but it looks like the old digital camera has decided to give up the ghost, new batteries, still no good.

RCinFLA:
I'm back on bench supply power now because the sun is fading now. At 34khz, and bench supply, still had the oscillations on the gate drive. Moved up to the 100khz and nice flat top now. On the bench supply there is 500ma being pulled for the lamp, not sure if that answers the coil charge question.

Will be picking up some more low ESR caps soon, so could experiment with larger/smaller input cap.

 

It could still be artifact from your scope's gournd wire, depending on how long that wire is. Either way, it doesn't look like a big deal to me.

__________________
You don't need a quadraphonic Blaupunkt -- you need a curve
ball.