Electronics > Beginners
Common ground ringing
garethw:
I have assembled a buck converter using an LM2594 DIP. It uses a 100uH inductor and has a switching frequency of 150kHz. The circuit is assembled on a breadboard and performs well.
I then tried the same thing with an MCP16301 (SOT-23 soldered to DIP breakout board). Assembling the circuit from the data sheet using a 22uH inductor, again on a breadboard. This IC uses a switching frequency of 500kHz.
Both circuits step down 12V DC to 5V. Both drive a small load of 60mA but I have tried higher to see if it helped.
My problem is ringing from the MCP16301 into ground and across the whole board. The first circuit had no issues at all.
Is this a classic breadboarding limitation? If so, I’m guessing the frequency limit (of the breadboard) must be between the 150kHz and 500kHz of the two circuits. I’m surprised the first circuit had no issues at all and the other is awful. The only major difference is the switching frequency.
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T3sl4co1l:
As in, you're probing anything, including even just ground itself, and you pick up the same ringing?
That's common mode noise, and indicates a voltage drop in ground itself, where the RF current (the ringing) is flowing through ground, inducing the voltage drop.
Shrink your switching loop. Use closer positions and avoid AC currents in and out of the ground rail.
Reminder that capacitors act as AC short circuits, and inductors act as opens.
A typical case for a buck converter might be:
1. Chip plugged in, GND jumpered to the ground rail.
2. Bypass cap between VIN and GND, right in front of the chip.
3. Diode between GND and SW, beside the chip. Jumper from SW pin to diode.
4. Inductor from diode to output filter cap.
#1 isn't a big deal in and of itself, because the regulator doesn't draw much ground current, if it is using an external diode. (Both devices you mentioned require external diodes.)
#2 can be okay in and of itself, but where it fails is #3. The switching loop includes the VIN bypass cap, regulator and diode.
If the diode were moved to a standing position and placed in front of the regulator itself (if there's room to), then the switching current flows all right in front of the regulator.
You should then use a modest inductance (~0.1uH, or a ferrite bead maybe) between the VIN pin, and the breadboard VIN (where you probably have a bigger like 100-1000uF electrolytic), to avoid switching currents being drawn in parallel with the bypass cap.
The output filter loop (inductor and cap) doesn't matter as much, because the inductor has high impedance at RF. It can still carry some, in which case the cap should again be returned to the diode anode, or VIN bypass cap ground, directly.
Tim
garethw:
Thank you for your reply. Answering your first question; Yes, I get a similar ringing everywhere, ground, 12v rail, 5v output of the smps, even the outputs of the LDOs. The common mode noise you mention sounds spot on. I will go through your suggestions and see if I can make some improvements.
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fourfathom:
Some of this could also be scope probe ground connection noise pick-up. Clip your probe ground to the probe tip and then probe your circuit ground. If you see a signal then your scope ground lead is too long and you are seeing inductive coupling. There are probing methods available to reduce this effect.
noname4me:
How many probes are you using?
Are you connecting the ground clip on each one? To different ground locations on the board?
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