DC Electronic Load induced noise

[Mad ID]

Hi,
I apologize if this was already discussed, quick search couldn't find it.

I'm using Maynuo M9711 for DC/DC converter verification. What I have noticed is that sometimes it induces a lot of noise into my power supply output. Only when I disconnect the load wires does the noise disappear. The noise is present even if the LOAD is turned OFF...

If I unplug the AC cable the switcher is still not working cleanly. I guess the load has some reactive input impedance which is hampering with my circuit.

Can someway explain what's behind this?

Here is an example of 50/100Hz noise.

[dom0]

Either ground currents (are you sure it's not connected in any way if you unplug the AC cable?) or EMI into the cables you use to connect it (how long are they? Do they run all iawjdiasdasdakd over your desk or did you twist them nicely?)

[Mad ID]

Either ground currents (are you sure it's not connected in any way if you unplug the AC cable?) or EMI into the cables you use to connect it (how long are they? Do they run all iawjdiasdasdakd over your desk or did you twist them nicely?)

The AC cable of the Electronic Load is unplugged.
The cables do not induce noise when they are connected to the DC/DC output. The problem starts only when I connect both + and - wires to the load's terminal.

Don't know how to explain this. Load is injecting noise somehow..

[dom0]

Well, connect a metal or cooper plate the size of your electronic load and see if it introduces similar noise.

[macboy]

I would try two things:
1. A capacitor at the electronic load input terminals (<=100 uF, and not low ESR).
2. A snap-on ferrite at the power supply end of the cabling, and maybe another at the load end.
Also, it should be obvious that the wires from the power supply to load should be twisted together. Two separate loose wires will act as a nice big loop antenna and will have undesirable inductance. Twisting them cancels out their inductance and reduces the effectiveness as an antenna.

[robrenz]

Looks like common mode noise. Try using two channels of your scope as the poor mans differential probe to make the measurement.

[Mad ID]

I would try two things:
1. A capacitor at the electronic load input terminals (<=100 uF, and not low ESR).
2. A snap-on ferrite at the power supply end of the cabling, and maybe another at the load end.
Also, it should be obvious that the wires from the power supply to load should be twisted together. Two separate loose wires will act as a nice big loop antenna and will have undesirable inductance. Twisting them cancels out their inductance and reduces the effectiveness as an antenna.

The first one is not a good idea. This changes my power supply and the tests are not valid.

What I know is that the Mayuno loads the circuit somehow. I have measured the input capacitance to be 4.5uF, which is not that bad but there is something else inside also.

Does anyone know what is the input impedance when it is not sinking any current?

[LaurenceW]

I have not seen behaviour like this from my own Maynuo 9712, and any power supply in my posession (switched or linear). When switched off, (or when on and the output dissabled) the outputs appear as a reverse biassed diode. This is a function of the main current sinking mosfets. When forward biassed (again, whether switched on or off, but with the output off), the foward current is so totally small as to be negligable.

The positive and negative poles of the Maynuo are not connected to ground (floating), though there is around 20nF capacitance to ground.

Depending on the testing frequency, I measure between 4 and 2 uf input capacitance and 400 down to 10mH of inductance. So there might be a slight resonant load, there. This may be my LCR meter confused by the diode effect of the load, when the output is off.

This same (mildly) confusing load may be having an effect on your SMPS, too. But to be honest, I think it is a design weakness in your supply, which ought to be stable enough to deal with mildly reactive loads such as this. A ferrite bead on the output should quieten things down.

Try a battery of a similar voltage across your Maynuo and look at the waveform - anything?

[David Balcaen]

I have run into this too and believe it is a natural result of the very short time spans (microseconds) of the switching devices used in the DC/DC converter. Then the circuitry rings like a bell and all sorts of high frequency crap is generated.  Even a load such as a cell telephone module exhibits current spikes and complex load pattern that place unusual momentary current demands on a power supply.  Even a battery.  I like the comment in the thread of connecting the DC/DC to a battery (mostly resistive internal resistance, little or no reactive/capacitive component) hook your scope up across the power leads and see what you have.  Check both ends of the power leads.  One of my switching power supplies is an excellent 121.5mhz spurious oscillator, really bad in some instances.  (especially around an airport)    Looking forward to what you find.

[nctnico]

It's 50Hz so it seems to me like it is mains related. The cables probably pick up some 50Hz noise.

[skennedy]

I have seen some quite strange EMI being produced by my Maynuo 9811. I was trying to see what sort of noise a power supply was producing and was a little horrified when I saw how bad it seemed to be. Testing with a resistor instead as the load improved things massively.

I had initially thought it might be the Maynuo trying to adjust for constant current with my the output ripple on the DC-DC converter but it happens in constant R as well. The load will be presenting a complex impedance but I would have thought that this would be hard to measure especially at the operating frequency of the DC-DC Converter (~1 MHz). Interested in seeing what people come up with.