Testing a power supply – Noise (III)

Switching transient noise

Switching transient noise is the noise generated when the PWM switches change state. The transient spike is a higher frequency spike or a sequence of spikes often in the form of a damped sine wave (Figure 3). This transient noise can cause high frequency interference internal to a system and/or cause EMI issues, which will not be discussed in this series.

Be aware that an electronic load may generate noise internally, adding to the output noise of the circuit under test. Also, the output wires may pick-up noise resulting in higher noise measurements than in reality. For this reason you may want to verify the switching transient noise measurements using a load resistor with very short leads. Keep in mind that if the system load is disconnected, you may be measuring noise higher than when a load is connected with high-frequency decoupling capacitors.

1. Before connecting the DC power supply to your power circuit, set the proper input voltage and verify correct polarity.
2. Connect the DC power supply to the input.
3. Set the electronic load to the maximum load expected and connect it to the output.
4. Turn on the input power.
5. Connect the oscilloscope probe with a short ground stub directly across the output capacitor and set the scope to normal trigger mode with the trigger level at midpoint of the waveform. (Ensure NO bandwidth limiting is enabled on the oscilloscope.)
6. Measure and log the maximum and minimum value (positive trigger for maximum and negative trigger for minimum).

Be aware that even with the best probes and probing techniques, some amount of high frequency noise will be coupled into the probe, resulting in a measurement worse than that actually in the circuit.

Switching waveform

It is always a good idea to observe the switch node of a switch-mode power supply. Verify that the switch voltage is not above a predetermined maximum or below a minimum value. Usually the limit is determined by either the power IC or, in the case of a power controller, the external transistor(s).

1. Before connecting the DC power supply to your power circuit, set the proper input voltage and verify correct polarity.
2. Connect the oscilloscope probe ground to the appropriate reference point being sure to isolate the load from the oscilloscope, if that point is not at ground potential. It’s always best to use a very short ground connection to ensure best accuracy. See earlier section on good probing technique.
3. Turn on the DC power supply.
4. Set the oscilloscope to trigger on the positive edge and turn off any bandwidth limiting.
5. Observe and log the most positive voltage on the node.
6. Set the oscilloscope to trigger on the negative edge.
7. Observe and log the most negative voltage on the node.

Thermal testing

Thermal testing and management deserves a thorough explanation of its own.  Here are some basic methods of testing component temperature within a power supply: Thermal probes: Thermal bi-metal probes with a meter can be used for accurate thermal measurements. A probe along with thermal grease allows measurement in the normal system operating conditions. Normally this can be performed with minimal changes to the mechanical structures of the system enclosure. Specifics should be obtained within the meter’s user’s manual.

Thermal IR probes: IR thermal probes are a cost-effective tool for gaining insight into the rough temperature of a circuit. For precise accuracy it may be necessary to use a thermal bi-metal probe or a thermal imager.