Power Supply Instability (2 of 3)

Solution

• At what operating points should analysis be performed and tests made?

• Is there a useful screen?

• Is this sufficient or is some follow-up necessary?

Analysis and Measurement

Nominal Measurements: You normally have to calculate one gain-phase plot and make one set of measurements so you can show your management or the customer that the system is stable with sufficient gain and phase margins, even though analysis and measurement at a single set of operating conditions is high risk. Typically the operating conditions are room temperature, nominal input and bias voltages, and either the expected or full load. This approach is at the lowest cost, highest risk extreme.

10% Increments: Another approach is measuring on a grid of intercepting operating points. This results in combination explosion. For example, using 10% increments with input voltage, one bias voltage, load current, and temperature, you would have to calculate 14,641 gain-phase plots and measure the same number. Even if you could or would want to do this, you probably are not going to have the patience to examine each one critically. This is the other extreme, maximum cost, but not necessarily the minimum risk, because no one is going to critically look at this many calculations and measurements. You increase the increments, say to 25%, but this is still 625 calculation/measurement pairs for the above example.

Coffin Corners: This approach, called the coffin corner approach here, is to calculate/measure at the worst case extremes only. For the above example this reduces the analysis/measurement pairs down to sixteen pairs for the above example. You've cut the cost, but you still have some uncomfortable risk, since you have not even measured at the nominal conditions that the power supply will most likely be used. To correct for this you add back in the min, nom, max load at nominal input, and the min, nom, max line at nominal load all measured at nominal bias and nominal temperature. This adds five more measurements for a total of 21 analysis/measurement pairs. This can be done, especially if you are using a computer controlled analysis/measurement system for this.

Coffin Corners Modified for Switching-Mode Power Supplies: The above may work for most feedback circuits, but for switching-mode power supplies you have a few more critical measurement points. The control loop characteristics change at the continuous/discontinuous current boundary. Typically a second order system in the continuous current mode reduces to a first-order system in discontinuous current, which is normally stabilizing. But other stability-related things happen near this boundary, for example see the Jang and Erickson paper in the discussion on Input Filter Interaction.

Finally, power supplies are often overloaded or shorted, and you don't want them to go unstable under these conditions. Hence more analysis/measurement pairs are called for and often you have to increase risk to get the number of pairs down to a reasonable number. Can you cut the pairs and do something else to reduce risk?

Oven Screen

The oven screen described below has proven extremely successful in finding operating conditions where stability margins in a switching-mode power supply deteriorate.

In essence, a square-wave voltage is applied across the reference and the output voltage response is observed as the load current is swept from no-load to full-load for various input voltages, biases, and temperatures.

Figure 1 shows some of the waveforms that can occur on the output.

The upper left waveform shows the overdamped response of a conservatively stabilized system where performance may be sacrificed for a robustly stable system.

The upper right waveform shows the ideal waveform. The output replicates the input waveform.

The lower left waveform shows the very beginning of ringing. Usually damping should occur within a cycle of the first overshoot/undershoot. Any less damping should be noted.

The lower right waveform shows a system about to go unstable.

The circuit is set up in an oven and for a given temperature, the load is swept for a fixed input voltage and bias voltage and any ringing or oscillations noted, along with frequency, on a plot of Vin vs. Iout as shown in Figure 2.

A grid of input voltages for a fixed bias is set up and the load sweeps made. Then repeated for various biases, and then repeated for a grid of temperatures.

Collapsing all the graphs on a single graph, such as Figure 2, shows the line and load conditions that are problem areas over the temperature range.

Typically, it takes a half day to set up the circuit in oven, and a half day to make all the sweeps. Most of the time is waiting for the circuit to reach thermal equilibrium with the oven ambient.

The theoretical foundation of this approach assumes a system no higher than second order, which is rarely the case. However, since you try to make the circuit behave like a first or second order system, it seems to work in practice. I have known a lot of power supplies to develop oscillation in the factory or the field, but never one that has passed this screen.

Variations. Inject white noise across the reference at various injected levels of noise. Noise can induce subharmonics and chaos in susceptible circuit.

Practical Advice

The signal can be injected from a square-wave generator through a blocking capacitor if the reference internal impedance is sufficient to develop a clean square wave across it. If a noise filtering capacitor is across the reference, then it may have to be removed in order to get a clean square-wave signal. More sophisticated injection approaches can also be used. Make sure that if capacitance or resistance at the reference node is modified, the break-points are outside the frequency range of interest. Always monitor the injected signal, to make sure it does not distort, as well as the output signal.

Scope probes melt before 125 C. Keep the probes outside the oven if the range will exceed the temperature rating of the probes, either hot or cold.

Make sure the circuit in the oven reaches thermal equilibrium with the oven ambient before you make the measurements. This will take most of the measurement time.

You can also make ripple, regulation, and other measurements of interest at various temperatures while you have the circuit in the oven.

Measuring your circuit alone is necessary but not sufficient to guarantee stability in the system. You need to include input filters, cable impedances, nonresistive and nonlinear load impedances, etc. in both your analysis and measurements.

Follow Up

Ultimately, your power supply must be stable in its operating environment over the life of the system. For that reason, an injection point and measurement point are often built into the power supply and brought out to board and system test connectors. You have to be very careful that the injection lines do not pick up noise and inject it into a critical point in your circuit. This approach lets you check your stability as the system builds and is deployed. An early version of this was the marginal test capability that was built into early computer power supplies that allowed the power supplies to be varied plus or minus some amount during maintenance tests in order to weed out weak integrated logic circuits. By observing the wave forms during this test, you could get a feel for the stability of the power supply.

One risk you will face is that some cost-reduction suggestion or "improved" part will be incorporated in your design during the manufacturing life. These often negate your hard work and make changes to your system that degrade it, including causing it to oscillate. You have to be continually vigilant.