How to Test and Troubleshoot DCDC Converter

Designing DCDC converter may not any more difficult by the help of online tutorials, simulation software and applications notes. However, the task will not end on the design. Checking if the design is working great is another challenge. Here is the step by step guide on how to test and troubleshoot DCDC converter.

How to Test and Troubleshoot DCDC Converter Comprehensively

1. Power up

The very first in the list on how to test and troubleshoot DCDC converter is power up. It is the very first time the DCDC converter circuit is being applied with power. During this time, do not apply load and a sudden full level of the input voltage. Instead, use the power supply knob and slowly increase the input voltage until the converter gives an output voltage. Monitor the output voltage using a DMM or an oscilloscope. Observe for any smoke, strange smell, arcing, pop, or explosion. Without any of these and the circuit is able to provide output voltage, congratulations, you have passed the first stage of the test and validation process. You can then power up using a full input voltage.

What to Do if Fails in Powerup?

This is the earliest stage of testing and failing on this stage means there is a fundamental issue on the design, wiring or setup. Once already failed, do not test another unit without doing the following:

a. Review the schematic diagram for correct connections.

b. Review the BOM versus actual part used.

c. Review the layout.

d. Review the actual connections.

e. Review the actual setup.

2. No Load Testing

This test should be done after the powerup. The tests that can be done at no load are regulation, ripple voltage and turn-on and turn-off. Attach an oscilloscope to the output rail. Apply power and monitor the behavior of the output waveform. The level of the output voltage must be within the desired range. The amount of ripple voltage must be within the specifications. On the other hand, there must be no issue when continuously perform turn on and turn off cycles.

What to Do if Fails at No Load Testing?

a. If failed at regulation and ripple voltage, check the loop compensation. Check if there is a need to add a pre-load.

b. If failed at turn-on and turn-off (like cannot turn-on right away after turning off), check delay circuits like capacitor networks.

c. If blows up, there showed be a fundamental error. Follow item 1 troubleshooting.

3. Light Load Testing

The purpose of a light load testing is to expose any abnormality when at light load. There is a tendency that a DCDC converter will behave oddly when there is only very small load but not a no load at all. Most issue observed is related to loop instability. The symptom is fluctuation in the output voltage or having a bad ripple.

Hook-up an oscilloscope to the converter output rail to monitor the voltage. Just run the converter for longer period. Set the scope mode to run mode to better capture waveform abnormality. Also explore any abnormality on the waveform during turn-on and turn-off with light load. Consider many turn-on and turn-off sequence.

What to Do if Fails at Light Load Testing?

a. If failed at regulation and ripple voltage, check the loop compensation. Check if there is a need to add a pre-load.

b. If failed at turn-on and turn-off (like cannot turn-on right away after turning off), check delay circuits like capacitor networks.

c. If blows up, there showed be a fundamental error. Follow item 1 troubleshooting.

4. Load Sweeping or Gradual Loading

Using an electronic load with a variable current, sweep the load or gradually increase it until reaches the full load. During the course, observe any abnormalities like shutting down unintentionally, loop instability that causes fluctuations and bad ripple and any catastrophic failures.

What to Do if Fails During Loop Sweeping or Gradual Loading?

a. If failed at regulation and ripple voltage or there is presence of fluctuations in the output waveform, check the loop compensation and the output filter.

b. If there is unintentional shutting down, this could be due to poor loop compensation, improper setting of protections, noisy over current sense lines or bad layout.

c. If blows up, there showed be a fundamental error. Follow item 1 troubleshooting.

5. Full Load Test

When the DCDC converter is tested to be working good from previous items, then test it with full load condition. Load with full load and doing nothing except monitoring the DCDC converter output with an oscilloscope and observing.

If the converter is design to have a cooling fan, ensure to have it before letting it run with full load. Measure the voltage regulation and ripple voltage. This stage is not yet a burn-in, so do not prolong the test. Few minutes is enough.

What to Do if Fails During Full Load Test?

a. If the issue is on regulation and ripple, it could be bad loop compensation and insufficient output filter.

b. If there is unintentional shutting down, this could be due to poor loop compensation, improper setting of protections, noisy over current sense lines or bad layout.

c. If the issue is blow up of components, there is electrical overstress. Need to perform recalculation of the power dissipations.

6. Burn-in and Thermal Test

This test must be performed when the DCDC converter is already proven working good. The intention of this is to know which component is getting high temperature. During this test, all heat generating components must be attached with a thermocouple. Monitor the temperature of the thermocouple using a thermal logger.

This test could be destructive. If the design is to use a cooling system, then put it to the converter before running the thermal test.

The thermal test will be done at full load and minimum and maximum ambient temperature. If the input voltage is wide range, it is also important to test with the minimum and maximum input voltage. Though the worst case is always with the lower input voltage as it will result to higher current in the input devices.

This test will be conveniently performed inside a thermal and humidity chamber.

All the components temperature must be within the safe operating area or as defined in the requirement.

What to Do if Fails in Thermals?

a. Avoid electrical overstress of any devices. Ensure the stress level is less than 70%.

b. Consider using components with lower RDSon, gate charge, COSS, rise time and fall time and saturation voltage to minimize power dissipations.

c. It could be the heatsink or cooling mechanism is not sufficient.

7. Step Load Test

Powerup the converter with no load. When it is running, apply a load. Apply first a small current then gradually increase until reaches full load. You can do 10% load step. This test could be easily done using an electronic load with constant current feature.

During this test, attach an oscilloscope to the output and monitor the voltage regulation and ripple voltage. The regulation must be within the specifications as well as the ripple. Observe also for any abnormal behavior and catastrophic failures.

What to Do if Fails Step Load Test?

a. Possible the loop compensation is not adequate.

b. The output filter could be insufficient.

c. There is also a tendency that the switch path to the ground is not big enough or not very low impedance.

8. Dynamic Loading

Load sweeping or gradual loading or step loading are ways to check the stability of the control loop. To achieve more margin, another essential test to do is the dynamic loading. This could be done by using an electronic load. In the electronic load, set the first load value and the second load value. For instance, the full load capacity of the DCDC converter is 100A, you can set the first load at 50A while the second load at 100A. Then also set the duty cycle and rise time. The test will continue to run until you terminate it.

Use an oscilloscope to monitor any abnormalities in the output voltage. Pay attention to any other observations like pop or explosion, strange smell, and smoke.

What to Do if Fails Dynamic Load Test?

a. Possible the loop compensation is not adequate.

b. The output filter could be insufficient.

c. There is also a tendency that the switch path to the ground is not big enough or not very low impedance.

9. Regulation Test

This test could be integrated to above items. The bottom line is to measure the voltage regulation at certain load conditions and compare to the requirements. Use an oscilloscope to do this.

Regulation voltage is a steady state voltage, not a dynamic or ripple in nature. For instance, the expected level of the output is 12V, the regulation (in DC) must very near to 12V.

What to Do if Fails Regulation Test?

a. Check the control loop compensation if sufficient

b. Check the feedback resistors if correct. Use very low tolerance resistors in the feedback circuit.

c. The output filter should be sufficient.

10. Ripple Voltage Test

This test could also be incorporated to above tests whichever applicable. The objective of a ripple test is to check if the output voltage swing is within the required limit. In this test, a special ripple probe must be used. Ripple probes have specific impedance to filter out noise and able to get a realistic data. A coaxial cable with terminals fit to a scope can be used also. Ripple test needs an oscilloscope. The scope must be set to AC coupling and bandwidth limited.

What to Do if Fails Ripple Voltage Test?

a. Check the control loop compensation if sufficient

b. The output filter should be sufficient.

c. It could be a setup issue, check it.

11. Gain and Phase Margin Test

The purpose of this test is to ensure to have sufficient gain and phase margins. These are indicators of a good control loop. This test is called by some as bode plot test. The equipment used in this testing could be scope with advance feature and specific equipment for frequency response.

What to Do if Fails Gain and Phase Margin Test?

a. Check the control loop compensation if sufficient

b. The output filter could be a factor. Find the sweet spot.

c. It could be a setup issue, check it.

12. Efficiency

To know how efficient the DCDC converter, an efficiency testing must be done. Efficiency is output divided by input. So, measure the output voltage and current (DC) and the input voltage and current (DC) using DMM or power meter. Then do the math.

What to Do if Fails in Efficiency?

Efficiency is

Efficiency = Output power / (Output power + Power loss)

If your design fails, you should minimize the Power loss. The power loss is dictated by the losses on the switching devices and diodes as well as magnetics.

Switching devices could be MOSFET, BJT or IGBT. In selecting MOSFET, ensure that the RDSon is low to minimize the conduction loss and low dynamic parameters (like gate charge, COSS, rise and fall times) to limit the switching loss.

In selecting BJT, ensure the VBEsat is small enough and the collector-emitter saturation voltage is also low to minimize the conduction loss. The dynamic parameters must be low enough to limit switching losses.

For IGBT, ensure the collector-emitter saturation voltage is very low to attain a low conduction loss. Make sure also to select a part with low gate charge for lower switching loss.

13. Reliability Testing

To predict the life of the DCDC converter, reliability testing could be done. This involves climatic testing together with humidity and vibration. The climatic chamber will be set to cycle from low temperature to high temperature and with equivalent humidity while also subjected to a vibration. The test may take a week to a month depending to individual requirements.

What to do if there is Failure during the Reliability Test?

It is normal to fail during the reliability test because its purpose is to test to fail to know the devices limits and somehow to extrapolate how many years the product will last in the field before any failure to occur. If the failure is too early, you should consider replacing components with more rugged specifications.