The element14 Experimenting with Polymer Capacitors Challenge is an opportunity to “experiment, test, breadboard, or just play around with Polymer Capacitors” and then show what was learned. In this 3rd and final post a Panasonic Polymer Capacitors from the element14 kit will be applied in a low ESR application where ceramic capacitors might otherwise be used.
Experiments to measure ESR and capacitance over a range of values which were completed and documented in Deep Dive into ESR, Introduction and Deep Dive into ESR, Testing. To illustrate one way polymer capacitor technology can be applied and ceramic capacitors replaced, bulk capacitance will be added to a TI SWIFT
Power Module to demonstrate output switching ripple reduction.
Background
The TPSM84A21 10A SWIFT Power Module is a buck converter which can take a 8V to 14V DC input and convert it to a 0.508V to 1.3 V DC maximum 10A output. The module has both input and output capacitors internal to the device and per the Operation Notes external capacitance is not normally needed.
But Section 9 of the datasheet does make the following statement:
If the input supply is located more than a few inches from the TPSM84A21, additional bulk capacitance may be required at the input pins. A typical recommended amount of bulk input capacitance is 47 μF - 100 μF.
The evaluation module provides footprints for external capacitors which are labelled C13 and C14 as shown in the photo below as well as provision for parallel ceramic capacitors (C1 through C11 located on the back of the PCB).
The datasheet states the typical Output Voltage Ripple with 20-MHz Bandwidth peak-to-peak to be 8mV. Additional external capacitors were not fitted during the RoadTest because the measured ripple met the datasheet specifications even though the power supply was rather noisy and located about a meter from the evaluation module.
But will low ESR polymer capacitance help? The Texas Instruments Application Report SLTA055, Input and Output Capacitor Selection state that extremely low ESR is necessary to reduce ripple voltage amplitude and generally recommends ceramic capacitors.
Adding Bulk Capacitance
As expected, the recommended capacitors for the module have low ESR. The module User Guide (SLVUAT2) recommends the following as an alternative to ceramic capacitors:
Woohoo! It recommends a Panasonic aluminum polymer capacitor for C13 with similar value and footprint to one in the kit provided by element14. In this case, a single aluminum polymer capacitor will replace what would otherwise be a number of ceramic capacitors. In the tests that follow, a limited number of conditions were evaluated at relatively low output power conditions. The setup used for measurement was similar to the original TPSM84A21 RoadTest as shown schematically below, differing only in that the current was read from the bench power supply.
See the RoadTest for more information on the instruments used and measurement technique for the power module. The basic setup for ripple measurement using the oscilloscope is shown in the photo below:
No External Bulk Capacitance
All ripple was in the range of 8 mV with no external bulk capacitance. This is identical to the results obtained in the TPSM84A21 RoadTest.
| Test No. | Input Voltage (V) | Input Current (A) | Output Voltage (V) | Output Current (A) | Output Load (Ohms) | Power Dissipated in Load (Watts) | Output Ripple (mV) |
|---|---|---|---|---|---|---|---|
| 1 | 10.52 | 0.036 | 1.005 | 0 | infinite | 0 | 8 |
| 2 | 10.52 | 0.046 | 1.004 | 0.1 | 10 | 0.1 | 8 |
| 3 | 10.52 | 0.138 | 1.005 | 1 | 1 | 1 | 8 |
| 4 | 10.51 | 0.247 | 1.005 | 2 | 0.5 | 2 | 8 |
Switching ripple noise occurred at the same frequency as the switching frequency of the module, i.e. 4MHz in the case of the TPSM84A21. A representative screenshot from the oscilloscope is shown below.
External Bulk Input Capacitance Added
A 120 uF Panasonic Conductive Aluminum Solid Capacitor surface mount OS*CON SVP was added for input bulk capacitance to footprint C13 as shown in the photo below.
Ripple was consistently around 5.6 mV with bulk capacitance added.
| Test No. | Input Voltage (V) | Input Current (A) | Output Voltage (V) | Output Current (A) | Output Load (Ohms) | Power Dissipated in Load (Watts) | Output Ripple (mV) |
|---|---|---|---|---|---|---|---|
| 5 | 10.04 | 0.046 | 1.005 | 0.1 | 10 | 0.1 | 5.6 |
| 6 | 10.05 | 0.138 | 1 | 1 | 1 | 1 | 5.6 |
| 7 | 10.05 | 0.247 | 1.005 | .5 | 0.5 | 2 | 5.6 |
In the following screenshots from the oscilloscope both power supply output and the TPSM84A21 output are shown for each of the tests shown above. The switching ripple is the upper yellow trace and the input voltage is the blue trace.
While the input ripple voltage deteriorates with increased load (blue trace), the output switching ripple from the TPSM84A21 (yellow trace) maintains a relatively constant 5.6 mV.
TPSM84A21 Ripple Reduction Conclusion
The TPSM84A21 module has both input and output capacitors internal to the device and gives remarkable performance with no external capacitors. The datasheet advises that additional bulk capacitance may be needed in some instances and recommends Panasonic polymer capacitors as a remedy. In the experiments performed here the module just met the datasheet specifications with a suboptimal power supply before bulk capacitance was added. Adding a 120 uF Panasonic Conductive Aluminum Solid bulk input capacitor reduced the ripple from 8 mV to 5.6 mV, well below the datasheet value. The voltage requirements can be demanding in some applications and the reduction in ripple from the additional bulk input capacitance could prove quite valuable. Note that the equipment and techniques used in the experiments were with enthusiast level equipment but the results are indicative.
Concluding Remarks
The following objectives were set for the Experimenting with Polymer Capacitors Challenge:
- Describe ESR and how it can be measured
- Obtain experimental ESR value for various capacitor types and values
- Obtain experimental capacitance values for various capacitor types and values
- Compare and summarize findings
- Evaluate application of polymer capacitors to reduce switching ripple in a DC-DC buck converter
In Deep Dive into ESR, Introduction the importance of ESR was described and there was an overview of the contents of the element14 kit. The theory and methods for measuring ESR with an oscilloscope were presented. A plan for testing the capacitors was outlined along with an error discussion.
In Deep Dive into ESR, Testing all of the Panasonic polymer capacitors in the element14 kit were tested as described in the test plan and all met their datasheet specifications for the conditions tested. A video demonstration of measuring capacitance with an oscilloscope was made. Methods for improving the test procedure and potential further areas for study were discussed.
In this final post Deep Dive into ESR, Application a Panasonic polymer capacitor from the element14 kit was used to successfully reduce switching ripple from a TI Swift Power Module where ceramic capacitors might ordinarily be used.
Thanks for the interest - your comments, suggestions and corrections are always appreciated.
Links
TI SWIFT Power Module EVM - RoadTest