Publication date: 15 December 2009
Switching power supplies (SMPS) are commonly found in many electronic systems. Important SMPS requirements are a stable output voltage with load current, good temperature stability, low ripple voltage and high overall efficiency. If the electronic system in question is to be portable, small size and light weight are also important considerations. One key component in switching power systems is the capacitor – used to store the charge and for smoothing - and therefore their careful selection plays a vital role in determining the overall parameters of the power supply. Different capacitor technologies – tantalum, ceramic (MLCC), niobium oxide (NbO) and aluminium - are suitable to meet different electrical requirements.
This article presents the results of an output capacitor benchmark study used in a step-down DC/DC converter design, based on a well-used control circuit (MAX 1537) with a 6-24V input voltage range and two separate voltage outputs of 3.3 and 5V. The behaviour of different output capacitor technologies was evaluated by measuring the output ripple voltage.
The selection of suitable output capacitor plays an important part in the design of switching voltage converters. “Some 99 percent of the ‚’design‘ problems associated with linear and switching regulators can be traced directly to the improper use of capacitors”, claims the National Semiconductor IC Power Handbook. The importance of output capacitor in switching DC/DC converters is related to the fact that it is (together with the main inductor) the reservoir of electric energy flowing to the output and it smoothes the output voltage. Frequency Dependence of Capacitance, ESR (effective serial resistance) and stability with operational temperature and DC bias voltage are the important parameters of output capacitors that define performance and functionality of the complete power system. Therefore, it is these key parameters that have been measured on different capacitor technologies for the purpose of benchmarking.
Notebooks are one of the most demanding electronics applications where DC/DC converters are typically used with high output current requirements. Notebook supply voltages usually fall into a range of between 15 and 22V with 3.3 and 5V internal power buses commonly seen. To satisfy market demand, semiconductor manufacturers offer integrated DC/DC controllers optimized for these voltage ranges. Such controllers, soldered on a PCB together with all necessary passive and discrete components function as DC/DC converters with maximal output currents of up to several amperes. One notebook power supply converter evaluation kit, based around Maxim’s MAX1537 has been chosen as the real application example for the evaluation of different capacitor technologies. The test connection diagram is shown in figure 1.
Figure 1: MAX1537EV evaluation kit measurement connection diagram
Initially, the frequency characteristics of capacitance and ESR of two capacitor groups was measured. The first group included different capacitors specified for the 3.3V output evaluation kit with capacitance C = 220µF; the second group contained capacitors for the 5V evaluation kit where C = 150µF. Temperature stability of the converter is one of industry’s most common requirements. Thus the second measurement concentrated on capacitance and ESR stability with temperature and DC voltage bias.
Figure 2: Capacitance vs. frequency of various capacitors for 3.3V output
Figure 3: ESR vs. frequency of various capacitors for 3.3V output
Figures 2 and 3 show the frequency characteristics of several different technology capacitors used for the 3.3V evaluation kit output with nominal capacitance C = 220µF (except MLCC where two 100µF were used). The capacitor technologies chosen were tantalum-polymer, tantalum-MnO2 (single and multi-anode construction), niobium oxide-MnO2, MLCC, and aluminium electrolytic.
We can observe a relatively small drop in capacitance in the frequency range 10 – 100kHz in the case of tantalum-polymer and tantalum-MnO2 multi-anode construction capacitors (see Figure 2), whereas tantalum-MnO2 and aluminium electrolytic capacitors exhibit a larger drop across the same range. The actual capacitance of the MLCC capacitor suffers due to its dependence on the DC bias voltage, which was applied during measurement. The very low ESR performance of the MLCC parts, and still relatively low ESR of the tantalum-polymer capacitors is shown in Figure 3. The ESR of aluminium electrolytic capacitors is relatively high over the complete measured frequency range.
Figure 4: Capacitance vs. frequency of various capacitors for 5V output
Figure 5: ESR vs. frequency of various capacitors for 5V output
Figures 4 and 5 show the frequency characteristics results for the selected different technology capacitors used with the 5V output evaluation kit (nominal capacitance = 150µF, except MLCC (100µF) and aluminium electrolytic (100µF).
Both tantalum-MnO2 single- and multi-anode capacitors retain a higher capacitance at higher frequencies (above 100kHz), whereas niobium oxide-MnO2 and Aluminium-electrolytic capacitors lose their capacitance faster at lower frequencies (see Figure 6). MLCCs exhibit very low ESR around the 100kHz frequency range; tantalum-MnO2 multi-anode and tantalum-polymer capacitors show low ESR in the same frequency range, whereas aluminium electrolytic devices have a high ESR over all frequency ranges.
The benchmark tests showed that the best overall capacitance stability is exhibited by the tantalum-MnO2 technology capacitor. The capacitance of niobium oxide-MnO2 devices is more sensitive to DC bias voltage and tantalum-polymer is more sensitive to temperature changes. The capacitance of MLCCs is very dependent on both actual temperature and DC bias. The capacitance of aluminium electrolytic capacitors is stable with DC bias but very dependent on temperature
ESR was shown to be relatively stable versus DC bias voltage for all capacitor types. Differences were observed when ESR stability versus temperature was considered. Tantalum-polymer and MLCC capacitors exhibit the most stable ESR, whereas the ESR of MLCC devices is very low over the whole temperature range. With tantalum-MnO2 and niobium oxide-MnO2 devices, ESR decreases as temperature increases. Aluminium electrolytic capacitors behave differently – ESR grows to very high values at low temperature (below 0degC), due to the limitation of wet electrolyte conductivity at low temperatures.
When comparing tantalum-polymer and tantalum-MnO2 capacitors, the ripple voltage using the tantalum-MnO2 device has a lower level of higher harmonic components for both 3.3 and 5V outputs. The basic frequency of the ripple voltage is naturally equal to the switching frequency of the converter fsw = 300kHz. When using MLCC capacitors, both 3.3 and 5V circuits exhibited undesirable oscillations with frequency approximately fosc = 50 kHz and a high AC Vrms due to the regulator instability. Aluminium electrolytic types did not perform well
Figure 6a: 3.3V output Vrms of ripple voltage benchmark, 6b magnified scale
Figure 7a: 5V output Vrms of ripple voltage benchmark, 7b magnified scale on right side
Aluminium electrolytic and MLCC capacitor Vrms behaviour across a wide Vrms range is highlighted in Figures 6 and 7. Figures 6b and 7b show a much smaller range in magnified scale. For both outputs and most of the capacitor technologies, the output ripple Vrms decreases with increasing temperature nearly linearly. Aluminium electrolytic and MLCC capacitors are exceptions due to their exponential change in capacitance and ESR with temperature. Aluminium electrolytic capacitors exhibit a too high level of ESR across the temperature range, so their smoothing ability is limited, as the output ripple voltage is much higher than with other technologies. When MLCC is used, the very low ESR levels cause circuit instabilities so output ripple voltage is also high. Among the other technologies we can observe that output ripple voltage at switching frequency will be lower when ESR is low and capacitance is high.
Table of output capacitor preliminary static measurements
Capacitor technology
Level of the ESR at fsw = 300 kHz
Capacitance stability vs. temperature
Capacitance stability vs. DC voltage bias
ESR stability vs. temperature
Ta-Polymer
Very good
good
Ta-MnO2 (single)
NbO-MnO2
Ta-MnO2 (multi)
MLCC
poor (too low)
neutral
poor
Al Electrolytic
poor (too high)
Table showing output capacitor application measurements
AC Vrms at 25 °C
Vrms stability vs. temperature
Case size
In our experiments, MLCC output capacitors had too low an ESR (in range of 1 – 2mΩ), which resulted in oscillations of the circuit and a relatively high ripple voltage. Therefore, they are not suitable for the test benchmarking detailed here, and should be considered after careful evaluation of their low ESR versus stability of the loop. In the benchmarks, the use of aluminium electrolytic capacitors resulted in high output ripple voltage and poor filtering due to their higher ESR characteristics, which also significantly deteriorates at lower temperatures. Based on measurements using the Maxim MAX1537EVKIT evaluation kit we can conclude that using low ESR output capacitors such as tantalum-Polymer and tantalum-MnO2 - especially with multi-anode construction - leads to the best results measured by AC Vrms of output ripple voltage and Vrms temperature stability. MLCC and aluminium electrolytic technologies can be used as long as attention is paid to the instability (MLCC) and output ripple (aluminium). Good cost versus performance value can be also achieved using NbO capacitors.
