Author:
Ivo Kopecký & Tomáš Zedníček, AVX Czech Republic
Date
12/30/2014
PDF
Here are the results of an output capacitor benchmark study used in a step-down DC/DC converter design, based on a well-used control IC (Maxim’s MAX 1537 [1]) with a 6-24V input voltage range and two separate voltage outputs of 3.3 and 5V.
“Some 99 percent of so-called ‘design’ problems associated with linear and switching regulators can be traced directly to the improper use of capacitors” [2]. The importance of the output capacitor in switching DC/DC converters is related to the fact that it is the reservoir of electric energy flowing to the output and it stabilizes the output voltage.
There are basically two types of regulators — linear LDO (low dropout) and step-down switch-mode DC/DC regulators — to convert voltage to lower level. Switching DC/DC regulators are preferred for applications that require a greater difference between input and output voltages because they are more efficient. This switching regulator option has been selected for our experimental measurements, as it is the most commonly used approach in today’s power supply circuits.
Frequency dependence of capacitance, ESR (effective serial resistance), stability with operational temperature, and DC bias voltage are the important parameters of output capacitors. 3.3 and 5V output DC/DC converters are one of the most common switching mode power supplies used in the electronic industry across the wide range of applications. One notebook power supply converter evaluation kit, based on Maxim's MAX 1537 has been chosen as a real application example for the evaluation of different capacitor technologies.
Switched-mode power supply (SMPS) – theory and simulations
Typical SMPS topologies are shown in Figure 1 and are well documented [3].
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Figure 1: Typical SMPS topologies: a) Step-down (buck) converter; b) Synchronous step-down (buck) converter; c) Step-up (boost) converter; d) Synchronous step-up (boost) converter; e) Synchronous step-down-up (buck-boost) converter; f) Single ended primary inductor converter (SEPIC); g) Flyback isolated-output converter; h) Inverting converter (inverter)
SMPS are used for VIN-to-VOUT transformation:
• VOUT > VIN, realized by step-up, flyback or SEPIC converters;
• VOUT < VIN, realized by step-down, flyback or SEPIC converters;
• VOUT = VIN, realized by flyback or SEPIC converters;
• VOUT = -VIN, realized by an inverting converter.
An SMPS circuit consists of these specific parts: one or more switching transistors, mainly enhancement-mode MOSFETs; input and output smoothing capacitors; low-loss passive device(s) accumulating electromagnetic energy (inductors, capacitors, transformers); non-linear rectifying devices (plain P-N or Schottky diodes); and control sub-circuitry for switching transistor management, feedback stability, and shut-down functionality.
The SMPS function actually lies in the periodic repetition of charging and discharging cycle parts (pulse-width modulation [PWM]) when the passive accumulating device(s) is (are) shortly connected to the input or output via the switching MOSFET. Switching duty cycle is the fundamental factor for VIN-to-VOUT transformation; i.e., it determines the VOUT value with respect to the VIN value. Voltage drops and switching times of both rectifying diodes and switching MOSFETs are critical due to thermal parasitic losses.
Figure 2 shows a basic simulation scheme with a step-down SMPS topology based on MAX1537 Evaluation Kit [4]. OrCAD simulations express output voltage ripple dependencies on switching conditions and output capacitor properties. The circuit parameters are as follows:
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Figure 2: SMPS simulation scheme
RINPUT= 0.1Ω, CIN = 10µF, LIN = 3nH, RIN = 0.1Ω, CINTER = 100nF, LSER= 6µH,
COUT = 1mF (basic value), LOUT = 3nH, ROUT = ESR = 10mΩ (basic value), RLOAD = 1Ω,
MOS Switch: RON = 25mΩ, ROFF = 1MΩ, tSWITCH = 2.5ns, tDELAY = 35ns.
Figure 3 shows that the output voltage ripple depends on the switching frequency and the duty cycle factor. VDC was set at 7V. Obviously, the optimal switching frequency lies between 100kHz and 1MHz, while the duty cycle factor linearly affects the DC output voltage.
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Figure 3: SMPS output voltage ripple (dependent on frequency and duty cycle)
Figure 4 shows the output voltage ripple dependency on output capacitor ESR and capacitance. VDC was set at 20V, switching frequency is 300kHz, and the duty cycle is 17%. The lowest ripple values can be obtained when 100µF < C < 1mF and ESR < 0.1Ω are used.
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Figure 4: SMPS output voltage ripple (dependent on output capacitor ESR and C)
Measurement set-up
Initially, Capacitance and ESR frequency characteristics of different capacitors were measured on 5V output rail using an HP 4194A impedance/gain-phase analyzer [5] in a frequency range of 120Hz to 1MHz (capacitance) and 120Hz to 10MHz (ESR). The 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.
Maxim Integrated Product’s MAX1537EV KIT [4] converter was used for the benchmarking tests. A photograph of the kit is shown in Figure 5. The recommended output capacitance, C, for the 5V output is 150µF. AC ripple voltage values and wave forms were used as the main indicator of filtering quality.
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Figure 5: MAX 1537EV evaluation kit
An output load was set up using resistors and capacitors to draw two thirds of the maximum current. For the 5V output, the value of R = 3.2Ω (see Figure 6). Voltage waveforms and relevant AC VRMS (effective value) were displayed using an Agilent Infiniium 54830B digital oscilloscope [6].
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Figure 6: MAX1537EV evaluation kit measurement connection diagram
Frequency characteristics of various capacitors chosen for 5V output
The graphs above show the frequency characteristics of different technology capacitors used with the 5V evaluation kit output with nominal capacitance (C) of 150µF, with the exception of the MLCC (100µF) and aluminum electrolytic (100µF).
Both tantalum-MnO2 single and multi-anode capacitors retained a higher capacitance at higher frequencies (above 100kHz), whereas the niobium oxide-MnO2 and aluminum electrolytic capacitors lost their capacitance faster at lower frequencies (see Figure 7). The MLCC exhibited very low ESR around the 100kHz frequency range, and the tantalum-MnO2 multi-anode and tantalum-polymer capacitors showed low ESR in the same frequency range, whereas the aluminum electrolytic devices had a high ESR over all frequency ranges (see Figure 8).
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Figure 7: Capacitance vs. frequency of various capacitors for 5V output
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Figure 8: ESR vs. frequency of various capacitors for 5V output
Capacitance stability vs. DC bias voltage and temperature
The best overall capacitance stability is exhibited by tantalum-MnO2 technology capacitors (see Figure 9). The capacitance of niobium oxide-MnO2 devices is more sensitive to DC bias voltage, while tantalum-polymer is more sensitive to temperature changes. The capacitance of MLCC devices is extremely dependent on both actual temperature and DC bias. Further, the capacitance of aluminum electrolytic capacitors is stable with DC bias, but extremely dependent on temperature.
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Figure 9: Capacitance stability of various capacitors for the 3.3V evaluation kit output
ESR stability vs. DC bias voltage and temperature
We can see that ESR is relatively stable vs. DC bias voltage for all capacitors. However, differences can be seen when we compare ESR stability versus temperature (see Figure 10). Tantalum-polymer and MLCC capacitors exhibit the most stable ESR, and the ESR of MLCC devices is very low over the whole temperature range. With tantalum-MnO2 and niobium oxide-MnO2 components, ESR decreases as temperature increases. Aluminium electrolytic capacitors behave differently: ESR grows to very high values at low temperatures (below 0°C), due to the limitation of wet electrolyte conductivity at low temperatures.
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Figure 10: ESR stability of various capacitors
DC/DC converter output ripple voltage waveform
Figure 11 shows the different waveform shapes that occur when different capacitors types are used. Comparing tantalum-polymer and tantalum-MnO2 capacitors shows that the ripple voltage using tantalum-MnO2 devices contains a lower level of higher harmonic components for both 5V output. The basic frequency of the ripple voltage is naturally equal to the switching frequency of the converter (fsw = 300kHz). When using MLCC capacitors, the circuit exhibited undesirable oscillations (fosc approximately = 50kHz) and high AC VRMS due to the regulator instability. Aluminum electrolytic types also did not perform well, as can be seen on the waveforms of both outputs measured by a relatively high AC VRMS.
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Figure 11: Output ripple current waveforms on the 5V rail with selected capacitors
Temperature effect on output ripple voltage
Aluminum electrolytic and MLCC capacitor VRMS behavior across a wide VRMS range is displayed in Figure 12. The output ripple VRMS decreased with increasing temperature in a nearly linear fashion. Aluminum electrolytic and MLCC capacitors were exceptions due to the exponential change in capacitance and ESR they exhibit with temperature (see Figure 9 and Figure 10.) Aluminum electrolytic capacitors also exhibit a too high level of ESR across the temperature range, so their filtering capability is limited, as the output ripple voltage is much higher than with other technologies. When MLCC is used with the very low ESR levels, circuit instabilities result, so output ripple voltage is also high. Among the other technologies, we can observe that ripple voltage at the output will be lower when ESR is low and capacitance at switching frequency is high.
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Considerations and Recommendations
Low output ripple voltage for DC/DC converters can be achieved using output capacitors with low ESR at the switching frequency — in our case, tantalum-polymer and tantalum-MnO2 multi-anode capacitors (see Tables 1 & 2). The rate of decrease of the actual capacitance with frequency in relation to the resonance frequency is also important. Tantalum-MnO2 capacitors are recommended in applications with variable output voltages because they offer the best capacitance stability versus DC bias voltage. It is strongly recommended that designers consider the capacitance and ESR temperature stability of output capacitors when deciding on the system’s operating temperature.
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Tantalum-polymer and tantalum-MnO2 capacitors were found to be the most stable, whereas the MLCC and aluminum electrolytic capacitors were the least stable in this experiment. The capacitance and ESR of the output capacitor can significantly influence the DC/DC converter regulator feedback loop, which defines the stability of the converter operation. The use of MLCC capacitors can be recommended only following a careful evaluation of their low ESR versus stability of the loop. Additionally, good cost versus performance value can be achieved using NbO capacitors.
References
1] Maxim MAX1537 datasheet: www.maxim-ic.com/quick_view2.cfm/qv_pk/4521
2] C. Simpson, Member of Technical Staff, Power Supply Design Group, National Semiconductor
3] SMPS datasheets from Maxim, National Semiconductor, Linear Technology, ON Semiconductor, Texas Instruments and other SMPS manufacturers, available at: www.maxim-ic.com/products/power/dc_dc_switchers/
www.national.com/analog/power/simple_switcher
www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1003,C1042
www.onsemi.com/PowerSolutions/parametrics.do?id=149
4] Maxim MAX1537EVKIT evaluation kit datasheet and product flyer: www.maxim-
ic.com/quick_view2.cfm/qv_pk/4546
5] HP Impedance analyzer 4192A www.testequipmentdepot.com/usedequipment/hewlettpackard/impedanceanalyzers/4194a.htm
6] Agilent Infiniium oscilloscope 54830B datasheet:
www.datasheetcatalog.org/datasheet2/9/0o4ptsp0alkuqg2rh3tp0wy3expy.pdf
