Pure tantalum capacitors – also in modern electronics design they stand for extremely high capacitance values for any given volume. But as beneficial as their constructive principle appears - it also reveals pitfalls that make engineers investigate suitable alternatives. Learn why Polymer capacitors could be an appropriate substitute.
We remember: Regularly, pure tantalum capacitors consist of a tantalum anode and a liquid or solid electrolyte as a cathode. This constitutes a certain behaviour that can be eventually outperformed by Polymer-based caps, that use conductive polymers to form the entire electrolyte – or conjunction with a liquid electrolyte, which is known as a hybrid capacitor. Let’s have a look at some key performance aspects:
Ripple current ratings and ESR:
Although pure tantalum capacitors are being offered in many different sizes, they do not come with a very high ripple current which disqualifies them for being employed in applications requiring different levels of current to be passed.
Additionally - albeit they do not suffer from any DC-Bias ageing – the relatively high ESR makes it hard for designers to use those types as smoothing capacitors. By contrast, Polymer caps achieve a very low ESR (e.g. 3mΩ for SP-CAP) with ripple currents up to 10.2A.
In recent years, there has been an increase in the use of low ESR capacitors including POSCAP. The ability to reduce the ripple voltage is introduced here as one advantage of low ESR capacitors.
The following figure below illustrates an example circuit of a general step-down DC-DC converter:
When a capacitor is used for the output smoothing capacitor “Cout”, there is some residual ripple voltage. The graphs above compare residual ripple voltage of different ESR POSCAP capacitors (Fig.1-2 and Fig.1-3) to that of conventional tantalum capacitors (Fig.1-4 and Fig.1-5).
That means, the smaller the capacitor’s ESR, the smaller the ripple voltage becomes.
Miniaturization:
Keeping that in mind will help designers to reduce part counts on the PCB where many tantalum capacitors are connected in parallel to deliver a specific output ripple. Consequently, this saves precious PCB space.
Safety:
Important: tantalum capacitors cannot resist excess voltages - and a very small spike might destroy them and even cause ignition which might affect the surrounding area on the PCB.
That’s why those types are not recommended for applications that are likely to be suffering from higher spikes.
Moreover, as conductive polymer – an organic material - is used as an electrolyte in POSCAPs, it acts non-conductive and as an insulator against leakage current at a temperature of approximately 300℃.
The conductive polymer used for the electrolyte of the POSCAP contains no oxygen molecules. In the unlikely event of a crack forming on the conductive oxide layer, a significant oxide reaction will not occur between sintered tantalum and electrolyte because there are no oxygen molecules. That means POSCAP won’t be exposed to ignition in case of overvoltage.
Frequency:
Polymer capacitors are ideal as decoupling capacitors to remove noises because their impedance shows ideal frequency characteristics. Using a high conductive polymer for the electrolyte greatly improves the ESR characteristics and enables the POSCAP to clearly outperform pure tantalum capacitors at higher frequency levels.
Conclusion:
To sum it up briefly: There is more than one rather clear reason for replacing the pure tantalum types with their polymer alternative in some cases: The specs in terms of ESR, ripple current and frequency characteristics are convincingly better while the overall level of circuit safety is undoubtedly higher.
And last but not least: The polymer caps are the first choice when it comes to saving space on the board – a true ace up the sleeve when acknowledging the unstoppable trend of keeping things safe in ever-smaller designs.