Countermeasures for Power Supply Troubles: Achieving Stable Operation by Using Capacitors!Follow article
Stabilisation of power supply
The first measure we will introduce is the use of conductive polymer capacitors. In many cases, using a conductive polymer capacitor will solve the problem, so please try it. Conductive polymer capacitors are capacitors that use polymer as electrolytes and are characterized by very low parasitic resistance (low ESR). If the parasitic resistance is small, a large voltage drop does not occur even if the stored power is released momentarily, and power can be supplied steadily. Panasonic offers a range of conductive polymer capacitors designed to meet the needs of customers' applications, such as size and reliability. We also have a design support tool "Characteristic Viewer" that allows you to select the best capacitor while looking at the graph. If you would like to know more about this conductive polymer capacitor, please refer to the links below.
Measures against momentary power outage
Next, what measures should be taken to prevent momentary (about microseconds) power outages? The data must be stored safely, and the capacitor must maintain power for the time that the CPU stops normally. If the power consumption of the system is not very high, increasing the number of capacitors in the power supply or using capacitors with high capacitance is a simple and reliable measure. However, if the power consumption is high, it may not be realistic due to constraints such as circuit board space and cost. In that case, consider using a booster circuit to increase the power stored in the capacitor as an alternative approach. The power stored in the capacitor is proportional to the square of the voltage. In other words, if you double the voltage, you can store four times more power. Considering the efficiency of the booster circuit, replacing a capacitor rated at 6.3 V with a capacitor rated at 25 V, for example, is equivalent to increasing the capacitance by approximately 15 times.
Then, what kind of capacitor is suitable for use to prepare for instantaneous power failure by combining with the booster circuit? Table 1 shows the characteristics of the capacitors. First of all, film capacitors are not suitable for high capacity. Electric double-layer capacitors are not suitable for high voltage. In addition, aluminium electrolytic capacitors do not have a long life because the capacity decreases with time since the electrolyte gradually evaporates and decreases (Figure 2, 3). Therefore, it is not proper for use as an instantaneous power failure countermeasure that requires reliability. Multi-layer ceramic capacitors are also not suitable as well. The reason is that, as shown in Figure 4, the multilayer ceramic capacitor has a drawback in that the capacitance decreases due to the influence of voltage and temperature. In this example, a 15 V DC voltage application reduces capacitance by as much as 80%. The capacitance also decreases by about 10% at high and low temperatures. Therefore, a conductive polymer tantalum solid electrolytic capacitor (POSCAP), which is mainly made of tantalum with high capacity, is the most suitable alternative.
For experienced engineers, the tantalum electrolytic capacitor may cause an ignition, but the Panasonic POSCAP is safe because of the self-healing function that repairs itself even when overvoltage is applied. Please use it at ease.
|Item||Tantalum Electrolytic||Multi-layer Ceramic||Aluminum Electrolytic||Electric Double||Film|
|Large capacity||Good||Not very good||Good||Very good||Poor|
|High voltage compatibility||Not very good||Good||Good||Poor||Very good|
|Temperature||Good||Not very good||Not very good||Not very good||Very good|
|Long life||Good||Very good||Not very good||Not very good||Very good|
Table 1: Characteristics of Capacitors
Figure 2: Mechanism of Electrolytic Capacitor Degradation
Figure 3: Electrolytic Capacitor Life
Figure 4: Voltage/Temperature Dependence of Capacitance of Multilayer Ceramic Capacitors
There has been a case in which a POSCAP was adopted for an enterprise SSD in order to prevent a memory write error from occurring when the power supply momentarily stops. In the past, this customer used electric double-layer capacitors and multilayer ceramic capacitors for SSDs, but as space is limited for SSDs, a POSCAP with features of low height and large capacity was adopted.
How do you like it? I hope this article will help you with your troubleshooting.