Leakage current & current leaks – part 1

Accounting for all of the obvious points of power consumption and their current levels for active and sleep states can be a difficult task. In every design there are also points of power consumption that are often overlooked. Every type of semiconductor device has some amount of “leakage current” that may or may not be called out in its datasheet.  Leakage current can make up the majority of the deep-sleep current draw for a modern micro when it shuts off power to most of its internal circuits. There may also be aspects of your design that can “leak” microamps to milliamps of current that you don’t take into account.

This is the first of a two-part posting on leakage currents and current leaks. The first part will cover leakage current in capacitors. The second part will cover leakage current in other types of devices along with common types of current leaks in designs.

Capacitor leakage current

Capacitors can be a major cause of unexpected current draw due to their leakage current. Some amount of leakage is unavoidable for any type of cap but to some degree leakage current can often be reduced through part selection.  This is what you should know about leakage current for the three most common types of capacitors.

Aluminum electrolytic – While hard to beat for capacitance versus cost, these are the leakiest commonly used caps with the highest number of factors that determine their leakage current. The formula typically used to calculate their leakage current is shown below.

AE leakageAs you can see, capacitance and the rated voltage are the primary factors in the leakage current. A couple of things not shown in this equation:

  1. With aluminum electrolytic caps the leakage current changes significantly as the difference between applied voltage and rated voltage increases. This works both ways, the leakage current decreases as the voltage is decreased below the rated voltage. The leakage current also increases significantly when the applied voltage exceeds the rated voltage – in this situation the cap can be physically damaged, causing a permanent increase in leakage current.
  2. The leakage current of aluminum electrolytic caps is also greatly influenced by temperature. The leakage current can increase 2X-3X with a temperature increase from 20°C to 60°C.
  3. There a number of other factors that influence the leakage current of electrolytic capacitors. These include age, storage conditions, type of electrolyte and the physical construction of the capacitor. Electrolytic caps that have been abused (over voltage, excessive current, etc.) may exhibit significantly higher leakage currents due to internal damage.

Tantalum – Often the primary choice when high capacitance is needed in a small space, tantalum caps also offer potentially much lower leakage currents than aluminum electrolytic caps. The formula typically used to calculate their leakage current is shown below.

tant leakage As you can see, capacitance and both the rated voltage and applied voltage are the primary factors in the leakage current. The applied voltage also comes into play and the usual voltage derating used for safety reasons with tantalum caps can provide a significant reduction in leakage current.

Ceramic – While not capable of the high capacitance density of aluminum electric and tantalum caps, ceramic caps by far suffer the least from leakage currents. The formula typically used to calculate their leakage current is shown below.

 ceramic leakage

 It appears that capacitance is not involved at all and this just the Ohm’s Law formula for current based on voltage and resistance. In ceramic caps, the leakage current is primarily due to insulation resistance between the part terminals. However the insulation resistance is typically based on an “Ohms Farads” value even if it is listed as a resistance in the cap datasheet. While rated voltage is also not included in the formula directly, insulation resistance is typically a function of the rated voltage. Insulation resistance typically increases with rated voltage so leakage current can be decreased by using parts with a higher rated voltage. The insulation resistance of ceramic caps is also dependent on temperature but since their leakage currents are so low the impact of temperature can often be ignored unless you are concerned with sub-microamp current draw or your device will be exposed to extreme heat.

 

The chart below shows examples of the calculated leakage current for these three types of caps and summarizes the discussion above. Two examples are presented for each type of cap. For example “A” the cap is 10uF, rated voltage is 10V and the applied voltage is 3.3V; for example “B” the cap is 47uF, rated voltage is 16 volts and the applied voltage is 5V.

Cap Type

Leakage

Current

Capacitance Impact

Voltage Derating impact

Temperature

Impact

Other considerations

Aluminum electrolytic

“A” = 3uA

“B” = 8.2uA

High

Medium

High

Storage conditions, electrolyte type,

 age, part abuse

Tantalum

“A” = 330nA

“B” = 2.3uA

High

High

Medium

N/A

Ceramic

“A” = 66nA

“B” = 100nA

Low

Medium

Low

N/A

The differences in leakage currents between the types of caps are obviously very significant for ultra low power design. Multiply these numbers by the number of caps in your design and the leakage current can become a significant part of the sleep state current of your device. This is particularly true for a device with a wireless radio where there may be several hundred microfarads of capacitance to prevent a voltage droop when the radio starts to transmit.

Several more thoughts on capacitor leakage currents:

  • Since one of the main factors in leakage current is the amount of capacitance, for a low power design is important to determine how little capacitance is actually required for the circuits to operate reliably. This is particularly important for electrolytic and tantalum caps.
  • For all three types of capacitors, the rated voltage is a determining factor in leakage current. For electrolytic and tantalum caps, selecting caps with rated voltages at least 2X to 3X the applied voltage will greatly reduce leakage current compared to the same value cap with a rated voltage only slightly higher than the applied voltage. For ceramic caps, caps rated at 50V or higher typically reduce leakage current by a factor of 10 compared to the same value cap with a 25V or lower rated voltage.
  • There is also a significant time aspect to leakage current. When voltage is applied to a discharged cap, the leakage current can spike to 50X to 100X the calculated value, remain there for a minute or two then gradually fall to the calculated level over several minutes. This is sometimes called “absorption current”. This spike is so high and lasts so long that leakage current tests on aluminum electrolytic are typically performed at least two to five minutes after voltage is applied to the cap. This really needs to be taken into consideration when selecting caps for circuits that are frequently turned on and off as part of the power management scheme and for caps used in timing circuits.
  • This may seem like a fine point but leakage current on capacitors down-stream from a voltage regulator carries an extra expense in terms of power since the energy stored in those caps was subject to the inefficiency of the voltage regulator. Be careful about throwing extra capacitance on the output of regulators just to be safe. Many low power LDO regulators can get by with just a few microfarads of input and output capacitance. The bulk capacitance on the output of switching regulators can often be reduced if your design can tolerate more ripple on the voltage.

 

The second part of this post will discuss leakage current in other types of semiconductor devices along with sources of current leaks commonly found in many designs.