Practical study about a power supply with supercap backup features

In this article a preliminary study will be described concerning a power supply capable of supplying energy for a sufficient time to perform a graceful shutdown on the system after a power loss occurs.

On the market there are a lot of devices useful to solve this kind of issue, very common in the MCU or CPU systems where a sudden power loss can corrupt the file system or create other problems. The current supercaps are cheap and more reliable than a battery of any chemistry. The system must be advised when the power goes down and it just requires a bunch of seconds to execute the normal shutdown procedure that syncs all the storage devices and detaches correctly all the peripherals. Let's explain a practical example where a 5V power supply is available to power up a 3.3V device. We need a regulator that supplies the device in usual operation and a backup source that stores enough energy to give to the system the time for a graceful shutdown. If the regulators is capable to work correctly in a wide range of input voltages, we can also use small supercaps.

Power regulator

Just because this issue is so common, it's easy to find a device that fits all our needs. Searching among the Texas Instruments documentation and developing tools (see link section for references) we found the TPS6302x buck-boost regulators family. The TPS63021 is the fixed voltage version.

As TI wrote on the datasheet:

The TPS6302x are high efficiency, low quiescent current, non-inverting buck-boost converters suitable for applications that need a regulated output voltage from an input supply that can be higher, lower or equal to the output voltage. Output currents can go as high as 2 A in boost mode and as high as 4 A in buck mode.

As the different application examples highlights, it has been designed to be used to get the maximum when batteries or super capacitors can still deliver enough power but with a voltage lower than required by the system, with high efficiency both at light load and at lower input voltage.

As shown in the graph below, it can still deliver an output current of 500mA @ 3.3V with an input voltage of 1.8V, draining a lot more energy from the backup source compared to buck only power regulator.

It has a good efficiency also when used in normal operation

Considering a market price of €1.2 @ 1000, small footprint and minimal BOM count, it can definitely be the right candidate.

Super capacitor

The market released a wide range of supercaps in the last few years. Differently from the ones available in the past, they have very low current leakage and ESR, allowing their use also in battery powered systems that require hundreds of mA. See also this study. Just as an example, searching in the Digikey catalog for supercaps in the 1F to 5F range, @ 5V with an ESR lower than 200mOhm, there are about 30 items, with a cost in a €1.2/€8.9 range @ 1000. The cost is influenced also by the reliability of the device, measured as the time it can resist at a given temperature. A good balance among price, reliability, features and availability, can be found in the AVX SCM series.

Schematic

Putting the previous consideration all together, we have the schematic below:

Using a footprint as shown below, we can choose a supercap between 0.47uF to 2.5F according to our needs.

Let's make some simple estimations on the performances. The input voltage is given at 5V. If the 5V power supply is under our control it can also be enhanced to 5.2V, compatible with all the other devices and compensating some voltage losses. In order to reduce the power losses it's strictly advised to use a very low forward voltage Schottky diode, even if its current limit is higher than we need. The graph below shows the Vf of the Diodes B320A rectifier vs If.

The resistor R1 limits the initial current to 50mA. This increases the time needed for the supercap to be fully charged but avoids a quasi-short circuit when it's empty. When the supercap is fully charged the current across DS2 is extremely low, as well as its Vf. At VCAP5 test point we can therefore consider a voltage very close to 5V, but in any case lower then 5V0, enabling the regulator to be powered by the main supply in normal conditions. As soon as the main power goes off, the voltage at VCAP5 TP2 becomes higher then 5V0, DS3 goes in conduction state powering up IC1 without glitches. DS1 and DS2 avoid the current to be wasted backward.

Energy available

We know that the energy available from the supercap is E=1/2 C ΔV^2 in Joule = W * s. If we consider a Vf of 0.2V across DS3 and a lower voltage limit of 1.8V for IC1, we have ΔV = 4.8 - 1.8 = 3V so, an energy in the range 2.2 watt per 1 second with a 0.47F capacitor to 22.5 watt per 1 second for a 5F capacitor at the IC1 input. In the worst case of an 85% efficiency we have about 2 to 20Ws @ 3.3V out.

In order to select the right capacitor for our own system, in the graph below is depicted a rough estimation of the time available to the CPU for shutdown in different conditions of current consumption and capacitance.

Shutdown trigger

In order to trigger the regular shutdown on the system, the divider realized with R3 and R4 resistors adapt the 5V power supply to a level compatible with the CPU/MCU. The SHDN_Trigger signal can be sent to a Schmitt triggered digital input, to an analog comparator, to an ADC input or whatever on the device is capable to trigger an high priority interrupt. This must run a safe procedure according to the peripherals involved when the input voltage drops below a given threshold.

Because the supercap keeps the 3.3V high for a long time with no or light load, the device could be not able to recognize the changing in the power supply when 5V0 power returns, failing the restart. To solve this issue the DD1 dual diode has been added to control the enable input of the regulator. The regulator is enabled when 5V0 power supply is up OR when SHDN out of the MCU is high. If 5V0 goes down, the 3V3 regulator keeps working, thanks to the SHDN signal, until the MCU is completely off. When the 5V0 returns, the enable pin rise high, the 3V3 power supply returns good as well and the MCU is restarted. An issue is still on. If the 5V0 returns while the SHDN is still high, meanwhile the shutdown procedure is executing, the regulator, as well as the MCU, is no more able to recognize the changing. The micro-interruptions can be bypassed adding at software level a slight delay before starting safe procedure.

A possible solution to this issue, not yet tested, could be modifying the schematic as shown below:

If 5V0 supply returns while the software shutdown procedure is running, C8 capacitor has the same voltage at both terminals, so it's empty. When SHDN goes low, it grounds C8 that starts charging with a time constant of about 1s. During this time 3V3_EN is low as well switching off the 3V3 power supply for a while. When C8 is full charged again, the enable is high and everything runs fine again after an hard reset.

Some sort of debouncing can be added both at hardware (C7) and software level in order to avoid false alarms.

Links

Guido Ottaviani (Author/Designer/Developer for electronics-firmware-robotics)
He deals with electronic design and embedded system firmware for Robotics and industrial and civil applications, as well as technical disclosure on the internet.
http://www.guiott.com - https://github.com/guiott - guido@guiott.com