There are multiple implementations of such converters for high power applications both in isolated and non-isolated versions [https://www.ti.com/lit/ug/tiducs2b/tiducs2b.pdf] [https://www.ti.com/lit/ug/tidud04/tidud04.pdf] . Quite often in my battery powered project I face a need to develop both a battery charger and the converter stabilizing the battery voltage. The possibility of joining those two converters allows for savings on power elements and size. I wanted to try to develop experimental board to test different current measurement circuits and try to push the limits of maximum power. In this article I will try to present how to build a simple bidirectional converter with fast primary current feedback loop and digital secondary voltage feedback all implemented with small 8bit microcontroller.

Topology

In presented design I’ve decided to join two classic non-isolated topologies the buck and boost converter. In this configuration the converter allows for transfer of energy between two voltage potentials in both directions.

In such converter control of current flow is not only dependent on the PWM signal but also on the ratio of input voltages. Creation of feedback loop which only maps input voltages to the PWM signal would not only be difficult but also hard to stabilize. Also it would omit control of the current where current is one of the most critical parameters in charging and discharging of batteries. Probably because of that problem most of the mass produce converters for charging include feedback loop measuring current flow trough small resistor in series with the power inductor. This arrangement allows to measure current rise and fall at each cycle of the converter. Considering different possibilities of measuring that current I sketched following schematic.

In presented circuit the main feedback element is the voltage comparator with hysteresis. By defining the converter phase and the state of main transistors it determines the cycle of increase and decrease of current in the main inductor. By simultaneously monitoring the current in the inductor trough the feedback resistors it limits the maximum rise and fall of that current. In this combination a negative feedback loop is created which creates cycle-by-cycle oscillations maintaining the inductor current within predefined range. The oscillation frequency is not fixed and it depends on inductor and resistors values, input voltages and and comparator hysteresis. Although the converter oscillates, in it’s default state the average current flow trough the inductor is null and no energy is transferred. Such oscillations maintain inductor in continues conduction state which might appear wasteful because of elements parasitic RI losses but surprisingly in presented circuit such losses represent only around 1Watt. While the comparator maintains the inductor current within predefined range additional control input (Ictrl) injects offset to the current measurement forcing average current to deviate. This allows the control of average current flow between input potentials and creates a abstraction block of voltage controlled current source.

Basic design

The main obstacle in designing bi-directional converter is not only a need for controlling the voltages at the input and output but also the decision of direction and amount of current flow. That is why most design are based on a digitally implemented feedback loop inside a microcontroller or a dedicated DSP. To simplified my design both in circuit and in software I decided to use a simple 8bit microcontroller, consciously sacrificing final precision and reaction speed for cost and simplicity. I decided to use the Attiny1616 because I contained all necessary peripherals in a simple package. Bellow is a diagram of the converter design in it’s initial form.

As I mentioned the main feedback loop creating oscillations and controlling inductor current is based on comparator with hysteresis. Using the build-in comparator...