OwnTech » OwnPower » single-phase

h1. Power Block Theory

The Power Block is the part that manages input/output power within the board. 

It consists of a single inverter leg, as illustrated in the figure below. 

p=. {{thumbnail(single_phase_topology.png, size=350, title=The power topology of the single-phase board)}}

_Figure 2 - The power topology of the single-phase board_

Figure 2 shows a Vlow, VHigh, T1, T2, D1, D2 and L. 

Vlow is the low-side voltage, VHigh is the high-side voltage, T1 and T2 are two transistors, D1 and D2 are two diodes and L is an inductor. 

This topology is current bi-directional. 

This means that its input can be either on the high-side or on the low-side. 

If the input is on the high-side, the circuit acts as a buck or step-down converter.

If the input is on the low-side, the circuit acts as a boost or step-up converter.

This topology allows the control of the current in the inductor L by controlling its charge and discharge using the switches. 

The figure below shows the switching and the current flow within the inductor. 

p=. {{thumbnail(states_converter_1.png, size=700, title=Current flow during switching)}}

_Figure 3 - The power topology of the single-phase board_

The switching leads to an average current flow between its input and output as shown in the image below.

p=. {{thumbnail(switching_states_1.png, size=700, title=Current flow during switching)}}

_Figure 4 - Left: average current in the inductor increases_

_Center: average current in the inductor stable_

_Right: average current in the inductor decreases_

As figure 4 shows, the key to control the current in the power converter is to control the duration of the signal that is sent to the transistors. 

This duration is called *duty cycle* .

A longer duty cycle will lead to a increase in current, while a shorter duty cycle will lead to a decrease in current. 

The duty cycle is the single most important control variable in a power converter. 

The presentation above is, obviously, not complete since there are further phenomena to be taken into account.

The instantaneous rise and fall in current will lead to abrupt variations in voltage at both the high and low sides. 

To filter these variations, power converters are equipped with capacitors in both sides, which effectively provide the instantaneous current needed by the converter. 

The relationship between high and low side voltages, high and low side currents, and the duty cycle is given by the equations below.

p=. !http://www.codecogs.com/eq.latex?\dfrac{V_{High}}{V_{Low}}=\dfrac{I_{Low}}{I_{High}}=\dfrac{1}{1-D}!

_Equation 1 - Relation between High and Low variables, and the duty cycle for a Boost Mode operation_

p=. !http://www.codecogs.com/eq.latex?\dfrac{V_{High}}{V_{Low}}=\dfrac{I_{Low}}{I_{High}}=\dfrac{1}{D}! [1]

_Equation 2 - Relation between High and Low variables, and the duty cycle for a Buck Mode operation_

These equations explain the need for all the other blocks of the system.

To estimate the correct duty cycle, it is necessary to measure either voltage or current, leading to a measurement block.

Once these measurements are made, they must be converted in digital values, treated through a mathematical control loop and calculate the duty cycle value, leading to a control block. 

Once the duty cycle has been correctly calculated, it must be converted in analog voltage pulses which are in turn used to drive the transistors, leading to a driver block.

Since all of these operations require some energy, the Feeder block is thus needed to power all of the other blocks.

Each block has its own specific challenges and technical difficulties, which are explained in their respective wiki page.