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Power Electronics: Inverters and DC-DC Converters

Introduction: The Challenge of Power Conversion

The solar modules are mounted, the battery is ready, the sun is shining. The foundation for a solar system is in place. But one crucial component is missing: the inverter and its diverse functions.

The challenge with solar systems:

  • Solar cells generate direct current (DC)
  • Household appliances require alternating current (AC)
  • Batteries store direct current (DC) again

Power electronics are employed to solve this. In this article, you will learn how the various components work.

Overview: Power Electronic Components

Whether in solar technology or any modern device, nothing works without power electronics. Various components are used in solar systems and batteries, each with specific tasks in power conversion:

Component Function
Inverter DC → AC (direct to alternating current)
Rectifier AC → DC (alternating to direct current)
Boost converter Low DC voltage → High DC voltage
Buck converter High DC voltage → Low DC voltage
BMS Battery Management System

Unidirectional and Bidirectional

These components can be unidirectional (one direction) or bidirectional (both directions):

  • Unidirectional: Only DC to AC conversion possible
  • Bidirectional: Both directions possible (essential for battery storage!)

The Inverter: Heart of the Solar System

Inverters convert the direct current generated by solar modules into alternating current. This occurs through electronically controlled switches, known as power switches.

Operating Principle: "Chopping"

  1. Direct current is switched on and off very rapidly
  2. Varying switching durations create a pattern
  3. The average values of the "DC fragments" produce alternating current
  4. The frequency is standardised at 50 Hz (Europe)

The result is grid-compliant alternating current suitable for all household applications.

Important Inverter Functions

Beyond pure power conversion, modern inverters perform further essential tasks for safe and efficient operation of the solar system:

Function Description
Grid synchronisation Frequency and phase are matched to the grid
Anti-islanding protection Disconnects during grid outages (protection for maintenance personnel)
Power limitation Software limitation (e.g. 70% rule)
Monitoring Surveillance and fault diagnosis

Efficiency

Modern inverters achieve 96–98% efficiency. Losses occur through:

  • Switching losses in semiconductors
  • Self-consumption of electronics
  • Heat generation

The Rectifier: The Counterpart

A rectifier is the opposite of an inverter: it converts alternating current into direct current.

Operating Principle

During rectification, alternating current is partially "clipped":

  • Only the positive "peaks" of the alternating current are used
  • The average produces approximately constant direct current
  • High-frequency switching smooths the result

Application in Solar Systems

Rectifiers are required when:

  • An AC-coupled battery is charged from the grid
  • Surplus grid electricity is to be stored

DC-DC Converters: Adjusting Voltage

DC-DC converters change the voltage level of direct current without converting it to alternating current.

Boost Converter

Converts low voltage to high voltage.

Components:

  • Low-voltage source
  • Inductor (coil)
  • Diode
  • Power switch
  • Capacitor

Operating Principle:

  1. Switch closed: Current flows through the inductor, magnetic field builds up
  2. Switch opens: Magnetic field collapses, generates current
  3. Capacitor charges: The increased voltage is stored in the capacitor

This process repeats at very high frequency for stable voltage.

Buck Converter

Converts high voltage to low voltage.

Operating Principle:

  1. Switch closed: Current flows to inductor and capacitor
  2. Switch opens: Magnetic field collapses, polarity reverses
  3. Alternating charge: Constant polarity changes produce lower voltage

Bidirectional DC-DC Converter (Buck-Boost)

Combines both functions – can step up and step down voltage. Important for:

  • Battery charging at various states of charge
  • Adaptation to fluctuating solar module voltage

Understanding the Building Blocks

For better understanding, here are the key components:

Inductor (Coil)

A wound electrical conductor that:

  • Generates a magnetic field when current flows
  • Briefly continues current flow after interruption

Analogy: Like a heavy flywheel that continues spinning after the power is switched off.

Diode

Allows current in one direction only. Functions like a non-return valve.

Capacitor

Stores energy in the form of an electric field. Comprises two opposing metal plates. Serves as a buffer for stable voltage.

Power Switch

Electronic switches based on semiconductors:

  • Extremely high switching speed
  • Compact size
  • Controlled by control current

MPPT: Maximum Power from the Solar System

The Maximum Power Point Tracker (MPPT) is often integrated into the inverter. Its task: to consistently extract maximum power from the solar system regardless of weather or load.

Why is This Necessary?

Electrical power is: P = U × I (voltage × current)

Each solar module has an individual characteristic curve that changes due to:

  • Shading
  • Temperature changes
  • Varying irradiance

The "Perturb and Observe" Algorithm

  1. Voltage is slightly increased or decreased (perturbation)
  2. The resulting power change is measured (observation)
  3. Was power higher? → Continue in this direction
  4. Was it lower? → Change direction

This way, the MPPT continuously finds the maximum power point – even under changing conditions.

Battery Management System (BMS)

Modern battery storage systems have an intelligent control and monitoring system for safe operation.

Core Tasks of the BMS

The BMS fulfils numerous tasks essential for safe and long-lasting battery operation:

Task Description
Monitoring Voltage, current, temperature of each cell
Cell balancing Even charging of all cells
State detection Calculating SoC, SoH, SoP
Protection Against overcharging, overheating, short circuit
Communication Sending data to other systems

Important Battery Metrics

The BMS monitors various metrics that indicate the current state of the battery. These standardised abbreviations appear frequently in technical documentation:

Abbreviation Meaning Question
SoC State of Charge How full is the battery?
SoH State of Health How healthy is the battery?
SoP State of Power How much power can it deliver?
SoS State of Safety How close to safety limits?
SoF State of Function How functional is the battery?

The BMS monitors these metrics continuously and decides on measures for continued operation.

Conclusion

Core Message: Power electronics form the link between solar modules, battery and household grid. Inverters convert DC to AC, rectifiers conversely AC to DC. DC-DC converters adjust voltage levels, the MPPT optimises yield and the BMS protects the battery. Without these components, no modern solar system would be possible.

What happens when all functions are combined in one device? More on this in the article The All-Rounder: Hybrid Inverters.

The Complete Article Series "Energy Storage for Solar Systems"

  1. From Frog Legs to Batteries: How Does an Energy Storage System Work? – Fundamentals
  2. Lithium vs. Lead: Which Battery for the Solar System? – Technology comparison
  3. Power Electronics: Inverters and DC-DC Converters – You are here
  4. The All-Rounder: Hybrid Inverters – Everything in one device
  5. AC or DC? System Topologies for Solar Systems – System concepts

Sources