Connections & Electronics – en.ghz-trading.com

1 Fundamentals: Current, Voltage and Power

1.1 Definitions

Current (I): Unit ampere (A). Indicates how much electrical charge flows per unit of time.
Voltage (V): Unit volt (V). Electrical potential difference between two points.
Power (P): Unit watt (W). Calculated as the product of voltage and current.

The rated power of a solar module is measured under Standard Test Conditions (STC). These are defined by IEC 60904-3 and include:

Air mass AM 1.5
1000 W/m² irradiance
25 °C cell temperature

Formula:

1.2 Examples of Typical Outputs

Connection	Voltage (V)	Current (A)	Power (W)
USB-A (standard)	5	2,0	10
USB-A (fast charge)	5	3,0	15
USB-C (PD)	9	2,0	18
USB-C (PD)	12	1,5	18
DC output	18	1,66	30

The typical output values (USB-A 5 V / 2 A, fast charge 5 V / 3 A, USB-C PD 9 V / 2 A or 12 V / 1.5 A) correspond to the specifications of the USB-IF standards (Battery Charging 1.2, USB Power Delivery).

1.3 Maximum Power of a Solar Panel

Multiple outputs only distribute the available power — they do not increase the total power.

The maximum output power of a solar panel is determined by:

Active cell area × efficiency

The rated power (Wp) is a fixed value defined under Standard Test Conditions (STC: 1000 W/m² irradiance, 25 °C cell temperature, air mass AM 1.5).

A panel with a rated power of 30 W can deliver a maximum of 30 W under STC, regardless of the number of available outputs.

2 Multiple Outputs Used Simultaneously

2.1 Basic Principle

A solar panel has a fixed maximum power output (Pmax), determined by active cell area and conversion efficiency.
Multiple outputs (e.g. 2× USB, 1× USB-C, 1× DC) share this total available power.
The power of individual ports is not additive.

2.2 Power Distribution

If only one output is used, almost the entire available power can be supplied to that port.
If multiple outputs are used simultaneously, the power available per port is reduced.
Electronic control circuits distribute the current and prevent overload.

Example: 30 W panel

Scenario	Number of devices	Power demand	Panel power	Result
DC only	1	30 W	30 W	Full power available
DC + USB	2	33 W	30 W	Power shared between devices, typically 15 W + 15 W or 18 W + 12 W
DC + USB + USB	3	40 W	30 W	Reduced power per port, charging time increases

2.3 Effects on Voltage and Current

Under high load, the output voltage may drop (e.g. from 5.0 V to 4.6 V).
The electronics limit the current per port (e.g. 1.5 A instead of 3 A).
In cases of significant overload, ports may be automatically disabled.

2.4 Electronic Complexity with Multiple Outputs

Each output requires its own voltage regulation stage (typically a DC-DC converter plus MOSFET).
More ports → more components → higher cost, higher losses, increased heat generation.
In low-cost panels, this increases the risk of simplified implementations, resulting in reduced stability and efficiency.

Modern protection ICs (e.g. from Texas Instruments or onsemi) continuously monitor the outputs and actively respond to overload conditions.
If the maximum output power is exceeded, they reduce the current or completely shut down the affected port.
Typical response times are in the range of microseconds to nanoseconds.

3 Safety Systems in End Devices

3.1 Basic Principle

Modern end devices incorporate their own protection mechanisms that monitor voltage and current intake.
This allows many devices to operate safely even when powered by fluctuating or limited energy sources.
However, such protection logic is not present in all device categories.

3.2 Typical Categories

Example:

Device category	Typical protection mechanisms	Risk with unstable power supply
Smartphones / tablets (from ~2010)	Charging IC, BMS, thermal protection	low
Power banks, laptops, cameras	Multi-stage protection systems	low
Simple consumers (LEDs, fans)	No dedicated protection ICs	high
Older devices (<2010), low-cost devices	Weak or missing protection logic	medium to high

3.3 Relevance for Operation on Solar Panels

Protection circuits in the panel are primarily important for devices without their own safety systems.
For modern end devices, they represent an additional safety barrier.
Unstable voltage (frequent regulation in/out due to changing solar irradiance) can increase stress on charging ICs.
Slow charging (e.g. reduced current draw) is generally uncritical or even gentle, as long as the voltage remains stable.

3.4 Effects on Different Device Classes

Modern smartphones:

Current draw is regulated by the charging IC.
Charging slows down at low available power, without creating a safety risk.
Slow charging (e.g. 1 A instead of 3 A) can even be beneficial for battery longevity due to reduced heat generation.

Older smartphones / low-cost devices:

In some cases, charging regulation is incomplete or poorly implemented.
Risk of overheating or unstable charging behavior.

Simple end devices (e.g. LED lamps, USB fans):

No integrated protection circuitry.
Direct reaction to voltage drops → flickering or overheating possible.

Smartphones and other modern devices automatically interrupt charging when the input voltage drops significantly.Typical result: charging errors (“charger not supported”) or repeated reconnection cycles under fluctuating solar conditions.

3.5 Influence of Charging Conditions

Slow charging:

Uncritical to beneficial for modern devices due to reduced heat generation.
The primary factor in battery aging is heat, not low charging power.

Unstable charging:

Problematic with constant voltage fluctuations (e.g. due to changing solar irradiance).
Increases stress on charging ICs in modern devices.
Potentially harmful for devices without protection circuits (e.g. LED lamps, simple fans).

Low charging currents (e.g. 1 A instead of 3 A) are not harmful as long as the voltage remains stable. On the contrary, lower currents generate less heat. Studies show that heat is the primary driver of battery aging, not reduced charging power.

4 Electronic Protection Circuits in the Panel

4.1 Types of Protection

Overcurrent protection: Limits the current when a connected load draws more current than intended.
Overvoltage protection: Prevents damage caused by increased open-circuit voltage under high irradiance.
Short-circuit protection: Detects a direct connection between positive and negative terminals and disables the output.
Thermal protection (optional): Reduces output power or shuts down the electronics in case of overheating.

4.2 Technical Implementation

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors):
- Act as ultra-fast electronic switches.
- Response times in the nanosecond range.
- Low conduction losses, therefore well suited for portable systems.
- Typical protection MOSFETs limit overcurrent within less than 2 µs and interrupt short circuits with virtually no delay.
Protection ICs:
- Monitor voltage and current.
- Usually operate in combination with MOSFETs.
- Typical limits: 2–3 A per port, voltage range 4.75–5.25 V.
Manufacturer examples: Texas Instruments, Analog Devices, onsemi.

4.3 EInfluence of the Number of Outputs

More outputs → more control components (ICs, MOSFETs, or multiplexers).
Higher circuit density leads to increased heat generation.
With simplified implementations (low-cost panels): reduced efficiency and limited protection performance.
USB specification: Output voltage must remain within 4.75 V to 5.25 V.

4.4 Example

30 W panel, 18 V DC output:

No damage to the panel or cable.
Short circuit → electronics immediately reduce the current to a few milliamps or shut down the output completely.

5 Practical Implications

5.1 Power Output

The maximum power of a solar panel is limited by active cell area and conversion efficiency.
Multiple outputs increase flexibility, but do not increase total power output.
When several outputs are used in parallel, the available power is distributed across all ports → charging speed per device decreases.

5.2 Safety

Modern end devices incorporate their own protection mechanisms.
Integrated protection circuits in the panel provide an additional layer of safety.
Particularly relevant for older or simple loads without internal protection logic.
Devices without their own protection circuits (e.g. simple LED lamps or USB fans) do not automatically shut down under unstable voltage conditions. This can lead to flickering or overheating.