What is the difference between a power supply and a driver for LEDs: theory and practice, all you need to know

Author's note: “The network has a fairly large amount of information about the power of LED products, but when I was preparing the material for this article, I found a lot of absurd information on sites from the top of search engine results. In this case, there is either a complete absence or an incorrect perception of basic theoretical information and concepts. ”

LEDs are the most efficient of all common light sources to date. Problems also lie behind efficiency, for example, the high requirement for the stability of the current that feeds them, the poor tolerance of complex thermal operating conditions (at elevated temperatures). Hence the task of solving these problems. Let's see how the concepts of power supply and driver differ. To begin with, let's delve into the theory.

Current source and voltage source

Power Supply is the generic name of a part of an electronic device or other electrical equipment that supplies and regulates electricity to power this equipment. It can be located both inside the device and outside, in a separate case.

Driver - the generic name of a specialized source, switch, or power regulator for specific electrical equipment.

There are two main types of power sources:

• Voltage source.

• Current source.

Let's look at their differences.

Voltage source - this is such a power source and the voltage at the output of which does not change when the output current changes.

For an ideal voltage source, the internal resistance is zero, and the output current can be infinitely large. In reality, the situation is different.

Any voltage source has an internal resistance. In this regard, the voltage may deviate somewhat from the nominal voltage when a powerful load is connected (powerful - low resistance, high consumption current), and the output current is determined by its internal device.

For a real voltage source, the emergency operation mode is short circuit mode. In this mode, the current increases sharply, it is limited only by the internal resistance of the power source. If the power supply does not have short circuit protection, it will fail

Current source - this is a power source whose current remains set regardless of the resistance of the connected load.

Since the purpose of the current source is to maintain a given current level. The emergency operation mode for it is idle mode.

If you explain the reason in simple words, then the situation is as follows: let's say you connected a 1 ohm load with a resistance of 1 Ohm to the current source, then the voltage at its output will be set to 1 Volt. A power of 1 W will stand out.

If you increase the load resistance, say, up to 10 Ohms, then the current will be 1A, and the voltage will already be set at 10V. So, 10W of power will be allocated. And vice versa, if you reduce the resistance to 0.1 Ohm, the current will still be 1A, and the voltage will become 0.1V.

Idling is the condition when nothing is connected to the terminals of the power source. Then we can say that at idle the load resistance is very large (infinite). The voltage will increase until a current of 1A flows. In practice, for an example of such a situation, you can bring the ignition coil of a car.

The voltage at the electrodes of the spark plug, when the power supply circuit of the primary winding of the coil opens, rises until its value reaches the breakdown voltage of the spark gap,after which a current flows through the spark and the energy accumulated in the coil is dissipated.

A short circuit condition for a current source is not an emergency operation. In the event of a short circuit, the load resistance of the power supply tends to zero, i.e. it is infinitely small. Then the voltage at the output of the current source will be appropriate for the flow of a given current, and the allocated power is negligible.

Let's move on to practice

If we talk about modern nomenclature or names that are given to power sources to a greater extent by marketers, rather than engineers, then power supply commonly called a voltage source.

These include:

• Charger for a mobile phone (in them the conversion of values ​​until the required charging current and voltage are achieved is carried out by converters installed on the board of the charging device).

• The power supply unit for the laptop.

• Power supply for LED strip.

A driver is a current source. Its main use in everyday life is the nutrition of individual LEDs and LED Arrays both of them are ordinary high power from 0.5 watts.

LED power

At the beginning of the article, it was mentioned that the LED has very high power requirements. The fact is that the LED is powered by current. It's connected with current-voltage characteristic of all semiconductor diodes. Take a look at her.

In the picture I-V characteristics of diodes of different colors:

This shape of the branch (close to a parabola) is due to the characteristics of the semiconductors and impurities that are introduced into them, as well as the characteristics of the pn junction. The current, when the voltage applied to the diode is almost less than the threshold, does not increase, or rather, its growth is negligible. When the voltage at the terminals of the diode reaches a threshold level, the current begins to rise sharply through the diode.

If the current through the resistor grows linearly and depends on its resistance and the applied voltage, then the increase in current through the diode does not obey this law. And with an increase in voltage by 1%, the current can increase by 100% or more.

Plus, in metals, resistance increases with increasing temperature, and in semiconductors, on the contrary, resistance decreases, and the current begins to grow.

To find out the reasons for this, you need to go deeper into the “Physical Fundamentals of Electronics” course and find out about the types of charge carriers, the band gap and other interesting things, but we will not do this, we briefly examined these issues in an article on bipolar transistors.

In the technical specifications, the threshold voltage is indicated as the voltage drop in forward bias, for white LEDs, usually about 3 volts.

At first glance it might seem that enough at the design and production stage of the lamp is enough current limiting resistors and set a stable voltage at the output of the power supply and everything will be fine. They do this on LED strips, but they are fed from stabilized power sources, and the power of LEDs used in tapes is often * small, tenths or hundredths of a watt.

* (if we are not talking about tapes and strips with 5730 LEDs, for more information on types of SMD LEDs, see the article - Types, characteristics and labeling of SMD LEDs)

Powerful LEDs, which are recommended to be powered by drivers, are heated quite strongly. For example, a 1W LED is heated to a temperature above 50 degrees in a few 5-15 seconds of operation without a radiator.

If such an LED is powered by a driver with a stable output current, then when the LED is heated, the current through it will not increase, but will remain unchanged, and the voltage at its terminals will decrease slightly for this.

And if from the power supply unit (voltage source), after heating, the current will increase, from which the heating will be even stronger.

There is another factor - the characteristics of all LEDs (as well as other elements) are always different.

Choice of driver: characteristics, connection

For the correct driver selection, you need to familiarize yourself with its technical characteristics, the main ones are:

• Rated output current;

• Maximum power;

• Minimum power. Not always indicated. The fact is, some drivers will not start if a load less than a certain power is connected to them.

Often in stores, instead of power, they indicate:

• Rated output current;

• The range of output voltages in the form of (min.) V ... (max.) V, for example 3-15V.

• The number of connected LEDs, depending on the voltage range, is written as (min) ... (max), for example 1-3 LEDs.

Since the current through all elements is the same when connected in series, therefore, the LEDs are connected in series to the driver.

In parallel, it is undesirable (rather impossible) to connect the LEDs to the driver, because the voltage drops on the LEDs may vary slightly and one will be overloaded, and the second, on the contrary, will operate in a lower nominal mode.

It is not recommended to connect more LEDs than determined by the design of the driver. The fact is that any power source has a certain maximum allowable power, which cannot be exceeded. And with each LED connected to a stabilized current source, the voltage at its outputs will increase by about 3V (if the LED is white), and the power will equal, as usual, the current to voltage.

Based on this, we draw conclusions to buy the right driver for LEDs, you need to determine the current that the LEDs consume and the voltage that falls on them, and select the driver according to the parameters.

For example, this driver supports connecting up to 12 high-power LEDs per 1W, with a consumption current of 0.4A.

This one produces a current of 1.5A and a voltage of 20 to 39V, which means that you can connect to it, for example, an LED at 1.5A, 32-36V and a power of 50W.

Conclusion

A driver is one of the types of power supply designed to provide LEDs with a given current. In principle, it doesn’t matter what this power source is called. Power supplies are called power supplies for LED strips of 12 or 24 Volts, they can produce any current below the maximum. Knowing the correct names, you are unlikely to make a mistake when purchasing goods in stores, and you will not have to change it.