Transformers vs LED drivers

This post aims to highlight the problems that arise when using constant voltage LED lamps with individual control items. In addition, it looks at the various constant voltage and constant current modules required given that power feed systems also differ.

LED lamps: Electromagnetic transformer, electronic transformer, dimmable driver or FAV. What is the right combination?

A. Constant voltage LED modules / lamps

Using LED lamps with a transformer:

A transformer converts the 230V input voltage to the output voltage, in this case 12V, which is considered to be safety extra low voltage (SELV) and thus prevents electric shock issues.

These transformers are the same as those used in halogen spot lights. Nowadays, they are also used for MR16 LED lamps, which are replacing halogen bulbs because the operating voltage is the same but with much lower power.

However, while halogen lamps use current in the same form as the input voltage, LEDs incorporate electronics to prepare (set up) the current. This means that current flowing through the transformer is distorted, which reduces the power factor, increases THD and increases power losses.

Waveforms with an halogen lamp and an LED lamp: Voltage (ch2-blue), current (ch1-yellow).

It is clear how with a halogen bulb, the current maintains the shape of the voltage perfectly, whereas with the LED lamp, the current is distorted, thus deteriorating PF, THD and increasing losses.

Additionally, each manufacturer uses different electronic solutions, so that lamp + transformer behaviour varies in each case.

In this regard, the best performance is achieved when the current behaves as similarly as possible as with a halogen.

Waveforms with LED lamps: Voltage (ch2-blue) and current (ch1-yellow).

*The second example shows a highly distorted current, which greatly increases THD and has significant peaks that may cause supply from the transformer to fail, thus leading to the lamps flashing or flickering.

Magnetic transformer:
As already mentioned, these are the same as those used for halogen lamps. They are very sturdy and have no difficulty in supplying the current peaks demanded by the lamp.

They can be operated with dimmers but only with leading-edge. Whenever a dimmer is used, losses increase and the power factor and THD worsen because the input waveform is clipped, so that it distorts and changes its harmonic content.

A magnetic transformer allows for the use of dimmers, but only with leading-edge; because it behaves like a strongly inductive load, sudden changes in current flow can lead to significant voltage surges that may damage the hardware.

Prior to implementing a full installation, we recommend that pre-compatibility tests be run.

Electronic transformer:
The principle behind the way this operates is similar, i.e. it converts input mains voltage from 230V to 12V so that it can be used by the LED lamp and ensure user safety. The difference is that in this case, transforming takes place at high frequency, following the 50/60 Hz form of the input voltage, which enables them to be much lighter and to have lower losses.

Waveforms with an electronic transformer: voltage (ch2-blue), current (ch1-yellow).
The envelope of the high frequency current follows the input voltage of 50Hz.

They also allow for the possible integration of different protections that magnetic transformers do not provide, such as:
- Protection against short circuits
- Protection against overloads
- Protection against overheating

However, they have the same problem with regard to LED lamps, which demand their own current, so that, as stated above, overall system operation deteriorates, THD increases and the power factor decreases.

Like with magnetic transformers, it is advisable to choose a lamp whose current characteristics are as uniform as possible in order to ensure improved performance and prevent flickering.

*LED lamp waveforms: voltage (ch2-blue), current (ch1-yellow)

Note that in the second case, peak current is much higher than in the first one, even though RMS current is lower. This means the transformer has to deliver high current peaks and short circuit protections may be “tricked”. On the other hand, there is an absence of current during large parts of the cycle, making it more likely that the lamp flashes, turns off or flickers because of those periods.

Moreover, due to such high current peaks, the total power of the lamps connected to the transformer is often much lower than its output power rating.

Operating with a dimmer:
Many dimmers are designed for halogen lamps that have much higher power, so they are not suitable for the extra low loads of LEDs. Furthermore, as already seen, there is an absence of current during large parts of the cycle in certain types of lamp, thus causing the dimmer to malfunction.

In that case, it is critical to choose a lamp with the most constant current possible and a compatible combination of transformer, lamp and dimmer.

The recommendation is to choose specific dimmers for LED systems and to test the entire unit prior to any installation. In this regard, we propose eDIM dimmers.

*eDIM dimmers are specially designed to work at low loads such as LED lamps; they are able to regulate a load range of between 1W and 440W.

(FAV) constant voltage control gear:

In this case, output voltage is 12V DC. They are designed for constant voltage LED modules. They are highly suited to operating LED lamps because their constant voltage produces flat current, which eliminates the problems of using lamps with poor (uneven) current characteristics.

Waveforms with constant voltage control gear: Output voltage (ch2-blue), Current (ch1-yellow).

Current is seen to be uniform, free of breaks or peaks, which means no flashing or flickering occurs, making for proper lamp operation. This system is the most appropriate for constant voltage LED modules / tapes.

This type of system does not allow for any kind of dimming.

B. Constant current LED modules

Dimmable constant current driver:

In this case, operation is different from the previous systems. No output voltage is supplied but rather a constant current is fed directly to the LED lamp, so that no lamp electronics are required. This means that intermediary transforming is avoided and greater efficiency is achieved.

This system is fed directly to the LED module and the driver has all the control and dimming electronics built in. These modules are specially designed to operate with constant current, so that constant voltage LEDs are not valid.

The problems of the lamp’s demand for current are avoided as the current is fixed directly by the driver. THD power factor is improved and losses are minimised, as it entails just one transforming stage.

*Current through the LED module – it is a constant DC current with minimal ripple that lengthens the life of the LED and reduces heat.

Operating with a dimmer:
In order to implement phase-trimmed dimming, a special driver that is prepared for the feature must be used – not all drivers are suitable for that purpose.

In this case, dimming presents no issues as the input current to the driver follows the voltage waveform due to the active PFC, so the dimmer operates properly and current control in the LED module is kept stable.

As mentioned above, it is advisable to use dimmers specifically designed for LEDs in order to achieve proper control at low power. We recommend the eDIM series.

 *Characteristic dimming curve of output current vs % of input voltage waveform

*The use of specific dimmers for LED loads is recommended, such as eDIM range

*Dimmers especially suitable for operating with LED lamps
**We recommend compatibility tests be made prior to implementing a complete system

Javier Milla, R+D Dept.