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A Product Line of

Diodes Incorporated

AN56

12Vac LED Driving without smoothing capacitors

Dr. Kit Latham, Senior Applications Engineer, Diodes Incorporated

Introduction

LED based architectural lighting is now coming of age, but there are still some problems to be

considered when designing luminaires to be fitted into existing installations.

This Application Note discusses some of the challenges and shows that the omission of the

traditional smoothing capacitors has advantages in saving cost, space and PFC problems.

Dimming will be discussed in a forthcoming note.

Description

This application note describes driver solutions developed using the Zetex ZXLD1360 LED driver

IC to drive a series of LEDs from 12Vac transformers. The solutions take advantage of the Super

Barrier Rectifier SBR2A40P1. This device has 2A 40V capability and high thermal stability. It is

housed in the proprietary PowerDI123 package, which offers a combination of small size and

good thermal conductivity.

Existing transformers

Ideally a LED based bulb should fit into existing sockets and be satisfactorily driven from the

existing 12V transformer. These are usually electronic transformers that produce a 30-40kHz

square wave modulated to >100% at 100Hz or 120Hz. They are designed to drive halogen bulbs,

which are resistive loads at power levels of 20 to 50W. Many of them will not properly drive a

load much less than 20W and most LED lamps are about 7W. Sometimes this problem can be

solved by driving more than one LED unit from each transformer.

AN56

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Characteristics of an LED lamp.

An LED differs from a halogen in many ways. Firstly it is polarity sensitive, secondly it’s IV curve

is very different from a halogen bulb and the way its lumen output varies with current even more

so. The Figures 1 & 2 below compare a 20W halogen with a string of 3 white LEDs in series.

Figure 1 - Comparison of the I-V characteristics of the 2 types of lamp.

Figure 2 - Comparison of the optical output characteristics of the 2 types of lamp

Perhaps the biggest difference is in the way that they generate light. The halogen produces light

as part of the spectrum of radiation generated due to the temperature of the filament. The

majority of the output is in the IR region and is radiated out of the lamp in the form of IR. The low

efficiency of the halogen bulb is a result of the majority of the radiation produced being outside

the visible range, if the filament could survive being run at a temperature of 6300K then its output

would be optimised in the visible range and the efficiency would be much higher. The visible

spectrum is smooth therefore it contains all visible colours and the variation of intensity with

colour is gradual. In other words, it has a high colour rendering index (CRI.) Some of the

radiation is converted into heat as it strikes the materials of the bulb and has to be conducted

away.

200

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024681012

LED

Halogen

I-V Characteristics

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02468101214

lx %

LED

Halogen

LED vs Halogen, 3GD in series, 20W MR16