Halogen lamp electronic transformer - Page 1
Forum section: electronics / projects

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 06, 2014, 11:14:40 am
content: Hi everyone, am working on a halogen lamp electronic transformer design that include short circuit protection, thermal protection. Well the usual concept is to use a half bridge structure. My first thought was to use a chip that drive this half bridge and include all protection feature and soft start feature too, i was surprised that the only design in use is based on some crappy self oscillating circuit with a couple of NPN driven by a high frequency pulse transformer, there is some sort of short circuit protection and nothing else.My question is if there is any reason why semi-conductor manufacturer are not producing chip for this type of lamp as it's done for LED and HID ( or my be am not able to find one so please advice ).Second am i wrong if i think that this kind of circuitry is not capable of all of those protection and feature ppl who producing it claims it do (buy one with all kind of certification logo stick in it) and i simply can't see how.Third, i thought i will use a general purpose half bridge driver but than i think again and i may be silly " there should be a reason why no one else is doing it ".and last i can't understand why in those thing, the DC voltage is not filtered and keeped as full rectified sin signal, i mean the purpose is to produce a sin-like 12V signal, it's more a HF square wave so why this output signal is not generated from a well shaped DC voltage. am kinf of lostplease advice

author: bktemp
author_rank: Super Contributor
timestamp: June 06, 2014, 11:39:21 am
content: Have a look at IR2161. It drives a half bridge and includes nearly all needed protection features.Without a big filtering cap the circuit has a power factor of almost 1. Otherwise you would nead a PFC wich is much more expensive, takes a lot of space and reduces the efficiency. The same applies to a rectifier at the output: It would waste at least 5-10% of the energy.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 06, 2014, 11:53:00 am
content: thanks a lot , i saw the IR chip (it's the only chip i found for this application) but it's too damn rare and i checked almost every supplier i know, this is not a design issue but it will be a problem once we start mass production, the main reason why am looking for an alternative solution either the stupid self oscillating circuit and the IR chip

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2014, 07:09:19 pm
content: Quote from: hamdi.tn on June 06, 2014, 11:14:40 amThat's probably because a self resonant converter is fairly efficient and the protection is adequate, given the device is fairly cheap to produce so there's no need to design an expensive purpose built IC.

author: Richard Head
author_rank: Frequent Contributor
timestamp: June 08, 2014, 06:20:22 pm
content: Hamdi.tnThe topology that is used for halogen electronic transformers is actually pretty good.It's is a self oscillating half bridge topology that is based on the Jensen (I think) converter.The way it works is that the little base drive core is a saturating core. It takes a certain amount of volt-seconds to saturate before base drive to the bipolar transistors is lost. The cross sectional area of the control core and the number of turns essentially determines the operating frequency. The core material is normally a square loop type material that transitions rapidly between normal magnetization and saturation. They generally oscillate around 50khz.The split capacitors across the rail are resonant with the leakage inductance of the power transformer. The transformers normally have a special split bobbin that allows the primary and secondary to be separated by an exact distance. This controls the leakage inductance quite accurately.Short circuit operation is quite interesting. When a short is present on the output no energy is absorbed by the load. Consequently the energy is returned to the DC bus via the anti-parallel diodes across the bipolar power switches. The Osram unit has a couple of low value 1206 SMD resistors in series with the anti-parallel diodes. When energy is being returned to the DC bus the series resistors heat up. This heat is sensed by an SMD thermistor located next to the resistors which changes value and switches the unit off for a minute or so.So why not just use MOSFET's and an integrated circuit driver and to hell with the complicated self resonating circuit? The reason is cost. Certainly, when I was involved with these units MOSFET's were much more expensive than bipolars of the same current rating. This is because MOSFETs have a much bigger silicon die compared to an equivalent rated bipolar transistor. It's that simple. It boils down to manufactured cost.But who cares now anyway. It's all about LED's now!Dick