Today I want to talk about one of my favorite topics in power electronics – Flyback converter !

Flyback Converter in General

You can see flyback topologies in many different areas, for example consumer electronics, industrial, medical and many more. The power ranges are usually from a few watts up to some 75W or so. Its very rare to see them above 100W because the transformer gets big and a flyback topology is not the most efficient one when it comes to higher power ranges. So why is it so popular in lower power ranges? Thats an easy answer – its all about simplicity and BOM – cost. With a flyback you have just two power switching semiconductors and a fairly simple transformer. This makes the design cheap and compact. Thats why its mostly the only type of power supply topology for notebook chargers, TV power supplies and so on.

The LT830x Family

LT8304 Flyback Converter Application Schematic

Lets come to the LT830x family consists mainly of 3 parts. First one is the LT8300 which is not a really new one but its the first one in this series. Lots of customers of mine use this one to get galvanic isolation on their sensors. Those things don’t need much power but do rely on a stable and reliable power supply, which the LT8300 delivers. To address the needs of “higher” power demands Linear Tech introduced the LT8303 and the LT8304. Both parts have the same input voltage but with a higher switching current capability. The LT8300 and the LT8303 are also pin compatible – one footprint 2 parts.

No-Opto Flyback Converter

When looking at the datasheet of the LT8300 you will recognize that there is one part missing, which is usually in every isolated power supply – the optocoupler. The optocoupler meanwhile became one of the weakest parts in a power supply and the reliability and isolation voltage often is directly related to this part and so semiconductor companies search a way around this part. A flyback topology allows the sensing of the secondary voltage right at the drain of the power MOSFet or at the AUX – winding of the transformer during the conduction phase of the secondary diode. The voltage that can be seen on the winding there is a direct relation of the output voltage and the transfer ratio of the transformer ( minus the diode forward voltage ). The LT830x – family applies some techniques to get this sensing done.

Of course the LT830x – family is not the only one who have that feature. The LT3798, which I mentioned in my post about the power factor correction , has this feature on board as well.

Key Applications

So, where is the key application for that parts. Mainly its industrial, transportation, medical and measurement equipment where things need to operate isolated, like sensors, ADC’s (for example using it together with the LTM2893 to get an ADC working isolated) or ADC’s combined with small FPGA’s or DSP’s (with the LT8304 up to 24W power consumption!). Due to the high input voltage that parts are suitable for a wide input voltage range which makes them reusable for many designs.

Meanwhile there is also a big choice when it comes to flyback transformers from all big magnetics companies – but if you like you can also wind your own transformer.