I am new to power supply circuits. I am studying datasheet of buck regulator LM22676 which generate 5V/3.3V dc for micro-controller supply in one of my system.The datasheet says it has a N-channel mosfet capable of driving upto 3A.

The place i first read about buck-boost converter is in Power electronics class in EE which uses a thyrsitor as switch with additional L,C filters.In power electronics the focus of study was driving high power dc loads like dc motors with dc-dc converters circuits with thyristor and gating circuit.

Is the buck converter principle used in LM22676 same compared to power electronic theory? Do integrated power converters exist in power electronics as well instead using thyristor based circuits?


All switching power converters are conceptually the same - they contains some energy accumulating devices - inductor(s) and capacitor(s) and controlled switch that commutate the input energy, switching it ON and OFF.

The big difference between different kinds of switching converters (buck, boost, etc.) is the topology of the elements.

While the different power inductors and capacitors are generally the same, for the switch different elements have to used. There are thyrsitors, BJTs, MOSFETs, IGBTs. In the old times even mechanical switches has been used.

Of course, the auxiliary electronics (the control schematic, feedback regulator, protections, etc.) highly vary depending on the output power and the destination of the converter. And it is obvious, that on high power schematics, all these auxiliary modules tend to be much more complex than in the low power converters.


A thyristor of the type you describe works on the rectified AC power waveform and that is not the case with buck converters - they usually operate on a reasonably smoothed DC voltage - that's the first difference I reckon.

I think the 2nd clear difference is that when using a thyristor (after a bridge rectifier), the switching frequency is going to be 2x the incoming power AC frequency - buck convertors tend to operate in the kHz to MHz range and so there is a significant difference here too.

The thyristor switching is synchronous with the AC (because it has to be) but some buck converter topologies can be made synchronous with a frequency reference (AC power derived or not) but this is usually to ensure that several buck converters can operate locked-in together. Not much of a similarity but it's the best I can think of!

"Buck", as the name kind of implies, takes it's name from the action of an inductor when it is open-circuited - the inductor in a buck DC converter is necessary as an energy storage device whereas the inductors and capacitors in a thyristor controller are usually there to cure secondary effects like EMI and possibly dV/dt problems in the thyristor.

I'm making this last point because of the types of loads thyristors tend to drive (such as heaters) - they are controlling the RMS voltage fed to a load. Should a thyristor be controlling an inductive load, the difference between the two types of circuit becomes more marked - zero crossing detection for current is needed to trigger the thyristor and this in my book does not constitute making any similarities with a regular buck converter.

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    \$\begingroup\$ Great info. I understand power electronics usually have converters circuits in power frequency.Why does LM22676 have switching frequency upto 500khz?What does it mean? \$\endgroup\$
    – Gopi
    Oct 21 '13 at 11:39
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    \$\begingroup\$ @Gopi higher frequencies are the objective because they lead to reduced output ripple voltage, decreased transient load response times, smaller component size, easier filtering but come with the cost of decreased efficiency however, modern buck regulators operate at up to 2MHz with 95% efficiency and that frequency will keep rising for a few years I reckon - it's all about controlling the power mosfets! Low frequency means bigger circuit and poorer performance. \$\endgroup\$
    – Andy aka
    Oct 21 '13 at 12:35

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