What is a Buck Regulator?

November 7, 2012
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We interviewed the VP responsible for the Picor product line, Robert Gendron, tapping into his knowledge about the design and performance considerations for buck regulators when powering embedded devices. Because it turned out to be such a great interview, with lots of great information, we’ll be posting his responses as a series over the next couple of months.

In the first part of the interview we quiz him on what a buck regulator is and the pros and cons of the different options available.

Q: What exactly is a buck regulator?

A:  That’s not as simple a question to answer as you would think! There are, in fact, three very different types of ‘buck regulator’.  First let’s define the term – a buck regulator takes an unregulated or quasi-regulated voltage and steps it down to highly regulated voltage for the point-of-load. 

The ground-up approach

At the simplest level the buck regulator is the IC controller itself. The engineer specifies the controller, FETs, an inductor, caps and resistors, to make a discrete buck regulator design. This is truly the ground-up approach to a buck regulator.

Whilst it’s the most basic, it’s not necessarily the simplest approach. Especially as the engineer has several component decisions to make and has to calculate the loop compensation and account for it. Not a surprise, then, that it’s becoming less common.

Integrated package

Recently the industry definition of ‘regulator’ has changed to be a device where all of the active devices – the controller and the power FETs – are integrated into one package.

There are definitely advantages over the traditional controller. Not least because of the impact it has on reducing the design complexity, simplifying as it does:

  • loop compensation;
  • reduction of parasitics;
  • sourcing (you really are only sourcing the one package, and then typically it’s already a prescribed inductor)

The module approach

The last type of a buck regulator is the module, sometimes called a micro module or simple switcher. Typically the most expensive type of buck regulator, the module is one up from the regulator as it integrates not only the controller and the FETs, but also the inductor.

For many this is a convenient option. But they have some drawbacks.

Firstly, they’re not all as self-contained as you would think. Yes, the expectation is that you would only have to add input and output ceramic caps. But, in reality, there are different modules available, and they vary as far as the surrounding support components that are needed. Their density – I should say the density of a module – can be on par with a regulator, so sometimes the densities quoted can be a little misleading if you take into account the total solution size.

One of the greatest drawbacks to using a module is the thermal capability of the device, as a direct result of having the inductor inside the module. Why? About 50% of the losses today are in the FETs, the other 50% of losses are in the inductor itself. So, by having that inductor inside the package, in effect you are almost doubling the heat that’s required to be dissipated through that package. This means that derating becomes very important when it comes to a module.

That was a very quick overview of the types of buck regulator available today. Dependent on what you are trying to achieve, each type has tradeoffs you need to consider. And even within each of the three categories the offerings aren’t consistent from supplier to supplier. So the next post will look into some of the performance considerations in more detail, particularly in terms of minimizing size, maximizing efficiency and the features that are available.

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