Controller vs converter vs module

Integration is the driving force behind many of the technologies we see from semiconductor manufacturers. Power devices are no different, whether it be integration of compensation, switching components, multiple outputs or inductors.

 

For this conversation we are going to define controller as, well, a switching controller. A converter would be a controller with integrated switches (we’ll focus on synchronous converters with both high and low side FETs). We are going to define a module by the inclusion of the output inductor, meaning in most cases all that is left for you to do is add input and output capacitors and your supply is ready to go. We will focus on a basic buck converter since it is most common.

Here is a very simplified model of the integration from a controller up to a module. Other features like power good, frequency synchronization, compensation, etc. can be found in all three types of devices. The core differentiators between the types of devices though are defined above, integration of switching elements and integration of output inductors.

 

The short list of advantages and disadvantages is typically smaller footprint, easier design but costs more. Let’s look at these tradeoffs a little closer

Advantages to a Module

The number one advantage to using a module is the ease of design. Place the device, add input and output capacitance and follow the layout guides for thermal relief and you are done. Because of the fixed footprint you should get very consistent results from your measureable testing. Transient response, load capacity, electromagnetic emissions are all tested by the manufacturer and should closely match the results you see in your own system. Obviously there are contributing factors that can influence your performance, but by using a module you cut down drastically on the potential sources of error.

 

Thermal performance is optimized inside the module. Variation over temperature is a big part of power design performance. One of the primary suspects of temperature related failure is the inductor. By using a module that has the inductor built in you get a design qualified to the temp range you need that includes the most temperature sensitive component. Using a module should give you a high degree of confidence in meeting your requirements over the entire temperature range of your product.

 

Footprint size and optimized layout are the other big benefits of a module design. The circuit is laid out inside the module to optimize size as well as thermal, EMC, and efficiency performance. The result is that you get an overall footprint about as small as possible without compromising performance. Take a look at the cutaway illustration below from Monolithic Power Systems of one of their power module devices to see an example of how the internal layout is optimized.

Disadvantages to a Module

Far and away the biggest negative to using a module is the cost premium over a discrete solution. The cost difference between discrete and module solutions used to be much more dramatic, however that delta is considerably smaller now making modules more attractive to a wider audience. Another thing to consider that impacts that cost delta is making sure to include all costs associated with the discrete design. Don’t compare the module to the converter; you have to add in the inductor to the converter BOM. You also have to add the component placement cost of the inductor during production as well as the cost for adding an additional line item to your production build. You then have to source 2 components from 2 different manufacturers to complete the design. I’m not trying to imply that these things are that big a deal, but they shouldn’t be ignored during the evaluation. Something I’ve had personal experience with is also having a production house substitute the inductor with a different part number they think is equivalent without qualification. That can lead to device failures or impact EMI performance if they aren’t truly equivalent. That’s also a fun one to try and figure out when inductors often aren’t easily identified visually, meaning it may take you a while to even realize the wrong part is on the board.

 

Lack of flexibility is another potential downside to a module approach. As you move from a controller to a module you lose the ability to optimize your design to specific parameters. If you know that your system runs 90% of the time at a certain load then you can optimize performance under those conditions using a discrete design. I like to think about it the same way as trying to fly a drone. I got one for my daughter and quickly realized I had no idea what I was doing. On one end of the spectrum you have controls like up, down, left, right, forward and back. At the other end you can set propeller angle, pitch, roll, all kinds of things that make no sense to me which is why I promptly crashed the drone about 200 yards down the neighborhood and had to do the walk of shame to retrieve it. Same concept, you can get as much control as you want, just ask yourself how much of that control you really want.

Vin, Vout and Iout Support

Vin ranges and supported output voltages can be a limiting factor for modules as well. You have to make sure the module will not only support your input voltage, but also the output you need at that input voltage over the temperature range you need. Efficiency typically goes down as the delta between Vin and Vout increases so if you need to go from 24V to 1V you are going to be dissipating more heat in the device than if you were going from 5V to 1V. You will find that there are many more options with a Vin max of about 5.5V when you look at modules, however the number of options supporting higher Vins is rising. MPS for example now has options supporting 18V and 36V inputs. Not surprisingly as your Vin increases the max output current capacity decreases. You can’t get around the physics - inside a given package you can only dissipate so much power.

 

To summarize modules are a much more attractive option than they were in the past. If you are a digital designer and don’t really want to spend time on the power design then a module approach may make a lot of sense. I’m a power designer so I am very comfortable designing my own discrete solutions. That being said, I’m giving heavy consideration to new modules coming to market for use on Avnet’s upcoming reference designs. The value proposition of modules continues to improve so if you haven’t considered them recently I highly recommend you take a look at what is out there.

 

Reference link

MPS power modules - http://www.monolithicpower.com/Products/Power-Modules