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So you hear it a lot, Digital PWM, so what does it really mean? is it better than Analog? What is a PWM? Those are the questions we will answer in this article.

What is a PWM?

Most people don’t know what a PWM is, some think a PWM is the whole VRM, and that is just not the case. PWM or Pulse Width Modulation on a motherboard’s power supply is a single controller IC or Chip, it is a single chip and it is the ONLY thing that determines whether the PWM is analog or digital. The VRM is the PWM+ MOSFETs+ Drivers+ Inductors/Chokes+ Capacitors.

Analog Vs Digital PWMs

This is a PWM, it is a tiny chip:

Analog Vs Digital PWMs

 

What is Analog PWM technology and what is Digital PWM technology? Which is better?

So which is better? First off let’s make something very clear, analog has dominated the scene of PWM controllers for a while now, however now that digital PWMs have been introduced and Intel has adopted them, it is a matter of time before they become required spec within the Intel VRM design guidelines (like VRD12 and VRD11.5). Now all PWMs incorporate some digital parts and some analog parts, for instance a few board makers have gone over the top with marketing and have started to call analog PWMs hybrid-digital PWMs as they use SVID. SVID is a digital bus which is used for the CPU to communicate with the PWM and thus VRM, however in analog PWMs SVID and DVID(DVID is digital too) are both immediately converted to analog signals. Sure there might be that digital and analog converter, but that definitely doesn’t make a PWM digital by any means. In fact what determines whether a PWM controller is analog or digital is the way in which it changes the output through its feedback loop. So just having the ability to use SVID and DVID (which is actually required by Intel for all LGA1155 boards and beyond) doesn't make a PWM digital. First of all analog refers to signals which are usually in their original form (like voltage) and digital refers to data in the terms of 1s and 0s.

Now let’s ask the basic question; how does a PWM controller, a tiny chip with less than 80 pins and an area less than a few mm2, control a circuit made up of 4-24 phases which is expected to deal with 100A+ load steps without budging the output voltage? Well the answer is that it uses a negative feedback loop to control the phases. For our sake I will simplify the loop for you and put it in basic terms.

The feedback loop and how the PWM processes the inputs from the system and produces the change for the output determines if it is an analog or digital PWM.

CPU is fed a voltage, that voltage is fed back to the PWM, and it is compared against a reference, then that difference between the reference and the real voltage, the error, is used to change the modulation scheme, duty cycle, or even switching frequency in some cases to change the output. That is how a PWM controls the VRM, through the error produced at the output, as there always will be an error. The system works in a loop which is why speed can be important, and why analog PWMs still exist.

In an analog PWM the reference and the output voltage are fed into an error amplifier which provides the difference, and then that difference is fed into a PWM generator/comparator along with the current balancing and a saw tooth waveform to generate a PWM signal which can change the output. This is very simplified we are only focusing on the voltage side of things, and not so much the logic and current monitoring to simplify things. Also different PWMs do things differently past the error amplifier, but the thing to remember is that most of the signal is analog all the way through. However in a digital PWM the output voltage and current monitoring are converted into digital signals through analog to digital converters, and then all the variables are fed into a control block, which uses a PID algorithm to determine what to do, and then the PWM generator generates the PWM signals and the output is the same as that of an analog PWM. This digitalization allows the PWM to better determine what to change and how much to change it, and thus gives it a much higher level of precision.

Below are two very simplified diagrams, you can see the difference.

Analog Vs Digital PWMs

Analog Vs Digital PWMs

 

Now you are probably thinking, so what is the difference in terms of actual operation which is better? So the basic principle for analog is for an output to deviate from the reference, that difference is then provided to a comparator and PWM generator and then the duty cycle or other factors are changed so that the new output shifts away from the old output, so if the voltage was too high the PWM would make it go lower until it was too low then it would make the voltage go back higher until it is too high.

Now in digital PWMs since everything is converted into 1s and 0s that means everything can be calculated, so there is no need to turn it up until it goes too high and turn it down until it goes too low, you can calculate exactly how long to change up the duty cycle or something else to raise or lower the voltage or whatever else needs to be changed. Then when it deviates again you change it exactly so much again. The result is more precision, and also can be less unwanted spikes, even though there is OVP and OCP in both of the PWM technologies, digital PWM’s give the user a ton of monitoring capabilities that come second nature to having everything in 1s and 0s ready for the OS.

Analog Vs Digital PWMs

 

Since the voltage will deviate by itself anyways, because of CPU load changes and other hardware related matters, if the PWM operates well enough then it can produce an output which can seem almost constant to the naked eye. Also please be advised that any reading you take of voltage with a digital multi meter is always an average of the true voltage, as the multimeter used to read voltages samples very slow, you need an oscilloscope to see the real voltage at any given time.

 

Analogy to Real Life:

So let’s say you get hungry, when you get hungry you eat some food and then a few minutes later your brain tells you that you are full. So you basically eat until you are full, and probably overdo it a little bit. That is how analog works, once you are full you stop eating until you are hungry again. However digital is a bit like knowing how much food you need to eat, so let’s say when you get hungry your brain tells you how hungry you are, and then you can calculate based on preset equations how much you need to eat to be full. It is more precise than the analog method, but not as fast because you will have to calculate it. It isn’t a perfect analogy, but maybe one you can relate too.

 

Benefits of Analog and Digital PWM Technologies:

Now you are probably thinking so isn’t analog crap? The truth is no, because while analog has to wait for certain voltage deviations, it also runs faster than digital because digital uses an analog to digital controller that cause a slightly delay, also digital has to wait for deviation as well. So let’s take a look at the table below to show the major differences and benefits:

Benefits of Analog

Benefits of Digital

100%hardware design is faster

Ability to predict transient loads

Pre-programmed is easier to implement

Higher precision and accuracy

No possible calculation errors

Ability to be re-programmed in the field, tailor to individual needs

 

Ability to control more of the VRM at any given time and process all the inputs.

Brands that use Digital or Analog PWMs, and Manufacturers that Implement them.

So which companies make analog PWMs? And which make Digital? Below is a list of the most common PWMs used on computer motherboards.

Analog

Digital

Intersil (Ziker Labs)

CHiL

uPI Semiconductor

International Rectifier(IR)

Analog Devices

Volterra

So what about manufacturers?

Below is a list of Z77 motherboard brands and what PWM brand they are using for their LGA1155 Z77 boards, however some brands have changed from the PWM they used before for Z68, such as GIGABYTE and ASRock. Listed below is what they will use for Z77, not what they used in the past.

ASUS

CHiL(Digital)

GIGABYTE

International Rectifier (Digital)

ASRock

Intersil (Analog) STMicroelectronics (Analog)

MSI

uPI (Analog)

Intel

CHiL (Digital)

ECS

uPI (Analog)

Analog Vs Digital PWMs

Analog Vs Digital PWMs

 

Additional information