Buck regulators are what provide DC power the CPU and DRAM and are the target of every Volt-modder. Volt modding in essence involves making available a voltage range beyond what is specified in the BIOS. While the BIOS allows a measure of voltage controlled vied the programmable nature of the buck regulator, going beyond the specs of the regulator itself is not possible in this manner. One could say we are overvolting the regulator or modifying the power supply loop characteristics to accommodate our needs. To attempt such a thing, we have to understand the basics of the component we are trying to mod. Linear voltage supplies/regulators have been around for quite a while. However, they bring with them several critical drawbacks such as low efficiency, which in turn necessitates the use of bulky heatsinks, cooling fans and isolation transformers. This in turn makes them unsuitable in today's world of compact electronic systems. The disadvantages of a linear power supply are greatly reduced by the use of an alternative scheme i.e regulated switching power supply. This brief note will discuss one such power supply, i.e the Buck-Regulator.
Linear regulators are o.k for powering very low powered devices and devices which are not sensitive to thermal fluctuations. They are easy to use and cheap, and therefore are very popular. However, due to the way they work, they are extremely inefficient. A linear regulator works by taking the difference between the input and output voltages, and just burning it up as waste heat. The larger the difference between the input and output voltage, the more heat is produced. In most cases, a linear regulator wastes more power stepping down the voltage than it actually ends up delivering to the target device! With typical efficiencies of 40%, and reaching as low as 14%, linear voltage regulation generates a lot of waste heat which must be dissipated with bulky and expensive heatsinks.Even the new LDO (low drop-out) regulators are still inefficient linear regulators; They just give you more flexibility with input voltage drops.
How is a switching regulator better?
A switching regulator works by taking small chunks of energy, bit by bit, from the input voltage source, and moving them to the output. This is accomplished with the help of an electrical switch and a controller which regulates the rate at which energy is transferred to the output (hence the term “switching regulator”).The energy losses involved in moving chunks of energy around in this way are relatively small, and the result is that a switching regulator can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they can power useful loads from higher voltage sources. Switch-mode regulators are used in devices like portable phones, video game platforms, robots, digital cameras, and your computer. Switching regulators are complex circuits to design, and as a result they are not very popular with hobbyists.
What can switching regulators do that linear regulators can't?
With high input voltages, driving loads over 200 mA with a linear regulator becomes extremely impractical. A switching regulator can easily power heavy loads from a high voltage.Certain kinds of switching regulators can also step up voltage. Linear regulators cannot do this. Ever. So, switching regulators are used to provide clean DC output to the CPU and DRAM.
How do I tell if I need a switching regulator?
As a general rule of thumb, if your linear voltage regulation solution is wasting less than 0.5 watts of power, a switching regulator would be overkill for your project. If your linear regulator is wasting several watts of power, you most certainly want to replace it with a switcher!
Here is how to calculate power losses:
The equation for wasted power in a linear regulator is:
Power wasted = (Input voltage – output voltage) * load current
For example, let’s say you have a 12V lead-acid battery and you want to power a microcontroller that draws 5 mA, and an ultrasonic rangefinder that draws 50 mA. Both the microcontroller and the ultrasonic rangefinder run off of 5V. You use an LM7805 (a very common linear regulator) to get the voltage down to 5V from 12V.
Power wasted = (12V – 5V) * (0.050A + 0.005A) = 0.385W
0.385W is not too bad for power losses. The LM7805 can handle this without a big heatsink. You could get more battery life if you used a switching regulator, but in this case the power consumption is so low that the battery life will be very long anyway.
Now let’s expand on this example, and add two servos that draw an average of 0.375A each, and also run off of the 5V supply. How much power is wasted in a linear regulator now?
Power wasted = (12V – 5V) * (0.050A + 0.005A + 0.375A + 0.375A) = 5.635W
5.6 Watts is a lot of waste heat! Without a large heatsink the LM7805 would get so hot it would desolder itself or melt your breadboard. Even with the heatsink, 5.6W is also a lot of life to suck out of your battery for no reason. A switching regulator such as a DE-SW050 would be very useful in this case, and would reduce power losses to around 0.5W.
Pulse Width Modulation Basics:All of the switching converters described here use a form of output voltage regulation known as Pulse Width Modulation (PWM). Put simply, the feedback loop adjusts (corrects) the output voltage by changing the ON time of the switching element in the converter.As an example of how PWM works, we will examine the result of applying a series of square wave pulses to an L-C filter.
When the switch turns on, the input voltage is connected to the inductor. The difference between the input and output voltages is then forced across the inductor,causing current through the inductor to increase.During the on time, the inductor current flows into both the load and the output capacitor (the capacitor charges during this time).When the switch is turned off, the input voltage applied to the inductor is removed. However, since the current in an inductor can not change instantly, the voltage across the inductor will adjust to hold the current constant. The input end of the inductor is forced negative in voltage by the decreasing current, eventually reaching the point where the diode is turned on. The inductor current then flows through the load and back through the diode. The capacitor discharges into the load during the off time, contributing to the total current being supplied to the load (the total load current during the switch off time is the sum of the inductor and capacitor current).
As explained, the current through the inductor ramps up when the switch is on, and ramps down when the switch is off. The DC load current from the regulated output is the average value of the inductor current. The peak-to-peak difference in the inductor current waveform is referred to as the inductor ripple current, and the inductor is typically selected large enough to keep this ripple current less than 20% to 30% of the rated DC current.In most Buck regulator applications, the inductor current never drops to zero during full-load operation (this is defined as continuous mode operation). Overall performance is usually better using continuous mode, and it allows maximum output power to be obtained from a given input voltage and switch current rating. In applications where the maximum load current is fairly low, it can be advantageous to design for discontinuous mode operation. In these cases, operating in discontinuous mode can result in a smaller overall converter size (because a smaller inductor can be used). Discontinuous mode operation at lower load current values is generally harmless, and even converters designed for continuous mode operation at full load will become discontinuous as the load current is decreased (usually causing no problems).
The I/O characteristics of all DC-DC converters can be examined by using the requirement that the initial and final Inductor current within a period/cycle should be constant (i.e net energy stored in an inductor is zero). So, average voltage per cycle is zero. Mathematically:
VLOND + VLOff (1-D) = 0, where D is the duty cycle.
Practical Constant ON-Time Buck Regulator (an example) :
The basic regulator IC consists of a comparator with input hysteresis that compares the output feedback voltage with a reference volt turning off the buck switch MOSFET.When the feedback voltage exceeds the reference voltage, the comparator output goes low, turning off the buck switch MOSFET. The switch remains off until the feedback voltage falls below the reference hysteresis voltage. Then, the comparator output goes high, turning on the switch and allowing the output voltage to rise again. It reacts extremely quickly to load and line transients due to its wide bandwidth control loop. Unlike a pulse width modulation (PWM) regulator, this loop does not require an error amplifier or frequency compensation.
Two PWM Control Techniques in Brief:
This is the most important part of a Buck regulator from an overclocker's standpoint. We had already hinted at this when calculating the output voltage and its dependence on the Duty cycle. In brief, by varying the ON time, the conduction of the transistor is increased and correspondingly the output voltage increases. The control circuitry senses any change in output voltage and adjusts the duty cycle to correct such changes. An oscillator sets the chopping frequency of the converter and a stable temperature compensated reference is used, to which, the output voltage is compared by a high gain error amplifier. An error voltage to the PWM is used to adjust the duty cycle. Feedback compensation techniques are nothing new and this is also known as a servo system. Usually a "servo" system has an error amplifier, integrator and a low pass filter in a feedback configuration.The two most common forms of control in dc/dc switching power converters are CM (current-mode) and VM (voltage-mode) control. Each method has its own advantages and disadvantages. CM control provides the ease of loop compensation and inherent line feed-forward, which makes this method a favorite among designers. VM control is more immune to noise. This characteristic is important in large-step-down-ratio applications in which the switch has a short on-time that is susceptible to noise pickup. The ideal approach that has been eluding designers is a practical CM-controlled regulator without noise-susceptibility challenges.
Voltage mode:
This is the more traditional mode of control in PWM switching converters. In a simple circuit, the components are an oscillator, an error amplifier and a comparator. The output voltage is sensed with respect to a reference and the error voltage is amplified by a high gain amplifier, this is followed by a comparator which compares the amplified error signal with a sawtooth waveform generated across a timing capacitor.
Let us take a look at the following schematic:
The circuit shown below is a voltage-mode PWM controller in which the error amplifier output is compared to a voltage ramp from the oscillator to determine the output pulse width. A current mode PWM replaces the oscillator ramp with a ramp that is proportional to the current in the magnetic element. A nice place to perform a mod would be to add a DC Bias voltage to the Voltage reference. It would be the easiest to do. Tweaking the compensation network would be a bit more involved as it can lead to huge output instability, resonance and ringing. It is not recommended that one messes with the feedback loop without any calculations.
Current Mode:
This is a more complicated multi-loop control technique, which has an AC current feedback loop (AC=oscillating current) in addition to the voltage feedback loop. The second loop controls the peak inductor current with the error signal rather than controlling the duty cycle of the switching waveform. This is an attractive option in high frequency switching applications.
To be continued....
References:
[1] http://www.dimensionengineering.com/switchingregulators.htm)
[2] http://www.elecdesign.com/Articles/ArticleID/12253/12253.html)
[3] PWM, Linear Technology App Notes)
[4] Power Electronics Design Handbook- Nihal Kulratna.
[5] Maxim-Dallas Semiconductor Application Notes
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