Power Supply Efficiency and Density Gains Provide Definitive Benefits for Customers
Improvements in efficiency and power density in power conversion allow customers to gain market share by reducing system size or increasing end system functionality.
Improvements in efficiency and power density in power conversion allow customers to gain market share by reducing system size or increasing end system functionality.
Once upon a time, the diode was an engineer’s friend. Today, it appears that the diode is the mortal enemy of the power supply engineer. This simple, yet robust silicon component has been increasingly replaced by low RDS(on) Field Effect Transistors (FETs) and digital control circuitry. The simple truth is that every design iteration of a power supply employs a greater and greater application of silicon content. The benefit to the end user is reduced size, higher efficiency, flexibility, and lower cost to operate.
Original industrial grade linear power supplies were perhaps 45% efficient, with the power factor of 0.65, and a power density of a mere 0.47 Watts/in³. Migrating to a switching power supply topology improved the efficiency into the 70% range. The addition of power factor correcting (PFC) circuitry improved the power factor of the supply to above 0.95 (and in many cases 0.99).
Following the lead of high efficiency 80Plus Front End power supplies for the Server market, current generation Industrial power supply design provides the user an entirely new experience. A case in point is the Power-One ABC 400–1012G. This high density 3×5 inch, open frame power supply provides 360 watts of power in a 1U form factor. This power density is 30x improvement over the old linear supplies, and nearly three times the density of what was available just a few years ago. Packaging techniques such as using the 1U channel chassis as a heat sink and integrated magnetics are part of the equation that yields this high density package. Energy efficient circuit design provides the second half of the equation. This high efficiency reduces thermal dissipation in customer’s systems which will improve reliability and reduce size.
Power supplies of greater than 65 W are required to have power factor correction. High efficiency designs such as the ABC 400–1012G employ an interleaved power factor correction circuit for efficiency and packaging density. The PFC circuits, operating at 180 degrees out of phase allows the input ripple current to be lower and of higher frequency, reducing input filter requirements. At lighter loads, one phase may be turned off to reduce system losses and maintain efficiency.
A significant improvement in efficiency occurs when the output diode is replaced with a synchronous rectifier. In this case, the diode is replaced with a FET and control circuitry. For example, let us look at the Power-One ABC400-1012G power supply. The output current of this 12 volt power supply is rated at 30 Amps. Using conventional diode rectification, with the rated VF of the diode at 1.1 volts (a common value for power rectifiers) the output dissipation is quite significant. Replacing the output diode with synchronous rectification employs a FET with RDS(on) of just 3.2 Milliohms. Even though the lost in that FET is a function of I²R, the loss in the output circuit is reduced by approximately 80%. See Figures 1A and 1B.
Figure 1A: Conventional Diode Rectification
Figure 1B: Higher Efficiency Synchronous Rectification
To properly drive synchronous rectifiers to offer this efficiency requires the use of sophisticated control circuitry. In the current and emerging generations of power supplies, it is not uncommon to find the use of microcontrollers and DSP Devices. The sophisticated control devices allow a greater control of the supply over the parameters of input voltage, output current and temperature.
Although circuit techniques such as synchronous rectification significantly add to the peak efficiency of the power supply, there is more to be considered. Careful control of synchronous rectification, along with sophisticated control of the primary side converter and the PFC circuitry leads to real world efficiency gains in the application of the supply. Part of the Total Cost of Ownership (TCO) of the power supply is the energy costs associated with the operation of the device. The work performed by the power supply changes over time and task such that the power supply rarely operates at any one load point indefinitely. As a general observation, when load decreases, power supplies become less efficient. This creates unwanted loss, i.e., heat dissipation in the assembly. The use of synchronous rectification control and other regulating devices in current generation power supplies allows for an extended efficiency curve at lighter loads.
Figure 2: Efficiencies by Generation and Application
The real thoroughbreds of the efficiency race are the power supplies designed for use in information technology servers. Attempts to attain high efficiency, Energy Star certification, and 80Plus certification have led to a classification known as Platinum efficiency. An example of an equivalent power rated product, the FCP 400-12G, has been tested by 80Plus to have Platinum efficiency. To attain this efficiency level, a single output, redundant capable power supply must have efficiency greater than 90% at 20% load, 94% efficiency at 50% load, and 91% efficiency at 100% load. The requirements for this include testing at 230 volt AC and with the fan power disconnected (the FCP 400-12G shown in the table above provides efficiency that does include the fan power). Even at 10% load, the FCP 400-12G achieves 87% efficiency that is higher than peak efficiency of many current designs.
Once again the humble diode takes a backseat to the FET’s used to increase power supply efficiency to the Platinum level. The OR-ing diodes that make redundancy possible in server supplies have been replaced with low RDS(on) FETs. In some instances, Platinum efficient supplies have replaced the venerable diode bridge with FETs as well. To properly drive these FET’s in a safe and efficient manner, it requires the addition of sophisticated digital controls. As the complex of the digital controls increase, they allow the power supply to work at an optimal efficiency over an ever increasing load range.
It has been said that the server power supply is nothing more than an industrial supply with OR-ing diodes. While there are some salient differences between an industrial supply and an information technology supply, the reality is that efficiency gains in one sector provide commensurate efficiency gains in the other.
