In every system and in a good number of boards there exists one or more devices used to transform either mains AC power or DC power to a different operating voltage.  Power supplies are a crucial and critical component of any system, they can and have made or broken a design.  Historically, power supplies were left as the last component of the system design and often improperly specified.  Modern power supplies are required to meet several stringent requirements for safety, EMI regulation and efficiency.

At Nuvation we have been called upon to design power circuits and supplies to provide the sometimes large array of DC voltages required by a particular system.  Often our clients have a limited understanding of the complexity of these designs and as a result assume that the process is simple and consumes little time or talent.

In this series of articles I hope to provide a better understanding of the complexities of design and some of the thought processes involved in the design of power supplies.  To do this I will describe the requirements and design process involved in designing an off line switching power supply, beginning at the wall plug and proceeding to the final output(s).

Modern power supplies are required to:

• Present a “real” load to the AC mains (power factor ≥ 92%)
• Not generate conducted interference (EMI noise or harmonics) on the AC mains in accordance with FCC part 15 regulations.
• In many applications to operate with overall efficiencies exceeding 85% and consequently, not generate large amounts of waste heat.
• To operate over an input voltage range of 87Vrms – 266Vrms 50Hz and 60Hz without requiring mechanical switches to select the input operating voltages (universal input power supply)
• Not present a hazard to either the end user or technician working on the supply

To accommodate item 2 in the list common mode and differential mode filters are employed between the AC mains and the rectifiers on the supply.  The nature and architecture involved in the design of this element will be covered at the end of this series of articles.  As stated in item 1 above, current regulations require that all power supplies providing an output power >15W must present a real load to the line and operate with a power factor ≥ 92%.

Power factor is the ratio between the DC equivalent power or Wattage used by an electrical load and the Volt*Amps presented to the AC line by an electrical load. VA is defined as the apparent power.  The AC generator providing the power to the wall must provide power equivalent to the Volt*Amps drawn whether or not it is used to do useful work. 

In order for the load to be using the energy provided, the voltage across and the current through the load must be in phase.  An AC voltage source connected to a resistor has its voltage and current in phase i.e. the current rises in the resistor is in step with the voltage, at each point in the voltage curve the current flowing in the load is exactly equal to the instantaneous voltage divided by the resistance (dI = dV/R), and the instantaneous power delivered to the load dP = dV*dI (where V and I are both functions of time). 

For a DC circuit and an AC source driving a pure resistance, the power used and delivered is simply P=VI.  In an AC circuit driving a general load we deal with the rms value of both current and voltage to calculate the “real” or equivalent DC power delivered to the load and the expression for power becomes, P = I·VCos(φ), where I and V are the rms values of each and φ is the phase angle between them.  Cos(φ) represents the power factor and is either expressed as a number between 0 and 1 or as a percentage with a value of 0 to 100%.  If φ = 0 than P = I·V the same as for a resistor or a DC load.  If  φ = 90°  than the power delivered to the load P = 0 even though there is voltage across the load and a current through it.  The generator providing the power must deliver I·V power even though none of it is being used to do anything useful.

Unfortunately, the input network in a normal non-corrected power supply is a diode bridge driving a large capacitor and as a result it presents a non-real load to the generator. Depending on the circuit components the power factor will be between 55% and 75% which implies, that the generator must provide between 125% to 145% more energy than is being used.  The difference either appears as heat or is reflected back to the AC mains. The most common means of correcting this condition is to employ a power factor controller.  In the case of the power supply the poor power factor is not caused by a reactive (i.e. capacitive or inductive) load but by a highly non-linear one.  A purely reactive load can be corrected by adding either an inductor, in the case of a capacitive load or a capacitor in the case of an inductive load.  In fact, this is what’s done when the load is an electric motor which looks like an inductor, when the load is non-linear as in the case of the power supply the solution is not quite so straight forward.

The non-linear load, in addition to reducing the power factor presents a considerable amount of “noise” to the mains in the form of harmonics of the line frequency, the time domain equivalent of the distortion caused by the rectifiers is to present a load, at the fundamental frequency of the line (50Hz or 60Hz) where the current and voltage are out of phase.  While the purely reactive load can be compensated for simply, to correct for the poor power factor due to a distorted current waveform requires a change in equipment design or expensive harmonic filters to gain an appreciable improvement.

Power Factor Correction (PFC) allows power distribution to operate at its maximum efficiency. A PFC appears resistive to its source. This implies that the input current must differ from the sinusoidal source voltage by only a scaling factor. Their waveforms must be identical, though scaled by the effective input resistance of the PFC, by Ohm's Law. There are two types of PFC, Active PFC and Passive PFC. I will be discussing the details of both Active and Passive PFC in the next issue. Stay tuned.

If you have any questions regarding power supply design, you can reach me at si@nuvation.com.