SWITCH-MODE POWER SUPPLIES
How switches work to control large amounts of power efficiently:
The well documented efficiency, size and weight penalties associated with conventional mains-frequency transformer driven a-c to d-c power supplies, has led to an extraordinary array of high frequency alternatives. Known generically as "Switching Power Supplies," SMPS, or "switchers," these designs rely upon the efficiency of a switch to control large amounts of power with relatively little loss.
The switch on your living room wall works in much the same fashion. When it is "off" the current through it is near zero, the open gap supports the full voltage of the utility feeding your home. When you flip it "on" the situation reverses. There is little or no voltage across it and the current flows unimpeded. In either position the switch itself dissipates little power. "On" there is current but no voltage and "off" there is voltage but no current.
If you modulated the ratio of on time to off time by vigorously manipulating the lever you'd be able to control the amount of light (or whatever was connected) without needing much more effort than is required to flip the switch. The switch itself would be controlling quite large amounts of energy with negligible dissipation. This is quite unlike the situation in linear series-pass control systems where power is deliberately dissipated away as the means of control. Switch-mode power supplies work like the wall switch in your living room. They use some form of pulse-width modulation to alter the on time relative to the off time to effect control.
Of course, given the constraints of frequency and the speed of the devices used for switching (now mostly power FETs), there is a limited dynamic range available. The ratio of the shortest pulse (constrained by the speed of the switch turning on and off) and the longest pulse (constrained by the frequency of operation) determines the available dynamic control range. This dynamic range is used for compensating the output against mains variations, load variations and a limited amount of setting control. Switch mode power supplies generally lack the large dynamic range that would make programming a practical matter. Switchers are generally used for fixed output or limited adjustment range output designs.
High frequency operation contributes to the small size of switch-mode power supply designs. Modern units typically operate in the 100-300 KHz range.
With the introduction of instrumentation switch-mode power supplies such as ABC and the BOP High Power series, Kepco has overcome this inherent limitation of switch-mode conversion to produce a true zero-up (to maximum) controllable switch. This allows Kepco to produce programmable d-c outputs in a pure switch- mode topology. The programmable RKW models, though not instrumentation power supplies, also offer zero-up control of a switch-mode power supply.
The use of power FETs with fast turn-on and turn-off characteristics have made switches that operate comfortably at frequencies above 100 KHz practical. Beyond this, the actual choice of operating frequency is driven by consideration of noise generation (EMI), safety issues and the trade-off of size vs. efficiency. Most of the designs in this catalog use one of two topologies, forward converters or flyback circuits operating at a fixed clocked frequency.
Forward converters are used in medium and high power applications. They can be recognized by their separate power transformer and output choke. Flybacks are favored for low power applications because they employ a transformer that doubles as the output choke thus saving on one large and expensive component.
TYPES OF SWITCHERS
Forward converters are used in medium and high power applications. They can be recognized by their separate power transformer and output choke. Flybacks are favored for low power applications because they employ a transformer that doubles as the output choke thus saving on one large and expensive component.
TRANSFORMERS STEP-DOWN
Power supplies need a transformer to provide isolation and to shift the level from the voltage of the utility (115V a-c -230V a-c) to the levels used by modern logic and their associated circuits (typically 5 volts to 48 volts).
The function of the switch, in addition to providing modulation (control) capability, is to convert d-c to a square-wave kind of a-c.
The square wave is necessary to meet two of a power supply's principal purposes as listed on page 141:the ability to change the output to a different voltage level than the input and to provide isolation. Transformers provide this function. In linear power supplies the transformer operates at mains frequency and is the heaviest and largest part of the unit. In switch-mode designs, the switch is made to operate at high frequencies. Transformers at 100+KHz can be a small fraction of the size of their mains-frequency counterparts.
The design that does all of this is a composite. The front end is a rectifier-filter that converts the low-frequency a-c to d-c. Operating on North American mains, this circuit functions as a doubler. Operating at European 230-250 volt mains, it functions as a simple rectifier. The resulting d-c is about 300-350 volts. The fast switch chops this so it can pass through a high frequency transformer to be re-rectified and filtered at the secondary to become the output.
OFF-LINE RECTIFIER FILTER
The input rectifier-doubler and filter is called an "off- line" circuit, that is, it operates directly off the line (mains). This "off-line" circuit is a source of a number of problems that have to be addressed by switch-mode power supplies.
For openers, there is little impedance between the filter capacitor that stores the high voltage d-c for the switch and the utility mains, little to impede the surge of current into this capacitor. To keep it under control, surge limiting circuits are required. In low power units this may take the form of a thermistor whose resistance is high when cold and low hen hot. In medium power design, a resistor is switched in for start-up and removed once the input capacitor is charged. In high power designs, a sort of soft-start is provided using thyristors to ramp up the voltage.
Another problem is the high voltage itself, which is in the range of 300-350 volts. This must be well isolated from the low (5V) output if users are to connect it to their expensive logic without trepidation.
A third problem is in the discontinuous way in which current is drawn by a capacitor-input filter. This produces both high peak currents and reflects harmonic distortion back to the mains. This is the problem known collectively as poor power factor. Power factor correction (PFC) is used to solve this. Please see the discussion of Power Factor.
The high voltage created when the a-c mains are directly rectified in off-line designs affords a unique advantage that is incidental to the process but nevertheless, quite valuable. Energy is stored in capacitors as 1/2 CV2.
This means that the energy is proportional to the square of the voltage and the voltage, as we have already seen, is quite high. Thus the energy stored in the input, off- line capacitor, is enormous, sufficient, indeed, to sustain operation of the whole power supply for some time when the mains are interrupted. When the time exceeds a full cycle (20 milliseconds at 50 Hz) a ride-through capability results. The power supply will function uninterrupted through the loss of a part or whole cycle.
The principal advantage of this is to be able to provide some warning to the load that a mains failure is about to occur. This warning can be issued if it is possible to recognize and provide a logic signal when the mains have failed. Since the power supply will continue to operate for some time on the stored energy of its off-line capacitor, this warning can be used to provide an orderly shut-down of the load.
The variety of a-c mains that confront power supplies are a logistical headache and sometimes a safety concern...not to mention a complicating factor in the initial design. Not too long ago, power supplies were produced in two varieties:115 volt models for the North American market and 230-250 volt models for Europe. Japan, with its 100 volt mains was accommodated either by a low tap on the 115V a-c connection or by extending the input range down to 85V a-c.
As we have seen, these designs use an input circuit that acts as a doubler when in the 115 volt mode and as a bridge rectifier when in the 230 volt mode. A simple jumper selects between them. While this simplified the logistical problem, it did give rise to a potential safety
UNIVERSAL OR WIDE RANGE INPUT
The variety of a-c mains that confront power supplies are a logistical headache and sometimes a safety concern...not to mention a complicating factor in the initial design. Not too long ago, power supplies were produced in two varieties:115 volt models for the North American market and 230-250 volt models for Europe. Japan, with its 100 volt mains was accommodated either by a low tap on the 115V a-c connection or by extending the input range down to 85V a-c.
As we have seen, these designs use an input circuit that acts as a doubler when in the 115 volt mode and as a bridge rectifier when in the 230 volt mode. A simple jumper selects between them. While this simplified the logistical problem, it did give rise to a potential safety issue...the selection of the wrong input voltage. If a unit set for 115 volts is connected to 230 volts, damage usually results. Also, the selector, though simple, does cost something and so a demand arose for power supplies that would work from any a-c input without user intervention. In low power flyback switchers, this could be accomplished by simply using more of the pulse width modulator's dynamic range for input accommodation and by sizing everything in the input for the extremes. The capacitor had to be sized for the high end voltage while the rectifiers had to be sized for the low end current. Kepco's MRW, KRW and FAW designs are examples of this.
This approach has been very successful up to about 150 watts. Beyond that power level, wide range input is achieved as a fallout of the effort at power factor correction. PFC designs now reach beyond 1.5 KW, so wide range input is available at those power levels too. The Kepco HSP, RKW and HSM designs are examples of this.
Kepco's MST design is a 200 watt product with wide range a-c input that is achieved by using a PFC front end. MST is a hybrid in that it is a switcher that has a linear post-regulator so that it may be programmed over 0-100 percent of its output range. MST are digitally controlled plug-in power supplies that may be combined in groups of up to 1800 watts.
Kepco's ABC design is a 100 watt product with wide range a-c input that is also achieved by using a PFC front end. Unlike MST, the ABC design functions as a "zero-up" controlled switch that can be adjusted to any voltage from 0 to its rated maximum. ABC are bench- style power supplies with keypad/GPIB control.
Kepco's BOP High Power are 1000 watt 4-quadrant products with a bi-directional PFC current that enables it to recuperate energy sinked from an active load.
