Designing for power: Using switchmode supplies Use the LT-1070 and LT-1270 switchmode ICs to make highly efficient power supplies for dc-powered RF equipment that save space. Proper circuitry adequately filters and regulates the voltage output.
With the steady demand for efficient output power, one area of amplifier design has been largely ignored. Amplifier designs may soon incorporate a “new” voltage-regulating technology, so let’s take a look at switchmode power supplies.
Computer manufacturers have been using switchmode supplies for more than a decade. These supplies are not particularly “clean” as far as spurious outputs may go, but they possess several worthwhile qualities. First, they are remarkably small. A 200W supply sits quietly in a corner of most 386 or 486 computers, occupying a footprint of perhaps 36 square inches.
Second, they do not require the utmost in input voltage regulation to perform well within their rated tolerances. If the supply is rated at 5V 610% at 20A, this rating holds true with an allowable input variation of 90Vac-130Vac. Third, these supplies use remarkably few components and a minimum of heatsinking. In short, they are efficient. With these three major attributes in mind, let’s take a closer look at “boost” or “step-up” switchmode power supplies.
‘Boost’ mode: the physics Historically, many electrical engineering pioneers made use of electromechanical switching power supplies to increase output voltages. In essence, they consisted of equipment diagrammed in Figure 1 above. A modified version of Figure 1 incorporates a diode in place of the series switch. (See Figure 2 above.) Operation of either version is identical. Dc rail voltage is applied to one end of an inductor. The other end of the inductor is alternately toggled between the source voltage and the load (represented in the figures as a capacitor). Output voltage from the capacitor consists of the rail voltage plus the voltage present across the coil as it discharges into the capacitor. Fully discharged, the coil merely represents a dc resistance in series with the load. Various ratios of S1 “on” time to S1 “off” time provide a simple voltage multiplier, as determined by the following formula: V(dc out) = [V(dc in)/(1)] – (duty cycle) Thus, a 50% duty cycle would result in: V(dc out) = [V(dc in)/(1)] – (0.5) or V(dc out) = V(dc in)/0.5 for a voltage doubler. Note that output voltage may be controlled entirely by changing the duty cycle of a “switcher.”
The inductor Achieving maximum performance from an inductor requires the solution to several formulas that find: the required value of inductance. the size of wire required to deliver a rated current. core size and permeability. the number of turns and insulation characteristics required to prevent failure of the inductor. All these issues are fully discussed in Application Note 19, available from Linear Technology, 1630 McCarthy Blvd., Milpitas, CA 95035, tel. 408-432-1900.
The major design factor is “delta-I,” the allowable maximum current change in the inductor as it moves from absorbing current to delivering current to the load. Typical “unregulated” supplies can allow this value to be as much as several amperes. (See Formula A on page 20.)
Formula B on page 20 provides a mathematical model representing the maximum available power using the inductance determined by Formula A. R (in the second term) symbolizes the effective resistance of the saturated transistor (emitter to collector) used in the LT-1070CK and LT-1270A. For the 1070, this value is typically 0.2V. For the 1270, it is typically 0.12V. The formula is used to determine the inductor and switching currents being removed from the maximum switching current that the transistor can maintain. This value is then multiplied by the input voltage to yield maximum power output from the inductor.
The diode The “steering” diode has to perform the function of S2 in Figure 1. Literally, it must be able to “toggle” as rapidly as the switching frequency-from full “on” to completely “off”-at a rate of 40kHz-60kHz. For this reason, Schottky diodes are recommended. Diodes from the 1N4000 series are ineffective at 60kHz, so do not use them. At higher output voltage levels, the “efficiency-factor” of the diode becomes less critical, allowing use of silicon fast turn-off diodes.
As with anything in electronics, where there is a voltage drop across a device and a current running through that device, there will be a power drop. Diodes are no exception. In this case, a typical Schottky diode will drop about 0.5V. This value is multiplied by the output current, resulting in: P = V(voltage drop) X I(load). Since this is a power loss, it must be included in the determination of overall efficiency, and it is subtracted from the available power at the inductor.
The ‘filter’ capacitor The “filter” capacitor component smooths the output voltage and should have a low “equivalent series resistance” (ESR) value. ESR values are used to determine the overall “effectiveness” of a capacitor, similar to the use of “Q” as a means of defining the relative merit of a coil. To achieve the lowest ESR, capacitors must use the shortest connections possible to their respective junctions. As the ESR value decreases, the output ripple across the capacitor declines, resulting in an output devoid of the usual “droops” and “sags.” In spite of the high frequency at which these “switchers” operate, output filtering is not just a simple matter of throwing a 0.1microfarad capacitor across the output and hoping for the best! Usually, values range from 150 microfarad to several thousand microfarads for these components, and capacitors lacking low ESR values should be avoided, if possible. In some cases it might be practical to parallel several capacitors of the same value to achieve acceptable results.
This component is at the “tail-end” of the switcher-but consider the work it must accomplish. First, it must maintain a steady dc voltage while absorbing energy from a coil that might be delivering some tens of watts at 40kHz-60kHz. It must also effectively bypass the switching frequency.
Switchmode power supplies often have output “ripple” that reaches several hundred millivolts peak-to-peak. To obtain cleaner outputs, an LC filter is added in series with the load. It often is possible to attain output ripple at less than 50mV peak-to-peak across the load using these small filters. Short circuit protection
These switchers have nothing “built-in” to respond to an output short circuit. With the input dc rail and output dc rail sharing an inductor and diode in common, there is nothing to prevent “melt-down” should the load or a capacitor become a dead short. About the only device that could prevent drastic failure would be a fuse placed in series with the “hot” side of the load. Better yet, a fuse placed directly after the diode would save the switcher and inductor should the filter capacitor(s) or load fail. The value for a fast-acting fuse can be calculated by the following equation: I(fuse) = I(out) X [V(out)/V(in)]. Provide a X2 “fudge-factor,” if necessary. Observations on the math After running through the math several times, you will find that “when I double the voltage, I halve the current.”
Welcome to energy conversion! Nothing is free. What is very nice about these chips is the built-in voltage regulator. Output voltage is constantly being monitored by the “feedback” pin, and any change in output voltage is immediately compensated for by a change in operating duty cycle to maintain the required voltage. Where to from here?
I was given an interesting problem. A warehouse had a new fleet of forklifts with a 12V battery power source. All of the data terminals and RF links used laser barcode readers and operated from 28Vdc-72Vdc with a rated current of 1.25A. Could I help?
Within a week, I delivered a working prototype. A week later, it was installed and running. My answer? The LT- 1070CK in a simple variable-voltage configuration. (See Figure 3 on page 24.)
Because this supply is enclosed within a grounded aluminum box, there is little need for extensive shielding. Further, the data-linking transmitter is shielded and dc decoupled from the 28Vdc-72Vdc switchmode supply, so additional filtering would have been redundant. But I built a filter anyway, just to see how effective it would be. A 10microhenry coil made of 12-gauge wire was placed in series with the dc output of the supply, along with a 100microfarad capacitor as the final output. My 150mV peak-to-peak ripple was reduced to less than 10mV peak-to- peak.
The voltage regulation is excellent. From no load to full-load, the voltage drops 0.02V! Not bad for a bit over an ampere of current! But even this regulation was improved.
Taking the “high” side of the 24K resistor to an unused terminal post allowed me to run a separate “sense” line directly to the load junction. Now the circuit was detecting the output voltage directly at the load and compensating for any I2R losses in the interconnecting power lines. The no-load to full-load output voltage remained absolutely stable.
Being somewhat of a skeptic, I decided (hesitantly) to touch the top of the LT-1070CK while it delivered output into a pair of parallel 24V bulbs, each drawing about 800mA at 28V. The case was barely warm. I let it run for 8 hours. It was still barely warm. I must’ve grinned for nearly 8 hours! What about RF?
With proper grounding, dc decoupling and shielding, it seems logical that such highly efficient supplies soon will find their place right next to RF stages. In a test circuit, the input voltage could drop to 10Vdc without affecting the output voltage. Voltage regulation from even my “homebrew” supply was superior, and designing it was a matter of following the AN19 notes from Linear Technology. Several other configurations of either the LT-1070CK or LT- 1270A probably will find applications in medium-power voltage regulators. With their high efficiency, the need for massive heatsinks is replaced with more space for the rest of the technology.
Read all about it! Application Note AN19 is in 1990 Linear Applications Handbook Volume 1 available from Linear Technology outlets (Mil-Rep Associates, 6111 F.M. 1960 West, Suite 213, Houston, TX 77069, tel. 713-444-2557). SwitcherCAD, an IBM-compatible program available on 3.5-inch or 5.25-inch diskettes, along with the SwitcherCAD User’s Manual, is available for $20 from Linear Technology, 1630 McCarthy Blvd., Milpitas, CA 95035, tel. 408-432-1900.