Electronic Design

Shed Some Pounds With This AC/DC Transformerless Power Supply

At The Heart Of This Programmable, High-Efficiency Design Is A High-Voltage, High-Current PWM Amplifier.

If there's one component that really defines a power supply, it's a transformer. But in some high-power applications, a transformer literally becomes a burden. Portable equipment is a good example. If you have to carry the equipment from one place to another, the elimination of a heavy-duty transformer makes the total package a lot lighter. In other applications, such as driving magnetic bearings, electric power in the order of kilowatts is required. A transformer at such power ratings is both heavy and expensive, which really makes you want to drive the bearings without one.

Fortunately, high-power pulse-width-modulated (PWM) amplifiers are now available in the 200- to 500-V range, with current ratings in the realm of 10 to 20 A. These amplifiers can be used to build transformerless ac-ac power supplies with an output ac voltage that's linearly proportional to an input control signal. The supply operates just like a linear amplifier that has a gain set by resistor values. Such an amplifier's efficiency is high, usually in the 90% range, because of the PWM technique.

Some applications require the combined functions of ac-ac conversion and amplification. One example is brush-type ac motors. You need an ac-ac power supply, plus an amplifier, to control the voltage across the motor or the current through the motor. The ac-ac power supply described here does the job of both. You obtain the power from a 115-V ac wall socket and control the motion of the ac motor directly.

Another instance is testing high-current devices, such as microprocessors, memory, and logic circuits with programmable V-I (voltage and current) sources. These kinds of sources are built into most automatic test equipment. At first glance, it seems like you can use a transformerless ac-ac power supply as a programmable V-I source. In practice, since this power supply uses PWM switching, the generated noise is usually too high and thus unacceptable for such applications.

To work around this problem, use the power supply to drive a linear amplifier, such as the Apex PA03, which in turn drives the real load (Fig. 1). A linear amplifier with power-supply rejection in the 60- to 100-dB range will suppress the switching noises from this programmable ac-ac power supply.

A key advantage of such an arrangement is that the internal power dissipation of the PA03 is kept at its minimum. The PA03 is capable of delivering 30 A of output current continuously. With a constant voltage at VCC and the same load current, the PA03's power dissipation increases as the load voltage drops. This programmable ac-ac supply allows the PA03's VCC to drop or increase in proportion to the load voltage, and thus keeps its internal power dissipation at a constant level. Because this supply uses PWM techniques, with efficiency in the 90% range, its internal power dissipation is minimal compared to that of the PA03.

To design a complete transformerless ac-ac power supply, we will first start out with a "paper-and-pencil" design. Using Spice simulation, we'll then verify the design and finally test a prototype to verify the Spice simulation (see "Safety Warning").

A functional diagram of a transformerless ac-ac power supply is shown in Figure 2. Power is taken from the 115-V ac wall outlet and goes through diode rectifier D1, which converts the input sinewave into a half-wave rectified output. Components L1 and C1 act as a filter to attenuate the harmonics of the half-wave signal and extract its ac component, which supplies a high voltage to the PWM amplifier.

The output of that amplifier is a pulse train whose duty cycle is controlled by its input voltage through a resistor divider made up of R1, R2, and R3. Components L2 and C2 form another filter that attenuates the harmonics of the PWM pulse train and extracts its ac component for use as a programmable high-voltage and high-current ac source. Therefore, the ac-ac power supply's output ac voltage is linearly proportional to the PWM amplifier's input control voltage.

The functional diagram of Figure 2 has no feedback or voltage regulation. Thus, the output ac voltage won't be very stable and will change with external environmental factors, such as load change, temperature, source voltage ripple, and so forth. In a real-world circuit, feedback control is necessary to compensate for such changes. Figure 3 is a complete circuit, which is made up of several blocks.

In the ac-ac block, D1 is a diode rectifier with a reverse voltage that must be at least 326 V (2 * 115 V * 1.4142). C1 is a smoothing capacitor with a value that affects the output ripple. The larger the capacitor is, the better—but it's also more expensive and bulky. We'll arbitrarily start out with C1 = 1000 µF because electrolytic capacitors with a voltage rating of 200 V are available at a reasonable price. Calculation of output ripple versus C1 is very complex, due to the PWM waveform. It's impractical, if not impossible.

Later, we'll use Spice to see the reduction in output ripple with increased C1. R2 is a bleeder resistor whose value determines how fast C1 will discharge after power is turned off. Its power rating must be equal or greater than the result of the square of 163 V divided by R2. For R2 = 10k, the rating must be 2.66 W or higher.

Block SA14 refers to the Apex SA14, a PWM amplifier with a 200-V, 20-A rating. The 115 V ac will provide a peak voltage of 163 V (115 * 1.4142), so the PWM amplifier must have a voltage rating of at least 163 V. In countries where the ac power source is 230 V rms, you might choose the Apex SA16, SA18, or other PWM amplifier boasting a voltage rating of 500 V.

L1, L2, and C4 form a 3-pole lowpass filter with Butterworth (maximum flatness) frequency response for the 8-O load. The corner frequency is set at 2.25 kHz, one decade below the SA14's 22.5-kHz PWM frequency. As the load changes, the filter's corner frequency will not budge. But its peaking, or Q factor, will. To achieve even lower ripple and noise, use higher pole filters. Information on the design of LC filters can be found in References 1 and 2.

The 2.25-kHz active low-pass filter block and the integrator block form a voltage feedback control for the ac-ac power supply. An active filter is used because this is the small-signal processing path, not the power-transmission path. It's also smaller and cheaper. Look for the design of active filters in Reference 3.

The SA14 is an inverted PWM amplifier. As the SA14's input increases, the duty cycle of its output decreases. We chose the multiple feedback active-filter configuration because of its ability to reverse the SA14's polarity. You can verify if your feedback loop has the correct polarity by using a method such as the one outlined below.

Start out with the SA14's +PWM input and arbitrarily assume that it's increasing. Because it's an inverted PWM amplifier, the SA14's output will decrease. Op amp X3's output will then rise, since the SA14's output drives the negative input of X3. Op amp X2's output will drop again, because X3's output drives its negative input. Since X2's output drives SA14's +PWM input, the former decreases while the latter was arbitrarily assumed increasing. Both go in opposite directions, which is negative feedback. If they were to go in the same direction, that would be positive feedback and the circuit wouldn't work.

The integrator block completes the voltage feedback loop when the filtered output of the SA14 is compared with an external voltage, EIN. In this example, the transfer function is given by:

Eload/EIN = 20 V/V
where EIN ranges from 0 to 8 V.

Each and every functional block described above is necessary for the functionality of the ac-ac power supply. The following protection components are highly recommended to guard the SA14 from accidental blowout. It's cheap insurance.

Diodes D6 and D9 are fast-recovery diodes used to keep the SA14 from inductive kickbacks. A UF1003 diode from Vishay Lite-On, Westlake Village, Calif., was chosen because of its 50-ns reverse recovery time and 200-V reverse-diode breakdown voltage. The circuit calls for diodes with at least 163-V breakdown voltage and speeds of 200 ns or faster.

Diodes D3 and D7 are Zener diodes, which prevent overvoltage at various inputs. D3 stops the +PWM input from going above 8.7 V and below -0.65 V. Diode D7 keeps the VCC input from rising above 16 V and dropping below 0.65 V.

Component D8 is a transzorb (transient absorber), which prevents over-voltage at the +VS terminal. It also absorbs energy from high-voltage spikes. The 1.5KE180AMSCT transzorb from Microsemi, Santa Ana, Calif., was chosen because of its 180 V rating. It had to be above the 163 V needed to operate the SA14, and equal to or below the SA14's rated voltage of 200 V.

As power-supply bypass capacitors, C7 and C8 must be located as close to the VCC and +VS pins as possible. In no case should these capacitors be more than 2 in. away from their respective pins. Use low ESR capacitors, such as ceramic.

Components D2, R4, R5, and R6 prevent the SA14 from entering a tri-state condition when powering on. R4 and R5 set the SA14's +PWM input at the midrange of 5 V, which puts the SA14's output at a 50% duty cycle immediately upon powering on. D2 keeps op amp X2 from sinking current, which should never happen under normal operation. R6 protects D3 and the +PWM input from overcurrent.

Finally, a 20-A slow-blow fuse should be placed in series with V1, the 115-V ac input power source.

TAGS: Components
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