[Design Application]
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.
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