What are the advantages of a digitally programmable AC power supply?

Programmable AC power supplies have been widely used to implement and support many product testing procedures over the years. They give test engineers complete control over the voltage, frequency and current of the unit under test (UUT). These products are used to simulate various power supply conditions and abnormal conditions that may occur in actual use of AC and DC powered products. They are also critical for providing the necessary 400 Hz or 800 Hz frequency AC power for military and avionics power subsystems.

Programmable AC power supplies offer design and/or test engineers the following features and benefits:

1. Safe isolation between AC grid input and UUT output;

2. 'Convert any grid voltage and frequency around the world to the specific desired output frequency of the UUT;

3. Precise control of output voltage, including line and load regulation;

4. Stable and controllable output power, not affected by any AC line input fluctuation or instantaneous voltage drop;

5. Phase conversion from single-phase to three-phase, single-phase to split-phase or three-phase to single-phase.

Traditional AC power topology and design

The vast majority of available AC power designs are based on pulse width modulation control circuits and use low frequency transformers to provide isolation between the input and output of the AC power source.

These PWM designs typically use analog control circuits to provide output regulation, current limit functions, and frequency conversion functions. While this is a proven design dating back to the early 1980s, it is fraught with drawbacks, to list a few:

1. Using a power frequency AC input transformer to provide galvanic isolation can significantly increase the size and weight of the product, especially as power levels increase. For example, considering transformer losses, a 15kVA AC power supply requires a 17kVA three-phase input transformer and weighs about 63.5kg, and the total weight of a power supply using this design can exceed 181.4kg.

2. The use of an output transformer to provide galvanic isolation results in a similar increase in size and weight and prevents the generation of DC output capability. Additionally, such output transformers must support the wide frequency range associated with programmable AC power supplies, typically from 45 Hz to 1000 Hz or higher, requiring more complex and expensive transformer designs.

3. The analog control loop is affected by discrete component changes and temperature drift. This negatively affects accuracy and stability and requires adjustment for calibration.

A simplified block diagram of this topology is shown in Figure 1. It shows a single-phase input, single-phase output source, but the three-phase version is similar in design.

What are the advantages of Gisley's all-digital programmable AC power supply?

Some manufacturers enhance this approach with some digital signal processor (DSP) controls, but these are usually part of the amplifier's external control loop and are therefore slower, or relegated to user interface and front panel controls. Variations of this topology can be found in designs such as a direct coupled output stage with dual inverters in series/parallel mode to eliminate the output transformer, which eliminates the need for an output transformer, but at a higher cost.

digital power conversion

The higher PWM switching speeds required to support a wide output frequency range (typically above 30kHz) of the AC mains make it difficult to use DSP to provide all the control functions. With recent advances in DSP technology in terms of performance and cost, all-digital implementations of AC power designs supporting these switching frequencies are now feasible.

Full digital control advantage

The use of fully digital control of key power conversion stages eliminates a large number of tolerances and variations associated with analog components and control circuitry. Additionally, the ability to monitor and control switching events in real-time on a cycle-by-cycle basis at the PWM switching frequency provides better protection against possible load or line induced anomalies. This level of protection is no longer affected by analog filtering and averaging, which can introduce delays in traditional power supply designs.

These improvements allow the following topology changes:

Replacing the AC input transformer for isolation with an AC/DC conversion stage with active power factor correction and then a VHF DC/DC conversion stage with galvanic isolation can significantly reduce the size and weight of the input stage components.

Replace any output transformer or series/parallel output inverter stage with a single high output voltage inverter that supports both low voltage/high current requirements as well as high voltage/low current requirements.