Utilizing digital technology developed by semiconductor pioneer iWatt Inc., a new type of power conversion controller not only achieves these goals but also provides unprecedented flexibility and programmability—capabilities unattainable with traditional PWM and other analog methods. These controllers significantly improve the overall cost and performance of AC/DC and DC/DC power conversion solutions, including isolated and direct-coupled converters.
Tests show that for a 60W single-stage, single-switch AC/DC converter with active power factor correction (PFC) circuitry, the new digital controller achieves efficiencies of 80% to 89% under various load conditions across the entire power frequency voltage range of 85–265V, as shown in Figure 1. This digital controller offers advantages such as eliminating the need for optocouplers; using only primary feedback for control without feedback circuit compensation; and adaptability to power supplies with any topology.
Figure 1 Power Supply Efficiency
Power supplies using PWM controllers typically experience a significant decrease in efficiency and stability under light loads. Pulse Train technology makes power supplies cheaper and lighter by eliminating the need for expensive optocouplers and related components. Unlike analog products, this digital solution also provides built-in active PFC characteristics for AC/DC applications below 250W.
Pulse Train technology features a pair of pulse generators (one power pulse generator and one detection pulse generator). In this scheme, the power pulse is used for energy transfer from the transformer to the load, while the detection pulse is used to monitor the load voltage. An optimization program is used to set the pulse on and off times. A pulse ratio controller (PRC) utilizes primary feedback to achieve voltage regulation by pulse-by-pulse gating of power pulses of fixed width and fixed period. (See Figure 2.)
Figure 2: Schematic diagram of pulse ratio control (PRC)
iWatt Chief Technology Officer Mark Telefus explains that in traditional analog PWM or pulse frequency modulation (PFM) converters, the size of each power pulse is determined based on making the output voltage exactly match the target.
In contrast, the pulse train converter operates more like a digital "switching" servo system. Each cycle, the pulse train samples a binary error signal to determine whether to issue a power pulse to shift the output voltage towards the target. Therefore, the output voltage is confined to a narrow range near the target voltage. For various practical applications, it has less ripple than traditional analog regulators.
Essentially, the digital pulse train controls the output voltage by issuing or blocking power pulses. For example, if the output voltage is below a set level, the pulse generator continuously issues power pulses until the desired level is reached. Similarly, if the output voltage is above the desired level, it issues a detection pulse instead of a power pulse. The turn-on time of the detection pulse is much shorter than that of the power pulse, and it transfers far less energy, as shown in Figure 3.
Figure 3 Power Pulse Controlled Output Voltage
Interestingly, this technique can also separate the pulse shape from the regulation process. This allows us to independently set the turn-on and turn-off times for various system optimizations. The turn-off time can be set to ensure that the switch in the flyback converter only turns on when the secondary current drops to zero. On the other hand, the turn-on time can be set to ensure that the peak primary current remains constant. In short, these optimizations result in a more efficient response from the power converter across the entire power frequency voltage and load range.
Unlike other PFC solutions that use complex circuitry and common PWM controllers, the iW2202's PFC and regulation are both provided by primary feedback in a single-stage, single-switch topology. In the same application, other PFC solutions would result in a voltage exceeding 1000V across a large capacitor, while the iW2202 solution does not exceed 400V. This not only reduces component stress and improves reliability but also enables a higher-performance system at a lower cost.
Measurements show that with a 120V AC input, the single-stage active PFC converter using the iW2202 achieves a power factor (PF) of 0.98 and provides a 19.0V DC output with an output current of 3.0A. The combination of high efficiency and high power factor enables the design to meet the harmonic distortion specifications of EN61000-3-2 and the no-load power consumption standards specified by BlueAngel.
Across the entire range of power frequency voltage and load variations, this design not only achieves an output regulation of less than ±1%, but also exhibits a dynamic response nearly 5 times faster than conventional circuits. The iW2202 is suitable for general power frequency voltages of AC 85–270V, 50–60Hz.
