Analysis of AC-DC Power Supply Design

Maximizing power efficiency in a small footprint is a challenge even for the most experienced power designers. There are many devices that require a small power supply design and may need to deliver hundreds of watts of power to the load at any given time. For systems with height restrictions less than 1U, forced air cooling may not be feasible, which means that costly, low-profile heat sinks with large surface areas must be used to manage heat.

AC/DC power supply is a power module with AC input and DC output. The module contains rectifier filter circuit, step-down circuit and voltage regulator circuit. In AC/DC power conversion applications, a wide input range is required, usually requiring: 85V~265V AC input, high output power conversion efficiency, and effective improvement of energy-saving performance. Full load efficiency is a major consideration in AC/DC power design. Improving the efficiency of AC/DC converters and achieving better energy-saving performance is the advocacy of green energy.

In most cases, AC-DC power supplies operating at these power levels require some type of active power factor correction (PFC). Instead of insulating and bolting the power semiconductors to the chassis, solder them directly to the PCB and then attach them to the chassis. Taking into account the cost of thermal paste materials, the overall assembly cost will be reduced. This also reduces the size of the power supply and reduces the temperature of the device connection by about 10 degrees Celsius, which can approximately double the mean time between failures. For AC-DC power supplies, a non-isolated offline boost pre-converter is generally used as the PFC stage, and its DC output voltage is used as the input of the downstream isolated DC-DC converter. Since these two converters are connected in series with each other, the overall system efficiency ηSYS is the product of the efficiency of each converter:

(1)

It is clear from equation (1) that a system solution with many high-efficiency features is to combine an interleaved dual critical conduction mode (BCM) PFC followed by an asymmetric half-bridge (AHB) and an isolated DC-DC converter, where the former uses a current doubler rectifier secondary with a self-driven synchronous rectifier.

Figure 1. A small, universal 12V, 300W AC-DC power supply.

For PFC converters in the 300W-1kW range, interleaved Boundary Conduction Mode (BCM) PFC should be considered as an option as it offers higher efficiency than continuous conduction mode (CCM) PFC control techniques at similar power levels. Interleaved BCM PFC is based on a variable frequency control algorithm where two PFC boost power stages are synchronized 180 degrees out of phase with each other. High peak currents typically seen in EMI filters and PFC output capacitors are reduced due to effective inductor ripple current cancellation. The output PFC bulk capacitor benefits from ripple current cancellation as the AC RMS current flowing through the equivalent series resistance (ESR) is reduced. In addition, efficiency is further improved as the boost MOSFET is turned off under AC line dependent zero voltage switching (ZVS) and turned on under zero current switching (ZCS). For a 350W interleaved BCM PFC design, the MOSFET heat sink can be eliminated as shown in Figure 1. On the other hand, the boost MOSFET used in a CCM PFC design is susceptible to frequency dependent switching losses which are proportional to the input current and line voltage. By turning off the interleaved BCM boost diodes at zero current, reverse recovery losses are avoided, allowing the use of low-cost fast recovery rectifier diodes and, in some cases, eliminating the need for a heat sink. The inherent characteristics of the PFC converter are that the steady-state duty cycle Du is constant (i.e., the on-time Ton is constant) when the output voltage is regulated using voltage-type PWM control, and the input current is close to a sine wave. Therefore, power factor correction can be achieved without the need for a multiplier and current control in the control circuit.

For isolated DC-DC converter design, half-bridge is a good topology choice because it has two complementary driven primary-side MOSFETs and the maximum drain-source voltage is limited by the applied DC input voltage. LLC achieves ZVS through variable frequency control techniques that exploit parasitic elements associated with power level design. However, since the regulated DC output uses only capacitor filtering, this topology is best suited for applications with low output ripple and high output voltage.

AHB is mainly used for the connection between high-performance modules (such as CPU, DMA and DSP, etc.). As the on-chip system bus of SoC, it includes the following features: single clock edge operation; non-three-state implementation; support for burst transmission; support for segmented transmission; support for multiple master controllers; configurable 32-bit to 128-bit bus width; support for byte, half-byte and word transmission. The AHB system consists of three parts: the master module, the slave module and the infrastructure (AHBInfrastructure). The transmission on the entire AHB bus is issued by the master module and responded by the slave module. The infrastructure consists of an arbitrator, a multiplexer from the master module to the slave module, a multiplexer from the slave module to the master module, a decoder, a dummy slave module, and a dummy master module.

AHB is an efficient choice for a 300W, 12V DC-DC converter. Since the primary current lags the primary voltage of the transformer, it provides the necessary conditions for ZVS of the two primary MOSFETs. Similar to LLC, the ability to achieve ZVS with AHB also depends on a thorough understanding of circuit parasitic elements, such as transformer leakage inductance, inter-turn capacitance, and junction capacitance of discrete devices. Compared to the variable frequency control method used in LLC control, the fixed frequency scheme can greatly simplify the task of self-driven synchronous rectification (SR) on the secondary side. The gate drive voltage of the self-driven SR is easily derived from the secondary side of the transformer. Adding a low-side MOSFET driver, such as the dual 4A FAN3224 driver shown in Figure 2, can accurately provide level shifting and high peak drive current through the Miller plateau of the MOSFETS.

Figure 2. FAN3224, using a current-doubler rectifier to implement self-driven synchronous rectification (SR).

This current doubler rectifier can be used in any two-terminal power supply topology and high DC current applications, and it has several outstanding features. First, its secondary side consists of a simple winding, which simplifies the transformer structure. Second, because the required output inductance is divided between the two inductors, the power loss caused by the large current flowing into the secondary side is distributed more efficiently. Third, as a function of duty cycle (D), the two inductor ripple currents cancel each other. The sum of the two canceled inductor currents has an apparent frequency of twice the switching frequency, which allows higher frequencies, and lower peak currents flowing into the output inductor.

The voltage asymmetry applied to the secondary rectifier can be one of the disadvantages of AHB. When AHB is operated near its limit D = 0.5, the loaded SR voltage is almost matched. However, a more reasonable solution is to design the transformer turns ratio so that D is kept at 0.25 during rated operation.

The regulator is followed by an asymmetric half-bridge DC-DC converter with a self-driven SR, as shown in Figure 1.

Table 1. Small AC-DC power supply design specifications.

The specifications in Table 1 are a brief summary of all the design requirements. The main design goals are as follows:

1. Achieve maximum efficiency over the widest possible range.

2. Achieve the smallest possible design size.

3. Minimize the use and size of heat sinks.

Achieving maximum efficiency over the widest possible load range requires careful consideration of material and component selection for each power level, especially in magnetic design. Since the frequency of interleaved BCM PFC can be as high as hundreds of kHz and vary by as much as 10:1, the boost inductor must be custom designed. Using the appropriate grade of equivalent stranded wire can minimize AC losses, which are the main part of copper losses in the BCM PFC boost inductor. Gapped ferrite materials suitable for high-frequency operation should be used, and the resulting PFC efficiency is shown in Figure 3.

Figure 3. Measured efficiency of interleaved BCM PFC (100%=330W).

For the 300W small AHB transformer, one solution is to use two horizontal core structures: the primary winding is connected in series and the secondary winding is connected in parallel. It is impossible to design a core with a traditional cross-sectional area of ​​150mm2 in a small component less than 20mm. The last important design step is to control the leakage inductance in the AHB transformer within the allowable range. For ZVS, certain specific leakage inductance values ​​are required, and for the self-driven SR, the timing delay needs to be adjusted. In this design, the effective leakage due to the transformer is optimized to 7μH, which is 1.5% of the total effective magnetic inductance. The measured efficiency results of the 300W AHB DC-DC converter are shown in Figure 4.

Figure 4. AHB 390V to 12V/25A, DC-DC measured efficiency (100%=300W).

The full load efficiency is dominated by the conduction losses at the converter power level, so there is little benefit from a controller under these conditions. However, to maintain high light load efficiency, there are several controller techniques to consider. The FAN9612 is an interleaved dual BCM PFC controller that utilizes an internal fixed maximum frequency clamp to limit the frequency-dependent Coss MOSFET switching losses at light loads and near the zero crossing of the AC input voltage. During the AC line voltage portion VIN>VOUT/2, a valley switching technique is used to sense the optimal MOSFET on-time, further reducing the Coss capacitive switching losses. On the other hand, when VIN

Figure 5. PFC phase management (1→2, 19%=64W; 2→1, 12%=42W).

The implementation of the AHB isolated DC-DC converter can be realized using the AHB controller FSFA2100. This advanced level of integration allows designers to achieve very high efficiency up to 420W with fewer external components. Integrating these three key functions in a single package avoids the task of programming the dead time required for ZVS and minimizes the gate drive parasitic inductance between the internal driver and the MOSFET. Most of the power dissipation in the SIP power package comes from the switching of the internal MOSFET, so a small extruded heat sink is required, especially for 300W designs without forced air cooling.

The total AC-DC system including input EMI filter, bridge rectifier, interleaved BCM PFC and AHB DC-DC achieves an overall efficiency as shown in Figure 6. The design has a peak efficiency of 91% at Vin=120VAC, 92% at Vin=230VAC, and greater than 90% when Vin=120VAC or 230VAC and POUT>38% (114W).

Figure 6. Measured total system efficiency (EMI filter included).

Magnetic design, power semiconductor selection, PCB layout, heat sink selection, and controller characteristics all must work together perfectly to successfully implement a small AC-DC power supply design that achieves high efficiency over a large load range. For a specific application, there may be more than one ideal solution depending on the specific requirements of the system.