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AC-DC power supply design analysis

In Electronic Infomation Category: A | on September 07,2011

Even for the most experienced power supply designers, in a small volume to achieve maximum power efficiency is not an easy task. Need to design small devices that have a lot of power in a given period of time, such equipment may need to load hundreds of watts of power. Less than 1U of height restrictions for the system, forced air cooling may be feasible, which means that the use of costly heat sink to achieve a large surface area thin thermal management.

AC / DC power supply is the AC input, output DC power supply module. Within this module which contains the rectifier filter circuit, step-down circuit and FDC6323L datasheet and the voltage regulator circuit. In the AC / DC power conversion applications, requiring a wide input range, usually require: 85V ~ 265V AC input, output requirements of high power conversion efficiency, while effective in improving energy performance, full-load efficiency in AC / DC power supply design is a major consideration. Improve the AC / DC converter efficiency, achieve better energy performance of the method, the green energy initiative.

In most cases, the level of work in these power AC-DC power requires some type of active power factor correction (PFC). The power semiconductors directly to the PCB board and FDC6323L price and then paste it in the end plate, and FDC6323L suppliers and not so insulated and put them on a plate with a bolt in the end. Taking into account the cost of thermal paste materials, the entire assembly costs will decline. This also reduces the power and reduce the size of the device junction temperature of about 10 degrees Celsius, which can mean trouble-free interval is approximately doubled. For AC-DC power, generally a non-isolated off-line to boost pre-converter as a PFC stage, the DC output voltage as the downstream isolated DC-DC converter input. Since the two converters are connected in series with each other, so the overall system efficiency SYS converter efficiency for each product of:

(1)

From (1) is clearly visible, a system with many features and efficient solution is a combination of staggered double critical conduction mode (BCM) PFC and isolated DC-DC converter, which, followed by the asymmetry of the former half Bridge (AHB), which uses a self-driven synchronous rectifier with a current doubler rectifier secondary side.

Figure 1. 12V, 300W, Universal AC-DC power small.

Range for the 300W-1kW PFC converter, it should consider selecting interlaced critical conduction mode (BCM) PFC, because in a similar power level, it is more efficient than continuous conduction mode (CCM) PFC control technology. Interleaved BCM PFC algorithm based on a variable-frequency control, in this algorithm, two PFC boost power stages 180 degrees out of phase with each other simultaneously. As with the effective elimination of the inductor ripple current, EMI filter and PFC output capacitor common to reduce the peak current. Output PFC bulk capacitor ripple current cancellation benefit is that flows through the equivalent series resistance (ESR) of the AC RMS current is reduced. In addition, because the boost MOSFET is dependent on the AC line zero voltage switching (ZVS) off Shimonoseki, in the zero-current switching (ZCS) under the conduction, it can further improve efficiency. For the interleaved BCM PFC 350W design, MOSFET heatsink can be removed, as shown in Figure 1. On the other hand, CCM PFC boost MOSFET used in the design is vulnerable to frequency-dependent effects of switching losses, and switching losses and the input current and line voltage proportional to. By zero-current turn-off interleaved BCM boost diode, reverse recovery loss can be avoided, allowing the use of low-cost fast recovery rectifier diode, and in some cases no heat sink. PFC converter work to the inherent characteristics: output voltage regulation using voltage-mode PWM control, steady-state duty cycle Du is a constant 9 (ie, on-time Ton is a constant), the input current close to sinusoidal. Therefore, the control circuit in the multiplier and without current control, power factor correction can be achieved.

For isolated DC-DC converter design, the half-bridge topology is a good choice because it has two complementary drive on the primary side MOSFET, and the maximum drain-source voltage is limited by the applied DC input voltage. LLC by variable frequency control technology, designed to use power level associated with the parasitic elements to achieve ZVS. However, due to a regulated DC output using only capacitor filter, this topology is the most suitable low output ripple, high output voltage applications.

AHB is mainly used for high-performance modules (such as CPU, DMA and DSP, etc.) the connection between, as the SoC on-chip system bus, which includes the following features: a single clock edge operation; non-tristate implementation methods; support Burst; support sub-transmission; supports multiple host controllers; configurable 32-bit 128-bit bus width; support byte, nibble, and word transfer. AHB system from the main module, from the module and infrastructure AHBInfrastructure) 3 parts, the AHB bus by the main module on the transmission issue, the response from the module is responsible. Infrastructure by the arbiter, the master module to the multiplexer from the module from the module to the main module of the multiplexer, decoder (decoder), the virtual from the module (dummy Slave), the virtual master module (dummy Master) of the composition.

For 300W, 12V DC-DC converters, AHB is an efficient choice. As the primary current lags behind the transformer primary voltage, so for two primary MOSFET to provide the necessary conditions of ZVS. Similar to the LLC, use the ability to achieve ZVS AHB also depends on a thorough understanding of the circuit parasitic elements, such as transformer leakage inductance, winding capacitance and junction capacitance of discrete devices. LLC control compared to the use of variable frequency control method, a fixed frequency plan can greatly simplify the secondary side of the self-driven synchronous rectification (SR) task. Self-driven SR gate drive voltage easily derived from the transformer secondary side. Adding a low-side MOSFET driver, such as shown in Figure 2 Dual 4A FAN3224 drives, you can accurately give Miller a flat area by MOSFST level conversion and peak drive current.

Figure 2. FAN3224, the use of a current doubler rectifier to achieve self-driven synchronous rectification (SR).

This Doubler rectifier can be used for any double-ended power supply topologies and a large DC current applications, it has several outstanding features. First, a simple winding its secondary terminus of composition, structure simplifies the transformer. Second, the output inductor required is allocated in the two inductors on the secondary side due to large current flows arising from the distribution of power more effectively. Thirdly, as the duty cycle (D) function, the two inductor ripple currents cancel each other. Offset of the two inductor currents, and has twice the switching frequency as the frequency (apparent frequency), thus allowing a higher frequency, in addition to the peak output inductor current flows lower.

Added to the secondary-side rectifier voltage on the asymmetry may be one of the shortcomings of AHB. When the AHB in the limit D = 0.5 in the vicinity of its work, the SR load voltage is almost up to match. However, a more reasonable solution is that by design the transformer turns ratio, so that D keep working at rated 0.25

Regulator followed by a self-driven SRs with asymmetrical half-bridge DC-DC converter, shown in Figure 1.

Table 1 small AC-DC power supply design specifications.

The specifications in Table 1 is a simple summary of all the design requirements. Main design objectives are as follows:

1. In the widest possible range for maximum efficiency.

2. Designed to achieve the smallest possible size.

3. Heatsink to minimize the use and size.

The widest possible range of loads for maximum efficiency on a power level required for each material and component selection to be carefully considered, especially in the magnetic design. As the frequency interleaved BCM PFC may be up to hundreds of kHz, and the changes up to 10:1, the boost inductor required custom design. Appropriate level equivalent stranded wire can minimize the AC losses, while the AC loss is the BCM PFC boost inductor copper loss in the main part. Work should be used for high-frequency open-gapped ferrite materials, the efficiency of the PFC was shown in Figure 3.

Figure 3. Interleaved BCM PFC measured efficiency (100% = 330W).

Small AHB transformer for 300W, one solution is to use two levels of core structure: the primary-side winding in series on the secondary side winding connected in parallel. Less than 20mm in a small design element on the 150mm2 cross-sectional area is the traditional shape of the core impossible. The last important design step is to AHB transformer leakage inductance volume control within the allowable range. For ZVS, the leakage inductance needs some specific value, for self-driven SR, need to adjust the timing delay. In this design, the effective leakage generated by the transformer is optimized for the 7H, which is generally effective magnetic inductance of 1.5%. 300W AHB DC-DC converter efficiency measured results shown in Figure 4.

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

Full load efficiency mainly by the power converter to determine the level of the conduction loss, therefore, under these conditions, almost no one controller helpful. However, to maintain high efficiency at light loads, do have several controller technology for consideration. FAN9612 is a staggered dual BCM PFC controller, the use of an internal fixed clamp to limit the maximum frequency under light load and AC input voltage zero crossing in the vicinity of the frequency-dependent Coss MOSFET switching losses. Part of the AC line voltage VIN> VOUT / 2 periods, the use of valley-switching technology to sense the best MOSFET on-time, further reduce Coss capacitive switching losses. On the other hand, when VIN

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

AHB isolated DC-DC converter controller FSFA2100 AHB can be used to achieve the program to achieve. This advanced level of integration allows designers to use fewer external components, you can get up to 420W of high efficiency. These three key features into a single package, you can avoid the dead time required for ZVS programming tasks, and the internal drive to the MOSFET gate driver between to minimize parasitic inductance. SIP power package in the most power from the internal MOSFET switch, requiring a small extruded heat sink, especially for the 300W with no forced air cooling design.

Total AC-DC system includes input EMI filter, bridge rectifier, interleaved BCM PFC and the AHB DC-DC, which received the overall efficiency shown in Figure 6. At Vin = 120VAC, the design peak efficiency 91%; Vin = 230VAC when 92%; Vin = 120VAC or 230VAC, as well as POUT> 38% (114W), the greater than 90%.

Figure 6 Measured overall system efficiency (includes EMI filter).

Magnetics design, power semiconductor selection, PCB layout, choice and control characteristics of the radiator, all of which must be fully work together to successfully implement a wide load range for high efficiency on the small AC-DC power supply design . For a specific application, according to the specific requirements of the system, there may be a more ideal solution.

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