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Posted on 10 July 2019

High Efficiency, Low-Profile AC-DC Power Supply Design

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There may be more than one ideal solution

By Steve Mappus, Principal Systems Engineer, Fairchild Semiconductor, Power Conversion America, PCIA, Bedford, NH

 

Maximizing efficiency in a low-profile form factor is a non-trivial challenge for even the most experienced power supply designers. Some examples of systems requiring low-profile power supply designs include: flat panel displays, rack mounted computer equipment and telecom and aerospace chassis-mounted assemblies. Equipment in this class can require several hundred watts of power delivered to the load at any given time. For example, a typical 12V, 300W power supply used in a 1U rack mounted application has a maximum height restriction of 1.75 inches (44.45 mm) and would include forced air cooling available from 1 or more fans. But for systems with height restrictions less than 1U, forced air cooling may not be possible, which means the heat dissipated must be managed using costly, lowprofile heat sinks with large surface area. Therefore, designing for the highest efficiency is critical because it has a direct impact on reducing the size and cost of the heat sinks and increasing the overall reliability of the design.

In most cases, AC-DC power supplies operating at these power levels will require some type of active power factor correction (PFC). The necessity for PFC can be driven by one or more criteria including: power level, end application, equipment class and geographical location and is usually guided by specifications such as EN2018-3-2 or IEEE 519. For an AC-DC power supply, a non-isolated, off-line, boost pre-regulator is normally used as the PFCstage where its DC output voltage is seen as the input to a downstream, isolated DC-DC converter. Since two converters appear in series with each other, the overall system efficiency, çSYS, is defined by the product of the individual converter efficiencies.

 

Equation 1

From equation (1) it is apparent that careful consideration must be given toward choosing the best power topologies and control techniques for both converter stages. One system solution that has many interesting high efficiency characteristics is the combination of an interleaved dual boundary conduction mode (BCM) PFC followed by an asymmetrical half-bridge (AHB), isolated DC-DC converter using a current doubler rectifier secondary with self-driven synchronous rectifiers (SR).

For PFC converters in the 300W-1kW range, interleaved boundary conduction mode (BCM) PFC should be considered due its higher efficiency compared to continuous conduction mode (CCM) PFC control at similar power levels. Interleaved BCM PFC is based upon a variable frequency control algorithm where two PFC boost power stages, are synchronized 180 degrees out of phase with respect to each other. The high peak currents normally seen by the EMI filter and PFC output capacitor are thereby reduced due to the effective inductor ripple current cancellation. The output PFC bulk capacitor benefits from ripple current cancellation because the AC RMS current flowing through the equivalent series resistance (ESR) is reduced. Further efficiency benefits are realized since the boost MOSFETs turn off under AC line-dependant zero voltage switching (ZVS) and turn on under zero current switching (ZCS). For a 350W interleaved BCM PFC design, MOSFET heat sinks can be eliminated as can be seen in Figure 1. Conversely, the boost MOSFET used in a CCM PFC design is subjected to frequency dependant switching losses that are proportional to input current and line voltage. By switching the interleaved BCM boost diodes off at zero current, there are no reverse recovery losses. Eliminating reverse recovery losses allows the use of less expensive, fast recovery rectifier diodes and can remove the need for heat sinking in some cases. For a CCM PFC design, reverse recovery losses are unavoidable and often dealt with by applying an RC snubber across the diode, which will lower efficiency or specifying higher performance, silicon carbide diodes, which have higher associated costs.

12V, 300W, Low-Profile, Universal AC-DC Power Supply

For the isolated DC-DC converter design, the half-bridge is a good topology choice since there are two complementary driven, primary side MOSFETs and the maximum drain-to-source voltage is limited to the applied DC input voltage. Two variations of the half-bridge, known as the LLC and asymmetrical half-bridge (AHB), are widely used partly due to the availability of power management control IC’s uniquely dedicated to these topologies. The LLC takes advantage of the parasitic elements associated with the power stage design to achieve ZVS using a variable frequency control technique. However, because the regulated DC output only uses capacitive filtering, this topology is best suited for lower output ripple, higher output voltage applications. As a general guideline for off-line, DC-DC applications the LLC tends to be favored when the output voltage is greater than 12VDC.

The AHB is an efficient choice for a 300W, 12V DC-DC converter. A fixed frequency control method is used, where the primary current naturally lags the transformer primary voltage, providing the necessary condition for ZVS of both primary MOSFETs. Similar to the LLC, the ability to achieve ZVS with the AHB relies upon a thorough understanding of circuit parasitic elements such as transformer leakage inductance, winding capacitance and junction capacitance of discrete power devices. Compared to the variable frequency control method used for LLC control, fixed frequency operation greatly simplifies the task of secondary side, self-driven SR. The self-driven, SR gate drive voltages are easily be derived from the transformer secondary. Adding a low-side MOSFET driver, such as the Dual 4A, FAN3224 shown in Figure 2, provides accurate level shifting and high peak drive current through the MOSFSTs Miller plateau region to assure fast, efficient SR switching transitions.

FAN3224, Self-Drive SR with Current Doubler Rectifier

The current doubler rectifier can be applied to any double-ended power topology and for high DC current applications, it has several noteworthy attributes. First, the secondary consists of a single winding, simplifying the transformer structure. Second, since the required output inductance is divided between two inductors, the power dissipated due to the high current flowing in the secondary is distributed more efficiently. Third, the individual inductor ripple currents cancel each other as a function of duty cycle (D). The cancelled sum of the two inductor currents has an apparent frequency equal to twice the switching frequency allowing higher frequency; lower peak current flowing into the output capacitor. And finally, in a symmetrical converter (push-pull, half-bridge, full-bridge), each current doubler inductor would carry half the output current but for the AHB this is not exactly the case.

If unaccounted for, the asymmetrical voltage applied to the secondary side rectifiers can be one of the AHB drawbacks. When the AHB is operated near its limit of D=0.5, the applied SR voltages are nearly matched. However, it is more reasonable that the transformer turns ratio be designed such that D is within the practical range of 0.25<D<0.35 during nominal operation. When D is within this range, the voltage stress between Q1 and Q2 and the applied voltage across L1 and L2 become imbalanced, resulting in an uneven current distribution between L1 and L2. Similarly, the voltage ratings for each SR MOSFET must also be considered. For this reason, it may be acceptable to use inductors L1 and L2 that are not equal in value and SR MOSFETs that have different voltage ratings. The transformer turns ratio can also be wound asymmetrically but these techniques require a detailed understanding of the circuit behavior under all operating conditions.

To demonstrate the feasibly of the recommended solution, the specifications shown in Table 1 were met using an interleaved dual BCM PFC boost, pre-regulator followed by an asymmetrical half-bridge, DC-DC converter with self-driven SR, as pictured in Figure 1.

Low-Profile, AC-DC Power Supply Design Specifications

The specifications shown in Table 1 are a simplified summary of the full design requirements. The primary design goals are to:
• Maximize efficiency over the widest range possible
• Achieve lowest possible design profile
• Minimize size and use of heat sinks

Maximizing efficiency over the widest possible load range requires careful consideration when choosing materials and components for each power stage, particularly in the area of magnetics design. Because the frequency for the interleaved BCM PFC can reach several hundred kHz, and vary by as much as 10:1, the boost inductors need to be custom designed. Using a properly rated, equivalent gauge litz wire gives best results for minimizing AC losses that can dominate copper loss in a BCM PFC boost inductor. A gapped ferrite material suitable for high frequency operation should be used and for this example, N87 material from EPCOS was chosen on a low-profile EFD30 ferrite core set. Measured efficiency results for the PFC are shown in Figure 3.

Interleaved BCM PFC Measured Efficiency

One solution for a 300W, low-profile, AHB transformer requires two horizontal core structures, where the primary windings are connected in series and the secondary windings connected in parallel. The use of two transformers is necessary because the cross sectional area, Ae, of each core is nearly half of the 150mm2 required to avoid saturation. Finding a single, conventional core shape with a 150mm2 cross section would not be possible in a less than 20mm low-profile component. Similar to the BCM PFC inductor design, litz wire and a high frequency ferrite core material are used to maintain high efficiency. A final important design step is controlling the amount of allowable leakage inductance in the AHB transformer. Some value of leakage inductance is required for ZVS and adjusting the timing delay for the self-driven SR. For this design the effective leakage due to both transformers was optimized to 7µH or 1.5% of the total effective magnetizing inductance. Measured efficiency results for the 300W AHB DC-DC converter are shown in Figure 4.

AHB 390V to 12V-25A, DC-DC Measured Efficiency

Full load efficiency is dominated by conduction losses through the converters power stage so there is little a controller can do to help under these conditions. However, there are several controller technologies that should be considered for maintaining higher light load efficiency. The FAN9612, an interleaved dual BCM PFC controller, limits frequency-dependent Coss MOSFET switching losses at light load and near the zero crossing of the AC input voltage by utilizing an internal fixed maximum frequency clamp. During the portion of the AC line voltage for VIN>VOUT/2, Coss capacitive switching losses are reduced through a valley-switching technique used to sense the optimal MOSFET turn-on time. Conversely, when VIN<VOUT/2, the PFC boost MOSFETs always turn-on under ZVS conditions. Light load efficiency improvements are further attained by introducing an automatic phase management feature that reduces operation from dual channel to single channel mode. The light load efficiency advantage from phase management can be seen in Figure 3 for 10%<POUT<20%, where the efficiency “curve” appears more flat. Operating in single channel mode minimizes the impact of switching losses on light load efficiency. The ability of the interleaved PFC to maintain synchronization during phase management is shown in Figure 5. The left-sided plot was recorded when transitioning from single channel to two channel operation as the load is increased from zero to 19% (64W). Similarly the right-sided plot was recorded when transitioning from dual channel to single channel operation while the load is decreased from full load to 12% (42W).

PFC Phase Management

The implementation of the AHB isolated DC-DC converter is achieved using the FSFA2100, AHB controller, which integrates the pulse width modulation (PWM) control, gate drive functionality and internal power MOSFETs into a single 9 pin SIP power package. This advanced level of packaging and integration allows designers to achieve very high efficiency up to 420W, using fewer external components Combining these three critical functions into a single package eliminates the task of programming the dead time required for ZVS and minimizes gate drive parasitic inductance between the internal driver and MOSFETs. Most of the power dissipated within the SIP power package is due to the switching internal MOSFETs, so a low profile extruded heat sink is required, especially for a 300W design with no available forced air cooling.

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

Total Measured System Efficiency

Magnetic component design, power semiconductor selection, pcb layout, choice of heat sinks and controller features all must work perfectly together for a successful low-profile AC-DC power supply design demonstrating high efficiency over a wide load range. Depending upon system requirements, there may be more than one ideal solution best suited for a particular application. The design discussed herein is just one example for achieving high efficiency from a universal AC input to 12V, 300W design requiring PFC and a low profile of only 18mm total height.

 

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