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Issue 3 2016 Power Electronics Europe www.power-mag.com

Improving Efficiency andPower Factor at Light LoadUniversal input AC/DC power supplies in the range of 80-800W can be found in a variety of applications.Within this power range, the single-stage boost PFC + half-bridge LLC is considered to be a very populartopology, mostly due to the relatively simple structure, mature IC solutions, general good performance, andcost. Multiple global regulatory agencies have proposed standards on efficiency and power factor (PF) forsuch power supplies. Agencies like DoE, CoC, and Energy Star typically cover a wide range of applications,and others such as 80PLUS are for specific applications, like PC power. Zhihong Yu, AC/DC & LightingProduct Marketing Manager, Monolithic Power Systems, Dallas, USA

For some of the standards, traditionally,only the efficiency and PF at full load werelisted, but the efficiency and PF at zero tolow load have become more important inrecent years for nearly all standards,whether it be for residential use orcommercial/industrial use. Some standardsdirectly point to low-load performance, andothers may specify the averagerequirement across all load ranges. Suchstandards force most original equipmentmanufacturers (OEM) to improve their endproducts1. It is very desirable for PC power

vendors to meet Gold, Platinum, or evenTitanium specs of over 90 % efficiency at50 % load without much additional cost.

As the new standards continue pushingfor higher performance limits, OEMsusually take different approaches, such asspending more to get better discrete FETsand diodes, or researching new topologies.However, in the PFC field, a recent marketresearch report that covers moresophisticated bridgeless or interleaved PFCsuggests that these topologies will alwayshave a small market share, even beyond

2019. This is mostly because they requiremore switches/magnetic, which adds tocomplexity and cost, among otherconcerns.

The most common PFC topology in themarket is by far the traditional, hard-switched, single-phase boost converter thatruns at either discontinuous conductionmode (DCM), critical conduction mode(CrM), or continuous conduction mode(CCM). These have all been well-studiedfor decades at most power IC and powersupply companies. However, based ontraditional topologies and control modes,they can still meet even the mostdemanding energy standards withoutpaying a higher price.

How to optimize efficiency at lightload for single-phase PFCsFor DCM, CrM, and CCM, the difference isif the inductor current reaches zero duringone switching cycle (see Figure 1).

For each operation mode, there arecertain pros and cons. Designers usuallychoose either CrM or CCM at full load. CrMusually applies constant on-time control,and the operating frequency changes tomaintain boundary mode operation.Although it has the benefit of softswitching, the high peak current leads to alarge magnetic design; therefore, thistopology is mostly used at <150 W. CCM

ABOVE: Figure 1. Various traditional Boost PFC operation modes (Source: Onsemi Power FactorCorrection Handbook)

Figure 2. TraditionalCCM PFC turn-ontime and frequencychange over one ACcycle

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has the benefits of fixed frequency and thelowest RMS current and ripple, whichensures the lowest amount of stress onthe power components. The continuouscurrent nature also makes filtering designeasier; therefore, this topology is preferredin >150 W applications.

Figure 2 shows the on time profile andswitching frequency profile within a halfline cycle of a CCM control scheme. The

PWM modulator maintains a constantswitching frequency regardless of load, sothe turn on time of the converter isdecreased at light load. One drawback ofCCM control, however, is that the high,fixed switching frequency at light loadleads to poor light-load efficiency. Onlywhen the load drops to minimum or noload is CCM control able to enter burstmode to save on efficiency. In general,

efficiency under CCM can be desirable at30-100 % or <1 % load, but notoptimized from 1-30 % load.

For the new control strategy we areproposing in Figure 3, the turn-on timeonly varies with the instant input voltage,but it remains the same at different load.With this strategy, the switching frequencyof the PFC converter is kept constant atheavy loads to keep the benefit of CCM,but reduces at lighter loads to enter DCMoperation (see Figure 3, Figure 4).

This control strategy was first realizedwith MPS’ new PFC + LLC combocontroller, the HR1200. This is also appliedin MPS’ stand-alone PFC controller, theMP44040. The efficiency of the HR1200PFC stage was measured on a universalinput, 12V, 20A output evaluation boardacross the entire load range.

Single IC solution to achieve the bestsingle-phase PFC performance In a PFC boost converter, there inevitably isa small capacitor beyond the input rectifierstage, which is used to supply the high-frequency portion of the inductor currentvia the shortest path and act as an EMIfilter. This capacitor must be chosen at theminimum rated input voltage. For CCM

Figure 3. New PFCcontrol strategy –fixed turn-on timeand variablefrequency control

Figure 4. Inductorcurrent waveforms ofadaptive on-time PFCat different loadconditions

Figure 5. Currentdistortions due toinput capacitor forboost converter

Table 1. Key Features and Benefits of MP44040 and HR1200

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Issue 3 2016 Power Electronics Europe www.power-mag.com

PFCs, only the inductor current (iL) issensed and controlled to be in phase withthe input voltage (VIN). However, as theinput current iin= iL+icz, where icz is the inputcapacitor current, there is inevitably somecurrent distortion created by the inputcapacitor (see Figure 5). At a higher VIN, ICZ

becomes larger. IL is smaller at high linecompared to low line, so under thecondition of the same load, PF is worse athigh line.

As most current flows through theinductor at mid-to-peak load, the inputcapacitor’s current distortion is also

proportionally smaller, so the PF can be >0.9with a universal input. However, since IC isnot load-dependent at light load, it becomesrelatively high and PF becomes worse. Suchbehavior is also commonly known in otherPFC ICs. The actual PF performance maydiffer from this figure for the actual design,but the trend is always valid.

Naturally, in order to improve PF at lowload and high line, we must provide ameans to compensate for the capacitorcurrent to the inductor current reference.Instead of the traditional way of matchingIL in the same sinusoidal shape as VIN, we

will match IIN with V . Since the analogcurrent sensing on the capacitor currentrequires more components and adds tocomplexity, we can introduce the digitalcompensation method without addingextra cost.

MPS has created a digital, stand-alone,CCM/DCM, mix-mode PFC controller(MP44040) and a digital PFC + analog LLCcombo controller (HR1200) that both applythe CCM/DCM mix mode and PFcompensation technologies and use a graphicuser interface (GUI) to configure all majorPFC functions. The features and benefits ofthese parts are summarized in Table 1.

An evaluation board (Figure 6) andvarious supporting documents are offeredto help customers become familiar withthis digital platform. The HR1200-basedPFC + LLC EVB is rated at 85-265 VAC atinput, and 12 V/20 A at output. The EVB isequipped with an I?C-to-USB adaptor,allowing customers to optimize theirdesigns by changing all GUI settings andmonitoring live performance differences.For mass production, the configuration canbe programmed at the factory with aspecial part number.

Figure 6. HR1200 PFC + LLC 240W evaluationboard

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