Resonant converters for high efficiency

Since individuals typically earn their incomes from working for companies whose requirements are constantly changing, it is quite possible that at any given time not all members of a country's potential work force will be able to find an employer that needs their labor. This would be less problematic in an economy in which such individuals had unlimited access to resources such as land in order to provide for themselves, but when the ownership of the bulk of its productive capacity resides in relatively few hands, most individuals will be dependent on employment for their economic well-being. It is typical for true capitalist economies to have rates of unemployment that fluctuate between 3% and 15%. Some economists have used the term "natural rate of unemployment" to describe this phenomenon.

Depressed or stagnant economies have been known to reach unemployment rates as high as 30%, while events such as military mobilization (a good example is that of World War II) have resulted in just 1-2% unemployment, a level that is often termed "full employment". Typical unemployment rates in Western economies range between 5% and 10%. Some economists consider that a certain level of unemployment is necessary for the proper functioning of capitalist economies. Equally, some politicians have claimed that the "natural rate of unemployment" highlights the inefficiency of a capitalist economy, since not all its resources – in this case human labor – are being allocated efficiently.

Some libertarian economists argue that higher unemployment rates are in part the result of minimum wage laws, as well as in part the result of misguided monetary policy, and are not inevitable in a capitalist economy. They also claim that if the value of the productive capacity of a given employee is worth less to the employer than the minimum wage, that person will become unemployed, and therefore unemployment will exist whenever the legal minimum wage exceeds the true economic value of the least productive members of the labor pool. Likewise, if the amount of money a person can obtain on welfare approaches or equals what they could make by working, that person's incentive to work will be reduced.

Some unemployment is voluntary, such as when a potential job is turned down because the unemployed person is seeking a better job, is voluntarily living on savings, or has a non-wage-earning role, such as in the case of a traditional homemaker. Some measures of employment disregard these categories of unemployment, counting only people who are actively seeking work and have been unable to find any.

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A meeting of the Council on regional policy and work with maslikhats in Majilis faction of Nur Otan was held in the Central Office of the party. The topic of the meeting was employment in the light of "Kazakhstan-2050 Strategy."

The meeting chaired by Majilis deputy Gulnar Seytmaganbetova took up the key aspects of the "Employment-2020" program that involves more than 200 thousand people. As a result the unemployment rate went down from 5.4 to 5.2% compared to 2011-2012. Many other positive aspects that allowed to support people, to give them a job, start a business, and to provide housing were named.

Unfortunately, there are still a number of unsolved problems in the area. However, work in this direction must keep going. MPs and secretaries of oblast maslikhats, including representative bodies of Astana and Almaty gathered there to resolve some complaints about bureaucracy and formalism.

In general, all directions of the "Employment-2020" were covered at the meeting. Vice-Minister of Labor and Social Protection Yergali Egemberdy detailed on their implementation.

The first direction - training and employment – covered more than 60 thousand people. 22 thousand of them got jobs. The least number of the employed is in Kyzylorda, Mangistau and Atyrau regions.

The second direction includes business development. More than eight thousand people have received micro-loans, that allowed to create over 12 thousand jobs. This year more than KZT 16 billion will be allocated for micro-crediting.

The third direction of the program is growth of labor mobility. 764 houses were constructed and 8,115 people were resettled. About 4 thousand of them are able to work, but 2,291 only are employed.

The fourth direction - rural areas’ development – is employment through an overhaul and current repairs of social facilities. A little more than 7 thousand Kazakhstanis got jobs. This year over KZT 24 billion is allocated for this direction.

In the end, the Council made recommendations on more efficient implementation of the employment program to the Government, local authorities and maslikhats.

Vladimir Osipo

Resonant converters for high efficiency

Several DC-DC power conversion topologies have been introduced to reduce switching losses, device stresses on the power MOSFETs and the radio frequency interference (RFI) while achieving high power density. Among them, resonant converters, which utilize body diode of MOSFETs for zero voltage switching, are proven to be very effective for higher efficiency. Especially, LLC resonant converter can achieve high efficiency at high input voltage and low voltage stress on secondary rectifier since there is no inductor on secondary. In addition, LLC resonant converter can guarantee zero voltage switching (ZVS) operation at even no load. The ZVS technique dramatically reduces the switching losses and significantly improves efficiency. The zero voltage switching also reduces the switching noise noticeably, which allows a small sized electromagnetic interference filter. Because of these unique characteristics, LLC resonant converters are becoming a popular topology for many applications including LED street lighting. FAN7621S offers everything necessary to build a reliable and robust LLC resonant converter. It simplifies designs and improves productivity by including a high-side gate drive circuit, an accurate current controlled oscillator, frequency limit circuit, soft start, and built-in protection functions.

The various protection functions it provides are over voltage and over current protections (OVP/OCP), abnormal over current protection (AOCP), and internal thermal shutdown (TSD). All protections are auto-restart because of the particular application requirement of LED street lighting. The high-side gate drive circuit has a common mode noise cancellation capability that guarantees stable operation with excellent noise immunity. Even with the latest resonant controller, converter operating point moves into zero current switching (ZCS) region under shorted output condition. Fig. 1 shows how an operating point moves. During this condition, ZVS is lost and the MOSFET conducts extremely high current. The most severe drawback of ZCS operation is hard switching at turn-on that leads to the MOSFET body diode reverse recovery stress.

The body diode turns off at a very large dv/dt and therefore, at a very large di/dt, generates a high reverse-recovery current spike. These spikes can be over ten times higher than the magnitude of the steady-state current. This high current causes considerable increase in losses and heats up the MOSFET. Then, dv/dt capability of MOSFET is degraded due to temperature rise in junction. In extreme cases, it may destroy the MOSFET and cause system failure.

Figure 1. Operating Points of LLC Resonant Converter according to Load Condition.
The latest MOSFET technology

The body diode of MOSFETs typically has very long reverse recovery time and large reverse recovery charge. In spite of its poor performance, the body diode has been utilized as freewheeling diode because it can make simple circuit without adding system cost in some applications such as resonant converters. As more and more applications use an intrinsic body diode as the critical component in the system, Fairchild designed a new highly optimized power MOSFET for resonant converters with deep analysis of MOSFET failure mechanisms. It has improved body diode ruggedness and less stored energy in output capacitance. As shown in Table 1, the reverse recovery charge (Qrr) of new UniFET™ II MOSFET family is dramatically reduced by 50% and 88% compared to alternate solutions.

Table 1. Critical Specification Comparison of DUT.

MOSFET's capacitance is nonlinear and depends on the drain-source voltage since its capacitance is essentially a junction capacitance. In soft switching applications, MOSFET output capacitance can be used as a resonant component. When the MOSFET is turned on, the current that extracted from the magnetizing energy stored in transformer flows to discharge the MOSFET output capacitance in order to allow ZVS condition. Therefore, if stored energy in output capacitance of MOSFET is small, less resonant energy required to achieve soft switching without increasing the circulating energy. The UniFET™ II MOSFET family has approximately 35% reduced stored energy in output capacitance than same on-resistance competitor's devices at for typical switching power supply bulk capacitor voltage. The benchmark of stored energy in output capacitance is shown in Fig. 2.

Figure 2. Stored energy in output capacitance

Date: 2014-12-22; view: 1363