Qsc - Pl90Wp 电路图.pdf

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1、1 The PowerLight 9.0PFC High-Power Innovations in Audio Amplifier Technology Sometimes the magnitude of a task requires tossing aside conventional technologies used for smaller, easier undertakings, and instead developing and adapting new ones to suit. QSCs engineers faced such a situation developin

2、g the Power-Light 9.0PFC, the first high-power professional audio amplifier to feature power factor correction, a very prominent feature among other “firsts” and “mosts.” Responding to the growing demand in the sound reinforce- ment industry for more powerful and reliable amplification in smaller an

3、d lighter packaging, our PowerLight 9.0PFC design team took on no small challenge: to develop an amplifier that delivers at least twice the power of one of the worlds most powerful amplifiersthe PowerLight 4.0without sacrificing the rack size and ratio of watts per pound. And of course, the team cou

4、ldnt compromise audio performance in any way. The design teams job was to develop a new class of amplifier whose power is measured in thousands of watts, rather than hundreds. The result is the most innovative and advanced amplifier on the planet, delivering up to 9000 watts of undistorted power. Th

5、e objective of this paper is to take the reader behind the curtains and explain the basic technology and its benefits to the pro audio user. The amplifier: new devices, new techniques Seeking to double the 4.0s power output in the same size package, we quickly saw that we could not simply scale up t

6、he familiar bipolar transistor designs that have served us well at lower powers. It would simply take too many devices and too much heat sink area to do the job. No, this required a new class of device. The most rapidly advancing class of power devices today is the N-channel power metal oxide semico

7、nductor field-effect transistor (MOSFET), currently used primarily for high-power switching supplies; this application requires devices with very high speed, capable of handling high currents and high volt- ages, and having low on-resistance. Coincidentally, these properties also make them suitable

8、for high-performance audio amplification. The power MOSFET supplies an unmatched combination of very high peak voltage and current capability, together with uniform gain. Among the broad choice of device ratings are large-die transistors that individually can replace several bipolar devices. Further

9、more, the power MOSFET has no “second breakdown” region, which has historically limited the maximum power potential of bipolar devices. The power handling capacity of the MOSFET is limited strictly by the thermal capacity of the device package and the heat sink structure it is attached to, which can

10、 be made as heavy as required. For the PowerLight 9.0PFC, we determined that eight latest-generation, large-die MOSFETs can replace a bank of 56 bipolar devices, with equal or better energy handling ability. Dont confuse the N-channel MOSFET with the lateral type that has been used for years in many

11、 traditional linear audio amplifiers. The lateral devices, although available in both polarities, have inherently lower current and efficiency limits. Although simple moderate-power-level designs utilizing lateral MOSFETs have been successful, their per-device power capacity is no better than tradit

12、ional bipolar devices, and 500110100 1 10 100 Collector current (I ) Drain current (I ) in amperes C D Collector-emitter voltage (V ) Drain-source voltage (V ) CE DS PowersMOSFETafe operating area (SOA) Power bipolar device safe operating area (SOA) SOA comparison of power devices RadioFans.CN 收音机爱

13、好者资料库 2 their efficiency is somewhat lower. The switching-type, N- channel MOSFET employed in the PowerLight 9.0PFC, on the other hand, has far lower on-losses and far greater current capacity. There are two main obstacles to using N-channel MOSFETs effectively: their turn-on thresholds vary widely

14、from one device to another; and they can only be produced effectively in N-channel polarity. The threshold variations would tend to result in uncontrollable idle current and possible crossover distortion, and the single polarity requires a new circuit concept to produce a fully complementary, matche

15、d positive and negative output signal. To solve the problems we developed a fundamentally superior amplifier architecture that we call Full-Bridge Current Cell topology. The current cell concept Current cell topology solves both problems by enclosing each FET within closed-loop active circuitry to c

16、reate a predictable relationship between input signal and output current. This support circuitry converts the real-world device, despite its variables, into an ideal circuit element with zero turn-on threshold and better than 1% linearity. In effect, it creates the ideal device that every audio engi

17、neer has dreamed of. Cell elements are matched to within 1%, making low- distortion arrays easy to configure. Deploying these cells in a conventional half bridge would require dual matching positive and negative power supplies in order to produce a symmetrical output signal. Because of the high powe

18、r requirements, though, the PowerLight 9.0PFC uses four dual current cells on each channel in an internally grounded, full-bridge configuration, which also simplifies the power supply structure, as well discuss later. The full-bridge topology has a balanced, fully symmetrical power flow, and it deli

19、vers positive and negative output voltages from a common power supply reservoir. Furthermore, it balances out certain residual distortion mechanisms from the audio signal. When combined with the four-step Class H rail control scheme described below, the result is a very high power linear output stag

20、e with an efficiency approaching that of a Class D (pulse- width modulation) amplifier. Class B signal conversion Getting good results even with ideal current cells required another innovation: a perfect Class A-to-B converter. Audio from the input stage is in the form of a Class A signal, a continu

21、ous voltage which varies symmetrically between positive and negative limits. For optimum efficiency, we need to split this single continuous signal into perfectly matched positive and negative halves, so the resulting Class B signals can be routed to their respective current cells. In a conventional

22、 amplifier, this polarity split is done with moderate accuracy using complementary positive- and negative- polarity transistors, carefully biased so they “just meet” at zero. The positive transistor handles the positive half of the wave- form and ignores the negative half, and the negative transisto

23、r does the opposite. In most cases, it works acceptably well, although the center matching is somewhat imprecise and thermally variable, requiring carefully designed thermal compensation circuits to maintain a good trade-off between acceptably low crossover distortion (zero-crossing errors) and exce

24、ssive idle current. Also, because the bipolar scheme uses two separate amplifying transistors, the cumulative component tolerances can cause a mismatch between the positive and negative halves. There are also breakdown situations in which both devices might amplify at the same time, leading to a cat

25、astrophic overload of both output polarities. And in a design involving many thousands of watts, the stakes in eliminating these problems are very high. The PowerLight 9.0PFC uses an innovative “current steering” scheme, which first converts the audio signal to a current using conventional conversio

26、n techniques. The current then enters a special cell carefully designed around the exact exponential characteristics of transistor junctions and separates the original current into positive and negative halves. Additional cells then route the signal current to the four quadrants of the full-bridge o

27、utput circuit, where the actual power amplification takes place. Processing a signal as current instead of voltage delivers exceptional high frequency bandwidth. Thus, current steering makes it possible for the PowerLight 9.0PFC to have both very high power and excellent performance over the full au

28、dio band and complete stability at high frequencies. Current steering is inherently linear because the total current remains constant, preventing unpredictable conditions that trig- ger destructive increases in idle current. Even a component failure cannot result in simultaneous positive and negativ

29、e conduction; at worst, only the point at which the positive and negative polarities split would be affected, and even then regular DC fault shutdown protection would guard against such a severe fault. As a result, the massive power of the output stage is smoothly and predictably controlled. Also, t

30、he circuit transfer function is uniformopen- and closed-loop gains remain constant even through the crossover transition. Moreover, the crossover operating point is completely free of thermal transient modulation because it is maintained instantly and electronically; therefore, crossover distortion

31、never creeps in even during the most severe musical dynamics. These are some reasons why the PowerLight 9.0PFC offers superb signal integrity similar to Class A designs, but without the characteristic high losses. RadioFans.CN 收音机爱 好者资料库 3 positive or negative limit. When the signal comes back into

32、scale there is a small delay before the internal circuitry recovers, so the amplifier remains in clipping slightly longer than necessary, followed by an abrupt recovery to linear operation. This is often termed clip sticking. The PowerLight 9.0PFC is immune to this problem because the signal clips o

33、utside of the feedback loop. A precision signal processing circuit tracks the available headroom and drives the amplifier with a signal that will clip just within the maximum possible linear amplitude. Therefore, the output cells themselves never actually clip, the feedback signal and internal circu

34、itry remain within scale, and clip recovery is instant, clean, and perfectly controlled. A further benefit: the amplifier maintains constant damping during clipping and never loses control of the load, no matter what the input signal does. Users say this results in cleaner, tighter subwoofer perform

35、ance. Multi-tier energy reservoir Several PowerLight models use multi-tier power supply rails to improve efficiency. For example, the PowerLight 4.0 has a bipolar, 3-tier design. 4-tier Class H rail switching in the PowerLight 9.0PFC “Smooth” clipping in the PowerLight 9.0PFC “Clip sticking” in a co

36、nventional amplifier The PowerLight 9.0PFC ups this to a four-tier design, which would normally require eight distinct rail voltages (four positive and four negative). However, the full-bridge output stage allows us to do the same thing with a single, much less complex four-tier supply that serves b

37、oth polarities of the output signal. But even multiple-rail designs will operate at less than optimal efficiency if the rail switching is slow or imprecise. In the PowerLight 9.0PFC, a predictive switching scheme guarantees that each supply voltage will be switched on just before the signal needs it

38、, and switched off as soon as possible after the signal falls, in order to squeeze maximum efficiency from the four-tier supply. The full-bridge topology also eliminates a subtle large- signal low frequency loading problem inherent in half-bridge designs. The extended period of a low frequency signa

39、l pulls current for a long time from first one supply polarity and then the other. As a result, the positive and negative supplies dont share the load as equally as with higher frequency signals. The effect is of power supply reservoir that shrinks as the frequency decreases, reducing low frequency

40、power bandwidth. The full bridge uses the same supply reservoir for both output polarities, so it loads the supply equally regardless of frequency to ensure a uniform low-frequency power bandwidth. “Perfect” clipping behavior Another notable feature is the “non-clipping feedback loop.” When ordinary

41、 amplifiers reach their voltage limits, the output devices become fully turned on, the feedback signal no longer tracks the input signal, and all internal circuitry slams to its 4 A 50 kHz Bessel filter and other signal processing circuitry completely prevent slew rate distortion by carefully tailor

42、ing rise time to just a little less than the PowerLight 9.0PFCs excellent 40 v/s slew rate, resulting in the most distortion- free high power high frequency performance available. These new approaches to audio power circuitry ensure that unlike with some other high power amplifiers, the 9.0s sound r

43、emains tight and controlled even if the audio signal tries to exceed its awesome power limits. The amp drives the most reactive loads with ease, and at least for now, the amplifier no longer needs to be a limiting element in system performance. Esoteric design for the real world Much of the value in

44、 QSC amplifiers comes from our time- tested solutions to the many mundane problems that exist in all high-power amplifiers. For instance, as in other QSC designs, the output power devices mount directly to their metal heat sinks to ensure the tightest possible coupling between the power chip and the

45、 cooling structure. Instead of lighter, high-density finned structures, QSC amps use aluminum extrusions for cooling, providing extra thermal mass to absorb musical peaks without short-term temperature rise. With four variable-speed fans providing cooling air flow, the PowerLight 9.0PFC boasts a hig

46、her thermal capacity than any other high-power amplifier. We also use the latest generation of components and production processes: 1% resistors are standard, with 0.1% resistors in critical areas. Surface mount technology (SMT) increases component density of small parts by 300%, with more reliable

47、solder connections. So how does it sound? User response to actual production units has been out- standing. We naturally assumed that such a large and expen- sive amplifier would only be considered for limited applications such as subwoofers, but users have reported that all types of speakers gain ne

48、w life and authority with the added headroom and control of the PowerLight 9.0PFC. The PowerLight 9.0PFC has been described as a “musical” amplifier, an evaluation that greatly pleases us. Much of this success is due to the audio design, but the power supply also plays a key role in the amplifiers p

49、erformance. The power supply story An amplifiers output section is the pump that supplies power to the speakers, but it can only work if its reservoir is full. This is the job of an amplifiers power supplyto maintain a controlled reservoir of energy at the right value to support the output stage. The problem, of course, is that commercial energy is supplied in the form of AC power at a somewhat constant voltage, which must be converted to a steady reservoir of DC that the output section can draw on. Conventional designs have performed this function acceptably but with noticeable limitatio