Crown-IT4000-pwr-sm维修电路原理图.pdf

Theory of Operation: I-Tech Power Supplies Eric Baker Last Edited 9/21/04 This work is intended to describe the operational theory behind the power supplies in the I-tech series of Crown amplifiers. Designers include: Eric Baker, Sergio Busquets-Monge, Ben Carroll, David Evans, and Gerald Stanley. Topology: Phase-modulated, full-bridge, series-resonant converter Control strategy: Constant line current for improved power-factor Circuit Level Block Diagram The basic block diagram appears below as Fig. 1. The stages of power processing include the following: 1) EMI filter: Reduces power line conducted noise from the power supply 2) Full-bridge rectifier: Rectifies the AC line providing a DC bulk supply for the full-bridge of switches 3) Impedance matching capacitor bank: Provides charge storage and a high-current low-impedance source 4) Full-bridge switch network: chops the DC bulk voltage in to discrete pulses 5) Series-resonant tank: Works as a constant current source for the main power transformer 6) Step-up transformer: Steps up the voltage from approximately 60V on the primary to the secondary voltage 7) Full-bridge rectifiers: Creates pulsating DC from the high frequency AC coupled through the transformer 8) Secondary energy storage: Serves as an energy storage reserve for the amplifier. Fig. 1. Block diagram of the power supplies EMI FilterSRTT1234, 5, 678EMI FilterSRTT1234, 5, 678RadioFans.CN 收音机爱 好者资料库Phase Shift Modulation The core of the converter is made up of two half-bridges, shown above in 4, which are connected between a positive bulk voltage (Upper Buss) and a return (Lower Buss). The switches, in each half-bridge, alternate in turning on and off, at high frequency (40-41.7kHz), in order to obtain a 50% duty cycle square waveform at each of the respective center points. One center point (A or the leading leg made up of Q21,23,25,27,29,& 31) connects to the series resonant tank (SRT), while the other center point (B or the lagging leg made up of Q33, 35,37,39,41, & 43) connects to the transformer (T). By varying the relative phase of these 50% duty cycle square waves, the effective duty cycle, seen differentially from one center point to the other, can range from 0% to 50% corresponding to phase relationships of 0o and 180o respectively. The diagram below, shown as Fig. 2, illustrates this point. In the first column both half-bridges are in phase, thus the effective differential voltage applied to the SRT and T is zero. In the second, third, and fourth columns the effective duty cycle has been increased to 16.7%, 33.3%, and 50%, respectively. This type of modulation is called phase shift modulation. Fig. 2. Phase shift modulation visualization 0%16.7%33.3%50%VbulkVbulk_rtnVbulkVbulk_rtnVbulk-VbulkABA-B0%16.7%33.3%50%VbulkVbulk_rtnVbulkVbulk_rtnVbulk-VbulkABA-BRadioFans.CN 收音机爱 好者资料库 Phase shift modulation is used, in conjunction with the series resonant tank, to provide a square wave of approximately 60V peak amplitude to the primary of the transformer, (T21). As the line voltage (50-60Hz) varies sinusoidally over each half cycle, the relative phase of the half bridges is also varying in order to try and maintain the voltage on the transformer primary. Another variable, that plays a role in the calculation of the effective duty cycle, is the load the amplifier places on the power supply. The larger the measured voltage drop on the power supply output rails, when compared to a preset reference, the larger the error generated in the control, and thus the duty cycle will also increase. Power Factor Correction In order to best use an AC lines full potential while minimizing the distortion cause by a product connected to it, the ideal load would be a resistive one. With a resistive load, the current is in phase with the line voltage and the harmonic content is defined only by the fundamental. Power factor is defined by the ratio of the real power to the product of the RMS voltage and RMS current consumed by a product. With a resistive load this leads to a value of unity. Most conventional power supplies with a simple transformer/rectifier combination have effective power factors in the 0.6-0.7 range, hence the AC line is called to deliver a larger RMS current than is actually ideally necessary to meet the power demands of a product. Fig. 3 , shown below, shows an ideal power factor and one of nearly constant current, as was the goal for the I-Tech series of power supplies. The main reasons non-sinusoidal power factor was chosen were available space, device utilization, and the desire to see high power factor and regulation achieved in a single stage of power processing. Fig. 3. Unity power factor compared to the improved power factor generated by the I-Tech power supplies VinIinPF = 1PF 0.95VinIinVinIinPF = 1PF 0.95VinIinControl Breakdown Now that some basics are defined, the control can be considered. Shown below, as Fig. 4 is the control block diagram for the power supply system. Definitions for the various inputs and node points are shown below in Table 1. Fig. 4. Control block diagram Table 1. Definitions of various terms and functional blocks in the control diagram Input or node point Description Vo diff. Scaled differential rail voltage feeding the amplifier (U102-B) Reference Known voltage used to compare to an input PI controller Proportional / Integral controller used in the main voltage feedback control loop (U108-D)Limiter Circuit which can vary the absolute limit of the PI controller thus keeping the error generated by the controller within set limits (U108-B, U108-C, U115) Lr Voltage Voltage across a single turn of the resonant inductor (I+_PSC to I-_PSC) Lr Current Scaled current through the resonant inductor found by performing mathematical integration on the voltage across it (TP101) Forward Current Integrator Integrates the inductor current when the converter is conducting in the forward direction in order to find the average AC line current (U101) PWM-to-PSM Pulse width modulation to phase shift modulation converter (U105 & U111) SRT Series resonant tank (C38-45, C79-80, L21) FBR Full bridge rectifier (D69-72) There are two operating modes for the power supply. Mode 1 is used initially every time the power is applied or for various other conditions such as when the line voltage drops too low, or the front panel switch is cycled, or the breaker is cycled, or if for some reason the output rail voltage drops below a preset minimum, Vo diff.ReferencePI controllerReferenceForward Current Reference+-+-Forward Current IntegratorLr VoltageIntegratorPWM-to-PSMSRTFBRVo diff.ABCDELimiterLr Current-+Vo diff.ReferencePI controllerReferenceForward Current Reference+-+-Forward Current IntegratorLr VoltageIntegratorPWM-to-PSMSRTFBRVo diff.ABCDELimiterLr Current-+indicating either a short circuit or amplifier problem. Mode 1 is basically a soft start operation mode used to reduce stress on the AC line when charging up the secondary side capacitors. In this mode the current limit is fixed at a suitably low level, and the control loops are disabled. With the current limit set low, the bulk of the capacitance can be charged at a reasonable rate such that AC line surge is minimized. This mode is disabled once the power supply output rails reach their nominal values. Mode 2 is defined as the normal operation mode for the supply. During this mode calculations are constantly being made to set the upper current limit in order to obtain the same max power output regardless of the AC input voltage. The I-Tech power supplies are universal input as these supplies can be run anywhere from 85Vac to 277Vac obtaining full power between 120Vac and 240Vac. The limiter circuit, as it is shown in Fig. 4 as A, contains an analog multiplier (U115) along with other circuitry. Here, the scaled product, of a constant and the differential output voltage, is divided by the scaled input voltage. In addition, the circuit compensates for the increased conduction angle on the AC line at higher input voltages and current related losses in the converter. Because the turns ratio of the step-up transformer (T21) and the output voltage is regulated, the voltage needed on the primary of the transformer can be achieved earlier in the AC half-cycle and maintained longer as the AC line voltage is increased. Conduction losses also increase with lower line voltages, due to the high currents necessary to achieve a regulated supply with a fixed output voltage. Normal operation involves sampling the average input voltage then calculating the correct upper current limit for the supply necessary to make bench power. By comparing the scaled differential rail voltage to a known reference, a current limit is derived from the feedback control B and C. As demand on the power supply increases, the rails will drop. The difference between the reference and the scaled rail voltage is the error which determines the average current limit. On a switching cycle basis, the input current is integrated in D until the average current limit is reached. The line current is found by first integrating the voltage across a single turn of wire on the resonant inductor. This yields a scaled waveform that has the shape of the current through the resonant tank. If this current is then integrated only over the portion of time that the phase shifted half-bridges overlap, the AC input current can be obtained. Once the integrated AC input current reaches the current limit, the switching cycle is terminated in E. The operation of this circuit generates a pulse width modulated signal, which is converted to phase shift modulation in order to control each of the two half-bridges. This action is done numerous times through the single AC half-cycle then continues over successive AC half-cycles. Just as a note, there can be a variance in the power dependant current limit during the half-cycle if the power demand is impulsive enough, but most likely the AC current waveform will look very square-like with the addition of rounded edges. Operational Details The following section describes the power-up sequence for various circuits throughout the power supply. There are five circuit boards that make up the power supply: I-tech panel 3 (PWA #s: 8K 137095, 6K 137098, 4K 137101) A) Power supply main B) EMI filter / LVPS C) Power supply control I-tech panel 2 (PWA #: 136541) F) Power supply gate drive CH-1 G) Power supply gate drive CH-2 Once the switch / breaker is closed the power flow begins in the EMI filter / LVPS board. The line voltage passes though the passive filter components. The LVPS, or low voltage power supply generates isolated +20V rails that power the controller and gate drive circuitry. The line voltage is also stepped down for sensing through the precision SIP (RN1) on this board. The scaled voltage is processed (U107-A,B,C,D, U102-C) and utilized by the controller for under voltage lock-out (U106-C) and calculation of the absolute maximum allowable current limit. Until the front panel switch is depressed, the controller holds LOW_ENERGY low which does not allow the power supply to function. As the front panel switch is pressed, the controller begins in soft-start mode. The PWM controller chip (U105) begins switching at a frequency determined by C107 and R120 (37-38kHz). The average line current is sensed indirectly, as described above, through the voltage across the resonant inductor (L21) and the circuitry centered around U101. As in normal operating mode, when the preset current level is reached (pin 5 of U104-A), the comparator driving U109-C changes state causing the switching pulse width to terminate. This in tern sets the phase angle between the two half-bridges. The greater the pulse width, the greater the phase shift. The conversion from pulse width modulation to phase shift modulation takes place in the circuit containing U109 and U111. As the rail voltages build up and reach their nominal value (as determined by the circuit surrounding U103-D), soft-start is released and the control circuit made up of U108-A takes over. The error-driven control loop constantly adjusts the average input current level in order to keep the rails as close as possible to the nominal values. The high-voltage rails are the main source of regulation for the converter. The low-voltage faster loop (U108-A) is used only to keep the low voltage rails from dropping below approximately 16.5V under extraordinary conditions. Once in normal operation mode, the synchronizing clock signal (CLOCK) is used to keep the power supply switching at a frequency that results in an integer when taking the ratio of the BCA switching frequency over the power supply switching frequency. In other words, the BCA switching frequency is a harmonic multiple of the power supply switching frequency. In addition, there also exists a circuit used to keep the high voltage rails from overshooting more than the 10% allowed by the over-voltage protection circuitry. This circuit is made up of U103-A and the surrounding circuitry. It simply reduces the voltage reference on the high-voltage regulator input (pin 12 U103-D) when the rails are too high such that the converter can quickly respond to rapidly changing power demands. Other circuits include over-voltage protection (U103-B), over temperature protection (U116-A) and the necessary crowbar protection, which clamps the high voltage rails and shuts the power supply down in the case of an amplifier problem. Summary The converter operates by utilizing a voltage controlled current loop in order to regulate the output voltage. Internal time constants are slow enough to allow almost constant current to be obtained from the AC line during the conduction period. Power factors above 95% can be had at medium to high power levels leading to much better utilization of the power provided from the AC mains. Due to the adaptive nature of the control, the power supply can operate at any voltage from 85VAC to 277VAC, with full power being achieved from 120VAC to 240VAC. I-TECH Theory of Operation 1 of 20 1. I-TECH THEORY OF OPERATION 1.1. Audio Signal Path 1.1.1. Input Stages (USP3T and Input PWA) Signal is presented to the I-TECH via an analog path XLR, or a digital path via the AES/EBU digital inputs to the USP3T. These paths can be selected either using the front LCD panel menu buttons or using the IQwik interface. The analog path is a specially designed low-noise balanced input to the A/D converters. The maximum input level (the level at which the ADC reaches full scale) may be set to +15dBu or +21dBu by using the front panel menu or IQwic. Optimal signal-to-noise ratio can be achived by setting the max input level to +21dBu and setting the amplifier gain to its minimum value. (Signal-to-noise ratio is specified and factory tested with gain set at 26dB). The balanced analog output design on the USP3T, U12 and U13 then provide final line-level amplification and filtering to drive the BCA