Design principles of foreign car audio

Whether the switching power supply adopts PWM or PCM, its energy output is formed by the electromagnetic conversion of the pulse transformer. When the switch is turned on, the magnetic energy is stored in the pulse transformer. When the switch is turned off, the magnetic energy is converted into electrical energy to provide voltage to the load. Even if the load current increases instantaneously and the output voltage drops, the voltage stabilization control system can only control the switch tube to extend the on time in the next conduction cycle.After the switch tube is loaded, the output voltage rises to compensate for the effect of the increase in load current . However, the fluctuation of music is ever-changing, and sometimes a large-scale rushing out of the signal is only an instant matter. It is limited, and the waveform distortion is generated. After the shock signal passes, the PWM circuit output signal rises, and the switching power supply reduces its output voltage to stabilize its output voltage. Unfortunately, all this is too late. In this process, the output signal is inevitably distorted. At the same time, the power supply ripple pulse is increased, which increases the noise of the amplifier.
The DC converter is different. The switch tube of the converter always works with the set pulse width. As long as the switch tube has enough switching current, it can provide voltage within its rated power at any time. From this point of view, there is no difference between DC converter and transformer rectified power supply, and the internal resistance of DC converter is lower and it is more adaptable to instantaneous large current. In fact, the converter is a switching circuit without a voltage stabilizing system. Any switching power supply removes pulse modulation, and the sampling error amplification part is essentially a DC converter.
According to the above principles, at the end of the last century, Europe began to configure DC converters on car audio to match with car power amplifiers. In 1980, the German-made Monacor HPB150 automotive power amplifier was equipped with 12V and ± 25V DC converters, and the maximum output current could reach 10 to 15A, making the effective output power of the amplifier up to 2X40W, or BTL connection, making the output power 150W. Another converter named "Jensen" is used in the car power amplifier, which can convert the 12V voltage into a dual power supply ± 30V / 15A output. It can supply power to the four-channel amplifier and output 4 * 60W of effective power. Monacor HPB150 is the earliest product. Its power amplifier converter uses discrete components to assemble a self-excited push-pull converter. A total of 13 transistors are used. The circuit is more complicated and it is not convenient to install and adjust. In addition, due to its operation, the self-excited oscillation circuit The frequency changes with the load current, and pulse interference suppression is also difficult.

The Jensen power converter uses a traditional switching power supply and its excitation driver to drive a parallel push-pull circuit composed of four MOS FET switches. The power amplifier converter circuit is shown in the figure. The use of MOS FET tubes as switching tubes in the automotive power amplifier can improve the working efficiency of the power transformer, help to suppress pulse interference, and at the same time reduce the size of the power transformer. The oscillator and control system of the converter are all integrated in the IC (TL494). TL494 was originally designed as its excitation switching power supply drive controller. In addition to the oscillator and pulse width modulator, it also has a reference voltage regulator circuit, a dead time control circuit and an error detection circuit composed of two sets of comparators. TL494 forms its excited converter in this circuit, and only uses its oscillator and drive circuit, which is used as the pulse signal source for driving the switch tube, so it is not used with conventional usage. In this circuit, the 5th and 6th pins of TL494 are connected to the time constant circuit (C3, R5), the oscillator generates a pulse signal of 80kHZ, which is controlled by the internal bistable trigger of TL494, and becomes two drive pulses with different timing Two sets of drive amplifiers. Two sets of driving stages in TL494 output positive pulses with different timings from pins 9 and 10. In order to avoid that the push-pull switch tubes VT3, VT5 and VT2, VT4 are turned on at the same time, the fourth pin of TL494 is externally connected to R6, C2, R4 to set the dead time. After a group of drive pulses makes one arm of the push-pull circuit conductive, there is a dead time interval before another set of drive pulses is sent to make the other arm conductive (the higher the voltage of the fourth pin, the longer the dead time). Pins 1 and 2 of TL494 are the in-phase and inverting input terminals of two sets of sampling amplifiers, which can control the duty cycle set by the pulse width modulator composed of the internal comparator. In this converter, the functions and applications of each pin of TL494 are as follows:

Pin 2 is the non-inverting input of the first group of error amplifiers. Connect 5V reference voltage by R7. When the second pin outputs a high level, the error amplifier output (pin 3) outputs a constant low level. This level controls the PWM modulator composed of the comparator within TL494, and the maximum pulse width is 45%, and the remaining 5% Dead time. In addition, the external connection of pin 4 to C4 is a soft start capacitor. At the moment of startup, C4 charging makes pin 2 low for a moment, and the error amplifier outputs high level. As the charging voltage of C4 increases, the voltage of pin 2 increases, and the third The voltage of the pin is reduced, so that the output pulse width of the PWM comparator is gradually increased to the rated pulse width, so as to avoid the damage of the switch tube caused by the inrush current of the startup.
The third pin is the output terminal of the error amplifier, externally connected to R3 and C1 to avoid oscillation of the error amplifier.
The fourth pin is the dead time control terminal, and the 0.05V dead time control voltage is obtained from the 5V reference voltage division through R6 and R4, so that there is a gap of 5% of the pulse width between the two sets of driving pulses. When the level of pin 4 reaches 0.3V, the drive pulse is turned off.
Pins 5 and 6 are the oscillation frequency control terminal. The external R5 and C3 set the oscillator to generate an oscillation pulse of about 80KHZ. The emblem R5 can make the oscillation frequency 100KHZ. The relationship between C3, R5 and the oscillation frequency is: f (kHZ) = 1.2 / R (kΩ) .C (μF).
Pin 7 is the public ground.