Application and Fault Analysis of PWM Circuit Designed by TL594 in High Frequency Machine

Application and Fault Analysis of PWM Circuit Designed by TL594 in High Frequency Machine

With the development of science and technology, traditional X-ray machines are gradually being eliminated by the market, while high-frequency machines are gradually entering the market. For this reason, this article mainly introduces a comprehensive use circuit based on TL594 pulse width modulator and high-frequency inverter technology, as well as related fault phenomena and troubleshooting ideas. In fact, whether it is Siemens, GE or Shimadzu in the world, or Neusoft, Wandong, Shangqi Factory and other related equipment in China, for various purposes, no longer provide specific circuit instructions. As a staff member of a teaching and maintenance institution, in the course of work, the author has repeatedly replaced blindly because he did not understand its complete working principle, which was time-consuming and laborious. In order to fully grasp its working principle and have a little guidance on maintenance, this article focuses on the working principle of the relevant circuits and the causes of failures of the HF50R film machine produced by Wandong. The basic concept of this machine is very similar to the early Spanish products.

As a domestic high-frequency machine, HF50R still cannot meet higher requirements in some hard indicators, such as the bulb still uses non-high-speed tubes, and the high-voltage inverter frequency (25 kHz) is still less than the required 30kHz (currently the international high frequency can reach more than 100 kHz), but its basic working concept has been in line with international standards. The high-voltage inverter electronic switch also uses the insulated gate bipolar transistor (IGBT) currently used internationally, and the filament inverter electronic switch uses the IFR540 transistor, and its drive signal comes from the PWM circuit composed of TL594.

1 Introduction to TL594

TL594 Application Circuit
1.1 Internal Composition of TL594

TL594 consists of a reference voltage generating circuit (Reference Regulayor), a rectangular wave oscillator (Oscillator), two error amplifiers (Error Amp), a dead time comparator (Deadtime comparator), a pulse width modulation comparator (PWM comparator) and related output circuits.

1.2 Pin Functions of TL594

The functions of each pin of TL594 are as follows:

Pins 1 and 2: the same direction and reverse input terminals of a group of error amplifiers;

Pins 16 and 15: the same direction and reverse input terminals of another group of error amplifiers;

Pin 3: the output terminals of two groups of error amplifiers;

Pin 14: 5 V power supply terminal, used as the reference voltage source of each comparator circuit, the maximum current of this pin is 10 mA;

Pin 7: GND;

Pin 13: working mode selection terminal. If pins 13 and 14 are connected, the two tubes are push-pull outputs, and the load current can reach 500mA;

Pin 6 (RT ₎ , Pin 5 (CT ₎ : Oscillator frequency setting terminal, the frequency is: fosc = 1.1/RTCT _. Pin ₄ : Dead time setting pin. The voltage range of this point is 0 ~ 3.3 V. (This machine is 1.0 V) _Pin 12: 7 ~ 41 V power supply (this machine is 12 V). Pins 9 and 10: Output terminal, can output two-phase pulses with a phase difference of 180°. 1.3 Basic working principle of TL594 In general, the clock signal of the flip-flop (FLIP-FLOF) will be selected only when it is low, and this signal is controlled by the OR gate, and its control signal comes from the dead zone comparator, PWM comparator, and undervoltage lockout comparator respectively. The positive sawtooth wave output by the oscillator is added to the inverting input of the dead zone comparator and the PWM comparator respectively. The dead zone control voltage has an input compensation voltage of 0.12 V, which limits the dead zone time to at least 4% of the sawtooth wave period, that is, the maximum output duty cycle is 96%. The input signal of the PWM comparator of TL594 comes from the combination of the output of the two error amplifiers and the feedback signal. In this way, when the sawtooth wave signal is higher than the control signal, the OR gate outputs a low level, the trigger is selected, Q1 and Q2 will receive the excitation signal and output a pulse signal whose pulse width varies with the level of the control signal. TL594 has a built-in 5 V reference power supply. The temperature drift in the range of 0 to 70 ° C is less than 50 mV, and the voltage can achieve an accuracy of ±1.5%. 2 HF50R PWM circuit The PWM circuit of the HF50R high-frequency machine is mainly used in kV and mA adjustment boards. The drive signals of the large and small focus filament circuits in the mA adjustment board are completed by two PWM circuits respectively. 2.1 HF50R Filament Principle The PWM circuit on the filament adjustment board (taking a small focus as an example) has R _T = 11 K, C _T = 0.01 μF, from which the oscillator frequency can be calculated to be 10 kHz. The dead zone voltage is sampled by the built-in 5 V power supply through VR3 to 1.0 ± 0.3 V. Pin 2 is the FILAlSET (filament setting) signal from the CPU, and pin 1 input is the primary sampling signal of the filament, and its output signal can control the pulse width. The reverse input terminal (pin 15) of the other error amplifier of TL594 is a +5 V power supply, and pin 16 is the potential after the primary sampling signal of the filament is compared with the reference voltage set by R54. If the sampling signal is less than 2.0 V, the TL594 can work normally (in the large focus driving circuit, the maximum sampling signal is set to 2.5 V by R74). According to the size of the tube current selected by the console, the PU will send out the corresponding filament setting signal, and the sampling signal will also be sent to the non-inverting input terminal of the error amplifier, so as to change the potential of pin 3 according to the difference between the setting signal and the sampling signal, so as to achieve the purpose of controlling the pulse width of FILAlDR1 and FILAlDR2, and finally change the signal obtained by the filament transformer. The frequency of the signal is fixed at 10 kHz, and the pulse width is adjustable. 2.2 kV adjustment board The oscillator frequency of the kV adjustment board can be determined by C4 (4700p) and R8+VR1 (adjustable resistor). After fine-tuning, Fosc=25 kHz can be made. The voltage of pin 15 is set to 2.5 V, and pins 13 and 14 are connected to determine the working mode of TL594. The dead zone voltage setting is the same as that of the filament board. The signal of the reverse input terminal of the error amplifier 1 on the adjustment board comes from the kV-SET signal of the CPU board (1 V of this signal corresponds to 33.3 kV). The non-inverting input terminal comes from the difference between the measured kV+ and kV- signals of the high-pressure oil tank, that is, the kV sampling signal also satisfies 1 V corresponding to 33.3 kV. In this way, if the sampling signal is lower than the kV-SET signal, the comparator output voltage drops, the output pulse width widens, and kV rises accordingly. The reverse input terminal of the error amplifier 2 is a fixed 2.5 V power supply, and the signal of the non-inverting input terminal comes from two channels. One channel comes from the CPU's /kV ON signal, and the other channel is the kV detection signal. If the kV detection signal is less than 4.8 V (4.8V corresponds to 160 kV), that is, kV does not exceed 160 kV. If the /kV ON signal arrives and kV does not exceed 160kV, T1594 works normally. If one of the two signals is abnormal, the 16th foot will be forcibly pulled to about 4.8 V, resulting in the 3rd foot potential being too high, the output pulse is prohibited or the output pulse width is zero, and there will be no kVDR1 and kVDR2 signals at this time, and the system will report an error. 3 Fault Analysis 3.1 Fault Phenomenon Analysis Now let’s take high voltage as an example to analyze the fault phenomenon. In general, damage to TL594 will prevent the HF50R high-frequency machine from receiving kVDR1 and kVDR2 signals, and eventually show no kV signal. However, no kV is not always caused by damage to TL594. The following will analyze the causes of no kV faults and when and how to test TL594. The causes of no KV can be roughly divided into the following three types: (1) High-voltage inverter power supply failure High-voltage inverter power supply generally comes from the three-phase rectifier BUS+ and BUS- signals. If the signal cannot arrive, PC7-3 of the CPU board should be able to detect the MPSFLT signal and report an error. (2) IPM failure If the kV DR1 and kV DR2 signals are normal but the IPM is damaged, PC3-1 on the CPU board of HF5003 should be able to detect the IPMFLT signal and report an error E25.










































(3) IPM drive failure

For this failure, we must first analyze whether the CPU board has given a kV ON signal. If there is no such signal, we need to start with the CPU, which is controlled by the P1.5 pin of EXP and U1. If the kV DR1 and kV DR2 signals are still not detected after the signal arrives, we need to check the TL594 and its peripheral components one by one (error E13 is reported at this time).

3.2 Simple method for distinguishing TL594

The simple method for distinguishing TL594 is as follows:

(1) Check whether the 12th pin (12 V), 13th pin (5 V), and 14th pin (5 V) of TL594 are normal. If not, disconnect the peripheral components and measure again. If the peripheral components are normal, remove the TL594 to determine whether it is damaged.

(2) Measure the waveform of the 5th and 6th pins of TL594. Normally, it should be a 25kHz sawtooth wave (the waveform of the 6th pin is slightly lower). The amplitude of the sawtooth wave is in the range of 0.4 to 4 V. If there is no such signal, it means that the oscillation circuit cannot start or the oscillation is poor.

(3) For the TL594 integrated circuit, a simple method can also be used for auxiliary judgment. That is, when the output pulse is measured, the VREF voltage is quickly shorted to the dead zone control voltage. At this time, the output pulse should disappear. In this way, by adjusting the size of the two input signals of the error amplifier, the pulse width change of the output waveform should be detected. At the same time, the voltage of the same-direction input terminal of the error amplifier can be increased by more than 3 V. At this time, the output pulse width should drop to zero, that is, no output.

3.3 Repair example analysis

The situation encountered by the author is not that the TL594 is damaged, but because the EXP signal cannot arrive, resulting in kV ON always being a high level. After measurement, the 16th pin of the TL594 is close to 4.8 V, which is much larger than the 2.5 V of the 15th pin. This causes the potential of the 3rd pin to rise, or the gate outputs a high level, so that Q1 and Q2 cannot get the excitation pulse and cannot work, and finally causes kV DR1 and kV DR2 to disappear and the machine reports an error.

4 Conclusion

This article mainly aims at the gaps in relevant information, and according to the actual maintenance needs, in order to reduce the blindness in the maintenance process, a rational analysis and summary is made after troubleshooting, so as to compile the working concept of related modules. Such a concept can not only be used in this small circuit, but also can be used for the internal structure, working principle and related fault phenomena of modules such as PM300DSA120, and the troubleshooting and maintenance of similar fault phenomena.