Application of TDA1 6846 in switching power supply and fault investigation

Abstract: TDA16846 is a new power supply controller with a wide range of applications. Starting from the functions and characteristics of TDA16846, this paper comprehensively introduces the application circuit of TDA16846 in the switching power supply, and makes a detailed analysis of the switching power supply composed of TDA16846. Finally, the fault types and maintenance techniques of the switching power supply composed of TDA16846 are deeply explored. This paper is very helpful for the majority of electronic enthusiasts, electronic product maintenance personnel, and teachers and students of electronic technology.
Keywords: TDA16846; switching power supply; fault

0 Introduction
TDA16846 is a new power supply controller launched by Infineon. It is often used in conjunction with field effect switch tubes to form a high-performance switching power supply circuit. This power supply has the characteristics of simple structure, large output power, strong load capacity, wide voltage regulation range, good safety performance, etc. It can be used in color TVs, color monitors, printers, copiers and other office equipment.

1 Introduction to TDA16846
1.1 Structural Features
The internal structure of TDA16846 is shown in Figure 1. It consists of a large number of comparators, triggers, and gate circuits, which can complete functions such as pulse oscillation, voltage regulation control, pulse driving, and various protections.

The operating frequency of TDA16846 can be fixed or freely adjusted, and it has a power correction function. Under light load, the power consumption is very low. Its starting current is small and the starting voltage is low, which can effectively avoid the impact on the field effect switch tube during the starting process. It has a series of protection functions inside, such as power supply overvoltage/undervoltage protection, switch tube overcurrent protection, etc. At the same time, it also has two error comparators for fault detection. It can use the internal comparator and external optocoupler feedback circuit to achieve dual voltage regulation control.

1.2 Pin Function Description
Pin 1: A parallel RC network is connected between this pin and the ground, which can determine the switch tube cut-off time and standby frequency.
Pin 2: Start-up terminal, also used for primary current detection. A resistor is connected between pin 2 and the primary winding of the switching transformer, and a capacitor (or RC series network) is connected between pin 2 and the ground.
During the period when pin 13 outputs a low level, the internal switch of pin 2 is turned on, and the external capacitor of pin 2 is charged to 1.5V; during the period when pin 13 outputs a high level, the internal switch of pin 2 is turned off, the external capacitor of pin 2 is charged, and the voltage of pin 2 rises. When the voltage of pin 2 rises to the control value, the voltage of pin 13 immediately jumps to a low level, causing the switch tube to cut off.
Pin 3: This pin is the input terminal of the error amplifier, and also serves as the zero-crossing detection input. When the pulse amplitude of pin 3 exceeds 5V, the internal error amplifier will output a negative pulse, and the voltage of pin 4 will drop, and the output voltage of the switching power supply will also automatically drop. When the pulse amplitude of pin 3 is lower than 5V, the internal error amplifier will output a positive pulse, causing the voltage of pin 4 to rise, and the output voltage of the switching power supply will also rise. The pulse at pin 3 is also sent to the zero-crossing detector ED1. When the voltage at pin 3 is lower than 25mV, it indicates that a zero-crossing phenomenon has occurred. The zero-crossing detector outputs a high level, and the switch tube is turned on again. The zero-crossing detection characteristics are shown in Figure 2.

Pin 4: used for soft start, with internal control voltage buffer (BCV) and external soft start capacitor. Immediately after power on, the internal 5V power supply charges the external capacitor at pin 4 through R2, the voltage at pin 4 rises slowly, and the output voltage of BCV also rises slowly. The BCV output voltage is provided to the on-time comparator (ONTC) to control the width of the switch pulse, so that the saturation time of the field effect switch tube gradually increases to a stable value, so that the output voltage of each channel also slowly rises to a stable value, achieving soft start. Soft start is not only beneficial to protect the components in the power supply circuit, but also to protect the load.
Pin 5: Optocoupler input terminal, by sampling the output voltage and sending the output voltage change information to pin 5, voltage regulation control can be completed. Since pin 3 already has a voltage regulation function, if pin 5 is used, the voltage regulation characteristics of the circuit will be better.
Pins 6 and 10: Input terminals of the error comparator, often used for fault detection. When the voltage at pin 6 is greater than 1.2V, the internal error comparator 2 will output a high level, and pin 13 will stop the pulse output. When the voltage at pin 10 is greater than 1V, the internal error comparator 1 will output a high level, and pin 13 will stop pulse output.
Pin 7: If a parallel RC network is connected between pin 7 and ground, the circuit works in fixed frequency mode, and the external RC time constant of pin 7 determines the frequency. If a synchronization pulse is input from pin 7, the circuit works in synchronization mode. If pin 7 is connected to a reference voltage (i.e., pin 9), the circuit works in automatic frequency adjustment mode.
Pin 9: This pin outputs a 5V reference voltage. If a resistor (51k) is connected between this pin and ground, the internal error comparator 2 at pin 6 can work effectively.
Pin 11: This pin is used for primary voltage detection to achieve overvoltage and undervoltage protection. When the voltage at pin 11 is less than 1V, the internal PVC circuit outputs a high level, thereby turning off the switch tube to achieve undervoltage protection. If the voltage at pin 11 is higher than 1.5V, the internal PVA circuit outputs a low level, thereby shortening the saturation time of the switch tube and reducing the output voltage of each channel, thereby achieving the purpose of overvoltage protection.

Pin 12: Ground terminal (hot ground).
Pin 13: This pin outputs a drive pulse, which is connected to the power switch tube through a series resistor.
Pin 14: This pin is used to start the power supply. After starting, a winding of the switching transformer will provide voltage to pin 14. The starting current required for pin 14 is very small, only 100μA. When the voltage of pin 14 reaches 15V, the internal circuit starts. After starting, as long as pin 14 is not lower than 8V, the circuit can work normally. If the voltage of pin 14 is lower than 8V, the internal SVC circuit (power supply voltage comparator) outputs a low level, which in turn causes pin 13 to output a low level, the switch tube is turned off, and the circuit enters a protection state. If the voltage of pin 14 is higher than 16V, the internal OVER circuit (overvoltage comparator) outputs a high level, which in turn causes pin 13 to output a low level, the switch tube is turned off, and the circuit enters a protection state. The starting characteristics of pin 14 are shown in Figure 3.

2 Application Circuit
Figure 4 is the application circuit of TDA16846 in Konka "K/N" series color TV switching power supply. The working process of this power supply is analyzed below.

2.1 AC input and rectification and filtering
After the 220V AC mains passes through the power switch and the mutual inductance filter L901, it is sent to the degaussing circuit on the one hand, so that the cathode ray tube is degaussed once every time the power is turned on. On the other hand, it is sent to the bridge rectifier VC901 through the mutual inductance filter L902. After rectification by VC901, it is RC filtered by R901 and C909, forming a DC voltage of about 300V on C909.
2.2 Oscillation process
The 300V voltage on C909 is sent to pin 2 through R918, and then charged to C913 outside pin 14 through the internal diode D1 of pin 2. The voltage on C913 begins to rise. After about 1.5s, the voltage on C913 rises to 15V, the internal circuit starts, and generates a switch pulse output from pin 13, which is sent to the field effect switch tube V901, so that V901 starts to work. After V901 works, pulse voltage will be continuously generated on the primary winding of the switching transformer
, so that pulse voltage will be continuously generated on each secondary winding. After the pulse voltage on each secondary winding is processed by its own rectification and filtering circuit, it outputs 130V (+B voltage), 15V and 13V DC voltage to supply power to the corresponding load. A
soft start capacitor is connected to pin 4. After the circuit is started, due to the charging effect of the external capacitor (C920) on pin 4, the width of the output pulse on pin 13 gradually widens and finally stabilizes at the design value. The output voltage of each channel also gradually rises to a stable value. This will greatly reduce the impact of the instantaneous surge current on the switch tube and the load at the start-up, and improve the reliability of the power supply. After
the circuit is started, the current required by pin 14 will increase greatly. At this time, the pulse voltage on the L2 winding of the switching transformer is rectified by VD902 and filtered by C913 to obtain a DC voltage of about 12V to supply power to pin 14 to continue to meet the needs of pin 14.

The external RC circuit of pin 1 determines the cut-off time of the switch. During the saturation period of the switch, the internal circuit charges C917, and C917 is charged to 3.5V. During the cut-off period of the switch, C917 is discharged through R907. Before C917 is discharged to the threshold voltage (the minimum value of the threshold voltage is 2V), the switch always remains cut off.
2.3 Voltage stabilization process
TDA16846 has two voltage stabilization circuits outside. The first voltage stabilization circuit is located outside pin 3, and the second voltage stabilization circuit is located outside pin 5.
The working process of the first voltage stabilization circuit is as follows: When the output voltage rises for some reason, the pulse amplitude on the L2 winding of the switching transformer also rises. After the voltage is divided by R919 and R909, the pulse amplitude of pin 3 is higher than 5V. After being processed by the internal circuit, the voltage of pin 4 is reduced, and then the width of the output pulse of pin 13 is narrowed, the saturation time of V901 is shortened, and the output voltage of each channel is reduced. If the output voltage of each channel drops due to some reason, the pulse amplitude of pin 3 will be less than 5V. At this time, the pulse width of pin 13 will become wider, the saturation time of V901 will increase, and the output voltage of each channel will rise. By adjusting the ratio of R919 and R909, the output voltage can be adjusted. Pin 3 also serves as the zero-crossing detection input. When the pulse of pin 3 jumps from high level to low level (lower than 2.5mV), it means that there is a zero-crossing phenomenon. The output pulse of pin 13 jumps from low level to high level, making the switch tube conduct again.
The working process of the second voltage stabilizing circuit is as follows: when the 130V output voltage rises due to some reason, the base voltage of V904 also rises, thereby increasing the emitter voltage of V902, while the base voltage of V902 has to remain unchanged, resulting in enhanced conduction of V902, increased luminous intensity of the LED in N902, enhanced conduction degree of the phototransistor, and decreased voltage at pin 5. After being processed by the internal circuit, the width of the output pulse at pin 13 is automatically adjusted to narrow the pulse width, shorten the saturation time of V901, and decrease the output voltage of each channel. If the 130V voltage drops due to some reason, the voltage stabilizing process is opposite to the above. The 130V output voltage can be adjusted by adjusting RP901.
It is worth mentioning that these two voltage stabilizing circuits do not work at the same time. The internal circuit always connects the voltage stabilizing circuit with a lower voltage stabilizing value to complete the voltage stabilizing control, while the voltage stabilizing circuit with a higher voltage stabilizing value is blocked. For example, the voltage regulator circuit outside the 3-pin can stabilize the +B voltage at 140V, while the voltage regulator circuit outside the 5-pin can stabilize the +B voltage at 130V. At this time, the internal
circuit uses the voltage regulator circuit outside the 5-pin to complete the voltage regulation control and stabilize the output voltage at 130V. Using two voltage regulator circuits can effectively improve the safety of the power supply. When one voltage regulator circuit is open, the other voltage regulator circuit will continue to stabilize the voltage, so that the output voltage will not rise significantly.
2.4 Protection process
Pin 11 is used for primary overvoltage and undervoltage protection. The 300V voltage on C909 is divided by R920 and R910 and added to pin 11. When the grid voltage is too low, the 300V voltage on C909 is also too low, so that the voltage on pin 11 is less than 1V. At this time, the internal circuit will stop the output of pin 13, and V901 is in the cut-off state to achieve undervoltage protection. If the grid voltage increases, the 300V voltage on C909 will also increase, and the voltage on pin 11 will be higher than 1.5V. After being processed by the internal circuit, the output pulse width of pin 13 will be narrowed, thereby shortening the saturation time of V901, reducing the output voltage, and achieving overvoltage protection.
Pin 14 has secondary overvoltage and overcurrent protection functions. When the pulse amplitude of each secondary winding is too high for some reason, the voltage on pin 14 must be greater than 16V. After being processed by the internal circuit, the pulse output of pin 13 will be stopped, and V901 will be cut off, thereby achieving secondary overvoltage protection. When the load is short-circuited, the voltage on pin 14 will be less than 8V. After being processed by the internal circuit, the pulse output of pin 13 will be stopped, and V901 will be cut off, thereby achieving secondary overcurrent protection.

3 Fault Exploration
3.1 The fuse is not blown, but the output voltage of each channel is 0V.
When this fault occurs, the voltage across C909 and the voltage on pin 14 of TDA16846 should be used as key detection points.
First, check whether there is 300V voltage at both ends of C909. If not, check the AC input circuit and the rectifier filter circuit; if yes, measure the voltage of pin 14 of TDA16846. By measuring the voltage of pin 14, the inspection direction can be determined.
If the voltage of pin 14 is 0V, check whether the external starting resistor R918 of pin 2 is open circuit, whether the external filter capacitor C913 of pin 14 is broken down, whether the external rectifier diode VD902 of pin 14 is broken down, whether D1 inside N901 is open circuit, etc.
If the voltage of pin 14 is lower than 15V, it means that the starting voltage is too low, resulting in the circuit not being able to start. Check whether the resistance of R918 increases too much, whether C913 is leaking, whether C918 is broken down, whether VD902 is leaking in reverse, etc.
If the voltage of pin 14 is above 15V, it means that the starting voltage has met the oscillation requirements, and there should be no problem with the external circuits of pins 2 and 14. At this time, we should focus on checking whether the external soft-start capacitor C920 on pin 4 is broken down, because when the external soft-start capacitor on pin 4 is broken down, the switch tube will always be in the cut-off state. If the external capacitor
on pin 4 is normal, we should check N901 itself. If the voltage on pin 14 swings between 8 and 15V (swings once for about 1.5s), it means that the internal oscillator circuit of TDA16848 has started oscillating, and the fault generally occurs in the +B voltage forming circuit or load. The +B voltage rectifier and filter circuit should be checked (i.e. check VD904, C924, C923 and other components). If there is no problem, check the output circuit.
3.2 Three Nos at Startup, Fuse Blown
Since the fault is manifested as a burnt fuse, it means that there is a serious short circuit in the circuit. The high-frequency filter capacitor, bridge rectifier circuit and capacitors connected in parallel with it, 300V filter capacitor C909, switch tube V901 and other components in the AC input circuit should be checked to see if there is any breakdown in these components. When the switch tube V901 is repeatedly broken down, the components such as R918, C918, R908 and C920 should be checked. Although R918 is a starting resistor, it has another important function. That is, when the circuit is started, the circuit formed by it, C918 and R908 will determine the saturation time of the switch tube. When the resistance of R918 or R908 increases or C918 leaks, the voltage rise speed on C918 will slow down, that is, the voltage rise speed of pin 2 will slow down, and the saturation time of the switch tube will be extended. Because when the switch tube is saturated, its collector current rises linearly. In this way, when the saturation time of the switch tube is extended, the current flowing through the switch tube will be too large, causing the switch tube to burn out.
C920 is a soft start capacitor. When it fails, the soft start function will be lost. After starting up, the saturation time of the switch tube V901 will immediately reach the design value, which will increase the impact on the switch tube at the moment of starting up and increase the possibility of the switch tube being broken down.
When you encounter a fault that breaks down the switch tube, do not rush to replace the switch tube. You should first remove the switch tube, then power on and measure the voltage at pin 14. If the voltage at pin 14 swings between 8 and 15V, and the swing takes about 1.5 seconds, it means that the TDA16846 is basically working normally; if the swing takes too long, it means that there is a problem with the external circuit at pin 2. After eliminating the fault of the external component at pin 2, install a new switch tube.
The switch tube can be replaced with 2SK1794, 2SK727, BUZ91A, 2SK2645, 2SK2488 and other models.
3.3 Output voltage is too high
The output voltage is too high, indicating that the saturation time of the switch tube increases, and the cause is a bad voltage stabilizing circuit. This power supply relies on the second voltage stabilizing circuit to stabilize the +B (+130V) voltage. When the second voltage stabilizing circuit fails, the first voltage stabilizing circuit will continue to stabilize the voltage. Because the voltage stabilization setting value of the first voltage stabilizing circuit is higher than that of the second voltage stabilizing circuit, the output voltage will increase. Therefore, when encountering an output voltage increase fault, you only need to check the second voltage stabilizing circuit (N902, V902, V904 and its peripheral components).