DIP-8 package single chip high voltage power switching power supply module

1 VIPer22A Device Function Introduction

The package form of VIPer22A type single-chip switching power supply power converter is DIP-8: D—positive terminal, that is, the drain of power MOSFET, 5, 6, 7, 8 pins (in parallel); S—negative terminal, 1, 2 pins (in parallel), that is, the source of power MOSFET; UDD—self-supplied power supply terminal, also the self-excited power supply terminal outside the chip, 4 pin; FB—output voltage feedback terminal, 3 pin. The package form is 8 pins, but there are only 4 pins in reality, which is simple and easy to remember and easy to make boards, as shown in Figure 1.

The internal circuit structure block diagram of the VIPer22A monolithic switching power supply power converter is shown in Figure 2. Since both the positive and negative ends of the device pass a large current, they are connected in parallel to increase the capacity. When drawing the printed circuit board circuit diagram, the two ends are made of a large area of ​​copper foil, and when the VIPer22A device is soldered, the bottom of the device is directly pressed against this large area of ​​copper foil, which is equivalent to adding a small heat sink.

Although the device is a DIP-8 package, it has a built-in high-voltage power MOSFET, a drain-source breakdown voltage of more than 730V, a typical limiting current of 0.7A, an on-state resistance of 15Ω, and can still output 12W of power when the input voltage fluctuates from 85VAC to 265VAC. The device also has overcurrent, overvoltage, and overheating protection functions with hysteresis characteristics (see Section 3 for details). Therefore, its working stability and reliability are excellent, and it can be easily powered by AC power to produce low-voltage and low-power DC power supplies of various specifications. As long as the parameters of the transformer and other parameters are designed correctly, there is almost no need for debugging, and the connected circuit can be put into normal operation.

2 Working principle of the overall circuit of 12V switching power supply made with VIPer22A device

2.1 Startup Overview

The 12V switching power supply made by the single-chip switching power converter VIPer22A is shown in Figure 3. When the AC220V mains power is turned on, the high-voltage current source in the VIPer22A converter N1 is put into operation through the primary winding W1 of the high-frequency transformer T1, and the self-sufficient power supply UDD inside the chip is automatically turned on. The power MOSFET is put into operation, and current flows through the primary winding W1 of T1. The current generates magnetic flux in the transformer core, and induced voltage appears in each winding, and its direction is shown by the name terminal symbol. The induced voltage in the auxiliary winding W2 is charged to the capacitor C9 through the rectifier tube VD6, and C9 is connected in parallel to the UDD end power supply. The UDD end becomes a continuous self-excited DC power supply and starts to power the chip. At this point, the VIPer22A converter has completed the startup procedure.

2.2 Current control mode participates in voltage regulation

2.2.1 Current Feedback

At the moment of VlPer22A startup, the PWM output pulse voltage drives the power MOSFET to turn on, and the primary of transformer T1 flows through a rapidly increasing current ID. When the current reaches the limit value, the voltage drop of the sampled current IS on RS will be greater than 0.23V, the overcurrent comparator outputs a high level, turns off the drive circuit, the power MOSFET is turned off, and the load current drops.

2.2.2 Converting voltage feedback into current feedback

When the voltage of the secondary winding of T1 is established, the FB terminal of N1 obtains a feedback current IFB proportional to the voltage of the winding of W2, which is superimposed on the sampling current IS to generate a comprehensive voltage on the resistor RS. The comprehensive voltage begins to act on the overcurrent comparator, adjusts the PWM, and thus stabilizes the output voltage.

2.2.3 Advantages of current feedback

The voltage regulation process of common power supply chips is only controlled by feedback voltage, and the feedback sampling current is only used for overcurrent protection; however, the voltage regulation process of this chip has both feedback sampling current and feedback voltage, and the source voltage effect is extremely good, and the load effect is also better than the switching power supply without current control, ensuring that the voltage regulation accuracy is higher than that of common power supply chips - it is suitable for occasions with large fluctuations in the mains power supply and for occasions with load fluctuations. The current feedback is directly displayed on the sampling resistor RS without passing through the second-order circuit, with fast response speed, large gain, good dynamic stability, high reliability, and both overcurrent and short-circuit protection functions. It is also suitable for multiple complete machines to run in parallel with current sharing.

2.2.4 It is a hybrid working mode of current and voltage dual loop control

From the above description, it can be seen that current-controlled PWM is not just current control, but actually dual-loop control. The current control is encapsulated in the inner loop of the chip, as shown in Figure 2, and does not need to be implemented externally. It mainly responds to source voltage (including power frequency rectifier voltage) fluctuations and T1 primary current fluctuations. The voltage control is in the outer loop, as shown in Figure 3. The feedback voltage is applied to the feedback terminal FB of the chip through components such as N2 and N3. Like ordinary power chips, it can respond to load fluctuations and source voltage fluctuations at the same time.

2.3 Self-contained power supply plus self-excited power supply

It is worth mentioning that the power switching power supply shown in Figure 3 does not have an auxiliary power supply like in a general switching power supply. When the voltage at the UDD terminal from the high-voltage current source reaches the turn-on voltage value Vdd (on) = 14.5V, the high-voltage current source is turned off; when the voltage at the UDD terminal drops to the chip turn-off value Vdd (off) < 8V, the high-voltage current source automatically turns on again. The self-sufficient power supply in N1 first works at the UDD terminal. After the power MOSFET is put into operation, the self-excited power supply composed of the auxiliary winding W2 of T1 outside N1 is connected to the UDD terminal. In this way, "self-sufficiency" plus "self-excitation" ensures the continuous oscillation of N1, but it is not the unstable self-excited oscillation frequency usually mentioned, but the stable externally excited oscillation frequency of 60kHz in N1, which uniquely establishes the distinctive characteristics of this integrated circuit, so the circuit structure is simple and the stability and reliability are high. The voltage range of the output voltage feedback terminal FB is between 0V and 1V.

2.4 Voltage stabilization process

2.4.1 Characteristics of single-ended flyback converter

In the circuit of Figure 3, at the moment when the power MOSFET is turned on, the same-name end of the winding W3 is opposite to W1, and the rectifier tube VD7 is in a reverse bias state; when the power MOSFET is turned off, VD7 is turned on, so this converter is called a single-ended flyback converter, also known as an inductive energy storage converter - charging capacitors C10 and C12, that is, the transformer T1 winding has the function of inductance, and the value of the smoothing inductor L1 is tens of µH to meet the requirements for ripple voltage, and L1 may not even be used. The rectifier pulse width of the single-ended flyback converter can exceed 1/2 cycle, so it can still maintain a good voltage regulation rate in places with large fluctuations in the mains.

2.4.2 Voltage regulation process when source voltage fluctuates

When the AC220V mains power fluctuates, the current amplitude in the primary winding W1 of T1 will also change accordingly, and will immediately appear on the sampling resistor RS in the chip. The overcurrent comparator will adjust the PWM pulse width and the output voltage accordingly. This process plays an absolutely dominant role in the entire voltage regulation process. At the same time, the voltage at the input pin 1 of the integrated adjustable reference regulator N3 changes accordingly, causing the voltage at the output pin 3 to change in the opposite direction. Then, the FB voltage at the control pin 3 of the integrated power converter N1 is adjusted through the optocoupler N2, so that the gate pulse width and output voltage of the power MOSFET in N1 change in the opposite direction, thereby restoring the output voltage to the value before the external voltage fluctuation to the maximum extent. That is, the fluctuation of the source voltage is promptly responded to by the current control working mode. The sampling circuit of the inner loop control is placed at the inverting input of the overcurrent comparator, and the source voltage effect is better than 0.01%. The voltage control working mode of the outer loop also participates in the response, but its role is smaller than that of the current control mode, and the response speed is also lower.

2.4.3 Voltage stabilization process during load fluctuation

(1) When the load fluctuates, the power frequency rectifier and filter voltage fluctuates accordingly

When the mains voltage remains unchanged and the load fluctuates, the voltage on capacitor C6 fluctuates accordingly, and the voltage on RS in Figure 2 changes accordingly. The overcurrent comparator and subsequent links implement pulse width regulation in a timely manner, and the regulation accuracy is better than 0.01%, that is, when the voltage on C6 changes, the output pulse width changes in the opposite direction, ensuring that the output voltage of the circuit in Figure 3 remains unchanged. This is actually the current-controlled voltage regulation process in Article 2.4.2. The load regulation rate of the VIPer22A power supply is conditionally better than that of ordinary converter-type switching power supplies.

(2) When the load fluctuates, the current in the primary winding of the high-frequency transformer fluctuates accordingly

When the mains voltage remains unchanged and the load fluctuates, the current of the W1 winding fluctuates accordingly, and the voltage at the non-inverting input terminal of the overcurrent comparator in Figure 2 also changes accordingly, and the pulse width is adjusted in time to maintain a stable accuracy better than 0.01%.

(3) When the load fluctuates, the drain-source voltage of the power MOSFET in VIPer22A fluctuates accordingly

The voltage drop fluctuation between the primary side of the transformer and the drain-source (on-resistance is 15Ω) of the power MOSFET caused by load fluctuation will inevitably cause current and voltage fluctuations on the sampling resistor RS, which are fed back to the inverting input of the overcurrent comparator. The inner loop controls PWM to minimize the impact and ensure the stability of the output voltage with an adjustment accuracy better than 0.01%. That is, the current control of the inner loop of VIPer22A reduces the impact of voltage fluctuations on the secondary side of the transformer and the previous secondary side to a negligible level.

(4) VIPer22A's automatic intermittent operation mode under low load conditions also optimizes load regulation

The chip also has an automatic intermittent working mode under low load conditions (see 3.2.4 below), which suppresses the output voltage rise under light load. The above four features of this circuit are not available in ordinary voltage-controlled switching power supplies, so it can ensure that its load effect is significantly better than that of ordinary voltage-controlled switching power supplies.

(5) The voltage control mode of the outer loop also helps stabilize the output voltage fluctuation caused by load changes.

At the same time, when the load fluctuates, the voltage of the W3 winding and its output terminal also fluctuates, so the voltage control mode in the outer loop of the chip is put into operation to keep the output voltage stable, but the accuracy and response speed are inferior to the inner loop.

(6) The load effect of VIPer22A is better than that of voltage-controlled switching power supply, but still inferior to its own source voltage effect.

It is a rule that the source voltage effect of the AC-powered converter-type switching regulator is better than the load effect. Although the load effect of the VIPer22A power supply is better than that of the ordinary converter voltage-controlled switching power supply, it is also inferior to its own source voltage effect and does not exceed this rule.

3. Overheat, overcurrent and overvoltage protection functions with automatic restart

3.1 Overheat protection

The switching power supply shown in Figure 3 has a chip N1 that encapsulates the main heat-generating component, the power MOSFET, and the overheat protection link. Once the chip reaches a high temperature of 170°C, the output signal of the protection link acts on the RS trigger, which cuts off the trigger pulse on the power MOSFET, as shown in Figure 2. After the chip is turned off, the temperature gradually drops, and it can resume operation only after it drops to 40°C. The hysteresis temperature is 40°C.

3.2 Overcurrent protection

3.2.1 Characteristics of current sampling

Conventional power MOSFET current sampling is done at the S-pole, with full current and high loss. VIPer22A samples current at the sensing pole near the S-pole. The induced current IS flowing through the resistor RS (as shown in Figure 2) is proportional to the D-pole current ID flowing through the power MOSFET, IS/ID=1/560, and the power consumption is very low, which is another major advantage of the device.

3.2.2 Overcurrent protection process

When the power MOSFET current ID increases to a certain value, the voltage on the current sampling resistor is ≥0.23V, that is,

The over-current comparator outputs a high level, and through the leading edge latch circuit and RS trigger, the power MOSFET gate pulse is turned off, achieving the purpose of over-current protection. After the over-current, the circuit automatically resumes operation.

3.2.3 Overcurrent protection process without feedback

When the FB terminal is grounded, that is, there is no external loop feedback, the output voltage increases, the current increases, and the drain current ID will be larger than that described in the previous section. As shown in Figure 2, it is equivalent to RS and R1 being connected in parallel, the resistance value decreases, and the current flowing through the D pole of the power MOSFET increases and reaches the limiting current, that is,

But it will not increase infinitely, the maximum is the chip's limit value of 0.7A.

3.2.4 Automatic intermittent mode under low load conditions

When the power supply is unloaded or the drain current flowing through the power MOSFET is less than or equal to 12% of the limit value - about 85mA, the chip N1 will automatically enter the intermittent working state, which can not only ensure normal operation under low load, but also reduce the power consumption of the whole machine, and the safety factor will be higher.

3.3 Overvoltage protection

When the output voltage rises suddenly for some reason, the voltage of the auxiliary winding W2 of T1 and the UDD terminal rises in the same proportion. If the voltage of the in-phase terminal on the overvoltage comparator (see Figure 2) exceeds 42V, that is, VDD ≥ 42V, the comparator flips, and under the action of the subsequent RS trigger, the power MOSFET gate trigger pulse is cut off, stopping the output voltage. After the overvoltage, the circuit automatically resumes operation.

3.4 Hiccup mode to prevent breakdown caused by output overload, short circuit or overvoltage

When the power supply is overloaded, short-circuited or over-voltage, the VIPer22A device protection action reduces the duty cycle, the output voltage, and the UDD terminal voltage. When it drops below 8V, the entire circuit is closed, and then the next intermittent startup process begins with the internal high-voltage constant current source. This process is called "hiccup" protection. The working time is very short, only xxμs, and the stop time is very long, with a cycle of about 260ms. Therefore, the average power is extremely low, protecting the power supply from damage. Once the fault is eliminated, the power supply will be put into normal operation for easy operation. It is not difficult to understand that this working mode belongs to overcurrent and overvoltage protection, which is the external manifestation of these two functions. Its cycle is similar to the intermittent cycle of the fault described in Section 4.2.1 below.

3.5 Undervoltage Lockout

When the grid voltage is too low, or the power supply fails and the UDD voltage is lower than 8V, the chip stops outputting the trigger pulse width and the power supply stops outputting. When the grid voltage rises or the power supply failure is eliminated and the UDD voltage returns to the range of (8-14.5)V, normal operation will automatically resume.

4. Main fault examples

4.1 DC output voltage is zero

4.1.1 The primary winding of the power transformer is disconnected

Due to the low power and high input voltage of the transformer, the primary winding wire is thinner, with a wire diameter of about 0.2mm. If the winding process is not perfect, the wire end is more likely to corrode or break at the connection with the skeleton terminal. Such failures account for about 15% of the total power failures, but the damage rate inside the transformer is zero. There are two reasons for the open circuit of the primary winding end.

(1) The enameled wire and the frame are disconnected shortly after leaving the factory

The wire winder is a novice. After fixing the end of the thin enameled wire on the bobbin terminal, he tightens it with a large force and then winds the wire, which makes the contact between the wire and the bobbin bear a large prestress. Therefore, the contact between the enameled wire and the bobbin is often disconnected shortly after leaving the factory - everyone can wind wires, but the quality varies greatly.

(2) The end of the enameled wire is corroded and broken by the paint stripper

For the sake of convenience, transformer manufacturers often use enameled wire paint strippers to remove the paint film on the ends of enameled wires. After the paint film is removed, the paint strippers remaining on the ends are not cleaned. The chemical reaction between the paint strippers and copper is also very strong - the paint strippers contain sulfuric acid, which will soon corrode the enameled wire ends and completely break. This must be paid attention to by transformer manufacturers.

4.1.2 Secondary circuit of power transformer is broken

The reason is roughly the same as 3.1.1.

4.1.3 Power converter module N2 internal burnout

This kind of low-power DC power supply made of VIPer22A device, when the power supply end is not connected to the protective neutral line (PE), the unexpected high voltage from the outside may act on the DC power supply circuit through the chassis, causing the N2 device and related components to be destroyed. When the power supply is only used as an auxiliary power supply for large circuits, especially as an auxiliary power supply for high-voltage power supplies, the auxiliary power supply will often be affected by problems in the use of the main power supply.

Therefore, in order to protect the reliable and safe operation of the power supply, it is necessary to add a protective neutral line (PE) at the mains input end. Today's mains distribution lines, whether for industrial or residential use, are all equipped with phase lines (L), neutral lines (N) and protective neutral lines (PE). Practice has shown that when the protective neutral line (PE) is added to the power supply end, the VIPer22A device has a very high degree of stability and reliability.

4.1.4 Rectifier tube VD7 is broken

The reasons for the burning of the rectifier tube VD7 are: ① The rated parameters were not tested before welding; ② The magnetic gap of the transformer was not adjusted properly, resulting in a large output peak voltage, and the reverse voltage of the VD7 tube exceeded or approached the rated reverse voltage, which was not discovered during debugging. In the switching power supply, the production and inspection of the transformer are very important.

4.2 The DC output voltage is pulsating, and the pulsation amplitude is equal to 12V

Under normal circumstances, when the power is turned on, the high-voltage current source in the VIPer22A converter starts running and automatically starts the power supply. When the voltage at the UDD terminal reaches the turn-on voltage value VDDON = 14.5V (typical value), the high-voltage current source is turned off, the power MOSFET starts working, and the auxiliary winding W2 starts to supply power to the chip. At this point, the VIPer22A converter completes the startup procedure.

When the auxiliary winding W2 circuit is broken, or the UDD terminal is short-circuited to the negative terminal (pins 1 and 2) of the N2 device, the power supply is automatically started intermittently by the high-voltage current source in the converter, and the voltage on the UDD terminal cannot be maintained within the range of (8-14.5) V. N1 will stop running when it is below 8 V, so the output voltage will pulsate, as shown in Figure 4. The reason why the pulsation amplitude is still equal to 12 V is that in addition to the transformer auxiliary winding W2 circuit being broken, the output winding W3 and the sampling voltage, reference voltage and optocoupler are still operating normally, limiting the amplitude of the output voltage.

The transformer auxiliary winding W2 loop includes: the auxiliary winding itself; the rectifier tube VD6; the capacitor C9; and the related printed circuit board copper foil connection. This also verifies the content of the previous working principle: the high-voltage current source in the VIPer22A converter is only put into operation when the AC220V mains power is connected and the UDD terminal voltage is as low as 8V, and has a certain working cycle; in other cases, it is in the off state.

5 Conclusion

The VIPer22A, a monolithic switching power converter module in a common DIP-8 package, has a built-in current control PWM, a self-sufficient power supply, and automatically adds a self-excited power supply after it is turned on; at the same time, it also has a built-in 730V/0.7A power MOSFET. The circuit structure is concise, and the protection functions such as overheating, overcurrent and overvoltage are complete. It has high voltage stabilization accuracy, fast response speed, high stability and reliability. It is suitable for occasions with large power grid fluctuations and load changes, and is also easy to run in parallel. The switching power supply made with this integrated circuit, like other electronic products, can standardize the following matters in the process of assembly, welding, debugging and use: ① Connect the protection neutral line (PE) at the mains input terminal, ② Standardize the transformer manufacturing process, ③ Stabilize the purchase channel of electronic components, ④ Increase inter-process detection, and ⑤ Perform full-power test and assessment of the whole machine, and the whole machine will run perfectly.