Monolithic switching power supply and high voltage power supply

andybeck

Monolithic switching power supply and high voltage power supply [Copy link]

With the rapid development of modern science and technology, the continuous improvement and updating of power devices, and the increasingly perfect development of PWM technology, the switching power supply is developing in the direction of short, small, light and thin. Since the advent of the monolithic switching power supply integrated circuit in 1984, it has now become a new product with development and influence. The switching power supply integrated chip represented by the TOPSwitch series integrates the control system drive circuit, power tube MOSFET, pulse width modulator, high-voltage startup circuit, loop compensation circuit, fault protection circuit, etc. in a monolithic IC with only three pins. All the functions required for a 100kHz pulse width modulation regulated power supply are integrated inside it, and a high-voltage bias current source, bias shunt regulator, error voltage amplifier, oscillator, and bandgap reference voltage source are added.
In general, the drain D of the TOP switch device is connected to one end of the primary of the transformer. The reverse peak voltage caused by the leakage inductance is reflected to the primary of the transformer and will be directly added to the drain. The reverse peak voltage is related to the output voltage, that is, the higher the output voltage, the higher the reverse peak voltage. For the TOPSwitch device with a withstand voltage of only a few hundred volts between the drain and the source, excessive voltage will easily cause it to break down. Therefore, most of the switching power supplies made with TOPSwitch series devices use low-voltage and low-power output. This article realizes the application of TOPSwitch devices in high-voltage switching power supplies by improving the circuit.
1. TOPSwitch series monolithic integrated switching power
supplies Since the application of switching power supplies in the 1970s, many fully functional integrated control circuits have emerged, making the switching power supply circuit increasingly simplified, the operating frequency continuously increased, the efficiency greatly improved, and providing broad prospects for the miniaturization of switching power supplies. The three-terminal offline pulse width modulation monolithic switching integrated circuit TOP (Three Terminal Off Line) combines the PWM controller and the power switch MOSFET into one package and has become the mainstream of the development of switching power supply ICs. Using TOP switch integrated circuits to design switching power supplies can greatly simplify the circuit, further reduce the volume, and significantly reduce the cost. Switching power supplies have significant advantages such as monolithic integration, minimal peripheral circuits, optimal performance indicators, and the ability to form a switching power supply without an industrial frequency transformer. The three-terminal offline pulse width modulation monolithic switching integrated circuit developed by the American Power Company (POWER) is the first in the world and is known as the "top switching power supply". Its first generation of products is represented by the TOP100/200 series launched in 1994, and later launched TOPSwitch II, TOPSwitch-FX, TOPSwitch-GX, TinySwitch, TinySwitch II, LinkSwitch, LinkSwitch-TN, DPA-Switch and other series of products. Although each series of products has the common advantages of monolithic switching power supplies, they each have their own unique characteristics.
The TOPSwitch-II series has the least external components and is the most convenient to design. Compared with the TOPSwitch series devices, its internal circuit has been improved in many ways, and the device's sensitivity to circuit board layout and input bus transients has been greatly reduced, so the design is more convenient, the performance has been enhanced, and the performance-price ratio is higher. Its power is increased from 0-100W of the TOPSwitch series to 0-150W. In the practical application of low voltage and medium power, the TOPSwitch-II series is the most widely used.
The internal structure of this series of products is shown in Figure 1. Its basic working principle is to use the feedback current Ic to adjust the duty cycle D to achieve the purpose of voltage stabilization. When the output voltage Uo rises, the optocoupler feedback circuit makes IC↑, D↓, Uo↓, and finally keeps Uo stable. This chip is used in PWM control mode with a switching frequency of 100k.
2. Working principle of TOPSwitch-II
The TOPSwitch-II series of single-chip power supply integrated circuits can be widely used in various general and special switching power supplies, standby power supplies, and switching power supply modules. It integrates all the functions of the PWM control system into a three-terminal chip. It contains a pulse width modulator, a power switch field effect tube (MOSFET), an automatic bias circuit, a protection circuit, a high-voltage startup circuit, and a loop compensation circuit. The output end is completely isolated from the power grid through a high-frequency transformer, which truly realizes the monolithic integration of reactive frequency transformers and isolated switching power supplies, and is safe and reliable to use. Because the manufacturing process uses BCD technology, the power consumption of the device is significantly reduced. TOPSwitch-II does not require an external high-power overcurrent detection resistor, and the external bias current does not need to be provided at startup. It is an AV/DC power converter with an open-drain output and uses current to linearly adjust the duty cycle, that is, a current-controlled switching power supply.
The basic principle of the pulse width modulation switching power supply is shown in Figure 1. The AC 220V input voltage is rectified and filtered to become a DC voltage V1, which is then chopped by the power switch tube VT (or MOSFET) and stepped down by the high-frequency transformer T to obtain a high-frequency rectangular wave voltage. Finally, the required DC output voltage Vo is obtained through the output rectifier filter VD and C2. The pulse width modulator is the core of this type of switching power supply. It can generate a drive signal with a fixed frequency and adjustable pulse width, control the on and off state of the power switch tube, and adjust the output voltage to achieve the purpose of voltage stabilization. The sawtooth wave generator provides a clock signal. The error amplifier and PWM comparator form a closed-loop regulation system. If Vo drops for some reason, the pulse width modulator changes the pulse width of the drive signal, that is, changes the duty cycle D, so that the average voltage after chopping increases, causing Vo to rise. Vice versa.
3. Application of TOPSwitch-Ⅱ
Based on the characteristics of the TOPSwitch circuit, we have realized a dual-output power supply with one high voltage and one low voltage. For the high-voltage output, we use the secondary boost and 6-fold voltage rectification method to obtain one of the DC high-voltage outputs: 2500V, 40mA. Since this article is a discussion of the high-voltage application of TOPSwitch, the output voltage accuracy of this channel is not required; the other low-voltage output is 6V/1A, with an accuracy better than 1%. Since the accuracy requirements of the high-voltage part are not high, it can be used as a direct output, but the accuracy requirements of the low-voltage part are higher. Therefore, at the voltage output end of this channel, the output voltage is sampled through a sampling resistor and input into TL431. TL431 is a three-terminal adjustable shunt reference source with good thermal stability. Its output voltage can be arbitrarily set to any value from Vref (2.5V) to 36V with two resistors. The sampled voltage is sent to the control end of TOPSwitch after being isolated by an optocoupler. The optocoupler supply voltage can be powered by the secondary output voltage of the transformer.
Since the Flyback converter has a simple circuit and uses fewer components, it is suitable for multi-output occasions. We use the Flyback converter based on TOPSwitch-II to achieve it. The required dual output voltage is obtained through multiple secondary windings of a transformer. In addition, the two output voltages are at different potentials, and the voltage difference between them is large, so the isolation of the transformer is very high, which is also a difficulty in the implementation of this power supply.
1. Selection of TOPSwitch chip
The input voltage of this power supply is 270VDC, which is within the voltage range required by this series. The selection of the chip is mainly based on the power requirement. The calculated output power Po is about 110W. Considering a certain margin, we choose the chip model TOP226Y. The specific implementation circuit diagram is shown below:
2. Feedback circuit design
The form of the feedback loop is determined by the output voltage accuracy. The "optocoupler + TL431" in this solution can control the output voltage accuracy within ±1%. The voltage feedback signal is guided into the Ref terminal of TL431 through the voltage divider network, converted into a current feedback signal, and input into the control terminal of TOPSwitch after optocoupler isolation.
The optocoupler works in a linear state and plays an isolation role. If the upper limit of the current amplification factor of the selected optocoupler exceeds 200%, it is easy to cause TOPSwitch overvoltage protection. On the contrary, if its lower limit is less than 40%, the duty cycle D will not decrease with the increase of the feedback current, resulting in overcurrent. Therefore, an optocoupler with a current amplification factor range close to 100% should be selected.
3. Transformer design
The design of the transformer is the most important part of this circuit design. The design of transformer T1 can be carried out according to the following steps:
① The core selection can be selected by the widely used AP method. First, calculate the AP value according to the formula, and then select a group of cores similar to it from the existing core models.
In fact, this method is very complicated to calculate, and many parameters need to be obtained by looking up tables or based on experience. Therefore, in practice, the size of the core is generally selected based on experience and the output power. Adjust it after the design is completed. For this example, although the output power is not large, the two output voltages are very different, and good insulation and isolation measures are required. Therefore, when selecting a core, a core with a slightly larger window area should be selected. We choose the TDK company model EPC-25 core. The main parameters of the core are: effective cross-sectional area: Ae=46.4, saturation flux density is selected as 0.35T.
② The primary peak current

is Pout=110W, Vin=270V, D=0.5, and Ip=1.63A is obtained.
③Primary inductance

Vin=270V, D=0.5, Ip=1.63A, f=100k Substitute Lp=0.83mH.
④Primary turns

In the above formula, Vinmin=270V, D=0.5, Ae=0.0000464, the working magnetic flux density B is 0.3T, f=100k, substitute Np=96.98, take Np=97.
⑤Secondary winding

For the high voltage output part, take Vo1=60V, Vinmin=270V, D=0.5, Np=97 to calculate Ns1=21.56, take Ns1=22. For the low voltage output part, taking into account the voltage drop of the diode, take Vo=7V, it can be known that Ns2=2.51, take Ns2=3.
⑥Air gap length

In which, combined with the known quantities above, it can be known that lg=0.06mm. In fact, it is difficult to grind such a small air gap accurately. A more practical method is to grind manually and measure at the same time until the inductance value meets the requirements.
⑦ Selection of coils
Since the insulation strength between the transformer windings is very high, we choose three-layer insulated wire instead of traditional enameled wire for winding. The outer wall thickness of the wire is 0.1mm, but it can withstand voltage of more than 7000V. The insulating tape can basically be omitted in the middle. For the input and high-voltage windings, due to the small current, we choose a single wire with an inner diameter of 0.16mm. For the low-voltage part, due to the large current, we use 5 wires of this specification in parallel. The three-layer insulated wire can not only enhance the isolation of the transformer, but also save the insulating tape used for isolation between the windings, saving volume.
For the design of the step-up transformer T2, the following parameters can be obtained by referring to the design of T1 above:
Np'=22, Ns'=152, lg=1.25mm.
4. Precautions for using TOPSwitch
In applications, the source pin lead of the TOP switch should be very short, the bypass capacitor should be as close to the source pin and the control pin as possible, and the source pin should be grounded at a single point.
When the switch is turned off, in order to reduce the drain peak voltage and the drain impulse excitation oscillation voltage, a damping resistor or a box position circuit must be added to the primary of the high-frequency transformer with large leakage inductance.
During testing, the TOP switch device cannot be directly inserted into the integrated circuit socket with high voltage. On the printed circuit board, the external capacitor of the control pin can transmit a large surge current to the trigger and the shutdown latch, thereby turning off the TOP switch.
When the switching power supply is lightly loaded or unloaded, the output MOSFET conduction time is very short. In order to keep the output voltage within the design range, the switching power supply must be added with a very small load.
IV. Test results and waveforms
The high and low voltages of the two outputs of this design meet the index requirements. The experimental waveforms are shown in the figure. The left figure is the waveform at both ends of TOPSwitch226Y, the left figure is the waveform at both ends of TOP226Y, and the right figure is the waveform at both ends of the primary side of transformer T1.
V. Conclusion
As an integrated chip, TOPSwitch-II greatly simplifies the design process of switching power supplies. It can be used not only for low-voltage power supplies but also for high-voltage power supplies.