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What is a switching power supply

Source:    Author:新望    Release time:2017-08-22    Views:3434
Switching power supply is through high-speed circuit control switch opening and closing. The direct current into high frequency AC transformer provided to the transformer to produce the required voltage of one or more groups! Into a high-frequency alternating current is due to high frequency AC transformer circuit in the transformer efficiency is much higher than 50HZ. Therefore, a small switching transformer can be done, and work
Switching power supply is through high-speed circuit control switch opening and closing. The direct current into high frequency AC transformer provided to the transformer to produce the required voltage of one or more groups! Into a high-frequency alternating current is due to high frequency AC transformer circuit in the transformer efficiency is much higher than 50HZ. So switching transformer can do small and not very hot at work! ! Low cost. If it is not the high-frequency switching power supply into 50HZ does not make sense! ! Switching transformer is not mysterious. Is an ordinary transformer! This is the switching power supply.

Switching power supply can be roughly divided into two kinds of isolated and non-isolated, isolation-type transformers must be a switch, rather than isolation may not necessarily have.

In short, switching power supply works:
1. AC power input into DC by the rectifier filter;
2. By high-frequency PWM (pulse width modulation) signal control switch, the DC will be added to switch on the transformer primary;
3. Switch induces a high frequency transformer secondary voltage, the rectifier filter supply the load;
4. The output of the circuit through a certain part of the feedback to the control circuit, control PWM duty cycle to achieve a stable output purposes.

AC power input Ecuador flow through the circle generally a class of things, the interference filter power-down line, but also filter out the interference power on the grid;
The same in power, the higher the switching frequency, the smaller the size of the transformer switches, but switch to the higher requirements;
Switch can have multiple secondary transformer winding or a winding more than one tap to get the required output;
General should also increase the number of protection circuits, such as no-load, short circuit protection, or they might burn down switching power supply.

Switching power supply that is above the general works.
In fact, now has a very high integration ASIC can make a very simple external circuit, or even be free debugging.
For example, switching power supply chip TOP series (or module), as long as with a number of RC elements, and a switching transformer, can make a basic switching power supply.
PWN-PFM switching power supply control circuit
Zouhuai An Zhang Rui Hu Rongqiang
College of Automation, Wuhan University of Technology

1. Introduction

Switching power supply due to the size, weight, efficiency and reliability, a lot of advantages, current computer, telecommunications, household appliances, radar, space technology and other fields have been completely replaced the traditional linear power supply. Switching power supply control technology there are three: (1) pulse width modulation (PWM); (2) pulse frequency modulation (PPN); (3) the frequency pulse width modulation (PWN-PFH). PWM which is currently the most widely used in switching power supply in a controlled way, it is characterized by low noise and high time at full load and can operate in continuous conduction mode, and now there are a variety of market performance, low price of PWM integrated chips, such as UCl842/2842/3842, TDAl6846, TL494, SGl525/2525/3525 etc.; PFM has the advantages of static power consumption, but it does not limit the function can not work in continuous conduction mode, with the integration of PFM function chip has MAX641, TL497, etc.; PWM-PFM combines the advantages of PWM and PFM, but the market does not function with the PWM-PFM single integrated chip to limit the sale of its applications.

This paper presents a PWM-PFM controlled by switching power supply, on which the control circuit - PWM-PFM circuit principle of a more detailed analysis, the control circuit component count and achieve simple.

2. Switching power supply and control circuit described PWH-PFH

The switch power supply input for the three-phase AC input voltage range of 320 ~ 575V, frequency 45 ~ 65Hz, output specifications for 24V/10A, weighing only about 1.4Kg. The main circuit switching power supply for the two-transistor flyback topology change type, by using two switches in series to reduce high-power MOS FET turn-off voltage requirements.

PWM-PFM control circuit consists of three digital chip (4011,4066,40106) and a few resistors, capacitors, diodes, transistors formed circuit shown in Figure 1. Among them, 4011 for two four-input NAND gate, four-way analog switch 4066, 40,106 for the six Schmitt trigger.

3. Principle Analysis

As shown in the schematic, which has four inputs, the signal output from coupler 1, coupler 2 outputs, the rectified high voltage DC terminal, and coupled with the switching transformer, an output to drive high-power MOS field Effect of tube. The core part of the circuit is composed of two NAND gates (IN3a/IN3b/OUT3 and IN4a/IN4b/OUT4) composed of RS flip-flop. Flip-flop has two inputs: 40ll 9 feet (IN3b) and 12 feet (IN4a); two output 401l 10 feet (OUT3) and 11 feet (OUT4). OUT4 output signal sent to the five parallel Schmitt trigger (I1/O1, I2/O2, I3/O3, I4/O4 and I5/O5). The role of Schmitt trigger NAND gate setting the output waveform, the circuit to prevent malfunction. Schmitt trigger with five parallel redundant design of the circuit's current drive to improve capacity and enhance the reliability of the entire circuit. After shaping the waveform formed by two complementary transistor totem pole high-power MOS FET drive, control the main circuit of the turn-on and off. Input for the on-time Ton IN3b control, input terminal for the off-time Toff IN4a control side, the two inputs are active low. When the low input IN4a trigger NAND gate 4 to output OUT4 as high electron, the Schmitt trigger switch after the shutdown; when IN3b low input NAND gate 3 when the trigger so that the output OUT3 high, low output OUT4 electronics, OUT4 low after by the Schmitt trigger is high, the switch conduction. OUT3 HIGH the analog switch 1 (IN1/OUTl/CONTROLl) conduction, the current through the coil Ll D5 and R17 to ground, to prevent magnetic saturation coil L1.

Starts, the coil Ll is no current, low on the ground, the NAND gate 1 (INla/IN1b/OUTl) and NAND gate 2 (IN2a/IN2b/OUT2) remains low after that input IN3b is low, the RS flip-flop and Schmitt trigger switch after the conduction, switching power supply output, the output voltage by comparing the feedback signal is amplified by the optocoupler 2 isolation, through R16, R17, R18, C5, C6, T4, R15 to IN4a, to switch off, PWM-PFM control circuit start-up.

Normal operation, the coil L1 switch transformer coupled signal (ie switch the signal) with the non-gate NAND gate l and after 2 to IN3b, the control switch on-time Ton, the output from the switching power supply feedback signal by coupler 2 isolation, through R16, R17, R18, C5, C6, T4, R15 to IN4a, the control switch in the off-time Toff. If the load and input voltage change, the switch RS flip-flop circuit by a constant frequency determined by the width and the cycle of on and off to maintain output voltage constant. If the load or input voltage changes, the coil Ll and optocoupler feedback signal at the same time 2 change, so that on-time Ton and the off-time Toff also change the control pulse width and frequency also changes, to achieve a PWM-PFM control.

Optocoupler l, output OVP, when the switching power supply output is normal, optocoupler 1 output is high, the low level after the Schmitt trigger, analog switch 3 (IN3/OUT3/CONTROL3) do not turn , OUT3 does not work; when the switching power supply output over-voltage, the optocoupler 1 output is low, the Schmitt trigger is high, after so CONTROL3 is high, analog switch 3 turns on, OUT3 terminal ground, IN4a terminal is low, switch off.

Rectified high voltage DC terminal for input over-voltage protection. When the AC input voltage is normal, T1 pipe saturated conduction, CONTROL4 is low, analog switch 4 (IN4/0UT4/CONTROL4) does not turn, OUT4 does not work; AC input voltage is too high, Tl tube cut-off, CONTROL4 is high, analog switch 4 turns on, OUT4 ground, IN4a ended output is low, switch off.

4. Conclusion

Switching power supply control circuit is the core of the control circuit to simplify the use of appropriate switching power supply design, and the excellent performance. With the PWM-PFM control circuit switching power supply, the output voltage ripple is very small (only tens of millivolts), AC input voltage range (320V ~ 575V in the context of work), high switching frequency (work when the frequency varies between 100 ~ 300KHz), small changes in duty cycle (from 15% to 35%), full time high (up to 90%), widely used in industrial electrical control cabinet, escalators and other power environment the place is relatively poor. Hard-switching technology of switching power supply is concerned, the PWM-PFM control circuit has good performance, measured also showed that when the load changes rapidly, the switching power supply stable output voltage drop, therefore, the circuit does not apply more severe changes in load situations.

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