TWI666860B - Flyback switching power supply with lps protection function - Google Patents

Abstract

A flyback switching power supply with a limited power supply (LPS) protection function includes a controller having a feedback pin and a driving pin. The feedback pin is coupled to the output terminal of the flyback switching power supply through a feedback circuit. The driving foot outputs a pulse width modulated (PWM) square wave, and the operating frequency of the PWM square wave in the heavy-load operating mode is greater than the operating frequency in the light-load operating mode. The controller includes an LPS detection module, which is coupled to the feedback pin and the drive pin. The LPS detection module is used to detect whether the voltage of the feedback pin is less than the voltage threshold, which is equal to the voltage of the feedback pin in the light load mode; If the feedback pin voltage is less than the voltage threshold, then it is detected whether the on-time of the PWM square wave is greater than the time threshold; and if the on-time of the PWM square wave is greater than the time threshold, the drive controller stops outputting the PWM square wave.

Description

Flyback switching power supply with limited power supply protection function

The present invention relates to a flyback switching power supply, and more particularly, to a flyback switching power supply with a Limited Power Source (LPS) protection function.

When the test of a switching power supply under normal conditions and single fault conditions meets the requirement that the output is not greater than 8A / 100W, the switching power supply is a kind of limited power supply (LPS). In the safety standard IEC 60950, information products are required to provide fire-resistant enclosures. However, when an information product is powered by a limited power supply (LPS), if the electronic parts are printed circuit boards installed above the flame retardant class V-1 , Then the information product may not need to provide a fire-resistant casing, that is, a flame-resistant HB casing may be used. In addition to the low price, the flame retardant HB shell material also has better physical properties and easily meets environmental protection requirements. Therefore, the switching power supply often adds some circuits for LPS protection, so that the test under normal conditions and single fault conditions can meet the requirement that the output is not greater than 8A / 100W, and then the flame-retardant HB enclosure can be used.

A single group of flyback switching power supply, such as the output 19V / 2A shown in Figure 1 to power the display. When the single-fault condition test of the short-circuit current detection resistor (ie, the resistor Rs) of the switching power supply is performed, since the controller U1 of the existing switching power supply does not protect against the short-circuit of the resistor Rs, the resistor Rs When the short circuit occurs, the voltage detected by the current detection pin CS of the controller U1 is always in a very small state, causing the controller U1 to always misjudge that the switching power supply is in a light load condition, so that the driving frequency of the driving pin DRI output of the controller U1 is relatively low A low (eg, around 20KHz) Pulse Width Modulation (PWM) square wave is applied to the power switch Q1 to reduce the power output to the load through the transformer T1, and the feedback pin COMP of the controller U1 detects the output terminal OUT The voltage returned will also be in a low voltage state at light load. When the resistor Rs is not short-circuited and the load is heavy, the drive pin DRI of the controller U1 will output a PWM square wave with a higher operating frequency (such as about 65KHz) to the power switch Q1 to increase the power output to the load through the transformer T1, and The COMP voltage of the feedback pin of the controller U1 will also be in a high voltage state during heavy load. In addition, the reciprocal of the operating frequency (represented by F) of the PWM square wave is the duty cycle (represented by T). Each duty cycle T consists of the on time (represented by Ton) and the off time (represented by Toff). The power switch Q1 is controlled to be turned on during the on time Ton, and the power switch Q1 is controlled to be turned off during the off time Toff. When the resistor Rs is short-circuited, the working frequency F of the PWM square wave is lower, that is, the working cycle T is longer, so the on-time Ton of the power switch Q1 will also be longer. Table 1 below is the test result of the flyback switching power supply on one of the existing power supply boards of the company under the single fault condition of normal condition and short circuit of the resistor Rs. 【Table 1】 Resistor Rs Input voltage Working frequency F Duty cycle T Power switch Q1 on time Ton Short-circuit fault 90V / 60Hz 20.4KHz 49μs 11μs 264V / 50Hz 20.4KHz 49μs 6.2μs No failure 90V / 60Hz 65.5.KHz 14.8μs 7.1 μs 264V / 50Hz 65.5.KHz 14.8μs 2.8μs

The longer the on-time Ton, the larger the current generated by the primary-side winding Np of the transformer T1. At the moment when the power switch Q1 changes from on to off, the auxiliary winding Na of the transformer T1 and its corresponding leakage inductance and the parasitic junction capacitance of the diode D1 undergo LC resonance, which will generate higher resonance energy through the diode D1. The rectifier charges the capacitor C1, so that the voltage of the capacitor C1 rises. When the output load becomes larger, the on-time Ton becomes larger, and the voltage of the capacitor C1 continues to rise. When the voltage of the capacitor C1 reaches the protection point of the internal overvoltage protection (OVP) of the power supply pin VCC of the controller U1, the controller U1 starts the protection function and does not work. Therefore, the flyback switching power supply of a single group output is supplemented with LPS protection by the OVP function inside the power supply pin VCC of the controller U1.

However, the flyback switching power supply with a single output is attached with LPS protection by the OVP function inside the power supply pin VCC of the controller U1, which has the following problems: 1. The voltage of the power supply pin VCC of the controller U1 is affected by the diode D1 itself The response speed of the transformer and the number of Na windings and the winding structure of the auxiliary winding of the transformer T1. When the diode D1 is replaced with a slow diode or the transformer T1 design is changed due to electromagnetic interference (EMI) and other problems, the output power before the controller U1 starts the OVP function will increase. Therefore, the LPS protection point is not accurate, and is easily affected by the characteristics of the diode D1 and the T1 design of the transformer, which makes it impossible to pass the LPS test. 2. In the existing design, when the LPS test of the resistor Rs short-circuit fault is performed under the input voltage of 264V, the output power when the LPS protection function is activated will usually fall around 80W ~ 100W. The output power from the LPS test standard of IEC 60950 requires output power. Not too close to 100W. If the diode D1 is replaced with a slow diode or the transformer T1 design is changed due to EMI and other problems during the test run, the output power will easily exceed 100W before the LPS protection function is activated.

Double-group output flyback switching power supply, for example, one group outputs 5V / 2.5A to power the display motherboard circuit, and the other group outputs 16V / 1A to power the display backlight boost driver circuit. In order to pass the single fault condition test of the short-circuit current detection resistor on the primary side of the switching power supply, it is common practice to connect a 4A / 250V slow-blow fuse in series at the 5V output terminal with a higher current and a 16V output terminal with a lower current. Connect a 0 ohm resistor in series such as model 1206 for fuse. However, the flyback switching power supply with dual output outputs protects the LPS by connecting a fuse in series at the output end, which increases the design cost.

The purpose of the present invention is to provide a flyback switching power supply with LPS protection function, in order to perform LPS protection against a single fault condition of a short-circuit of a primary-side current detection resistor of a switching power supply, and the LPS protection point is accurate and the design cost is low.

In order to achieve the above or other objectives, the present invention provides a flyback switching power supply with an LPS protection function, which has an input end and an output end. Flyback switching power supply includes power switch, transformer, resistor, feedback circuit and controller. The power switch has a first terminal, a second terminal, and a control terminal. The control terminal controls the first terminal and the second terminal to be turned on or off. The transformer has a primary-side winding and a secondary-side winding, two ends of the primary-side winding are respectively coupled to the input end and the first end of the power switch, and two ends of the secondary-side winding are respectively coupled to the output end and the ground. The two ends of the resistor are respectively coupled to the second end of the power switch and the ground. One end of the feedback circuit is coupled to the output end. The controller has a feedback pin, a driving pin and a current detection pin, the feedback pin is coupled to the other end of the feedback circuit, the driving pin is coupled to the power switch control end, and the current detection pin is coupled to the second end of the power switch. The controller outputs a PWM square wave from the driving pin, and the operating frequency of the PWM square wave in the heavy-load working mode is greater than the operating frequency in the light-load working mode. The controller includes an LPS detection module. The LPS detection module is coupled to the feedback pin and the driver pin, and is used to detect whether the voltage of the feedback pin is less than the voltage threshold, which is equal to the voltage of the feedback pin in the light load working mode. If the pin voltage is less than the voltage threshold, then it is detected whether the on-time of the PWM square wave is greater than the time threshold; and if the on-time of the PWM square wave is greater than the time threshold, the drive controller stops outputting the PWM square wave.

In an embodiment of the present invention, if the feedback pin voltage is not less than the voltage threshold, the driving controller normally outputs a PWM square wave.

In an embodiment of the present invention, if the on-time of the PWM square wave is not greater than the time threshold, the driving controller normally outputs the PWM square wave.

The present invention adds an LPS detection module inside the controller of the flyback switching power supply. The LPS detection module determines whether to perform LPS protection by detecting the voltage of the feedback pin and the on-time of the PWM square wave output by the driving pin. With this LPS protection function, the LPS detection module will make a correct decision to activate the LPS protection function when a single fault condition test of the primary-side current detection resistor of the switching power supply is shorted. Adding the LPS detection module is easy to implement inside the existing controller, and it basically does not affect the design cost of the controller. The invention overcomes the problem of inaccurate protection points caused by the existing single-group output flyback switching power supply through the OVP function inside the power supply foot of the controller, which also brings inaccurate protection points, and also overcomes the existing dual-group output flyback type The switching power supply causes LPS protection by connecting a fuse in series at the output end, which causes an increase in design costs.

In order to make the description of this disclosure more detailed and complete, the following presents an illustrative description of the implementation modes and specific embodiments of the present invention, but this is not a limited form of implementing or applying the specific embodiments of the present invention. In addition, some conventional components in the circuit that are relatively unrelated to the technical features of the present invention will be omitted from the description, but will still be marked with the following component symbols in the drawings. As used herein, when an element is referred to as being "connected" or "coupled" to another element, it can be that the element is directly connected or coupled to the other element; or that there is a Or more additional elements, that is, one element is connected to another element via one or more additional elements. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no additional elements there.

Please refer to FIG. 2, which is a circuit diagram of a single output flyback switching power supply according to an embodiment of the present invention. The flyback switching power supply has an input terminal IN and an output terminal OUT. The input terminal IN receives AC mains power, such as 90V ~ 264V / 50Hz or 60Hz, and the output terminal OUT outputs 19V / 2A power to the display. The flyback switching power supply includes a power switch Q1, a transformer T1, a resistor Rs, a feedback circuit and a controller U2. The power switch Q1 has a first terminal, a second terminal, and a control terminal. The control terminal controls the first terminal and the second terminal to be turned on or off. In this embodiment, the power switch Q1 is an n-channel metal oxide semiconductor field effect transistor. (Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)), the first terminal, the second terminal and the control terminal are respectively a drain terminal, a source terminal and a gate terminal. Transformer T1 has a primary-side winding Np and an auxiliary winding Na on the primary side, and transformer T1 has a secondary-side winding Ns on the secondary side. The two ends of the primary-side winding Np are respectively coupled to the input terminal IN and the first end of the power switch Q1. The two ends of the stage-side winding Ns are respectively coupled to the output terminal OUT and the ground (the ground on the secondary side). The two ends of the resistor Rs are respectively coupled to the second end of the power switch Q1 and the ground (the ground on the primary side). One end of the feedback circuit is coupled to the output terminal OUT. The controller U2 has a feedback pin COMP, a driving pin DRI, a current detection pin CS and a power supply pin VCC. The feedback pin COMP is coupled to the other end of the feedback circuit, the driving pin DRI is coupled to the power switch Q1 control terminal, and the current detection pin CS is coupled. Connected to the second end of the power switch Q1, and the auxiliary winding Na is coupled to the rectifier filter circuit composed of the diode D1 and the capacitor C1 to supply power from the power supply pin VCC to the controller U2. The controller U2 outputs a PWM square wave from the driving pin DRI, and the operating frequency of the PWM square wave in the heavy-load operating mode (such as about 65KHz) is greater than the operating frequency in the light-load operating mode (such as about 20KHz). Compared with the existing flyback switching power supply shown in FIG. 1, the flyback switching power supply of the present invention adds a LPS detection module inside the controller U1 shown in FIG. 1 to form the controller U2, and the LPS detection module is coupled The feedback pin COMP and the driving pin DRI are connected to detect the ON time of the feedback voltage of the COMP pin and the PWM square wave output from the driving pin DRI to determine whether to activate the LPS protection function.

Please refer to FIG. 3, which is a flowchart of an LPS protection function activation mechanism of the controller U2 shown in FIG. 2. In step S1, the LPS detection module detects the COMP voltage of the feedback pin. In step S2, the LPS detection module determines whether the COMP voltage of the feedback pin is smaller than a voltage threshold, where the voltage threshold is equal to the COMP voltage (eg, 1V) of the feedback pin in a light load working mode. When it is judged that the COMP voltage of the feedback pin is not less than the voltage threshold value, it means that the flyback switching power supply is not in the light load working mode, so step S3 is performed, and the LPS detection module drive controller U2 normally outputs a PWM square wave. When it is determined that the COMP voltage of the feedback pin is less than the voltage threshold, it indicates that the light-load operation mode or the short-circuit of the primary-side current detection resistor Rs has occurred, so step S4 is performed for further judgment. In step S4, the LPS detection module detects the on-time of the PWM square wave output from the driving pin DRI. In step S5, the LPS detection module determines whether the on-time is greater than a time threshold (eg, 4 μs). When it is judged that the on-time is not greater than the time threshold, it indicates that it is in the light-load working mode, so step S6 is performed, and the LPS detection module driving controller U2 normally outputs a PWM square wave. When it is judged that the on-time is greater than the time threshold, it means that the primary-side current detection resistor Rs is short-circuited, so step S7 is performed, the LPS detection module starts the LPS protection function, and the drive controller U2 stops outputting a PWM square wave.

Please refer to FIG. 2 to FIG. 6 at the same time, FIG. 4 and FIG. 5 are the internal circuit diagrams and the waveforms of the internal nodes of the controller U2 shown in FIG. 2, and FIG. Graph of wave operating frequency F. The working principle of the LPS protection function of the controller U2 is described below.

When the flyback switching power supply is in the heavy-load working mode, the feedback pin COMP of the controller U2 receives the feedback signal from the feedback circuit to learn the output terminal OUT voltage state, so the feedback pin COMP voltage Vcomp (hereinafter referred to as the feedback pin voltage Vcomp) In a high-voltage state under heavy load, for example, the voltage Vcomp of the feedback pin is greater than 1.2V, and the operating frequency F of the switching power supply is at the highest 65KHz (as shown in Figure 6). The negative input terminal voltage Va = (Vcomp-V _D ) × Ra2 / (Ral + Ra2) of the internal PWM comparator of the controller U2 is also high, where V _D is the on-voltage drop of the diode Da. The positive input terminal voltage Vb of the PWM comparator in the controller U2 receives the voltage signal sampled from the current detection pin CS processed by the Leading-Edge Blanking (LEB) module and the slope compensation module. When Vb> Va, the PWM comparator outputs a high level trigger signal Vc, so that the PWM square wave output from the driving pin DRI changes from a high level to a low level, that is, the power switch Q1 changes from on to off, and The voltage Vcs of the current detection pin CS = (Vin × Ton / L + Ip0) × Rs, where Vin is the voltage across the primary-side winding Np when the power switch Q1 is on, Ton is the on-time of the power switch Q1, and L is the primary-side winding Np inductance, Ip0 is the initial current of the primary winding Np when the power switch Q1 is turned on, Ip0 ≧ 0A. When the switching power supply output load is heavier, the voltage Va will be higher, the PWM square wave high level of the driving pin DRI and the on-time Ton of the corresponding power switch Q1 will also be longer, and the transformer T1 will output more power in a unit cycle To the load, when the transformer T1 works in the discontinuous mode, the energy output per unit period E = Vin ² × Ton ² / 2L. Conversely, when the output load of the switching power supply becomes lighter, the on-time Ton of the power switch Q1 will become shorter, and the output power of the transformer T1 within a unit period will decrease. Therefore, when the switching power supply is in the heavy-load working mode, because the feedback pin voltage Vcomp is greater than 1.2V, that is, greater than the voltage threshold set by the LPS detection module (such as 1V), the LPS detection module performs the steps shown in FIG. 3 S1, S2, and S3 without activating the LPS protection function.

When the output load of the flyback switching power supply changes from heavy to light, under the condition that the input voltage of the switching power supply does not change, the PWM square wave high level of the driving pin DRI of the controller U2 and the corresponding on-time of the power switch Q1 Ton will get smaller. When the load is getting lighter and lighter, in order to reduce the switching loss of the power switch Q1, the controller U2 will work in a frequency reduction manner. When the output load of the switching power supply is reduced to a certain value, the operating frequency F of the switching power supply is changed from 65KHz in the original heavy-load mode to about 20KHz in the light-load mode. As shown in Figure 6). The on-time Ton of the power switch Q1 will change from 2.8μs at 264V heavy load to 1.6μs at light load, and from 7.lμs at 90V heavy load to 2.7μs at light load (as shown below) Table 2). When the output load continues to decrease, the operating frequency F of the switching power supply will also enter Burst Mode, a more energy-efficient working mode. Therefore, when the switching power supply output load is changed from heavy load to light load, although the voltage Vcomp of the feedback pin will be reduced to less than lV, the on-time Ton of the power switch Q1 also becomes very small, such as 1.6 μs when the input voltage is 264V And 2.7μs at 90V input voltage, which is less than the time threshold set by the LPS detection module (such as 4μs), so the LPS detection module performs steps S1, S2, S4, S5 and S6 shown in Figure 3 without Activate the LPS protection function.

When the single fault condition test of the primary-side current detection resistor Rs of the flyback switching power supply is short-circuited, the impedance of the current detection pin CS to ground becomes very small. When the output load changes from light to heavy, The voltage detected by the current detection pin CS is very small, so that the voltage Vcomp of the feedback pin is always at a minimum value, such as about 0.6V. Under the condition of Vcomp = 0.6V, the controller U2 will mistakenly think that the switching power supply is in the light-load operating mode, and the operating frequency F will be reduced to within 20KHz (as shown in Figure 6). Under the condition that the output load is unchanged, When the operating frequency is reduced, in order to ensure that the transformer T1 can still output the same energy at a higher operating frequency in a unit time, the drive pin DRI of the controller U2 will provide a longer on time Ton to drive the power switch in a unit cycle. Q1 is on. Table 2 below shows the test results of the flyback switching power supply of the present invention under a single fault condition of a normal condition and a short circuit of the resistor Rs. 【Table 2】 Resistor Rs Input voltage Working frequency F Duty cycle T Power switch Q1 on time Ton Short-circuit fault output 19V / 2A 90V / 60Hz 20.4KHz 49μs 11μs 264V / 50Hz 20.4KHz 49μs 6.2μs No fault output 19V / 2A 90V / 60Hz 65.5KHz 14.8μs 7.1 μs 264V / 50Hz 65.5KHz 14.8μs 2.8μs No fault output 19V / 0.3A 90V / 60Hz 20.4KHz 49μs 2.7μs 264V / 50Hz 20.4KHz 49μs 1.6μs

Therefore, when the single-fault condition test of the primary-side current detection resistor Rs of the flyback switching power supply is short-circuited, as long as the output has an appropriate load, the protection conditions of the LPS detection module can be achieved, such as the feedback pin voltage Vcomp = 0.6V <voltage threshold = 1V, on-time Ton = 6.2μs> time threshold = 4μs, so the LPS detection module executes steps S1, S2, S4, S5, and S7 shown in FIG. 3 to activate the LPS protection function.

In this embodiment, the working frequency in heavy load and light load working modes are about 65KHz and 20KHz, respectively, but it is not limited to this. The working frequency can be different according to the needs of different controllers; the voltage of the LPS protection point The threshold is set to 1V and the time threshold is set to 4 μs, but it is not limited to this. The voltage threshold and time threshold can be set according to the actual situation of the flyback switching power supply. In addition, although this embodiment is a flyback switching power supply with a single group output, the controller therein can still be applied to a flyback switching power supply with a dual group output.

Although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications and retouching without departing from the spirit and scope of the present invention. The scope of protection shall be determined by the scope of the attached patent application.

IN‧‧‧Input

OUT‧‧‧output

C1 ～ C5‧‧‧Capacitors

D1, D2, Da‧‧‧ Diodes

L1‧‧‧Inductor

Q1‧‧‧Power Switch

R1 ～ R3, Rs, Ra, Ra1, Ra2‧‧‧ resistors

T1‧‧‧Transformer

Np‧‧‧Primary side winding

Ns‧‧‧Secondary winding

Na‧‧‧ auxiliary winding

U1, U2‧‧‧ controller

COMP‧‧‧Feedback

DRI‧‧‧Drive feet

CS‧‧‧Current detection pin

VCC‧‧‧Power supply pin

Vcomp‧‧‧Feedback pin voltage

Va, Vb, Vc, Vd, Ve‧‧‧ Voltage

Vref1, Vref2‧‧‧ reference voltage

F‧‧‧Working frequency

T‧‧‧ duty cycle

Ton‧‧‧on time

Toff‧‧‧off time

S1 ～ S7‧‧‧‧Steps for activating LPS protection function

FIG. 1 is a circuit diagram of a conventional single-output flyback switching power supply; FIG. 2 is a circuit diagram of a single-output flyback switching power supply according to an embodiment of the present invention; FIG. 3 is a controller according to an embodiment of the present invention Flow chart of the LPS protection function activation mechanism; Figure 4 is an internal circuit diagram of a controller according to an embodiment of the present invention; Figure 5 is a waveform diagram of internal nodes of a controller according to an embodiment of the present invention; The graph of the feedback pin voltage and the PWM square wave operating frequency of the controller of the embodiment.

Claims (3)

A flyback switching power supply with a limited power source protection function has an input end and an output end. The flyback switching power supply includes a power switch having a first end, a second end, and a control end. The control terminal controls the first terminal and the second terminal to be turned on or off. A transformer has a primary-side winding and a primary-side winding. The two ends of the primary-side winding are respectively coupled to the input terminal and the first terminal. The two ends of the secondary-side winding are respectively coupled to the output terminal and the ground; a resistor, two ends of which are respectively coupled to the second terminal and the ground; a feedback circuit, one end of which is coupled to the output terminal; and a controller, There is a feedback pin, a driving pin and a current detection pin, the feedback pin is coupled to the other end of the feedback circuit, the driving pin is coupled to the control end, the current detection pin is coupled to the second end, and the controller A pulse-width-modulated square wave is output from the driving pin, and the pulse-width-modulated square wave has a higher operating frequency in a heavy-load operating mode than in a light-load operating mode. The controller includes a limited power Power detection module, the limited power detection The group is coupled to the feedback pin and the driving pin for detecting whether the voltage of the feedback pin is less than a voltage threshold, the voltage threshold is equal to the voltage of the feedback pin in the light load working mode; if the voltage of the feedback pin is less than the voltage Threshold value, it is then detected whether a conduction time of the pulse width modulated square wave is greater than a time threshold; and if the conduction time of the pulse width modulated square wave is greater than the time threshold, the power-limited power protection function is activated, The controller is driven to stop outputting the pulse width modulated square wave. For example, the flyback switching power supply with a limited power source protection function described in item 1 of the scope of the patent application, wherein if the voltage of the feedback pin is not less than the voltage threshold, the controller is normally driven to output the pulse width modulated square wave. The flyback switching power supply with limited power source protection function described in item 1 of the scope of patent application, wherein if the on-time of the pulse width modulated square wave is not greater than the time threshold, the controller is driven to normally output the Pulse width modulated square wave.