A secondary side feedback circuit and switching power supply

Abstract

The utility model provides a kind of secondary side feedback circuit and switching power supply, secondary side feedback circuit includes controller, error amplifier, photo-coupler, diode and capacitor;The positive input end of error amplifier is sampled by sampling resistance output current of switching power supply, and reference voltage is input in negative input end;The anode of diode is connected with switching power supply secondary side winding, and one end of capacitor is connected with the cathode of diode;The other end of capacitor is connected with secondary side ground;The connection node of diode and capacitor is connected with the power supply end of error amplifier;The diode anode of connecting photo-coupler is connected with the output end of error amplifier;The diode cathode of photo-coupler and the ground end of error amplifier are connected with secondary side ground;The triode of photo-coupler is connected with controller;Controller output control signal controls the output power of switching power supply.The utility model provides secondary side feedback circuit, provides stable power supply voltage for error amplifier, even when the output voltage of switching power supply is zero, error amplifier can also work stably, avoid the problem of loop out of control.

Description

Technical Field

[0001] This utility model relates to the field of feedback control technology for switching power supplies, and in particular to a secondary-side feedback circuit and a switching power supply. Background Technology

[0002] The feedback circuit of a conventional constant voltage output isolated DC-DC switching power supply consists of an error amplifier, an optocoupler, and a controller, such as... Figure 1 As shown, the error amplifier on the secondary side detects the output voltage and transmits the error signal to the controller on the primary side via an optocoupler. The controller then adjusts the frequency or peak current of the main power supply to adapt to changes in the output load. Generally, the output voltage of the error amplifier is linearly related to the frequency and peak current of the controller. Figure 1 In a circuit, a smaller load resistance requires a larger output power. If the primary side maintains the same switching frequency / duty cycle of the main power transistor, the output power will not meet the demands of the downstream load. If the load resistance decreases, the output power increases, the output voltage drops slightly, the voltage difference across the error amplifier input decreases, the current of the optocoupler diode controlled by the error amplifier decreases, the voltage at the controller's COM pin increases, and the frequency or peak current of the main power circuit controlled by the controller increases. This results in higher output power on the transformer secondary side, achieving a negative feedback automatic adjustment effect. The controller typically has over-power / short-circuit protection. When the load is too heavy or an output short circuit causes the error amplifier to open-loop, the current of the optocoupler diode becomes very small or zero. At this point, the voltage at the COM pin becomes very high, reaching the over-power threshold. The controller then triggers the over-power / short-circuit protection to ensure the safety of the main power circuit.

[0003] In existing technologies, error amplifiers are typically powered directly by the main power output voltage, such as... Figure 1 As shown, in a constant current output isolated DC-DC switching power supply, the output voltage varies. In some demanding applications, constant current output is required even when the output voltage is 0V. In constant current output applications, the error amplifier detects the output current. When the output voltage decreases, the output power decreases, and the error amplifier controls the current of the optocoupler diode to increase, thereby controlling the voltage at the COM pin of the controller to decrease. This reduces the frequency or peak current of the main power supply, thus reducing the output power and achieving the effect of negative feedback automatic adjustment. In this application, if the error amplifier is still powered by the output voltage, it will cause additional problems. When the output voltage changes from a high level to 0V, the impact on the optocoupler current can be summarized in three ways:

[0004] like Figure 2 As shown, when the error amplifier is working normally, the output voltage decreases and the output power decreases, the current of the optocoupler diode increases, and the main power frequency / peak current decreases. This is normal negative feedback regulation.

[0005] When the output voltage drops to a level where the error amplifier cannot operate, the error amplifier has no output, the optocoupler diode current is 0, the COM pin voltage of the controller is very high, triggering the short circuit protection, and the main power is in a protected state.

[0006] When the output voltage is near the critical value of the error amplifier's normal operating voltage, or when the output voltage is too low to maintain the optocoupler current, an abnormal transition state will exist. The current of the optocoupler diode decreases as the output voltage decreases, leading to an increase in the COM pin voltage and thus an increase in output power. This state indicates that the loop has run out of control, but has not yet reached the over-power / short-circuit protection state. The output current will be much greater than the constant current value. Prolonged operation in this state will result in severe overheating and a risk of burnout. Therefore, in constant current output applications, the power supply method of the error amplifier is crucial.

[0007] It should be noted that the information disclosed in the background section is intended only to enhance the understanding of the overall background of this application and should not be construed as an admission or implication in any way that the information is prior art known to those skilled in the art. Utility Model Content

[0008] The purpose of this invention is to provide a secondary-side feedback circuit that can solve the problem of powering the error amplifier by the output voltage in the prior art.

[0009] The objective of this utility model is achieved through the following technical solution:

[0010] In a first aspect, the present invention provides a secondary-side feedback circuit, including a controller, an error amplifier, and an optocoupler, and further including a diode and a capacitor; the positive input terminal of the error amplifier samples the output current of the switching power supply through a sampling resistor, and the negative input terminal receives a reference voltage; the anode of the diode is connected to the secondary winding of the switching power supply, and the cathode of the diode is connected to one end of the capacitor; the other end of the capacitor is connected to the secondary-side ground; the power supply terminal of the error amplifier is connected to the junction of the diode and the capacitor; the output terminal of the error amplifier is connected to the anode of the diode connected to the optocoupler; the cathode of the diode of the optocoupler and the ground terminal of the error amplifier are connected to the secondary-side ground; the transistor of the optocoupler is connected to the controller; the controller outputs a control signal to control the output power of the switching power supply.

[0011] Furthermore, the secondary-side feedback circuit also includes a voltage conversion circuit, the input of which is connected to the negative terminal of a diode, and the output of which is connected to the power supply terminal of an error amplifier.

[0012] Furthermore, the secondary-side feedback circuit also includes a transistor and a first resistor; the negative terminal of the diode is connected to the positive terminal of the optocoupler diode through the first resistor; the output terminal of the error amplifier is connected to the base of the transistor, the collector of the transistor is connected to the negative terminal of the optocoupler diode, and the emitter of the transistor is connected to the secondary-side ground.

[0013] Secondly, the present invention provides a switching power supply, including a transformer and a primary power transistor, and also including the aforementioned secondary feedback circuit; the controller outputs a control signal to the gate of the primary power transistor, the source of the primary power transistor is connected to the primary ground, the drain of the primary power transistor is connected to one end of the primary winding of the transformer, and the other end of the primary winding of the transformer is connected to the input voltage; the two ends of the secondary winding of the transformer output voltage.

[0014] Furthermore, the switching power supply is an isolated DC-DC switching power supply.

[0015] The secondary-side feedback circuit of this invention provides a stable power supply voltage to the error amplifier in constant current output switching power supply applications. The error amplifier can also work stably when the output voltage of the switching power supply is zero, avoiding reliability issues such as loop runaway. Attached Figure Description

[0016] Figure 1 This is a structural diagram of the secondary feedback circuit in the prior art.

[0017] Figure 2 A graph showing the relationship between the supply voltage and the optocoupler current when supplying power to the output voltage in the prior art.

[0018] Figure 3 This is one of the circuit schematics of the secondary-side feedback circuit of this utility model applied to a flyback DC-DC switching power supply.

[0019] Figure 4 This is the second circuit diagram of the secondary-side feedback circuit of this utility model applied to a flyback DC-DC switching power supply.

[0020] Figure 5 This is the third circuit diagram of the secondary-side feedback circuit of this utility model applied to a flyback DC-DC switching power supply. Detailed Implementation

[0021] The embodiments of this disclosure will now be described in detail with reference to the accompanying drawings.

[0022] The following specific examples illustrate the implementation of this disclosure. Those skilled in the art can easily understand other advantages and effects of this disclosure from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of this disclosure, and not all of them. This disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of this disclosure. It should be noted that, in the absence of conflict, the following embodiments and features in the embodiments can be combined with each other. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0023] Example 1

[0024] A secondary-side feedback circuit, such as Figure 3 As shown, the system includes a controller, error amplifier OP1, transistor Q1, resistor R3, optocoupler OTP1, diode D2, and capacitor C2. The positive input of error amplifier OP1 samples the output current of the switching power supply through sampling resistor R1, and the negative input receives the reference voltage REF. The anode of diode D2 is connected to the secondary winding of the switching power supply, and the cathode of diode D2 is connected to one end of capacitor C2, with the other end of capacitor C2 connected to the secondary ground. The cathode of diode D2 is also connected to the power supply terminal of error amplifier OP1, supplying power to OP1. The cathode of diode D2 is also connected to the anode of the diode in optocoupler OTP1 through resistor R3, supplying power to the optocoupler. The output of error amplifier OP1 is connected to the base of transistor Q1, and the collector of transistor Q1 is connected to the cathode of the diode in optocoupler OTP1. The emitter of transistor Q1 and the ground terminal of error amplifier OP1 are connected to the secondary ground. The collector of optocoupler OTP1 is connected to the COM pin of the controller, and the emitter is connected to the primary ground. The controller's control pin outputs a control signal to control the frequency and / or peak current of the primary-side power transistors of the switching power supply, thereby controlling the output power of the switching power supply.

[0025] The working principle of the secondary-side feedback circuit of this invention, applied to a constant-current output flyback isolated DC-DC switching power supply, is explained with reference to the accompanying drawings. R1 is a current sampling resistor, and R2 is the load. The output current passes through the sampling resistor R1 to form a sampling voltage. The error amplifier OP1 samples the voltage across resistor R1, and after error amplification, it is output from its output terminal to the diode of the optocoupler, generating current in the optocoupler diode. The sampling and feedback process of this invention is the same as that of existing technologies and will not be described in detail here.

[0026] The main innovation of this invention lies in the power supply to the error amplifier. When the controller controls the primary-side power transistor TR1 of the switching power supply to switch, the voltage waveform at the anode of diode D2 is a square wave voltage, and its high-level voltage value is:

[0027] Formula (1)

[0028] Among them, V IN V is the input voltage of the switching power supply. O N is the output voltage of the switching power supply. PS Let V be the turns ratio of the primary and secondary windings of the transformer in the switching power supply. From formula (1), we can obtain that even if the output voltage V... O When the voltage is 0V, the voltage at the anode of diode D2 is V. IN / N PS After passing through capacitor C2, a stable voltage can be obtained to power the error amplifier and optocoupler, thus avoiding the loop malfunction problem caused by insufficient power supply to the error amplifier.

[0029] This invention does not have any special requirements for the controller; theoretically, any controller that can control the switching of power transistors in power management is applicable.

[0030] Example 2

[0031] Based on Example 1, a voltage conversion circuit is added. The input terminal of the voltage conversion circuit is connected to the negative terminal of diode D2, the output terminal is connected to the power supply terminal of error amplifier OP1, and is connected to the positive terminal of diode of optocoupler OTP1 through resistor R3.

[0032] In Example 1, the voltage at the anode of diode D1 changes with both the input and output voltages, meaning the voltage supplied to the error amplifier is not completely stable. Therefore, to make the supply voltage to the error amplifier more stable, a voltage conversion circuit is added, such as... Figure 4 As shown. The voltage conversion circuit can be one of the following: buck circuit (BUCK, LDO), boost circuit (BOOST), buck-boost circuit (BUCK-BOOST), or other voltage regulator devices, as long as they can provide a stable output voltage to power the error amplifier.

[0033] Example 3

[0034] To further simplify the circuit, based on Embodiment 1 or Embodiment 2, transistor Q1 is removed, and the optocoupler OTP1 is directly driven by error amplifier OP1. After removing transistor Q1, the circuit connections are slightly different. The output terminal of error amplifier OP1 is directly connected to the anode of the diode in optocoupler OTP1, and the cathode of the diode in optocoupler OTP1 and the ground terminal of the error amplifier are connected to the secondary ground. Other connections are the same as in Embodiments 1 and 2, and will not be repeated here.

[0035] The working principle of Embodiment 3 is the same as that of Embodiments 1 and 2. However, to simplify the circuit, transistor Q1 is removed and the optocoupler is directly driven by error amplifier OP1.

[0036] In this invention, "connection" refers to electrical connection, which can be a direct electrical connection or an electrical connection through other components.

[0037] Example 4

[0038] Based on embodiments one to three above, a switching power supply is provided, including a transformer T1, a primary-side power transistor TR1, and a secondary-side feedback circuit as described in any one of embodiments one to three. The control terminal of the controller is connected to the gate of the primary-side power transistor TR1, controlling the switching frequency and / or peak current of TR1. The source of the primary-side power transistor TR1 is connected to primary-side ground, and its drain is connected to one end of the primary winding of the transformer T1. The other end of the primary winding of the transformer T1 is connected to the input voltage V. IN The output voltage V across the secondary winding of the transformer O The switching power supply in this embodiment can be any type of switching power supply other than an isolated DC-DC switching power supply. This invention does not limit the main power circuit structure of the switching power supply.

[0039] The above description is merely illustrative of the embodiments of this utility model and is not intended to limit the scope of this utility model. For those skilled in the art, any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this utility model without creative labor should be included within the protection scope of this utility model.

Claims

1. A secondary-side feedback circuit, comprising a controller, an error amplifier, and an optocoupler, characterized in that, It also includes diodes and capacitors; the positive input terminal of the error amplifier samples the output current of the switching power supply through a sampling resistor, and the negative input terminal receives the reference voltage; the anode of the diode is connected to the secondary winding of the switching power supply, and the cathode of the diode is connected to one end of the capacitor; the other end of the capacitor is connected to the secondary ground; the power supply terminal of the error amplifier is connected to the junction of the diode and the capacitor; the output terminal of the error amplifier is connected to the anode of the diode connected to the optocoupler; the cathode of the diode of the optocoupler and the ground terminal of the error amplifier are connected to the secondary ground; the transistor of the optocoupler is connected to the controller; the controller outputs a control signal to control the output power of the switching power supply.

2. The secondary-side feedback circuit according to claim 1, characterized in that, It also includes a voltage conversion circuit, whose input terminal is connected to the negative terminal of a diode and whose output terminal is connected to the power supply terminal of an error amplifier.

3. The secondary-side feedback circuit according to claim 1 or 2, characterized in that, It also includes a transistor and a first resistor; the negative terminal of the diode is connected to the positive terminal of the diode of the optocoupler through the first resistor; the output terminal of the error amplifier is connected to the base of the transistor, the collector of the transistor is connected to the negative terminal of the diode of the optocoupler, and the emitter of the transistor is connected to the secondary ground.

4. A switching power supply, comprising a transformer and a primary-side power transistor, characterized in that, It also includes the secondary-side feedback circuit as described in any one of claims 1 to 3; the controller outputs a control signal to the gate of the primary-side power transistor, the source of the primary-side power transistor is connected to the primary-side ground, the drain of the primary-side power transistor is connected to one end of the primary winding of the transformer, and the other end of the primary winding of the transformer is connected to the input voltage; the two ends of the secondary winding of the transformer output voltage.

5. The switching power supply according to claim 4, characterized in that, The switching power supply is an isolated DC-DC switching power supply.

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