TW201737606A - Flyback power supply system - Google Patents

Abstract

The invention provides a flyback power supply system, which comprises a transformer and a control chip, wherein the control chip comprises a first detection circuit and a second detection circuit; one end of an auxiliary winding of the transformer is directly grounded, and the other end is grounded via a voltage dividing circuit; the first detection circuit and the second detection circuit are connected onto the same pin of the control chip and are connected onto a wiring end of voltage dividing resistors in the voltage dividing circuit via the pin and a thermistor; and within demagnetization time of the transformer, the first detection circuit and the second detection circuit alternatively carry out over voltage protection detection and over temperature protection detection.

Description

Flyback power system

This invention relates to the field of circuits, and more particularly to flyback power systems.

Generally, the flyback power supply system isolates the primary input and the secondary output through a transformer, and the output voltage information is fed back to the control wafer on the primary side through the optocoupler, and is controlled by the different pins of the control chip respectively. Over Voltage Protection (OVP/Under Voltage Protection, UVP) sensing circuits and Over Temperature Protection (OTP) sensing circuits to trigger overvoltage/undervoltage protection and overtemperature protection.

Figure 1 is a circuit schematic of a conventional flyback power system. In Fig. 1, T1 is a transformer, M1 is a power switch such as a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), or a bipolar transistor, and Rs is a current. Sampling resistor; the OVP/UVP sensing circuit is connected to the dem pin of the Pulse Width Modulation (PWM) controller (ie, the control chip), and the OTP sensing circuit is connected to the otp pin of the PWM controller; The amplification and error isolation circuit generates a feedback voltage V _FB fed back to the PWM controller's FB (Feedback voltage) pin based on the output voltage V _O ; the feedback voltage V _FB controls the current at the CS (Current Sensor) pin of the PWM controller The peak value of the voltage V _CS is sampled, thereby controlling the output voltage V _O .

As shown in Figure 1, the OTP sensing circuit is grounded via a thermistor R _L (for example, a negative temperature coefficient (NTC)); the otp pin of the PWM controller flows out of the fixed current I _OTP ; Normally, the resistance of the thermistor R _L is high, the voltage value on the otp pin of the PWM controller is higher than the first threshold voltage OTP_ref set internally by the PWM controller, and the OTP sensing circuit does not trigger over-temperature protection; When the temperature rises, the resistance value of the thermistor R _L decreases, the voltage value on the otp pin of the PWM controller is lower than the first threshold voltage OTP_ref, and the OTP sensing circuit triggers the over temperature protection.

As shown in Figure 1, the OVP/UVP sensing circuit senses the output voltage information through the auxiliary winding of the transformer T1; when the power switch M1 is turned on, the energy is stored in the transformer T1; when the power switch M1 is turned off, the transformer The energy stored in T1 is released to the output; during the demagnetization time _Toff of the transformer T1 (ie, the energy stored in the transformer T1 is released to the output), the relationship between the auxiliary winding voltage Vaux and the output voltage V _{O is} as follows Equation (1) shows the relationship between the voltage V _DEM at the dem pin of the PWM controller and the auxiliary winding voltage Vaux and the output voltage V _O as shown in equation (2); when the dem pin of the PWM controller When the voltage V _{DEM is} greater than the second threshold voltage OVP_ref set by the PWM controller, the OVP/UVP sensing circuit triggers overvoltage protection; when the voltage V _DEM at the dem pin of the PWM controller is less than the internal setting of the PWM controller When the three threshold voltage UVP_ref, the OVP/UVP sensing circuit triggers undervoltage protection: Vaux = n . ( V _O + V _F ) (1)

Where n is the ratio of the number of turns Naux of the auxiliary winding of the transformer T1 and the number of turns Nux of the secondary winding Naux/Nsec, that is, n=Naux/Nsec; VF is the voltage drop of the output diode D1; k=R2/( R1+R2) is the feedback coefficient.

Fig. 2 is a circuit diagram showing an OVP sensing circuit and an OTP sensing circuit applied to the flyback power supply system shown in Fig. 1. Fig. 3 is a circuit diagram showing an OVP sensing circuit, a UVP sensing circuit, and an OTP sensing circuit applied to the flyback power supply system shown in Fig. 1. In the circuits shown in FIGS. 2 and 3, the OVP/UVP sensing circuit and the OTP sensing circuit are completely separated, and need to be respectively connected to the two pins of the PWM controller, and the peripheral circuits are complicated.

The present invention provides a novel flyback power system.

A flyback power supply system according to an embodiment of the present invention includes: a transformer; a control chip including a first sensing circuit and a second sensing circuit, wherein one end of the auxiliary winding of the transformer is directly grounded, and the other end is via a voltage dividing circuit Grounding, the first sensing circuit and the second sensing circuit are connected to the same pin of the control chip, and are connected to the terminal between the voltage dividing resistors in the voltage dividing circuit via the pin and the thermistor, During the demagnetization time of the transformer, the first sensing circuit and the second sensing circuit alternately perform overvoltage protection sensing and overtemperature protection sensing.

A flyback power supply system according to another embodiment of the present invention includes: a transformer; a control chip including a first sensing circuit and a second sensing circuit, wherein one end of the auxiliary winding of the transformer is directly grounded, and the other end is via a divided piezoelectric Grounding, the first sensing circuit and the second sensing circuit are connected to the same pin of the control chip, and are connected to the terminal between the voltage dividing resistors in the voltage dividing circuit via the pin and the thermistor, During the demagnetization time of the transformer, the first sensing circuit and the second sensing circuit alternately perform undervoltage protection sensing and overtemperature protection sensing.

In the flyback power supply system according to the present invention, overvoltage/undervoltage protection sensing and overtemperature protection can be simultaneously realized by controlling the same pin of the wafer connected to the first sensing circuit and the second sensing circuit. Sensing, thus saving the pin resources of the control chip.

T1‧‧‧ transformer

M1‧‧‧ power switch

Rs‧‧‧ current sampling resistor

Dem, otp‧‧‧ pin connection

V _O ‧‧‧Output voltage

V _gs ‧‧‧ gate voltage

V _FB ‧‧‧ feedback voltage

V _CS ‧‧‧current sampling voltage

R _L ‧‧‧Thermistor

I _OTP ‧‧‧fixed current

OTP_ref‧‧‧first threshold voltage

OVP_ref‧‧‧second threshold voltage

UVP_ref‧‧‧ third threshold voltage

T _off ‧‧‧Demagnetization time

R3‧‧‧ resistance

V _aux ‧‧‧Auxiliary winding voltage

V1‧‧‧ voltage

D1‧‧‧ output diode

S1, S2, S3, S4, S5‧‧ ‧ switch

FB, CS, PRT‧‧‧ pins

R1, R2‧‧‧ voltage divider resistor

Sovp/uvp‧‧‧Closed by control signal

Sotp, OTP_switch‧‧‧ control signals

V _{sample_1} , V _{sample_2 ‧‧‧Sampling} voltage

UVP_signal‧‧‧ alarm undervoltage

OVP_signal‧‧‧Alarm overvoltage

OTP_signal‧‧‧Over temperature protection signal

C1, C2‧‧‧ capacitor

V _OTP ‧‧‧Fixed threshold voltage

C _O ‧‧‧ output capacitor

Other features, objects, and advantages of the invention will be apparent from the description of the accompanying drawings.

Figure 1 is a circuit schematic of a conventional flyback power system.

Fig. 2 is a circuit diagram showing an OVP sensing circuit and an OTP sensing circuit applied to the flyback power supply system shown in Fig. 1.

Fig. 3 is a circuit diagram showing an OVP sensing circuit, a UVP sensing circuit, and an OTP sensing circuit applied to the flyback power supply system shown in Fig. 1.

4 is a circuit schematic diagram showing a first sensing circuit and a second sensing circuit according to an embodiment of the present invention applied to the flyback power supply system shown in FIG. 1.

Fig. 5 is a waveform diagram showing control signals for controlling the turning off and closing of the switches S1, S2, and S3 shown in Fig. 4, and the gate voltage Vgs of the power switch M1.

Fig. 6 is a view showing a specific circuit diagram of the second sensing circuit shown in Fig. 4.

Figure 7 shows the auxiliary winding voltage Vaux of the transformer T1 and the gate voltage Vgs of the power switch M1 in the first sensing circuit shown in Fig. 4 and the flyback power supply system of the second sensing circuit. Waveform.

Features and exemplary embodiments of various aspects of the invention are described in detail below. In the following detailed description, numerous specific details are set forth However, it will be apparent to those skilled in the art that the present invention may be practiced without some of the details. The following description of the embodiments is merely provided to provide a better understanding of the invention. The present invention is in no way limited to any specific configurations and algorithms presented below, but without departing from the spirit and scope of the invention. In the drawings and the following description, well-known structures and techniques are not shown in order to avoid unnecessary obscuring the invention.

In view of one or more of the problems described above, the present invention provides a novel fly-back power system that has, in addition to the parts mentioned below and the fly-back power system shown in FIG. The other parts are the same as the flyback power supply system shown in Fig. 1. For the sake of brevity and ease of understanding, only the portion of the flyback power supply system according to the embodiment of the present invention is different from the portion of the flyback power supply system shown in Fig. 1 will be described below.

4 shows a first sensing circuit (ie, an OVP/UVP sensing circuit) and a second sensing circuit (ie, an OVP/UVP sensing circuit) according to an embodiment of the present invention applied to the flyback power supply system shown in FIG. , OTP sensing circuit) circuit schematic. As can be seen from FIGS. 1 and 4, in the flyback power supply system according to the embodiment of the present invention, one end of the auxiliary winding of the transformer T1 is directly grounded, and the other end is composed of a voltage dividing resistor R1 and a voltage dividing resistor R2. The voltage dividing circuit is grounded; the control chip (ie, the PWM controller) includes a first sensing circuit and a second sensing circuit; the first sensing circuit and the second sensing circuit are connected to the same pin of the control chip (ie, , PRT (Protection) pin), and connected to the terminal between the voltage dividing resistor R1 and the voltage dividing resistor R2 via the pin and the thermistor R _L ; by controlling the switches S1 to S3 to be turned off and closed, A sensing circuit and a second sensing circuit alternately perform overvoltage/undervoltage protection sensing and overtemperature protection sensing during the demagnetization time of the transformer.

It should be understood by those skilled in the art that the first sensing circuit can perform overvoltage protection sensing alone, undervoltage protection sensing alone, or simultaneous overvoltage protection sensing and undervoltage protection sensing without affecting. The second sensing circuit performs over temperature protection sensing.

Fig. 5 is a waveform diagram showing control signals for controlling the turning off and closing of the switches S1, S2, and S3 shown in Fig. 4, and the gate voltage Vgs of the power switch M1. In the flyback switch system according to the embodiment of the present invention, the turning off and closing of the switch S1 is controlled by the control signal Sovp/uvp, the turning off and closing of the switch S2 is controlled by the control signal Sotp, and the turning off and closing of the switch S3 is performed by Control signal OTP_switch control. Specifically, when the control signal Sovp/uvp is at a high level, the switch S1 is closed, when the control signal Sovp/uvp is at a low level, the switch S1 is turned off; when the control signal Sotp is at a high level, the switch S2 is closed, and when the control signal Sotp is at a low level, the switch S2 is closed. Shutdown; when the control signal OTP_switch is at the high level, the switch S3 is closed, and when the control signal OTP_switch is at the low level, the switch S3 is turned off.

As shown in Fig. 5, during the nth demagnetization time of the power switch M1 (the transformer demagnetizes during the time when the gate voltage Vgs of the power switch M1 is at a low level), the control signal Sovp/uvp changes from a high level to a low level. The control signals Sotp and OTP_switch are always at a low level, the switch S1 is changed from the closed state to the off state, the switches S2 and S3 are always in the off state, and the first sensing circuit is turned on when the switch S1 is changed from the closed state to the closed state. The voltage at the PRT pin of the control wafer is sampled to obtain a sample voltage V _{sample_1} . That is, during the nth demagnetization time of the power switch M1, the first sensing circuit performs overvoltage and/or undervoltage protection sensing by sampling the voltage at the PRT pin of the control wafer, the second sense The measurement circuit does not perform over-temperature protection sensing.

As shown in Fig. 5, during the (n-1) demagnetization time of the power switch M1, the control signal Sotp changes from a high level to a low level, the control signals Sovp/uvp and OTP_switch are always at a low level, and the switch S2 is closed. The state changes to the off state, the switches S1 and S3 are always in the off state, and the second sensing circuit samples the voltage at the PRT pin of the control wafer when the switch S2 changes from the closed state to the off state to obtain the sampling voltage V. _{Sample_1} . That is, during the (n-1)th demagnetization time of the power switch M1, the second sensing circuit performs the first over-temperature protection sensing by sampling the voltage at the PRT pin of the control chip, A sensing circuit does not sense overvoltage and/or undervoltage protection.

As shown in FIG. 5, during the (n+1)th demagnetization time of the power switch M1, the control signal Sotp changes from a high level to a low level, and the control signal OTP_switch changes from a high level to a low level later than the control signal Sotp. The control signal Sovp/uvp is always at the low level, the switch S1 is always in the off state, the switch S2 is changed from the closed state to the off state, and the switch S3 is changed from the closed state to the off state later than the switch S2, and the second sensing circuit is The sampling voltage V _{sample_2 is} obtained by sampling the voltage at the PRT pin of the control wafer when the switch S2 changes from the closed state to the off state and the switch S3 is still in the closed state. That is, during the (n+1)th demagnetization time of the power switch M1, the second sensing circuit performs the second over-temperature protection sensing by sampling the voltage at the PRT pin of the control chip, A sensing circuit does not sense overvoltage and/or undervoltage protection.

When the first sensing circuit performs overvoltage and/or undervoltage protection sensing or when the second sensing circuit performs the first overtemperature protection sensing, the switch S3 is in an off state, and the _IOTTP does not control the chip. The PRT pin is flowing out. Therefore, in both cases, the sampling voltage sampled by the first sensing circuit and the sampling voltage sampled by the second sensing circuit are equal, that is, the sampling voltage V _{sample_1} , and the sampling voltage is equal to the voltage dividing resistor R1 And the voltage V1 of the terminal between the voltage dividing resistor R2.

As mentioned above, the auxiliary winding voltage Vaux of the transformer T1 can be derived from equation (1): V _aux = n . ( V _O + V _F ) (1)

Where n is the ratio Nux/Nsec of the number of turns Naux of the auxiliary winding of the transformer T1 and the number of turns Nsec of the secondary winding, that is, n=Naux/Nsec; VF is the voltage drop of the output diode D1.

The sampling voltage V _{sample_1} can be obtained by the following equation (3):

If V _{sample_1} <UVP_ref, the first sensing circuit outputs a high level UVP_signal alarm undervoltage, thereby triggering undervoltage protection. If V _{sample_1} > OVP_ref, the first sensing circuit outputs a high level OVP_signal alarm overvoltage, thereby triggering overvoltage protection.

When the second sensing circuit performs the second over-temperature protection sensing, the switch S3 is in the closed state, and the I _OTP flows out from the PRT pin of the control chip. The sampling voltage V _{sample_2} sampled by the second sensing circuit in this case can be obtained by the following equations (4) and (5):

Wherein V ₁ is the voltage at the terminal between the voltage dividing resistor R1 and the voltage dividing resistor R2 when the switch S3 is in the closed state.

In the flyback power supply system according to the embodiment of the present invention, the second sensing circuit can obtain the voltage difference ΔV _sample between the two sampling voltages V _{sample_1} and V _{sample_2} by using the following equation (6), and at the voltage The over-temperature protection is triggered when the difference ΔV _{sample is} smaller than the OTP_ref set in the control chip.

Specifically, when the temperature is normal, the resistance of the thermistor R _L is high, and V _{sample_2} is a higher voltage; when ΔV _sample > OTP_ref, the second sensing circuit outputs a low level of OTP_signal, and does not trigger over-temperature protection. When the temperature rises, the resistance of the thermistor R _L decreases, and V _{sample_2} decreases. When ΔV _sample <OTP_ref, the second sensing circuit outputs a high level of OTP_signal, triggering over temperature protection.

In some embodiments, the second sensing circuit can be implemented as the circuit shown in FIG. 6, and the voltage difference ΔV _sample between the sampling voltages V _{sample_1} and V _{sample_2} can be controlled by the opening of the switch S4 and the switch S5. Turn off to get.

As shown in Fig. 6, during the (n-1) demagnetization time of the power switch M1, the switch S2 and the switch S5 are closed, the switch S4 is turned off, and no current (ie, zero current) is received from the PRT pin of the control chip. After flowing out, the capacitor C1 samples the voltage on the PRT pin of the control chip to obtain the sampling voltage V _{sample_1} , and increases the sampling voltage on the capacitor C1 by a fixed threshold voltage V _OTP and inputs it to the same input terminal of the OTP sensing comparator. The voltage V _{sample_1} +V _{OTP is} input to the _{non-inverting} input of the OTP sensing comparator. During the (n+1)th demagnetization time of the power switch M1, the switch S2 and the switch S4 are closed, the switch S5 is turned off, the current I _OTP flows out from the PRT pin of the control chip, and the capacitor C2 is on the PRT pin of the control chip. The voltage is sampled to obtain the sampled voltage V _{sample_2} , and the sampled voltage on the capacitor C2 is input to the inverting input of the OTP sense comparator.

At normal temperature, V _{sample_2 is} greater than V _{sample_1} +V _OTP , that is, the second sensing circuit outputs a low level OTP_signal, and does not trigger over temperature protection. At high temperature, V _{sample_2 is} less than V _{sample_1} +V _OPT , that is, the second sensing circuit outputs a high level of OTP_signal, triggering over temperature protection. That is to say, at normal temperature, ΔV _sample =V _{sample_2} -V _{sample_1} >V _OTP , the second sensing circuit does not trigger over-temperature protection; at high temperature, ΔV _sample =V _{sample_2} -V _{sample_1} <V _OTP , second sense The measurement circuit triggers over temperature protection. Here, V _{OTP is} the OTP_ref that controls the internal setting of the wafer.

Figure 7 shows the auxiliary winding voltage Vaux of the transformer T1 and the gate voltage Vgs of the power switch M1 in the first sensing circuit shown in Fig. 4 and the flyback power supply system of the second sensing circuit. Waveform. As shown in FIG. 7, in the flyback power supply system according to the embodiment of the present invention, the OVP/UVP sensing and the OTP sensing are alternately performed during the power switch M1 off; after the power switch M1 is turned off, the transformer T1 The auxiliary winding voltage Vaux will oscillate for a short period of time; by controlling the sampling delay time t, this oscillation time can be avoided, and the auxiliary winding voltage Vaux is stabilized when it is stabilized for OVP/UVP sensing and OTP sensing. The measured voltage is sampled; this allows the OVP protection to be falsely triggered by the turn-off ringing of the power switch M1, which makes the OVP protection more accurate.

In some embodiments, since the resistance value of the thermistor R _L is large at normal temperature, which affects the demagnetization waveform of the power switch M1 and affects the accuracy of the output voltage V _O , the auxiliary winding voltage Vaux is divided by the voltage dividing resistor R1 and After the voltage resistor R2 is divided, it can be input to the PRT pin of the control chip via a parallel circuit composed of the thermistor R _L and the resistor R3. Connecting the thermistor R _{L in} parallel with the resistor R3 can greatly reduce the influence of the large value of the thermistor R _L at normal temperature on the protection accuracy of overvoltage protection and/or undervoltage protection, and does not affect the overtemperature protection. Features.

The invention may be embodied in other specific forms without departing from the spirit and essential characteristics. The present embodiments are to be considered in all respects as illustrative and not limiting, and the scope of the invention All changes in the scope of the invention are thus included in the scope of the invention.

V _aux ‧‧‧Auxiliary winding voltage

V _gs ‧‧‧ gate voltage

R1, R2‧‧‧ voltage divider resistor

R3‧‧‧ resistance

PRT‧‧‧ pin

R _L ‧‧‧Thermistor

I _OTP ‧‧‧fixed current

V _OTP ‧‧‧Fixed threshold voltage

V1‧‧‧ voltage

S1, S2, S3‧‧‧ switch

OTP_ref‧‧‧first threshold voltage

OVP_ref‧‧‧second threshold voltage

UVP_ref‧‧‧ third threshold voltage

UVP_signal‧‧‧ alarm undervoltage

OVP_signal‧‧‧Alarm overvoltage

OTP_signal‧‧‧Over temperature protection signal

Claims (13)

A flyback power system includes: a transformer; a control chip including a first sensing circuit and a second sensing circuit, wherein one end of the auxiliary winding of the transformer is directly grounded, and the other end is grounded via a voltage dividing circuit, a first sensing circuit and the second sensing circuit are connected to the same pin of the control chip, and are connected between the voltage dividing resistors in the voltage dividing circuit via the pin and the thermistor And the first sensing circuit and the second sensing circuit alternately perform overvoltage protection sensing and overtemperature protection sensing during the demagnetization time of the transformer. The flyback power supply system of claim 1, further comprising: a resistor connected in parallel with the thermistor between the pin and the terminal. The flyback power supply system of claim 1, wherein the first sensing circuit performs overvoltage protection sensing during an nth demagnetization time of the transformer, the second sensing circuit Over-temperature protection sensing is performed during the (n+1)th and (n-1) demagnetization times of the transformer, and n is an integer greater than zero. The flyback power supply system of claim 3, wherein the second sensing circuit is based on the (n+1)th and (n-1) demagnetization times of the transformer, Overcurrent protection is sensed by the different currents flowing out of the pin, ie the zero current flowing from the pin during one demagnetization time and the current I _OTP flowing out of the pin during another demagnetization time. The flyback power supply system of claim 3, wherein the second sensing circuit acquires a voltage at the pin sensed during the (n+1)th demagnetization time a difference between a voltage at the pin sensed during the (n-1)th demagnetization time, and the difference is less than a first threshold voltage set inside the control wafer In case of over temperature protection is triggered. The flyback power supply system of claim 3, wherein the first sensing circuit senses that a voltage at the pin is higher than a second threshold set inside the control chip. Overvoltage protection is triggered at voltage. The flyback power supply system of claim 3, wherein the first sensing circuit senses that the voltage at the pin is lower than the internal setting of the control chip. Undervoltage protection is triggered at three threshold voltages. A flyback power system includes: a transformer; a control chip including a first sensing circuit and a second sensing circuit, wherein one end of the auxiliary winding of the transformer is directly grounded, and the other end is grounded via a voltage dividing circuit, a first sensing circuit and the second sensing circuit are connected to the same pin of the control chip, and are connected between the voltage dividing resistors in the voltage dividing circuit via the pin and the thermistor And the first sensing circuit and the second sensing circuit alternately perform undervoltage protection sensing and over temperature protection sensing during the demagnetization time of the transformer. The flyback power supply system of claim 8, further comprising: a resistor connected in parallel with the thermistor between the pin and the terminal. The flyback power supply system of claim 8, wherein the first sensing circuit performs undervoltage protection sensing during an nth demagnetization time of the transformer, the second sensing circuit Over-temperature protection sensing is performed during the (n+1)th and (n-1) demagnetization times of the transformer, and n is an integer greater than zero. The flyback power supply system of claim 9, wherein the second sensing circuit is based on the (n+1)th and (n-1) demagnetization times of the transformer, Overcurrent protection is sensed by the different currents flowing out of the pin, ie the zero current flowing from the pin during one demagnetization time and the current I _OTP flowing out of the pin during another demagnetization time. The flyback power supply system of claim 10, wherein the second sensing circuit acquires a voltage at the pin sensed during the (n+1)th demagnetization time a difference between a voltage at the pin sensed during the (n-1)th demagnetization time, and the difference is less than a first threshold voltage set inside the control wafer Triggered in case Temperature protection. The flyback power supply system of claim 10, wherein the first sensing circuit senses that a voltage at the pin is lower than a third threshold voltage set inside the control chip Trigger undervoltage protection.