CN109298224B

CN109298224B - Primary side detection circuit and primary side detection method

Info

Publication number: CN109298224B
Application number: CN201811065698.XA
Authority: CN
Inventors: 赵永宁; 肖华
Original assignee: Shenzhen Nanyun Microelectronics Co ltd; Mornsun Guangzhou Science and Technology Ltd
Current assignee: Shenzhen Nanyun Microelectronics Co ltd; Mornsun Guangzhou Science and Technology Ltd
Priority date: 2018-09-13
Filing date: 2018-09-13
Publication date: 2020-12-18
Anticipated expiration: 2038-09-13
Also published as: CN109298224A

Abstract

The invention provides a novel primary side detection circuit and a primary side detection method, which are suitable for a secondary side feedback control mode circuit. The sampling delay circuit is used for shielding the oscillation which occurs after the primary side main MOS tube is turned off, the oscillation can be falsely detected by a circuit behind the sampling delay circuit to cause abnormal control, the rising edge judging circuit is used for judging whether the voltage of the auxiliary winding has a voltage rising slope or not, the falling edge judging circuit is used for judging whether the voltage of the auxiliary winding has a voltage falling slope or not, and the time detecting circuit is used for detecting the time from the rising edge to the falling edge. The invention does not need additional series and parallel diodes, reduces the occupied area of the board, reduces the cost, enhances the anti-interference capability and improves the reliability of the product.

Description

Primary side detection circuit and primary side detection method

Technical Field

The invention relates to a method for detecting a primary side signal, in particular to a primary side signal detection method and a primary side detection circuit applied to secondary side feedback control.

Background

In a primary side control scheme of the flyback isolation converter, output voltage or current information needs to be fed back to a primary side controller to realize closed-loop control. Common feedback techniques are secondary feedback and primary feedback. The task of secondary side feedback is completed by an isolation amplifier composed of a device TL431, an optical coupler and an auxiliary device. The output voltage of the converter and the reference voltage are compared and amplified through TL431 to give an error signal, the error signal flows through the input end of the optical coupler in a current mode, the output end of the optical coupler draws current from a primary side controller FB port to generate corresponding error voltage, and the voltage is used for adjusting the duty ratio of a primary side power tube, so that the output voltage of the converter is stabilized at a set value. The feedback technology has the characteristic of high precision, but the TL431, the optocoupler, the auxiliary device and the like increase the space of a converter system board, and the optocoupler can not work at high temperature and is easy to age.

In contrast, primary side feedback (PSR) technology has no secondary side feedback device, and obtains information on the converter output voltage by simply detecting the voltage on the auxiliary winding. Because the voltage on the auxiliary winding is proportional to the voltage on the secondary winding, specifically to the turn ratio of the winding, the duty ratio of the power tube can be adjusted according to the voltage on the auxiliary winding, so that the output voltage of the converter is stabilized at a set value. However, primary side feedback has inherent drawbacks: firstly, under the influence of voltage drop of a rectifier device, the voltage on an adopted auxiliary winding is not the output voltage of the converter in the true sense; secondly, the turn ratio of the auxiliary winding and the secondary winding is influenced, and the turn ratio changes to a certain degree along with the production process; and thirdly, under the influence of the primary side sampling circuit, the primary side controller cannot accurately sample the voltage of the auxiliary winding. Therefore, converters employing primary-side feedback techniques have limited output voltage accuracy.

The chinese patent application publication No. CN 105610306A proposes a secondary feedback control method shown in fig. 1 to overcome the above-mentioned disadvantages of the feedback technique. Specifically, the secondary side controller samples the output voltage of the converter and compares the output voltage with a reference voltage through a comparator, and the comparison result reflects that the output voltage is below or above the reference; the secondary side controller selects two different resistance states of the switch unit according to the comparison result, so that the feedback information is pressurized to the secondary side winding in a variable voltage drop mode; the voltage of the secondary winding is reflected to the auxiliary winding, and the primary side controller detects the voltage change on the auxiliary winding to judge whether the output voltage of the converter is higher than the reference or lower than the reference; if the output voltage of the converter is higher than the reference, the turn-on duty ratio of the primary side switching tube is reduced cycle by cycle until the output voltage is detected to be lower than the reference, otherwise, the turn-on duty ratio of the primary side switching tube is increased cycle by cycle until the output voltage is detected to be higher than the reference, and the cycle is repeated in such a way that the output voltage is stabilized at a set value.

The change of the resistance state of the secondary side proposed by the Chinese patent application with the publication number of CN 105610306A is coding, and the change of the detection voltage of the primary side is decoding. The encoding process occurs in a demagnetization stage, the demagnetization current generates voltage drop on different resistance states and is added to the secondary winding, the larger the impedance difference of the two resistance states is, the larger the difference of the two voltage drops is, and the better the detection is by the primary side controller. The primary side detection method is to compare the auxiliary winding voltage sampled in the current period with the previous period, if the auxiliary winding voltage is larger than the previous period, the secondary side selects a high-resistance state, and otherwise, the secondary side is in a low-resistance state. Because the whole control scheme provided by the patent can enable the output voltage of the converter to have certain low-frequency ripples, when the output voltage changes of two adjacent periods reach the set threshold value, wrong judgment can be caused on the primary side, and the loop is out of control. In order to improve the reliability of primary side detection, on the basis of the invention patent application CN 105610306A, a chinese invention patent application with publication number CN107612334A is generated, which judges whether the output voltage is higher or lower by detecting whether a voltage rising slope exists in the degaussing time period of the voltage FA after voltage division of the auxiliary winding in the same period, as shown in fig. 2, the method can be used in the second and fourth embodiments of the invention patent CN 105610306A, but is ineffective for the third embodiment because a rising slope occurs at least once in each period, in practical application, the second embodiment rectifies the output voltage higher by only connecting an external diode without switching on a synchronous rectification MOS transistor, which has a significant disadvantage of high cost and low efficiency, the fourth embodiment is realized by connecting a diode in series, which is also low efficiency, the scheme is that the scheme can be realized only by one synchronous rectification MOS tube without any additional external rectification device, the secondary side feedback signal is generated by switching off the synchronous rectification MOS tube at different current points, and then the secondary side feedback signal is reflected on the auxiliary winding through the transformer transmission and finally received by the FA of the primary side chip, but the scheme does not have a primary side detection scheme which can well realize secondary side control in Chinese inventions with patent application publication numbers of CN 105610306A and CN 107612334A.

Disclosure of Invention

In order to implement the secondary side feedback control scheme in an optimal manner, a suitable primary side signal detection circuit needs to be matched, so that on the basis of the invention of China with the patent application publication numbers of CN 105610306A and CN107612334A, the invention provides a new primary side detection circuit, and the invention extends the specific application of the patent technology. As described in the background art, the realization mode of the secondary feedback control circuit is mainly the generation and the receiving of signals, the generation of the signals needs to use a synchronous rectification circuit, the reason of the initial synchronous rectification is that the efficiency is much higher than that of diode rectification, based on the characteristic, if a rectification diode or a parallel diode is connected on a synchronous rectification MOS tube in series, the secondary winding voltage is pressurized by judging whether the synchronous rectification tube is not switched on in a certain period, thereby the signal required to be transmitted is generated to realize the secondary feedback control, the mode of using the synchronous rectification tube to be switched off at different times to realize the pressurization is the most economic and most effective mode, when the output voltage is detected to be higher than the set voltage, the synchronous rectification tube is controlled to be switched off at higher secondary current, therefore, residual winding current flows through a body diode of the synchronous rectifier tube after the synchronous rectifier tube is turned off, so that the voltage of two drain-source ends of the synchronous MOS is increased, and pressurization is realized, and the MOS tube is turned off under lower secondary side current when the output voltage is lower, so that smaller winding current flows through the body diode of the synchronous rectifier tube, and the condition of pressurization also occurs because current flows through the body diode, but the amplitude value is possibly slightly lower, based on the control scheme, the proposed primary side signal detection circuit comprises the following components:

a primary side detection circuit is suitable for a secondary side feedback control mode circuit and comprises a sampling delay circuit, a rising edge judgment circuit, a falling edge judgment circuit and a time detection circuit. The sampling delay circuit is used for shielding the oscillation which occurs after the primary side main MOS tube is turned off, the oscillation can be falsely detected by a circuit behind the sampling delay circuit to cause abnormal control, the rising edge judging circuit is used for judging whether the voltage of the auxiliary winding has a voltage rising slope or not, the falling edge judging circuit is used for judging whether the voltage of the auxiliary winding has a voltage falling slope or not, and the time detecting circuit is used for detecting the time from the rising edge to the falling edge.

The input end of the sampling delay circuit is connected with the voltage signal after voltage division of the auxiliary winding, and the output end of the sampling delay circuit outputs a voltage signal after voltage division of the auxiliary winding after shielding; the input end of the rising edge judging circuit is connected with the shielded auxiliary winding voltage dividing signal, and a control signal Vctrl-1 is decoded and output; the input end of the falling edge judgment circuit is connected with the shielded auxiliary winding voltage division signal, and a control signal Vctrl-2 is decoded and output; the input end of the time detection circuit is connected with a control signal Vctrl-1 and a control signal Vctrl-2, and outputs a duty ratio and frequency control signal Vctrl.

A primary side detection method comprising the steps of:

when the voltage of the secondary winding rises, the input end of the sampling delay circuit samples a rising voltage signal after voltage division of the auxiliary winding, the rising voltage signal is output to the rising edge judgment circuit after being shielded, the rising edge control circuit detects the slope and the voltage change amplitude and outputs a high-level or low-level control signal Vctrl-1, the time detection circuit starts timing after receiving the high-level signal Vctrl-1, and the time detection circuit does not time when receiving the low-level signal Vctrl-1;

when the voltage of the secondary winding is reduced, the input end of the sampling delay circuit samples a reduced voltage signal after voltage division of the auxiliary winding, the reduced voltage signal is output to the falling edge judgment circuit after being shielded, the falling edge judgment circuit detects the slope and the voltage change amplitude and outputs a control high-low level control signal Vctr-2, the time detection circuit stops timing after receiving the high level Vctr-2 signal, and the timing time is recorded as Tx;

a fixed time Tc is set in the time detection circuit, and if Tx is more than Tc, the time detection circuit outputs a control signal Vctrl to be high level; if Tx < Tc, the time detection circuit outputs a control signal Vctrl at a low level.

The specific working principle of the invention is as follows:

when the output voltage in the secondary feedback control circuit is higher than a set value, the control circuit on the secondary side can control the synchronous rectification MOS tube to turn off the MOS tube when the current of the secondary winding is larger after the detected output voltage is higher, the residual current passes through a body diode of the synchronous rectification MOS tube, the voltage of the secondary winding is increased because the voltage drop of the diode is larger than the voltage drop caused by rds (on), a rising slope appears, the voltage of the secondary winding is reflected to the auxiliary winding according to the turn ratio through the coupling of the transformer, then the voltage is input into the sampling delay circuit after the voltage division of the divider resistor, and the sampling shielding time is ended when the rising slope comes, so the rising edge and the falling edge judging circuit are in place, the rising edge judging circuit can detect the generation of a rising edge, and the rising edge judging circuit also needs to detect the amplitude, when the voltage variation amplitude reaches a set value VT1, a high level control signal Vctrl-1 is output, the time detection circuit starts timing after receiving the high level control signal Vctrl-1, if the voltage variation amplitude does not reach VT1, the timing is not performed, the timing time is marked as Tx, at this time, Tx is 0, when the secondary side demagnetization is finished, the secondary side winding voltage starts to drop, a falling edge appears, the voltage is detected by the falling edge judgment circuit after the transformer turns ratio conversion and the voltage division by the voltage division resistor, the falling edge judgment circuit also needs to detect the amplitude when the short circuit is detected, when the voltage variation amplitude reaches a set value VT2, the high level control signal Vctr-2 is output, the time detection circuit stops timing after receiving the high level Vctr-2 signal, the timing time is marked as Tx, then the timing time is compared with a fixed time in the time detection circuit, the fixed time is Tc, if Tx > Tc, the output voltage is judged to be high, the output control signal Vctrl of the time detection circuit is high level, the state is recorded as 1, the primary side control IC reduces the primary side driving duty ratio and the working frequency, the output voltage is reduced, if Tx < Tc, the output voltage is judged to be low, the output control signal Vctrl of the time detection circuit is low level, the state is recorded as 0, because the output synchronous rectification is turned off under the higher current of the secondary side, the primary side Tx is longer than Tc, and therefore the output voltage can be reliably detected to be high.

When the output voltage is low, the secondary control IC detects that the output voltage is lower than a set value, controls the synchronous rectification MOS tube to be switched off when the current of the secondary winding is small, and inputs the synchronous rectification MOS tube into the sampling delay circuit after the synchronous rectification MOS tube is subjected to the turn ratio conversion of the transformer and the voltage division of the auxiliary winding divider resistor, and the sampling shielding time is finished when the rising slope comes, so that the rising edge and falling edge decision circuits are already in place, the rising edge decision circuit will detect that a rising edge has occurred, the rising edge judgment circuit also needs to detect the amplitude, when the voltage variation amplitude reaches a set value VT1, a high level control signal Vctrl-1 is output, the time detection circuit starts to time after receiving the control signal to the level Vctrl-1, if the magnitude VT1 cannot be reached, the control signal remains low, and the timer time Tx is 0 without timing. When the secondary side demagnetization is finished, the voltage of the secondary side winding begins to drop, a falling edge appears, the voltage is detected by a falling edge judgment circuit after the voltage is subjected to transformer turn ratio conversion and voltage division by a voltage division resistor, the falling edge judgment circuit also needs to detect the amplitude, when the voltage variation amplitude reaches a set value VT2 (the amplitude at the end of demagnetization is certain to be very large and can reach a set value VT2), a high-level control signal Vctr-2 is output, the timing is stopped after the time detection circuit receives the high-level Vctr-2 signal, the timing time is marked as Tx, the timing time is compared with a fixed time in the time detection circuit, the fixed time is Tc, if Tx is more than Tc, the output voltage is judged to be high, the time detection circuit outputs a control signal Vctrl with a high level, the control signal is marked as a state 1, the primary side control IC reduces the driving primary side duty ratio and the working frequency, the output voltage is reduced, if Tx is less than Tc, the output voltage is judged to be low, the time detection circuit outputs a control signal Vctrl with low level, the level is recorded as state 0, the primary side control IC increases the primary side driving duty ratio and working frequency, the output voltage is increased, and at the moment, the secondary side synchronous rectification is turned off when the secondary side winding is small, Tx is small, so that the primary side can reliably detect that the output voltage is low.

The invention has the beneficial effects that:

1. the voltage of the auxiliary winding can be detected in each period, judgment is made, the influence of the output of the converter system is avoided, and the reliability is improved;

2. the synchronous rectification of the secondary side has long working time, and higher efficiency is ensured.

3. Extra diodes in series connection and parallel connection are not needed, the occupied area of the board is reduced, and the cost is reduced.

4. The reliability is greatly improved, the anti-interference capability is enhanced, and the control scheme of secondary feedback can be perfectly matched to carry out normal decoding on the coded signal.

Drawings

FIG. 1 is a schematic circuit diagram of a secondary feedback control method proposed in CN 105610306A;

fig. 2 is a schematic block diagram of a primary side detection circuit proposed in CN 107612334A;

fig. 3 is a schematic block diagram of a primary side detection circuit according to the present invention;

FIG. 4 is a schematic diagram of a partial node waveform of the detection circuit of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more clearly understood, the important elements of the present invention are further described in detail below with reference to fig. 3 and 4. As shown in fig. 3, the primary side detection circuit of the secondary side feedback control includes a sampling delay circuit 204, a rising edge determination circuit 205, a falling edge determination circuit 206, and a time detection circuit 211.

As shown in fig. 3, the connection relationship of the modules of the primary side signal detection circuit is as follows: the auxiliary winding NA is connected with a voltage division pull-up resistor RFA1 in a homonymy mode, and is connected to a lower voltage division resistor RFA2 and a sampling delay circuit module 204 after passing through an RFA1, the other end of the lower sampling resistor RFA2 is connected with a primary side reference ground, the output of the sampling delay circuit module 204 is connected with a rising edge judgment circuit 205 and a falling edge judgment circuit 206, the output of the rising edge judgment circuit is connected with a time detection circuit 211, the output of the falling edge judgment circuit is connected with the time detection circuit 211, and an output signal Vctrl of the time detection circuit is finally input to a duty ratio control module and a frequency control module of a primary side chip.

As shown in fig. 4, a waveform diagram of some key nodes Is shown, where VA Is a voltage waveform after voltage division of the auxiliary winding, SRGT Is a synchronous rectification driving waveform, Is a secondary current waveform, and in the diagram, a waveform corresponding to a state 0 in a first synchronous rectification driving period Is shown, at this time, an output voltage Is lower than a set value, the synchronous rectification tube Is turned off at a lower secondary current Is1, at this time, a voltage protrusion occurs in VA, that Is, a rising slope occurs in VA, and then a voltage drop occurs after demagnetization ends, a falling slope Is generated, a protrusion Is formed, a voltage amplitude Is also small, a time Tx1 corresponding to the protrusion Is very short, Tx1 Is less than Tc, so that Vctrl outputs a high level, and a primary IC Is controlled to increase a duty ratio and a working frequency, so that the output voltage rises to reach the set value; the second synchronous rectification driving period IS the corresponding waveform under the state 1, at this time, the output voltage IS higher than the set value, the synchronous rectification tube IS turned off at a larger secondary side current IS2, similarly, a voltage protrusion appears in VA, the time Tx2 corresponding to the protrusion IS longer, Tx2 IS greater than Tc, so that Vctrl outputs a low level, the primary side IC IS controlled to reduce the duty ratio and the working frequency, and the output voltage IS reduced to reach the set value.

The detailed description of the specific principles is omitted as it is described in detail in the summary of the invention.

Based on the above, according to the common technical knowledge and conventional means in the field, the detection circuit of the present invention has other embodiments without departing from the concept of detecting the rising edge of the present invention; therefore, the present invention may be modified, replaced or changed in various other ways, which fall within the scope of the appended claims.

Claims

1. A primary side detection circuit is suitable for a secondary side feedback control mode circuit, and is characterized in that: the circuit comprises a sampling delay circuit, a rising edge judgment circuit, a falling edge judgment circuit and a time detection circuit; the input end of the sampling delay circuit is connected with the voltage signal after voltage division of the auxiliary winding, and the output end of the sampling delay circuit outputs a shielded voltage signal after voltage division of the auxiliary winding; the input end of the rising edge judging circuit is connected with the shielded auxiliary winding voltage dividing signal, and a control signal Vctrl-1 is decoded and output; the input end of the falling edge judgment circuit is connected with the shielded auxiliary winding voltage division signal, and a control signal Vctrl-2 is decoded and output; the input end of the time detection circuit is connected with a control signal Vctrl-1 and a control signal Vctrl-2, and outputs a duty ratio and frequency control signal Vctrl.

2. A primary side detection method using the primary side detection circuit according to claim 1, characterized in that:

when the voltage of the secondary winding rises, the input end of the sampling delay circuit samples a rising voltage signal after voltage division of the auxiliary winding, the rising voltage signal is output to the rising edge judgment circuit after being shielded, the rising edge control circuit detects a rising edge and outputs a high-level control signal Vctrl-1 after the voltage change amplitude reaches a set value, the time detection circuit starts timing after receiving the high-level control signal Vctrl-1, and the time is not timed if the Vctrl-1 is detected to be a low level;

when the voltage of the secondary winding is reduced, the input end of the sampling delay circuit samples a reduced voltage signal after voltage division of the auxiliary winding, the reduced voltage signal is output to the falling edge judgment circuit after shielding, the falling edge judgment circuit outputs a high-level control signal Vctr-2 after detecting a falling slope and the voltage change amplitude reaches a set value, the time detection circuit stops timing after receiving the high-level control signal Vctr-2, and the timing time is recorded as Tx;