CN103763820B - The load sample circuit of isolated LED drive circuit - Google Patents

Abstract

A load sample circuit for isolated LED drive circuit, belongs to LED illumination drive circuit technical field.Comprise LED drive circuit and sample circuit, the former limit winding switching of described sample circuit and LED drive circuit, sample circuit comprises the first resistance R1, the second resistance R2 and the first electric capacity C1, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.Advantage: without the need to auxiliary winding or optic coupling element, acquisition precision is high, structure is simple, be easy to realization, and can be used for the drive circuit of continuous current mode or discontinuous current mode.

Description

The load sample circuit of isolated LED drive circuit

Technical field

The invention belongs to LED illumination drive circuit technical field, relate to a kind of load sample circuit of isolated LED drive circuit, particularly relate to the load sample circuit that a kind of nothing assists winding or the isolated LED drive circuit without photoelectric coupled circuit.

Background technology

Traditional LED drive circuit or DC/DC electric power management circuit need adopt auxiliary winding or photoelectric coupled circuit to gather load voltage.As shown in Figure 5, LED drive circuit is made up of driving chip U1, the second flyback transformer T2, the first metal-oxide-semiconductor M1, the 3rd resistance R3, the 4th resistance R4, the 6th resistance R6, the 7th resistance R7, the second electric capacity C2, the 3rd electric capacity C3, the 4th electric capacity C4, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5 and LED string DL, wherein, first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4 form a rectifier bridge, connect external communication power supply.The second described flyback transformer T2 has three windings: former limit, secondary and auxiliary winding.Former limit is the switching circuitry for providing energy to be fed to, and the inductance on former limit, in the switching process of the first metal-oxide-semiconductor M1, completes from power supply energy storage and the function to secondary delivery of energy; The effect of secondary carries out rectification to the electromotive force responded to from former limit, is supplied to LED string DL or other loads after filtering; The effect of auxiliary winding is the collection of secondary voltage and the overvoltage protection of secondary.The effect of driving chip U1 is the switching time that size by gathering secondary current or voltage controls former limit first metal-oxide-semiconductor M1, reaches the constant electric current of LED string DL or the object of other load output voltage constant with this.But, assist winding to gather load voltage V in above-mentioned traditional employing _lEDreverse excitation circuit structure in, the existence of auxiliary winding causes circuit to have following defect: more or less there is leakage inductance, the precision of voltage regulation is declined, consistency variation; Auxiliary winding is used in circuit, except increase manufacturing cost, also can occupying volume outer volume, and the circuit board finite volume that LED lamp tube can be held, if particularly certain parameter of circuit board length, width and height is mated not good with the size of LED lamp tube, also can affect the heat dispersion of circuit board, thus reduce the life-span of driving power; In addition, auxiliary winding also can make the structure of transformer become complicated, and the restriction be subject to during circuit design increases, and the difficulty of light source design also becomes large thereupon.Fig. 6 illustrates a kind of electrical schematic diagram being gathered load voltage by optocoupler, this circuit comprises: driving chip U1, first flyback transformer T1, first metal-oxide-semiconductor M1, 3rd resistance R3, 4th resistance R4, second electric capacity C2, 3rd electric capacity C3, 4th electric capacity C4, first diode D1, second diode D2, 3rd diode D3, 4th diode D4, 5th diode D5, LED string DL and optocoupler OC, wherein the difference of the first flyback transformer T1 and the second flyback transformer T2 is not assist winding, this circuit gathers load voltage V by optocoupler OC _lED, but same existing defects: cost of manufacture rises, and increases larger power consumption, thus affects power-efficient, in addition, the sensitivity of optocoupler is poor, and the aging of optocoupler also can affect the accuracy of sampled voltage and the stability of overvoltage protection.

In view of above-mentioned prior art, be necessary to be improved, for this reason, the applicant has done useful design, and technical scheme described below produces under this background

Summary of the invention

The object of the present invention is to provide a kind of load sample circuit of isolated LED drive circuit, without the need to auxiliary winding or optic coupling element, acquisition precision is high, structure is simple, be easy to realization, and can be used for the drive circuit of continuous current mode (CCM) or interrupted (discontinuous) current-mode (DCM).

The object of the invention is to reach like this, a kind of load sample circuit of isolated LED drive circuit, comprise LED drive circuit and sample circuit, the former limit winding switching of described sample circuit and LED drive circuit, described LED drive circuit comprises driving chip U1, first flyback transformer T1, first metal-oxide-semiconductor M1, 3rd resistance R3, 4th resistance R4, 5th resistance R5, second electric capacity C2, 3rd electric capacity C3, 4th electric capacity C4, 5th electric capacity C5, first diode D1, second diode D2, 3rd diode D3, 4th diode D4, 5th diode D5, 6th diode D6 and LED string DL, the negative pole of the first described diode D1 and the negative pole of the second diode D2, one end of 3rd resistance R3, one end of 3rd electric capacity C3, one end of 5th electric capacity C5, one end of one end of 5th resistance R5 and the former limit winding of the first flyback transformer T1 connects, positive pole and the negative pole of the 3rd diode D3 of the first diode D1 are connected one end of external ac power source jointly, positive pole and the negative pole of the 4th diode D4 of the second diode D2 are connected the other end of external ac power source jointly, the other end of the 3rd resistance R3 is connected with one end of the second electric capacity C2, and connect driving chip U1, the other end of the 5th electric capacity C5 is connected with the negative pole of the other end of the 5th resistance R5 and the 6th diode D6, the positive pole of the 6th diode D6 and the drain electrode of the first metal-oxide-semiconductor M1, the other end of the former limit winding of the first flyback transformer T1 connects, and connect an input of sample circuit, the grid of the first metal-oxide-semiconductor M1 connects driving chip U1, the source electrode of the first metal-oxide-semiconductor M1 is connected with one end of the 4th resistance R4, and connect driving chip U1, one output of the first flyback transformer T1 vice-side winding is connected with the positive pole of the 5th diode D5, and the negative pole of the 5th diode D5 connects one end of the 4th electric capacity C4 and the positive input terminal of LED string DL, the positive pole of the 3rd diode D3, the positive pole of the 4th diode D4, the other end of the second electric capacity C2, the other end of the 3rd electric capacity C3, the other end of the 4th resistance R4, another output of the vice-side winding of the first flyback transformer T1, the other end of the 4th electric capacity C4 and the negative input end common ground of LED string DL, is characterized in that: described sample circuit comprises the first resistance R1, second resistance R2 and the first electric capacity C1, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.

In one particular embodiment of the present invention, described sample circuit comprises the first resistance R1, the second resistance R2 and the first electric capacity C1, one end of first electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.

In another specific embodiment of the present invention, described sample circuit comprises the first resistance R1, second resistance R2, first electric capacity C1 and the second metal-oxide-semiconductor M2, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.

In another specific embodiment of the present invention, described sample circuit comprises the first resistance R1, second resistance R2, first electric capacity C1 and the second metal-oxide-semiconductor M2, one end of first electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.

The present invention is owing to have employed said structure, compared with prior art, the beneficial effect had is: described sample circuit only with the former limit winding switching of the first flyback transformer T1, thus the isolation of circuit is not affected, this sample circuit can be sampled at the load voltage of the state of continuous current mode to isolated LED drive circuit, in addition, by increasing by the second metal-oxide-semiconductor M2, also can sample to the load voltage of the isolated LED drive circuit of discontinuous current mode; It gathers secondary voltage from former limit, and to determine secondary load condition, acquisition precision is high, and described sample circuit does not need auxiliary winding and optocoupler, and thus structure is simple, easily realize, and cost of manufacture is lower.

Accompanying drawing explanation

Fig. 1 is the electrical schematic diagram of one embodiment of the invention.

Fig. 2 is one embodiment of the invention current-voltage waveform figure at steady state.

Fig. 3 is the electrical schematic diagram of another embodiment of the present invention.

Fig. 4 is another embodiment of the present invention current-voltage waveform figure at steady state.

Fig. 5 is that utilization of the prior art assists winding to gather the electrical schematic diagram of LED drive circuit load voltage.

Fig. 6 is the electrical schematic diagram utilizing optocoupler to gather LED drive circuit load voltage of the prior art.

Embodiment

Technical spirit of the present invention and beneficial effect can be fully understood in order to make the public; applicant will describe in detail the specific embodiment of the present invention below by reference to the accompanying drawings; but applicant is not the restriction to technical scheme to the description of embodiment, any changing in the form rather than substance according to the present invention's design all should be considered as protection scope of the present invention.

Embodiment 1:

Refer to Fig. 1, a kind of load sample circuit of isolated LED drive circuit, comprises LED drive circuit and sample circuit, the former limit winding switching of described sample circuit and LED drive circuit.Described LED drive circuit comprises driving chip U1, the first flyback transformer T1, the first metal-oxide-semiconductor M1, the 3rd resistance R3, the 4th resistance R4, the 5th resistance R5, the second electric capacity C2, the 3rd electric capacity C3, the 4th electric capacity C4, the 5th electric capacity C5, the first diode D1, the second diode D2, the 3rd diode D3, the 4th diode D4, the 5th diode D5, the 6th diode D6 and LED string DL, wherein, the first diode D1, the second diode D2, the 3rd diode D3 and the 4th diode D4 form a rectifier bridge.Described sample circuit comprises the first resistance R1, the second resistance R2 and the first electric capacity C1, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with one end of the second resistance R2 and driving chip U1, and become the output (sampled point shown in figure) of sample circuit, the other end ground connection of the second resistance R2.

Please continue to refer to Fig. 1 and composition graphs 2, the operation principle of the present embodiment is described.Described sample circuit only with the former limit winding switching of the first flyback transformer T1, thus the isolation of circuit is not affected, this sample circuit can be sampled at the load voltage of state to isolated LED drive circuit of continuous current mode or discontinuous current mode, in the present embodiment, be described for the state of continuous current mode.When described LED drive circuit is in stable condition, if the first metal-oxide-semiconductor M1 conducting, electric current from the positive output end of rectifier bridge, successively through the former limit winding of the first flyback transformer T1, the first metal-oxide-semiconductor M1 and the 4th resistance R4(sampling resistor) get back to the negative output terminal of rectifier bridge.After electric current increases to certain threshold value, the first metal-oxide-semiconductor M1 ends, the secondary loop conducting of the first flyback transformer T1.When the electric current in secondary loop is reduced to zero, the first metal-oxide-semiconductor M1 conducting again.Especially, suppose the first metal-oxide-semiconductor M1 conducting when secondary current zero passage, end when the voltage of the 4th resistance R4 reaches certain threshold values, namely circuit working is also suitable for the present embodiment in the critical condition of continuous current mode and discontinuous current mode.The first metal-oxide-semiconductor M1 is set again to keep the time durations of conducting as T ₁, the time durations of the first metal-oxide-semiconductor M1 remain off is T ₂, the former limit inductance size of the first flyback transformer T1 is L, and the current peak on former limit is I _pEAK, the DC input voitage of external communication voltage after rectifier bridge rectification is V _iN, the voltage at the first resistance R1 two ends is V _r1, the voltage at the second resistance R2 two ends is V _r2, the voltage at the first electric capacity C1 two ends is V _c1, the output voltage values of LED string DL is V _lED, former limit and the secondary turn ratio of the first flyback transformer T1 are N, obtain

$T_1=\frac{L\·I_{\mathrm{PEAK}}}{V_{\mathrm{IN}}}---\left(1\right)$

$T_2=\frac{L\·I_{\mathrm{PEAK}}}{N\·V_{\mathrm{LED}}}---\left(2\right)$

To T ₁, T ₂ask ratio, obtain

$\frac{T_1}{T_2}=\frac{N\·V_{\mathrm{LED}}}{V_{\mathrm{IN}}}---\left(3\right)$

$\frac{T_2}{T_1+T_2}=\frac{V_{\mathrm{IN}}}{N\·V_{\mathrm{LED}}+V_{\mathrm{IN}}}---\left(4\right)$

Because at T ₂in period section, in figure, the voltage at A point place is V _iN+ NV _lED, and at T ₁in period section, A point and the earth equipotential, be about 0, the voltage V therefore on the first electric capacity C1 _c1the mean value of A point voltage:

$T_{C1}=\frac{T_2}{T_1+T_2}\·(V_{\mathrm{IN}}+N\·V_{\mathrm{LED}})=\frac{V_{\mathrm{IN}}}{V_{\mathrm{IN}}+N\·V_{\mathrm{LED}}}(V_{\mathrm{IN}}+N\·V_{\mathrm{LED}})=V_{\mathrm{IN}}---\left(5\right)$

If RC time constant (R1+R2) C1 of described sample circuit is far longer than the switching frequency of the first metal-oxide-semiconductor M1, then the voltage V on the first electric capacity C1 _c1stabilize to V _iN, again due to

V _R1+V _R2+V _C1=N·V _LED+V _IN（6）

Therefore,

V _R1+V _R2=N·V _LED（7）

Because the voltage on the first resistance R1 and the second voltage R2 is according to the proportional distribution of their resistance value, the voltage that therefore can obtain on the second resistance R2 is

$V_{R2}=\frac{R_2}{R_1+R_2}\·N\·V_{\mathrm{LED}}---\left(8\right)$

As shown from the above formula, described sampled voltage, i.e. the voltage V at the second resistance R2 two ends _r2with the voltage V at secondary load LED lamp string DL two ends _lEDlinear, the coefficient of this linear relationship is only relevant with the resistance of the first resistance R1, the second resistance R2 and the turn ratio N of the first flyback transformer T1, therefore, it is possible to realize the precise acquisition of load end output voltage.

Embodiment 2:

Refer to Fig. 3, described sample circuit comprises the first resistance R1, second resistance R2, first electric capacity C1 and the second metal-oxide-semiconductor M2, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and become the output (sampled point shown in figure) of sample circuit, the other end ground connection of the second resistance R2.

Please continue to refer to Fig. 3 and composition graphs 4, the operation principle of the present embodiment is described.In this embodiment, can sample at the load voltage of the state of discontinuous current mode to isolated LED drive circuit by setting up the second metal-oxide-semiconductor M2.When described LED drive circuit is in stable state, when the electric current of vice-side winding reduces to zero, the first metal-oxide-semiconductor M1 not conducting immediately, but conducting after a period of time has passed.When the first metal-oxide-semiconductor M1 normally and cut-off, suppose that the first metal-oxide-semiconductor M1 keeps the time durations of conducting to be T ₃, the time durations of the first metal-oxide-semiconductor M1 remain off is T ₄, the time durations reducing to the 0 to the first metal-oxide-semiconductor M1 conducting from the first flyback transformer T1 secondary current is T ₅, in addition, with embodiment 1, supposing that the size of the former limit inductance of the first flyback transformer T1 is L, is I by the peak current on the first former limit of flyback transformer T1 _pEAK, circuit input voltage is V _iN, the voltage at the first resistance R1 two ends is V _r1, the voltage at the second resistance R2 two ends is V _r2, the voltage at the first electric capacity C1 two ends is V _c1, the output voltage of load LED lamp string DL is V _lED, former limit and the secondary turn ratio of the first flyback transformer T1 are N, obtain

$T_3=\frac{L\·I_{\mathrm{PEAK}}}{V_{\mathrm{IN}}}---\left(9\right)$

$T_4=\frac{L\·I_{\mathrm{PEAK}}}{N\·V_{\mathrm{LED}}}---\left(10\right)$

To T ₃, T ₄ask ratio, obtain

$\frac{T_3}{T_4}=\frac{N\·V_{\mathrm{LED}}}{V_{\mathrm{IN}}}---\left(11\right)$

$\frac{T_4}{T_3+T_4}=\frac{V_{\mathrm{IN}}}{N\·V_{\mathrm{LED}}+V_{\mathrm{IN}}}---\left(12\right)$

If at T ₅in period, the grid voltage of the second metal-oxide-semiconductor M2 becomes low level, then at T ₄voltage V in period on the first electric capacity C1 _c1for

$V_{C1}=\frac{T_4}{T_3+T_4}\·(V_{\mathrm{IN}}+N\·V_{\mathrm{LED}})=\frac{V_{\mathrm{IN}}}{V_{\mathrm{IN}}+N\·V_{\mathrm{LED}}}(V_{\mathrm{IN}}+N\·V_{\mathrm{LED}})=V_{\mathrm{IN}}---\left(13\right)$

With the deriving analysis in embodiment 1, can at T ₄in period,

V _R1+V _R2=N·V _LED（14）

$V_{R2}=\frac{R_2}{R_1+R_2}\·N\·V_{\mathrm{LED}}---\left(15\right)$

As shown from the above formula, identical with embodiment 1, described sampled voltage, i.e. the voltage V at the second resistance R2 two ends _r2with the voltage V at secondary load LED lamp string DL two ends _lEDlinear, the coefficient of this linear relationship is only relevant with the resistance of the first resistance R1, the second resistance R2 and the turn ratio N of the first flyback transformer T1, therefore, it is possible to realize the precise acquisition of load end output voltage.

Embodiment 3:

In the present embodiment, described sample circuit comprises the first resistance R1, the second resistance R2 and the first electric capacity C1, one end of first electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.All the other all with the description of embodiment 1, repeat no more herein.

Embodiment 4:

In the present embodiment, described sample circuit comprises the first resistance R1, second resistance R2, first electric capacity C1 and the second metal-oxide-semiconductor M2, one end of first electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.All the other all with the description of embodiment 2, repeat no more herein.

Claims (4)

1. a load sample circuit for isolated LED drive circuit, comprises LED drive circuit and sample circuit, the former limit winding switching of described sample circuit and LED drive circuit, and described LED drive circuit comprises driving chip U1, first flyback transformer T1, first metal-oxide-semiconductor M1, 3rd resistance R3, 4th resistance R4, 5th resistance R5, second electric capacity C2, 3rd electric capacity C3, 4th electric capacity C4, 5th electric capacity C5, first diode D1, second diode D2, 3rd diode D3, 4th diode D4, 5th diode D5, 6th diode D6 and LED string DL, the negative pole of the first described diode D1 and the negative pole of the second diode D2, one end of 3rd resistance R3, one end of 3rd electric capacity C3, one end of 5th electric capacity C5, one end of one end of 5th resistance R5 and the former limit winding of the first flyback transformer T1 connects, positive pole and the negative pole of the 3rd diode D3 of the first diode D1 are connected one end of external ac power source jointly, positive pole and the negative pole of the 4th diode D4 of the second diode D2 are connected the other end of external ac power source jointly, the other end of the 3rd resistance R3 is connected with one end of the second electric capacity C2, and connect driving chip U1, the other end of the 5th electric capacity C5 is connected with the negative pole of the other end of the 5th resistance R5 and the 6th diode D6, the positive pole of the 6th diode D6 and the drain electrode of the first metal-oxide-semiconductor M1, the other end of the former limit winding of the first flyback transformer T1 connects, and connect an input of sample circuit, the grid of the first metal-oxide-semiconductor M1 connects driving chip U1, the source electrode of the first metal-oxide-semiconductor M1 is connected with one end of the 4th resistance R4, and connect driving chip U1, one output of the first flyback transformer T1 vice-side winding is connected with the positive pole of the 5th diode D5, and the negative pole of the 5th diode D5 connects one end of the 4th electric capacity C4 and the positive input terminal of LED string DL, the positive pole of the 3rd diode D3, the positive pole of the 4th diode D4, the other end of the second electric capacity C2, the other end of the 3rd electric capacity C3, the other end of the 4th resistance R4, another output of the vice-side winding of the first flyback transformer T1, the other end of the 4th electric capacity C4 and the negative input end common ground of LED string DL, is characterized in that: described sample circuit comprises the first resistance R1, second resistance R2 and the first electric capacity C1, one end of first resistance R1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first resistance R1 is connected with one end of the first electric capacity C1, the other end of the first electric capacity C1 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit, the other end ground connection of the second resistance R2.

2. the load sample circuit of isolated LED drive circuit according to claim 1, it is characterized in that one end of the first described electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with one end of the second resistance R2 and driving chip U1, and becomes the output of sample circuit.

3. the load sample circuit of isolated LED drive circuit according to claim 1, it is characterized in that described sample circuit comprises the second metal-oxide-semiconductor M2, the other end of the first electric capacity C1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and becomes the output of sample circuit.

4. the load sample circuit of isolated LED drive circuit according to claim 1, it is characterized in that described sample circuit comprises the second metal-oxide-semiconductor M2, one end of first electric capacity C1 connects another input of the former limit winding of the first flyback transformer T1 as an input of sample circuit, the other end of the first electric capacity C1 is connected with one end of the first resistance R1, the other end of the first resistance R1 is connected with the drain electrode of the second metal-oxide-semiconductor M2, the grid of the second metal-oxide-semiconductor M2 connects driving chip U1 as another input of sample circuit, the source electrode of the second metal-oxide-semiconductor M2 is connected with one end of the second resistance R2 and driving chip U1, and become the output of sample circuit.