Current sampling circuit for inductive load driving circuit

Abstract

A current sampling circuit for an inductive load driving circuit comprises a sampling resistor Rs, an operational amplifier module, a voltage source Vs, and first and second resistor networks. The first resistor network comprises a resistor R3 and a resistor R1 connected in series, and the second resistor network comprises a resistor R4 and a resistor R2 connected in series; one end of the resistor R3 and one end of the resistor R4 are commonly connected to the voltage source Vs, the other end of the resistor R3 and the common point of one end of the resistor R1 are connected to the non-inverting input end of the operational amplifier module, and the other end of the resistor R1 is respectively connected to the output end of the inductive load driving circuit, the second end of the freewheeling circuit and one end of the sampling resistor Rs; the other end of the resistor R4 and the common point of one end of the resistor R2 are connected to the inverting input end of the operational amplifier module, and the other end of the resistor R2 is respectively connected to one end of the inductive load L and the other end of the sampling resistor Rs. The present invention expands the application lower limit range of the common mode input voltage of the operational amplifier module at a relatively low cost.

Description

Technical Field

[0001] The invention relates to a current sampling technology for an inductive load driving circuit. Background Art

[0002] When driving an inductive load L (such as a solenoid valve, motor, etc.), it is generally necessary to use the following Figure 1 The current sampling circuit shown collects the output current output to the inductive load to perform constant current control or overcurrent protection on the output current. The output current of the inductive load driving circuit 1 flows through the precision resistor R12, and a weak voltage drop is established at both ends of the precision resistor R12. The voltage drop is amplified by the operational amplifier module U1, and a voltage signal proportional to the current flowing through the precision resistor is output to the control circuit 2. Figure 1 The freewheeling diode D in the circuit can also be replaced by a MOSFET for freewheeling (i.e. synchronous rectification).

[0003] Figure 1 4 positions where precision resistors can be placed are shown in FIG. In addition to resistor R12, precision resistors can also be placed at the positions where resistors R11, R13 and R14 are located. The position where resistor R12 is located as a precision resistor has the following advantages: 1. It can detect the same current as the output current; 2. It can detect overcurrent caused by short circuit of ports P1 and P2, while the other three positions do not have the above advantages at the same time.

[0004] The voltage S+ and S- at both ends of the precision resistor R12 is a periodic fluctuating signal when driving an inductive load. The highest voltage can reach the input voltage of the driver, and the lowest voltage can be lower than the on-state voltage drop of a diode D (or MOSFET) at the reference ground. Therefore, there are some special requirements for circuit components when using precision resistor R12 to collect current. It requires the operational amplifier module U1 to have a large common-mode rejection ratio to amplify the weak differential-mode voltage Vd (Vd = Vs+-Vs-) at both ends of the precision resistor in the required original signal, and filter out the common-mode square wave voltage Vc (Vc = (Vs++Vs-) / 2) in the original signal. Some common current detection operational amplifier modules on the market generally have the characteristics of the large common-mode rejection ratio required above. However, the current detection op amp module has a limited range for the input common-mode voltage. The general current detection op amp module allows the input common-mode voltage to be greater than the power supply voltage of the op amp module, but does not allow too low common-mode voltage input. Too low common-mode input voltage will cause the input ESD protection diode inside the op amp module to turn on, thereby affecting the normal operation of the device. Severely too low common-mode input voltage may even cause device failure. The general current detection op amp module specification limits the absolute minimum value of the common-mode input voltage to negative 0.3V. Figure 1 In the example, when the freewheeling diode D is freewheeling, the common-mode input voltage is -V drop , Vdrop is the conduction voltage drop of the freewheeling diode D; when MOSFET is used to replace the diode D for freewheeling, the common-mode input voltage is -V drop , V drop is the conduction voltage drop of MOSFET. According to the selection of freewheeling diode D or MOSFET, current in actual operation, temperature and other conditions, the voltage of S+ and S- at both ends of precision resistor R12 will be lower than negative 0.3V, so the general current detection amplifier module cannot be directly used in the current sampling at the R12 position.

[0005] To solve the above problems, the common solutions are:

[0006] 1) Choose a current detection op amp module with a lower common-mode input voltage. This type of op amp module generally has a special design for the internal analog input stage or internal power supply circuit to achieve a lower common-mode input voltage range, but this will increase the cost;

[0007] 2) Add a filter circuit consisting of resistor R5, resistor R6 and capacitor C to the input end of the operational amplifier module U1, such as Figure 2 As shown, Figure 2 The internal structure of the operational amplifier module U1 is also shown. During the freewheeling process of the freewheeling diode D or MOSFET, the voltage across the precision resistor Rs, S+ and S-, is -V drop When the ESD protection diodes D1 and D2 in the operational amplifier module U1 are turned on, their conduction currents pass through resistors R5 and R6 respectively, and a voltage drop is established on resistors R5 and R6. Resistors R5 and R6 limit the current value passing through the ESD protection diodes D1 and D2, and raise the voltage of the input terminals IN+ and IN- of the operational amplifier module U1, protecting the device in the case of too low common-mode input voltage. However, the disadvantage of doing so is that the conduction of the ESD protection diodes D1 and D2 clamps the input signal at the input terminal of the operational amplifier module U1, causing the input differential signal to be inconsistent with the voltage drop across the precision resistor Rs. Therefore, the amplified signal cannot reflect the actual current size, resulting in a large sampling error. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a current sampling circuit for an inductive load driving circuit, which expands the application lower limit range of the common mode input voltage of the current detection operational amplifier module at a relatively low cost and has good sampling accuracy.

[0009] The embodiment of the present invention provides a current sampling circuit for an inductive load driving circuit, the current sampling circuit includes a sampling resistor Rs and an operational amplifier module; the sampling resistor Rs is connected in series between one end of the inductive load L and the output end of the inductive load driving circuit, the other end of the inductive load L and the first end of the freewheeling circuit are grounded together, and the second end of the freewheeling circuit is connected to the common point of the output end of the inductive load driving circuit and the sampling resistor Rs. The characteristic is that the current sampling circuit also includes a voltage source Vs, a first resistor network and a second resistor network; the first resistor network includes resistors R3 and R4 connected in series with each other. Resistor R1, the second resistor network includes a resistor R4 and a resistor R2 connected in series; one end of the resistor R3 and one end of the resistor R4 are commonly connected to the voltage source Vs, the other end of the resistor R3 and the common point of one end of the resistor R1 are connected to the non-inverting input end of the operational amplifier module, and the other end of the resistor R1 is respectively connected to the output end of the inductive load driving circuit, the second end of the freewheeling circuit and one end of the sampling resistor Rs; the other end of the resistor R4 and the common point of one end of the resistor R2 are connected to the inverting input end of the operational amplifier module, and the other end of the resistor R2 is respectively connected to one end of the inductive load L and the other end of the sampling resistor Rs.

[0010] The present invention has at least the following technical effects:

[0011] 1. The current sampling circuit of the embodiment of the present invention raises the input voltage of the non-inverting input terminal and the inverting input terminal of the operational amplifier module U1 by setting the voltage source Vs, the first resistor network and the second resistor network, so that a low-cost current detection operational amplifier module can be used to sample the original signal that is lower than the lower limit of the common-mode input voltage of the operational amplifier module;

[0012] 2. The output signal after amplification by the operational amplifier module of the current sampling circuit in the embodiment of the present invention is consistent with the real current, thereby ensuring the sampling accuracy of the current. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1 A circuit schematic diagram of an existing current sampling circuit for an inductive load driving circuit is shown.

[0015] Figure 2 A circuit schematic diagram of another existing current sampling circuit for an inductive load driving circuit is shown.

[0016] Figure 3A circuit schematic diagram of a current sampling circuit for an inductive load driving circuit according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0017] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] See also Figure 3 The current sampling circuit for an inductive load driving circuit according to an embodiment of the present invention includes a sampling resistor Rs, an operational amplifier module U1, a voltage source Vs, a first resistor network and a second resistor network.

[0019] The sampling resistor Rs is connected in series between one end of the inductive load L and the output end of the inductive load driving circuit 1, the other end of the inductive load L and the positive electrode of the freewheeling diode D are grounded together, and the negative electrode of the freewheeling diode D is connected to the common connection point of the output end of the inductive load driving circuit 1 and the sampling resistor Rs.

[0020] The first resistor network includes a resistor R3 and a resistor R1 connected in series, and the second resistor network includes a resistor R4 and a resistor R2 connected in series.

[0021] One end of the resistor R3 and one end of the resistor R4 are commonly connected to the voltage source Vs, the other end of the resistor R3 and the common point of one end of the resistor R1 are connected to the positive input terminal IN+ of the operational amplifier module U1, and the other end of the resistor R1 is respectively connected to the output end of the inductive load driving circuit 1, the negative electrode of the freewheeling diode D and one end S+ of the sampling resistor Rs.

[0022] The common point of the other end of the resistor R4 and one end of the resistor R2 is connected to the inverting input terminal IN- of the operational amplifier module U1, and the other end of the resistor R2 is respectively connected to one end of the inductive load L and the other end S- of the sampling resistor Rs.

[0023] Furthermore, the current sampling circuit also includes a capacitor C, one end of which is respectively connected to the positive input end of the operational amplifier module U1 and the common point of the other end of the resistor R3 and one end of the resistor R1, and the other end of the capacitor C is respectively connected to the negative input end of the operational amplifier module and the common point of the other end of the resistor R4 and one end of the resistor R2. The capacitor C has a filtering function.

[0024] Optionally, the first resistance network and the second resistance network are both linear resistance networks.

[0025] As another alternative embodiment, MOSFET can also be used to replace Figure 3 The freewheeling diode D in.

[0026] According to the superposition principle, the open circuit voltage V IN+(that is, when the first resistor network is not connected to the input resistor load of the operational amplifier module U1) is the original signal voltage V s+ (i.e. the voltage at point S+) and the open circuit voltage of the voltage source Vs at point IN+, that is, V s is the output voltage of the voltage source Vs. From this formula, we can know that when the freewheeling diode D is freewheeling (or MOSFET replaces the freewheeling diode D for freewheeling), the voltage V s+ When it is negative, the resistor R3 and the voltage source Vs can turn the voltage V at point IN+ to IN+ By adjusting the resistance of resistors R1 and R3, V IN+ When the freewheeling diode D (or MOSFET) is freewheeling, it is greater than or equal to 0V. Similarly, the open circuit voltage at point IN- When the freewheeling diode D (or MOSFET) is freewheeling, the voltage can be increased to greater than or equal to 0V through the resistor R4 and the voltage source Vs.

[0027] When the first and second resistor networks are connected to the input load of the operational amplifier module U1, because the operational amplifier module U1 has a large input impedance, the resistor network can be adjusted to make its output impedance much smaller than the input impedance of the operational amplifier module U1. In engineering, the effect of connecting the operational amplifier module on the voltage V IN+ and V IN- impact.

[0028] Optionally, the resistance value of resistor R1 is equal to the resistance value of resistor R2, and the resistance value of resistor R3 is equal to the resistance value of resistor R4. By making the resistance value of resistor R1 equal to the resistance value of resistor R2, and the resistance value of resistor R3 is equal to the resistance value of resistor R4, it can be ensured that the output of the operational amplifier module U1 is proportional to the input and output amplification factor of the operational amplifier module U1.

[0029] Furthermore, the resistance values ​​of the resistors R3 and R4 are both less than or equal to V s *R1 / V drop , V drop is the voltage drop of the freewheeling diode D (or MOSFET) during freewheeling, and the voltage of the voltage source Vs is V s , thereby ensuring that the common-mode input voltage of the operational amplifier module U1 is greater than or equal to 0V when the freewheeling diode D is turned on for freewheeling, thereby avoiding an excessively low common-mode input voltage.

[0030] Optionally, R1=R2=100ohm, R3=R4=V s *R1 / V dropIf the resistance of resistors R1, R2, R3 and R4 is too large, the output resistance of the original signal reaching the op amp module will be too large, causing additional errors. If the resistance of resistors R1 and R2 is too small, the loss of the resistor network will be too large. Choosing the resistance of resistors R1 and R2 to be 100ohm can achieve a balance in practical applications.

[0031] In addition, after adding resistors R3, R4, and voltage source Vs, when R1=R2 and R3=R4, whether the freewheeling diode D is in the freewheeling state or the power switch tube M of the inductive load driving circuit 1 is in the on state, the output voltage of the operational amplifier module U1 (i.e., the output voltage of the current sampling circuit) is equal to The current sampling circuit has a consistent amplification factor in different states, where A is the input and output amplification factor of the operational amplifier module U1, V d is the differential mode voltage in the original signal.

[0032] The current sampling circuit of the embodiment of the present invention enables a general operational amplifier module to be used in applications where the original signal voltage is lower than the lower limit of the common-mode input voltage of the operational amplifier module, thereby reducing costs. In addition, when the inductive load driving circuit outputs a driving signal and is in a freewheeling state, the current sampling circuit has a consistent amplification factor, thereby ensuring good sampling accuracy.

[0033] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A current sampling circuit for an inductive load driving circuit, the current sampling circuit comprising a sampling resistor Rs and an operational amplifier module; the sampling resistor Rs is connected in series between one end of an inductive load L and an output end of the inductive load driving circuit, the other end of the inductive load L and a first end of a freewheeling circuit are grounded together, and the second end of the freewheeling circuit is connected to a common point between the output end of the inductive load driving circuit and the sampling resistor Rs, characterized in that: The current sampling circuit also includes a voltage source Vs, a first resistor network and a second resistor network; The first resistor network includes a resistor R3 and a resistor R1 connected in series, and the second resistor network includes a resistor R4 and a resistor R2 connected in series; One end of the resistor R3 and one end of the resistor R4 are commonly connected to the voltage source Vs, the other end of the resistor R3 and the common point of one end of the resistor R1 are connected to the non-inverting input end of the operational amplifier module, and the other end of the resistor R1 is respectively connected to the output end of the inductive load driving circuit, the second end of the freewheeling circuit and one end of the sampling resistor Rs; The common point of the other end of the resistor R4 and one end of the resistor R2 is connected to the inverting input end of the operational amplifier module, and the other end of the resistor R2 is respectively connected to one end of the inductive load L and the other end of the sampling resistor Rs.

2. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The resistance value of the resistor R1 is equal to the resistance value of the resistor R2 , and the resistance value of the resistor R3 is equal to the resistance value of the resistor R4 .

3. The current sampling circuit for an inductive load driving circuit according to claim 2, characterized in that: The resistance of resistor R3 and resistor R4 are both less than or equal to V s *R1 / V drop , where V drop is the conduction voltage drop of the freewheeling circuit.

4. The current sampling circuit for an inductive load driving circuit according to claim 2, characterized in that: The resistance values ​​of resistors R1 and R2 are both 100 ohms, and the resistance values ​​of resistors R3 and R4 are both equal to V s *R1 / V drop , where V drop is the conduction voltage drop of the freewheeling circuit.

5. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The current sampling circuit also includes a capacitor C, one end of which is respectively connected to the non-inverting input end of the operational amplifier module and the common point between the other end of the resistor R3 and one end of the resistor R1, and the other end of the capacitor C is respectively connected to the inverting input end of the operational amplifier module and the common point between the other end of the resistor R4 and one end of the resistor R2.

6. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The first resistance network and the second resistance network are both linear resistance networks.

7. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The freewheeling circuit is a freewheeling diode, the anode of the freewheeling diode is the first end of the freewheeling circuit, and the cathode of the freewheeling diode is the second end of the freewheeling circuit.

8. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The freewheeling circuit is a MOSFET.

9. The current sampling circuit for an inductive load driving circuit according to claim 1, characterized in that: The inductive load driving circuit is a solenoid valve driving circuit or a motor driving circuit.

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