JP5612024B2 - High side current detection circuit - Google Patents

Description

  In the present invention, when detecting a load current flowing in a load connected to the high side of a DC power source by constant current control of an LED (light emitting diode) or the like, a current detection voltage generated in a high side shunt resistor (current detection resistor) is detected. The present invention relates to a high-side current detection circuit that can compensate for the influence of voltage fluctuations of a DC power supply when a current mirror circuit is used to guide the DC power supply to the low side.

  In order to stably drive a load with constant voltage characteristics such as an LED when the LED is lit with constant brightness, constant current drive that keeps the load current constant even when the power supply voltage changes is appropriate. is there. In order to drive a load having high efficiency such as an LED, a switching DC-DC (direct current-direct current) converter having high power conversion efficiency is used. A typical FET (field effect transistor) as a switching element of a DC-DC converter is usually placed on the low side of a DC power supply, and a control IC for controlling and driving the FET is also placed on the low side. Since the LED as a load is connected to the power supply in series with the FET, it is placed on the high side of the DC power supply.

  In order to carry out constant current control of the current of the load, it is necessary to detect the current of the load, and a current detection resistor for detecting the load current is placed on the high side with the load. Since the voltage drop of the current detection resistor, that is, the detection voltage proportional to the load current is based on the high side, in order to supply it to the control IC that controls the on / off of the switching element, the low side is referenced using a level shift circuit. It is necessary to level shift to voltage. In this case, the level shift circuit must be free from the influence of power supply voltage fluctuations.

  Since the voltage drop of the current detection resistor causes power loss, it is desirable that the voltage drop be small, and a low resistance that is usually 1 volt or less is used. The power supply voltage of the DC power supply is a potential difference between the high side and the low side, and a typical value is about 100 volts. Therefore, in order to supply the voltage of the current detection resistor to the control IC with 1% accuracy, the influence of the power supply voltage received by the level shift circuit needs to be 0.1% or less. A highly accurate circuit that is less affected by the power supply voltage is desirable.

  As such a level shift circuit that is less affected by the power supply voltage and has high accuracy, a current mirror circuit is generally used in which resistors are connected to the emitters of two transistors, and one of the resistors is a current detection resistor. However, in the current mirror circuit, the reference current changes with the power supply voltage, and in addition, the current mirror circuit is affected by the power supply voltage fluctuation due to the Early effect. Simple and effective compensation is desirable to compensate for the effect of such power supply voltage fluctuations.

[Conventional circuit]
FIG. 5 shows a DC power supply, a DC-DC converter, a load such as an LED, and a conventional high-side current detection circuit for detecting a load current flowing through the load. A load current is detected by a conventional high-side current detection circuit, and the load current is controlled at a constant current. In FIG. 5, 1 is a PWM control circuit, 2 is a DC-DC converter, and 3 is a high-side current detection circuit using a current mirror circuit.

  In the conventional high side current detection circuit 3, the load current I0 of the DC-DC converter 2 is detected by a current detection resistor R1 which is a shunt resistor on the high side. The DC-DC converter 2 includes a switching element Qsw, a freewheeling diode D1, a reactor L1, and a smoothing capacitor C1, and supplies controlled electric power to a load L. The power supply voltage of the DC power supply V1 is V1, and the high side is represented by P and the low side is represented by G.

  In the constant current control of the load current, the PWM control circuit 1 including the control IC controls the on / off of the switching element Qsw so that the detection output V2 detected by the high side current detection circuit 3 is constant. The current detection resistor R1 is a shunt resistor that detects the load current Io and can be included in a part of the current mirror circuit.

  The current detection circuit 3 includes a reference transistor Q1 and an output transistor Q2 of a current mirror circuit. If the load current Io does not flow through the current detection resistor R1, the output current I2 is proportional to the reference current I1. In the high-side current detection circuit 3, a current detection resistor R1 connected to the emitter of the reference transistor Q1 is used as a shunt resistor for current detection, a proportional resistor R2 is connected to the emitter of the output transistor Q2, and a collector resistor R1c is connected to the reference transistor Q1. Causes the reference current I1 to flow, and the collector current I2 of the output transistor Q2 gives the detection output V2 by the collector resistor R2c.

  The circuit condition is that the resistance value R1 of the current detection resistor R1 is sufficiently smaller than the resistance value R2 of the proportional resistor R2 so that the reference current I1 and the output current I2 are sufficiently smaller than the load current Io, and R1 << R2. The voltage drop Io · R1 at the current detection resistor R1 is sufficiently smaller than the power supply voltage V1 to be Io · R1 << V1, and the transistors Q1 and Q2 both have a large current amplification factor and the base current is negligible compared to the emitter current. And

  The basic operation of the current mirror circuit is that the output current I2 flows so that the emitter voltages of the two transistors Q1 and Q2 as seen from the high side are equal. Therefore, the voltage drops at the resistors R1 and R2 are equal. Is established.

Io · R1 = I2 · R2 (1)
From the equation (1), the collector current I2 of the output transistor Q2 is given by the following equation (2).

I2 = (R1 / R2) Io (2)
As a result, the collector current I2 of the output transistor Q2 is proportional to the load current Io, and the detection output V2 with respect to the low side G is obtained for the collector resistance R2c of the output transistor Q2. The detection output V2 is expressed by the following equation (3 ).

V2 = I2 · R2c = (R2c / R2) Io · R1 (3)
From the equation (3), the reference current I1 is not included in the detection output V2, but becomes a current for giving an appropriate operating point to the transistors Q1 and Q2.

  Since the circuit is I1 ≠ I2, the two transistors Q1. There is a difference in the base-emitter voltage of Q2, and strictly speaking, equation (2) does not hold and an error occurs. In particular, when the influence of the fluctuation of the power supply voltage V1 appears in the detection current I2, it is not preferable because the detection current I2 changes depending on the power supply voltage V1 when the load is controlled at a constant current. Hereinafter, the operation of FIG. 5 will be described in some detail.

[Analysis of conventional circuit]
Since the bases of the transistors Q1 and Q2 are connected in common, the potentials at both bases viewed from the high side P are equal, and the reference current I1 flowing through the current detection resistor R1 is ignored.
VBE1 + IoR1 = VBE2 + I2R2 (4)
However, VBE1: Q1 base voltage, VBE2: Q2 base voltage,
Since the voltage across the collector resistor R1c is substantially equal to the power supply voltage V1, the flowing reference current I1 is given by the following equation (5), where the resistance value of the collector resistor R1c is R1c.

I1 = V1 / R1c (5)
When the expression (4) is arranged and the output current I2 is expressed by I2R2, the following expression (6) is obtained.

I2R2 = IoR1 + VBE1-VBE2 (6)
The first term IoR1 on the right side in Equation (6) is a signal component. VBE1-VBE2 represents an error.

  The base voltages VBE1 and VBE2 of the transistors Q1 and Q2 are proportional to the logarithm of the collector currents I1 and I2, respectively, and the following equations (7) and (8) are established.

VBE1≈ (kT / q) ln (I1 / IS1) (7)
VBE2≈ (kT / q) ln (I2 / IS2) (8)
However, (kT / q): constant, Is1: Q1 saturation current, Is2: Q2 saturation current, k: Boltzmann constant, T: absolute temperature, q: electron charge.

  Substituting the equations (7) and (8) into the equation (6), the following equation (9) is established.

I2R2 = IoR1 + (kT / q) ln (I1 / IS1)
-(KT / q) ln (I2 / IS2)
= IoR1 + (kT / q) {ln (I1) -ln (I2)
−ln (IS1) + ln (IS2)} (9)
The Early effect is a phenomenon in which the collector current changes depending on the collector voltage of the transistor, and is explained by the change in saturation current as follows. The saturation current IS2 of the output transistor Q2 is a function of the collector voltage VBC. When expressed as IS2 when VAF: Early voltage and IS2 (0): VBC = 0, the saturation current IS2 changes as shown in the following equation (10). .

IS2 = IS2 (0) (1 + VBC / VAF) (10)
Both transistors Q1 and Q2 have the same characteristics, IS1 = IS2 (0) = IS, I2R2 << V1 and V2 << V1, and V1≈VBC, so that I1 in Equation (9) is changed to Equations (5) and IS2. Substituting equation (10), the following equation (11) is established.

I2R2 = IoR1- (kT / q) ln (R1C)
-(KT / q) ln (I2) + (kT / q) ln (V1)
+ (KT / q) ln (1 + V1 / VAF) (11)
The first term on the right side of equation (11) is a signal component, the second term is a constant value, the third term is a function of I2, so it is a nonlinear component, the fourth term is the fluctuation component of V1 through R1c, and the fifth term is It is a fluctuation component of V1 due to the Early effect. The value of R1c is not related to the fluctuation component of V1. Non-linearity has no adverse effect on constant current control.

[Conventional circuit characteristics]
As an example of the current detection characteristic of the conventional circuit, FIG. 6 shows the V2-Io characteristic at V1 = 90 V ± 20 V, with R1 = 0.5Ω, R2 = 1 kΩ, R2c = 1 kΩ, and R1c = 100 kΩ in FIG. FIG. 6 shows current detection characteristics in which the horizontal axis represents the load current Io and the vertical axis represents the detection output V2. In FIG. 6, the current detection characteristics when the power supply voltage V1 is medium during V1, large V1 when the power supply voltage V1 is large, and small V1 are when the power supply voltage V1 is small. As shown in FIG. 6, it can be seen that the influence of V1 appears in the detection output V2 when the magnitude of the power supply voltage V1 varies from large to medium to small.

Some conventional circuits reduce the influence of the power supply voltage V1 generated through the collector resistor R1c by obtaining the reference current of the current mirror circuit from the constant current circuit. (Patent Document 1, FIG. 7)
However, the constant current source shown in the circuit of Patent Document 1 has a complicated circuit configuration or a large loss, and the influence of the power supply voltage V1 due to the Early effect of the output transistor Q2 is the constant current source. It cannot be removed at the source.

JP-A-11-160368

  The present invention has been made in view of the above, and an object of the present invention is to compensate for the influence of fluctuations in the power supply voltage appearing in the detected output voltage of the current mirror circuit with a simple circuit configuration.

In order to achieve the above object, a high-side current detection circuit according to the first aspect of the present invention is a DC-DC converter in which a load is connected to the high side of a DC power supply , and a current detection resistor is provided between the high side of the DC power supply and the load. The emitter of the first transistor is connected to one end of the current detection resistor of the current mirror circuit inserted and commonly connected to the bases of the first and second transistors and the collector of the first transistor, and the collector is the first The collector of the second transistor is connected to the low side of the DC power source through the collector resistor, the emitter of the second transistor is connected to the high side of the DC power source together with the other end of the current detection resistor through the proportional resistor, and the collector of the second transistor is , through a second collector resistor connected to the low side of the DC power supply, the current to be detected flowing before Symbol current detection resistor A high-side current detection circuit for obtaining a proportional detection output voltage at the second collector resistor, wherein a compensation resistor is connected between an emitter of the second transistor and a low side of the DC power supply, and the compensation resistor A compensation current proportional to the power supply voltage is applied to the proportional resistance with a predetermined resistance value, thereby compensating for the influence of fluctuations in the power supply voltage of the DC power supply.

The high-side current detection circuit according to the second aspect of the present invention is a DC-DC converter in which a load is connected to the high side of a DC power supply, and a current detection resistor is inserted between the high side of the DC power supply and the load . In the current mirror circuit in which the base of the two transistors and the collector of the first transistor are connected in common, the emitter of the first transistor is connected to one end of the current detection resistor, and the collector is connected to the low side of the load through the first collector resistor. And the emitter of the second transistor is connected to the high side of the DC power supply together with the other end of the current detection resistor through a proportional resistor, and the collector of the second transistor is connected to the DC through a second collector resistor. connected to a power supply low-side, the detection output voltage proportional to the detected current flowing before Symbol current detection resistor a A high-side current detection circuit, wherein the obtaining of the collector resistor, by connecting a compensation resistor to the low side of the emitter and the DC power source of said second transistor, given to the detection output voltage Compensating for the influence of power supply voltage fluctuations of the DC power supply.

As a reference circuit, the first transistor is passed through a resistor from a load terminal under constant current control without using a constant current circuit which has been conventionally used in order to eliminate the fluctuation of the reference current which is the collector current of the first transistor. There are some which reduce the influence of power supply voltage fluctuations by supplying a collector current. The second aspect of the present invention applies the voltage compensation to the reference circuit, and connects the compensation resistor between the emitter of the second transistor and the low side of the direct current power supply to provide the direct current power supply to the detected output voltage. To compensate for the effects of power supply voltage fluctuations.

  According to the present invention, the high-side current of the DC power supply can be detected as a low-side voltage, and the influence on the detection output when the voltage of the DC power supply fluctuates can be eliminated.

1 is a circuit diagram of a high-side current detection circuit according to a first embodiment of the present invention. The figure which shows the electric current detection characteristic of FIG. The circuit diagram of the high side current detection circuit which concerns on a reference circuit . The circuit diagram of the high side current detection circuit concerning a 2nd embodiment. The circuit diagram of the conventional high side current detection circuit. The figure which shows the electric current detection characteristic of FIG. The conventional circuit diagram which supplies a reference current with a constant current circuit.

  Hereinafter, a high-side current detection circuit according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[First Embodiment]
With reference to FIG. 1, a high-side current detection circuit according to a first embodiment of the present invention will be described. In FIG. 1, V1 is a DC power source, 1 is a PWM (pulse width modulation) control circuit, 2 is a DC-DC converter, and 3 is a high-side current detection circuit including a current mirror circuit.

  The PWM control circuit 1 is composed of a control IC having a built-in stabilized power supply.

  The DC-DC converter 2 includes a switching element Qsw made of a MOSFET, a freewheeling diode D1, a reactor L1, and a smoothing capacitor C1, and supplies controlled electric power to a load L. The drain of the switching element Qsw is connected to a connection node between the anode of the freewheeling diode D1 and one end of the reactor L1, and the cathode of the freewheeling diode D1 is connected to the high side P. The source of the switching element Qsw is connected to the low side G. The other end side of the reactor L1 is connected to a connection node between one end side of the smoothing capacitor C1 and one end side of the load L, and the other end side of the smoothing capacitor C1 is connected to the high side P.

  The DC-DC converter 2 is not limited to the illustrated example, and includes a switching element, and PWM control of on / off of the switching element is performed so that the detection output V2 detected by the high-side current detection circuit 3 is constant. Any device can be used as long as it can be controlled by the circuit 1 to perform DC-DC conversion. The same applies to the following embodiments.

  The current mirror circuit includes a PNP-type reference transistor (first transistor) Q1 having a base collector connected thereto, and a PNP-type output transistor (second transistor) Q2 having a base connected to the base of the reference transistor Q1. Including.

  The emitter of the reference transistor Q1 is connected to the high side P via the current detection resistor R1. The emitter of the reference transistor Q1 is also connected to the other end of the load L. The collector of the reference transistor Q1 is connected to the low side G via the collector resistor R1C.

  The emitter of the output transistor Q2 is connected to the high side P via a proportional resistor R2, and the collector is connected to the low side G via a collector resistor R2C. The collector of the output transistor Q2 is also connected to the input section IN of the PWM control circuit 1. The output part OUT of the PWM control circuit 1 is connected to the gate of the switching element Qsw of the DC-DC converter 2.

  In the current mirror circuit, the emitter of the reference transistor Q1 is connected to one end of the current detection resistor R1, the collector is connected to the low side of the DC power supply V1 through the collector resistor Rc1, and the emitter of the output transistor Q2 is connected to the current detection resistor R1 through the proportional resistor R2. Is connected to the high side of the DC power source V1 together with the other end of the output transistor Q2, the collector of the output transistor Q2 is connected to the low side of the DC power source V1 through the collector resistor Rc2, and the current detection resistor R1 is inserted in series with the load L.

  In the constant current control, as described above, the PWM control circuit 1 is a control IC with a built-in stabilized power supply, and can operate stably even when the power supply voltage V1 fluctuates. The on / off of the switching element Qsw of the DC-DC converter 2 is controlled so that V2 (the collector potential of the output transistor Q2) becomes constant. The current detection resistor R1 is a shunt resistor that detects the load current Io flowing through the load L, and is also a part of the current mirror circuit.

  If the load current Io does not flow through the current detection resistor R1, the output current I2 flowing out from the collector of the output transistor Q2 is proportional to the reference current I1 flowing out from the collector of the reference transistor Q1, and forms a current mirror circuit. The reference current I1 flows from the collector of the reference transistor Q1 to the collector resistor R1C, and the output current I2 flows from the collector of the output transistor Q2 to the collector resistor R2c to give the detection output V2. The detection output V2 is input to the input unit IN of the PWM control circuit 1.

  In the present embodiment, a compensation resistor Rcmp is additionally connected to the high-side current detection circuit 3 using a current mirror circuit, and a compensation current I3 proportional to the power supply voltage V1 is passed through the compensation resistor Rcmp to the proportional resistor R2, thereby supplying the power supply voltage. It is characterized in that the fluctuation of V1 is compensated.

  When expressed by the equation, since the output current I2 on the left side of the above equation (11) is (I2 + I3), the following equation (12) is established.

(I2 + I3) R2 = IoR1- (kT / q) ln (R1C)
-(KT / q) ln (I2) + (kT / q) ln (V1)
+ (KT / q) ln (1 + V1 / VAF) (12)
Substituting I3 = V1 / Rcmp into equation (12), the following equation (13) is established.

I2R2 = IoR1- (kT / q) lnR1C- (kT / q) lnI2
+ (KT / q) ln (V1) + (kT / q) ln (1 + V1 / VAF)
-V1R2 / Rcmp (13)
The fourth and fifth terms on the right side of the expression (13) are (+) with respect to the power supply voltage V1, but the term of the sixth term Rcmp on the right side is (−) with respect to the power supply voltage V1, so the compensation resistance Rcmp If the value is set to a predetermined value, the fluctuation of the power supply voltage V1 can be compensated.

  When the equation (13) is differentiated by V1 and dI2 / dV1 = 0, the following equation (14) is established.

R2 / Rcmp = (kT / q) {1 / V1 + 1 / (VAF + V1)}
... (14)
The optimum value of the compensation resistance Rcmp is determined by this equation (14). If V1 = 90V and VAF = 100V with the above constants, the optimum value of the compensation resistor Rcmp is 2.2 MΩ.

  FIG. 2 shows current detection characteristics when the resistance value of the compensation resistor Rcmp is 2.2 MΩ, with the load current Io on the horizontal axis and the detection output V2 on the vertical axis. In FIG. 2, V1 indicates current detection characteristics when the power supply voltage V1 is medium, V1 large indicates current detection characteristics when the power supply voltage V1 is large, and V1 small indicates power supply voltage V1. The current detection characteristic when the magnitude of is small is shown. In FIG. 2, even if the power supply voltage V1 varies from large to medium to small, the current detection characteristic does not change. Therefore, in this embodiment, the compensation resistor Rcmp is provided so that the influence of the power supply voltage V1 is almost unchanged. It is clear that it is fully compensated.

  When V1 = 90V ± 20V, when the load current Io is 1 (A), the amount of fluctuation due to the power supply voltage V1 is the same as the fourth term on the right side of the detected voltage 500 mV in the first term on the right side of Equation (13). Through R1c, 12 mV, the fifth term Early voltage on the right side is 6 mV, and the compensation voltage based on the sixth term Rcmp on the right side is −18 mV. Thus, the fluctuation due to the power supply voltage V1 is compensated by causing the compensation current I3 proportional to the fluctuation of the power supply voltage V1 to flow through the compensation resistance Rcmp to the proportional resistance R2.

[ Reference circuit ]
FIG. 3 shows a reference circuit . The connection point of the collector resistor R1c through which the reference current I1 of FIG. 5 that is the conventional circuit flows is changed from the low side G of the power source to the low side of the load voltage Vo (a connection node between the smoothing capacitor C1 of the DC-DC converter 2 and the reactor L1). It is a thing. The load voltage Vo is substantially constant not only when the load L is driven at a constant voltage but also when driven at a constant current, and the influence of the power supply voltage V1 is small. Even when the power supply voltage V1 varies, the reference current I1 becomes a constant current.

  In Expression (13), V1 in the fourth term on the right side is the load voltage Vo, and the fluctuation due to the power supply voltage V1 is only in the fifth term on the right side. The fluctuation of the detected amount I2R2 with respect to V1 = 90V ± 20V is 6 mV of the Early voltage, which is about 1/3 of the conventional circuit.

In this reference circuit , since the connection point of the collector resistor R1c is only changed to the low side of the load voltage Vo, the circuit complexity and cost do not increase.

Second Embodiment
Figure 4 shows a second embodiment of the present invention. A compensation resistor Rcmp that compensates for fluctuations in the power supply voltage V1 is additionally connected to the reference circuit of FIG.

In the previous embodiment, the variation of the fourth term on the right side of Equation (13) is eliminated, and the variation of the fifth term on the right side is compensated for by the sixth term on the right side. The value of the compensation resistance Rcmp is calculated from the equation (14): R2 / Rcmp = (kT / q) / (VAF + V1) (15)
It becomes.

  Under the above conditions, the fluctuation of the power supply voltage V1 can be compensated with Rcmp = 7.3 MΩ.

  In the embodiment, the reference transistor Q1 and the output transistor Q2 are both PNP type transistors, but the same applies to NPN type transistors. In the embodiment, the current mirror circuit 3 is composed of a bipolar transistor, but may be composed of an FET.

DESCRIPTION OF SYMBOLS 1 PWM control circuit 2 DC-DC converter 3 High side current detection circuit Q1 Reference transistor (1st transistor) of a current mirror circuit
Q2 Output transistor of the current mirror circuit (second transistor)
R1 Current detection resistor (shunt resistor)
R2 proportional resistance R1c Q1 collector resistance (for reference current)
R2c Q2 collector resistance (for detection output)
Rcmp Compensation resistor Io Load current (Detected current)
I1 Reference current of current mirror circuit I2 Output current I3 of current mirror circuit Compensation current V1 Power supply voltage V2 of DC power supply V1 Detection output voltage Vo Load voltage P High side G of DC power supply V1 Low side of DC power supply V1

Claims (2)

In a DC-DC converter in which a load is connected to the high side of the DC power supply, a current detection resistor is inserted between the high side of the DC power supply and the load, and the bases of the first and second transistors and the collector of the first transistor are connected. The emitter of the first transistor of the current mirror circuit connected in common is connected to one end of a current detection resistor, and the collector is connected to the low side of the DC power supply through the first collector resistor, and the emitter of the second transistor is connected with the other end of the current detection resistance via the proportional resistance to the high side of the DC power source, the collector of the second transistor, through a second collector resistor connected to the low side of the DC power supply, pre-SL current detection A high-side current that obtains a detection output voltage proportional to the current to be detected flowing through the resistor at the second collector resistor. A detection circuit,
By connecting a compensation resistor between the emitter of the second transistor and the low side of the DC power supply, and giving a compensation current proportional to the power supply voltage to the proportional resistor with a resistance value of the compensation resistor as a predetermined value A high-side current detection circuit that compensates for the influence of power supply voltage fluctuations of the DC power supply. In a DC-DC converter in which a load is connected to the high side of the DC power supply, a current detection resistor is inserted between the high side of the DC power supply and the load, and the bases of the first and second transistors and the collector of the first transistor are connected. In the commonly connected current mirror circuit, the emitter of the first transistor is connected to one end of a current detection resistor, the collector is connected to the low side terminal of the load through the first collector resistor, and the emitter of the second transistor is the connected with the other end of the current detection resistor in the high side of the DC power source through a proportional resistor, the collector of the second transistor is connected to the low side of the DC power source through a second collector resistor, before Symbol current detection resistor A detection output voltage proportional to a flowing current to be detected is obtained at the second collector resistor. A Ido current detection circuit,
A high side compensates for the influence of the power supply voltage fluctuation of the DC power supply on the detected output voltage by connecting a compensation resistor between the emitter of the second transistor and the low side of the DC power supply. Current detection circuit.

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