JPH069607Y2 - Power supply for gradient magnetic field - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as NMR-CT).
(Hereinafter referred to as)) for a gradient magnetic field power supply device for supplying a current to a triaxial gradient magnetic field coil.

(Prior Art) In the NMR-CT, a gradient magnetic field amplifying device at an output stage of a gradient magnetic field current for applying a gradient magnetic field to three axes of x, y, and z has conventionally been configured as shown in FIG. there were. In the figure, a driver 1 drives FETs 2 and 3 which are n-channel depletion type MOS FETs for power. The FETs 2 and 3 constitute a class AB push-pull amplifier and supply a current to the gradient magnetic field coil 4. The gradient magnetic field coil 4 is a coil having an inductance L and a resistance R for giving a gradient magnetic field to the NMR-CT. 5 is FE
A protection diode connected in parallel between the source and drain of T2 protects FET2 by short-circuiting the source and drain of FET2 when a positive voltage is applied to the source.
The protection diode 6 is a diode that also protects the FET 3. The output power required for this gradient magnetic field amplifier is the maximum current I _M flowing in the gradient magnetic field coil and the maximum slew rate. Can be determined by. Gradient magnetic field coils 4 as the load for a series circuit of the inductance L and the winding resistance R, the maximum output voltage V _M applied to the gradient magnetic field coil 4 is as follows (considering the positive side only).

Now, when the steady current Idc is flowing, the power consumed by the FET2 is as follows. Voltage drop in the FET2, when the margin for the power supply voltage change and V _A, the power supply voltage V _C required is as follows.

V _C = V _M + V _A (2) When the DC current I _D is supplied to the gradient magnetic field coil 4, FE
Power consumption W in T2 is _{W = I D (V C -I} D · R) ... (3) ( challenge invention is to solve) However, _{V C} is (1), the maximum slew from (2) Of rate Therefore, a high voltage is required, and from equation (3), FE
The power consumption W of T2 increases, and the heat generation of the FET2 also increases. Therefore, use a large FET,
Depending on the case, the size of the device is increased due to the need for cooling
Therefore, the cost is increased.

Also, if you want to suppress heat Therefore, the scan time must be extended, which increases the burden on the subject and deteriorates the patient throughput.

The present invention has been made in view of the above problems, and its purpose is to supply a high power supply voltage only when a high voltage is required due to a large slew rate, and to supply a low power supply voltage when the output voltage of the power supply may be low. It is to realize a power supply device for a gradient magnetic field that can supply a voltage.

(Means for Solving the Problem) The above-mentioned problem is a power supply device for a gradient magnetic field for supplying a current to a gradient magnetic field coil of a nuclear magnetic resonance imaging apparatus, and a low power supply for supplying a constant current to the gradient magnetic field coil. A voltage power supply, a high voltage power supply for supplying a high slew rate current to the gradient magnetic field coil, and a current supply semiconductor element that supplies a voltage received from a power supply input terminal as a current from an output terminal to the gradient magnetic field coil. A diode connected in a forward direction so as to supply a low voltage from the low-voltage power supply to the power supply input terminal of the current supply semiconductor element, and the gradient magnetic field by dividing the voltage of the gradient magnetic field coil and the high-voltage power supply. A drive means for generating a drive voltage when a high slew rate current is required for the coil; and a power supply input terminal for receiving a drive voltage of the drive means at a control terminal. To the high voltage power supply, the output terminal is connected to the power supply input terminal of the current supply semiconductor element, a high voltage supply semiconductor element driven by the drive voltage Is solved by the power supply device for use.

(Operation) Normally, a steady current is supplied from the low-voltage power supply to the gradient magnetic field coil by the current supply semiconductor element via the diode. When a high slew rate current is required, a drive voltage is generated according to the voltage rise of the gradient magnetic field coil, and this drive voltage causes the high voltage supply semiconductor element to operate smoothly and the output terminal voltage to change continuously. To do. This allows
The voltage of the power supply input terminal of the current supply semiconductor element is changed from the low voltage from the low voltage power supply to the high voltage, and the current is supplied from the high voltage power supply side to the gradient magnetic field coil through the current supply semiconductor element. .

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention. In the figure, the same reference numerals are used for the same parts as in FIG. In the figure, 10 and 10 'are low-voltage power supplies that supply current in a steady state, and 11 and 11' are high-voltage power supplies that supply current when a high voltage is required. Reference numeral 12 is a positive voltage side FET for supplying a necessary gradient magnetic field current to the gradient magnetic field coil 4, which is an n-channel depletion type MOS FET.
Numeral 13 is a FET for conducting and supplying a current from the high voltage power source 11 when a high gradient magnetic field voltage is required due to the high slew rate. 14 is a FET for supplying a negative current
An FET that performs the same operation as 12 and a negative FET 15 that performs the same operation as the FET 13. FET 12-14
Is an n-channel depletion type MOS FET, and the FET 15 is a P-channel depletion type MOS FET. 1
6 is a Zener diode which is inserted in order to adjust the gate voltage of the FET 13, 17 is a Zener diode which is inserted likewise for the gate voltage control of the FET 15, the voltage drop is V _F. Also, V ₁ is a low voltage power source 10, ₁
0 'output voltage, _{V 2} is a high-voltage power supply 11, 11' which is the output voltage of. Since parts such as resistors are the same in positive and negative and the voltages appearing are the same, only the positive side will be described below. The negative side is distinguished by adding a dash to the sign of the positive side. Rg ₁ is the gate resistance of the FET 13, and the resistance Rg ₂
And the voltage drop V _F of the Zener diode 16 determine the gate voltage of the FET 13 according to the change of the gradient magnetic field coil voltage. Rs ₁ is the source resistance of FET 13,
It is connected to the drain of the FET 12 and the diode D connected to the low voltage power supply 10. Rs ₂ is a source resistance of the FET 12 and is connected to a connection point between the gradient magnetic field coil 4 and the resistance Rg ₂ . The positive drive voltage from the driver 1 shown in FIG. 2 is input to the gate of the FET 12,
The timing of the current flow is controlled.

Vg is the gate voltage of the FET 13, Vgs is the gate-source voltage of the FET 13, and Vds ₁ is the drain-source voltage of the FET 13. V _O is the drain voltage of the FET 12,
Vds ₂ is a drain-source voltage, and _VL is a gradient magnetic field voltage applied to the gradient magnetic field coil 4.

The operation of the apparatus configured as above will be described. First, the voltage V ₁ of the low voltage power supply 10 is ignored and the voltage of each part is calculated. When the Vg calculated by the equivalent circuit of FIG. 3, the current flowing through the circuit and i, if the anode voltage of the Zener diode 16 and _{V R, i = (V 2} -Vg) / Rg 1 V R = V _{L + i · Rg 2 = V} L + {(V 2 -Vg) / Rg 1} · Rg 2 Vg = V R + V F = V L + (Rg 2 / Rg 1) (V 2 -Vg) + V F ∴Vg = {Rg ₁ (V _L + V _F ) + Rg ₂ V ₂ } / (Rg ₁ + Rg ₂ ) ... (4) If the current flowing through the FET 13 is I and the voltage between the gate and the source at that time is Vgs, then Vds ₁ = _{V 2 -Vg + Vgs = Rg 1} (V 2 -V L -V F) / (Rg 1 + Rg 2) + Vgs ... (5) V O = Vg-Vgs-I · Rs 1 = {Rg 1 (V L + V F) _{+ Rg 2 V 2} / (} Rg 1 + Rg 2) -Vgs-I · R _{1 ... (6) Vds 2 =} V O -V L -I · Rs 2 = {Rg 1 V F + Rg 2 (V 2 -
_{V L)} / (Rg 1} + Rg 2) -Vgs -I (Rs 1 + Rs 2) ... (7) where, considering the voltage _{V 1,} is _{V 0} which (6) _{V O}
<When the V ₁ was, V ₁ was actually drain voltage of FET12 to pass diode D does not decrease to less to maintain V _1. The condition for V _O ≧ V ₁ is {Rg ₁ (V _L + V _F ) + Rg ₂ V ₂ } / (Rg ₁ + Rg ₂ ) −Vgs−I · Rs ₁ ≧ V ₁ ...
(8) Therefore, V _{0 in the} equation (6) is actually as follows.

After all, under the condition of the expression (8), the voltage V of the gradient magnetic field coil 4 is
_{When L} is high, Vg becomes high from the equation (4), so F
ET13 is turned on and the voltage V _{2 of the} high voltage power supply 11
Is fed through the FET 13, and V _{O in the} equation (6) is the FET
12 drains. At this time, the diode D
Becomes a reverse bias state and becomes non-conductive. For this reason,
The low voltage power supply V10 is in a state of being disconnected from the FET 12.

On the other hand, when the voltage _VL applied to the gradient magnetic field coil 4 decreases and the equation (8) is no longer satisfied, Vg decreases and the FET 13 turns "off", and the supply of the voltage V ₂ from the high voltage power supply 11 is stopped. Disconnected, output voltage V ₁ of low voltage power supply 10
Is supplied to the drain of the FET 12 via the diode D, and the FET 12 is driven at a low voltage V ₁ in the end. In formula (8), V _L is V _O when V _O ≧ V ₁
Let _L (TH) and the current I flowing through the FET 13 be I ₀ , then V _L (TH) = {(Rg ₁ + Rg ₂ ) / Rg ₁ } (V ₁ + Vgs + I ₀ · Rs ₁ ) − (Rg ₂ / Rg ₁ ). a _{V 2 -V F ... (10)} . Further, when ΔV _O / ΔV _L is calculated from the equation (6) and is set to k, k = ΔV _O / ΔV _L = Rg ₁ / (Rg ₁ + Rg ₂ ) ... (11) k is an increase in the gradient magnetic field coil voltage V _L Is a value that defines the operating point of the FET 13, that is, the rate of increase of V ₀ with respect to As is clear from the equations (10) and (11), V _L
And (TH) k can be set by determining appropriately the _{Rg 1, Rg 2, V F} and _{I 0.} I ₀ is set while measuring beforehand from the required load current. However, k must be selected so that the FET 12 does not saturate when V _L takes the maximum value. Further, V _L (TH) is set to a value such that the FET 13 does not conduct in a steady state, and is selected so as to reduce the heat generation amount of the entire device.

As described above, in the steady state in which the gradient magnetic field coil voltage V _L is low, only the FET 12 is driven by the low voltage V ₁ ,
The heat generated by T is stopped only by the FET 12. When high slew rate current is passed and a high voltage power supply is required, FET1
3 begins to conduct and brings V _O to an appropriate voltage. Therefore, when the power supply voltage is switched from V ₁ to V ₂ , V _O continuously and smoothly changes, so that noise hardly occurs in the gradient magnetic field coil 4 on the output side. In addition, the switching of the current supply from the high voltage power supply is performed using the voltage of the gradient magnetic field coil, and the low voltage power supply is disconnected by the diode when using the high voltage power supply. No circuit or the like is required.

(Effect of the Invention) As described above, according to the present invention, when a steady current is supplied, a low voltage operates, and when a high voltage is required, a semiconductor element for supplying a high voltage operates to generate a high slew rate current. By responding to the demand, it is possible to suppress wasteful power consumption by the semiconductor element in a steady state and suppress heat generation, which has a large practical effect.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of a conventional power supply device, and FIG. 3 is an explanatory diagram for calculating Vg. 1 ... Driver 2, 3, 12, 13, 14, 15 ... FET 4 ... Gradient magnetic field coil 10, 10 '... Low voltage power supply 11, 11' ... High voltage power supply

Claims (2)

[Scope of utility model registration request] 1. A gradient magnetic field power supply device for supplying a current to a gradient magnetic field coil of a nuclear magnetic resonance imaging apparatus, comprising: a low voltage power supply for supplying a constant current of the gradient magnetic field coil; A high-voltage power supply for supplying a high slew rate current, a current-supplying semiconductor element that supplies a voltage received from a power supply input terminal as a current from an output terminal to the gradient magnetic field coil, and a current supply from the low-voltage power supply. A high slew rate current is required for the gradient magnetic field coil by dividing the voltage of the diode connected in the forward direction to supply a low voltage to the power input terminal of the semiconductor device Drive means for generating a drive voltage at the time of operation, the control terminal receives the drive voltage of the drive means, the power input terminal is connected to the high-voltage power supply, Power input terminals of the current supply for a semiconductor element to the terminals are connected, the gradient magnetic field power supply apparatus characterized by comprising a high voltage supply for a semiconductor element driven by the driving voltage. 2. A depletion-type FET is used as the high-voltage supplying semiconductor element, and a voltage value of an output terminal of the high-voltage supplying semiconductor element depends on a voltage value of a driving voltage generated by the driving means. The power supply device for a gradient magnetic field according to claim 1, wherein the power supply device is configured to change continuously.