JPH10323030A - Initial start circuit for power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce power consumption at no-load or light-load time, perform stable operation against overvoltage and overcurrent, and improve efficiency, by providing a start part in an output of a switch and a minus end of a transformer so as to apply counter electromotive force to the start part through the switch. SOLUTION: A switch Q is turned on, and voltage is applied to an output part 500 through a transformer T. Here, by the applied voltage through a rectifying part and a smoothing part constituting the output part 500, output voltage V0 required in a device is generated and outputted. By a power supply device and an external condition when overvoltage and overcurrent are generated, output voltage V0 of the output part 500 is detected for overvoltage and overcurrent by a detector circuit, the detected overvoltage and overcurrent are fed back to the switch Q through a control part 21 to put the switch Q off, and the power supply device is protected from the overvoltage and the overcurrent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an initial start circuit of a power supply device, and more particularly, to a method of reducing power consumption at no load or light load (stand-by) and preventing overvoltage or overcurrent. The present invention relates to an initial startup circuit of a power supply device which operates stably and increases the efficiency of the power supply device.

[0002]

2. Description of the Related Art In general, a main switch which performs switching so as to be subjected to pulse width modulation in accordance with a signal to which power is applied and which is fed back from an output, and the main switch described above. And a transformer formed by a primary winding and a secondary winding so that the voltage switched by the transformer is induced in the output, and an output voltage obtained by rectifying and smoothing the output from the secondary winding of the transformer. A switch mode power supply (SMPS), which is generated from the output voltage and detects an overcurrent and an overvoltage from the output voltage and feeds it back to the input to stably output a voltage required for the device.
r Supply, hereinafter referred to as a power supply device), a main switch is formed by an integrated circuit (IC) formed of a bipolar transistor or a field effect transistor, and the main switch is fed back from an output to apply a control signal applied thereto. Control is performed so that pulse width modulation is performed accordingly. The main switch is used as a power source for an element that constitutes the main switch when a voltage output from the output unit is normally generated during a short period of an initial startup period in which power is applied to the power supply device. Power is not supplied to the main switch composed of the transistor before
A case occurs in which the normal operation of the main switch is not performed. To prevent this, the power supply is initially started by operating the main switch normally using the power applied to the input of the power supply during the initial startup period when power is applied to the power supply. An initial start-up circuit is used to cause this.

FIG. 1 is a circuit diagram of a power supply device using a control integrated circuit according to the prior art.

A power supply device using a control integrated circuit according to the prior art is controlled in accordance with a signal to which an input voltage (Vi) is applied and fed back from an output unit, and the input voltage (V) is controlled.
i), and a transformer (T) composed of a primary winding and a secondary winding so that the input voltage (Vi) switched at the input unit 100 is induced at the output unit 500. ) And the secondary winding (T) of the transformer (T).
An output unit 500 that senses an overcurrent and an overvoltage from an output voltage (V0) generated by rectifying and smoothing the output from 2) and feeds it back to the input unit 100 to output a voltage required for the device; Consists of

[0005] The input unit 100 includes a rectifier 15 which is a bridge diode (Bridge Diode) for rectifying an input voltage (Vi), and an output of the rectifier 15 and a primary winding of a primary winding of a transformer (T). A switch unit (Q) connected between the winding (T1a) and the output of the rectifying unit 15 for switching;
And a sensing signal that senses an overvoltage from the output voltage (V0) output from the output unit 500 is fed back, and pulse width modulation (PWM) is performed according to the sensing signal fed back from the output voltage (V0).
n) The integrated circuit 11 that controls the switch unit (Q) with the output thus obtained, and the output of the rectifier 15 is a resistor (R1)
And a capacitance (C1), is applied to the power supply terminal (Vcc) of the integrated circuit 11, and a signal output from the auxiliary winding (T1b) of the primary winding of the transformer (T) is a diode (D1). ) And a starter 10 applied via a capacitance (C1).

In the power supply apparatus using the control integrated circuit according to the related art, a switch is controlled by pulse width modulation, and an input voltage switched by the pulse width modulated switch is changed by a transformer (T). ). In the power supply device, when the power supply device is turned on and is initially activated, the normal output voltage (V
0) occurs, first, the switch section (Q) is turned on by the integrated circuit 11, and the transformer section (T) is turned on.
A voltage is induced in the output unit 500 through the output voltage (V).
0) occurs.

Accordingly, the input voltage (Vi) applied to the input unit 100 is rectified by the bridge diode 15 of the rectifier 15, and the rectified voltage is applied to the starter 10 for initial operation of the integrated circuit 11. You. The starting unit 10
Applied to the power supply terminal (Vc) of the integrated circuit 11 is distributed by the resistance (R1) and the capacitance (C1).
When the voltage is applied to c), power is supplied to the integrated circuit 11 and the integrated circuit 11 operates.

The integrated circuit 11 to which power is supplied to the power supply terminal (Vcc) receives the signal fed back from the output unit 500 and controls the output voltage (V0) generated from the output unit 500. A switch (Q), which is a field-effect transistor to which a gate is connected, is turned on /
Turning off, switching control is performed so that the input voltage is pulse width modulated. Therefore, the field effect transistor constituting the switch section (Q) is turned on / off, and the voltage switched via the transformer section (T) is induced in the output section 500, rectified and smoothed, and the output voltage is changed. (V0). Thus, the output voltage (V
When (0) occurs, a signal sensed from the output voltage (v0) is fed back to the integrated circuit 11 of the input unit 100, and the switch unit (Q) is controlled to generate a stable output.

A power supply device using such a control integrated circuit is constituted by an initial start-up circuit and an integrated circuit when there is no load or a light load in which only a remote control unit operates since it is remotely controlled. Since the power consumption used in the circuit becomes larger than the power consumption used in the load, there is a problem that the power consumption increases as a whole. Also, in the power supply device using the control integrated circuit, when the overvoltage or the overcurrent is applied to the input unit, the overvoltage or the overcurrent is directly applied to the integrated circuit 11 via the resistor (R1). There is a problem that the integrated circuit 11 for controlling (Q) is damaged.

FIG. 2 shows an RCC (Ringing Ch.) According to the prior art.
1 shows a power supply device based on the oke converter method.

The power supply device includes an input unit 200, a transformer (T), and an output unit 500. The input unit 200 includes a rectifying unit 15 that is a bridge diode that rectifies an input voltage (Vi), a control unit 21 to which a sensing signal such as an overcurrent and an overvoltage from the output unit 500 is fed back and input, and a transformer unit. The voltage of the auxiliary winding (T1b) of the primary winding of (T) is connected to the gate of the switch unit (Q) via the resistor (R4) and the capacitance (C4), and the output of the control unit 21 is gated. A switching unit (Q) connected to the switching unit (Q) for controlling the OFF time; and an activation unit (20) connected to the gate of the switching unit (Q) by dividing the output of the rectifying unit 15 by resistors (R1b, R2b). Consists of

The RC according to the prior art constructed as described above.
The power supply device based on the C system is a generally used circuit of a circuit configured in the RCC system. When the power supply device performs an initial operation, the output of the rectifying unit 15 includes resistors (R1b, R2
b) and is connected to the gate of the switch unit (Q), so that the output unit 500 outputs the output voltage (V0).
The switch section (Q) is activated by the activation section 20 before the occurrence of
The operation voltage is applied to the gate of the power supply device, and the power supply device is stably initial-started.

However, the switch section (Q), which is a field effect transistor, maintains the turn-on time of the switch section (Q) while a voltage of about 3.7 volts or more, which is a turn-on condition, is supplied to the gate. . The switch section (Q) in which the turn-on time is maintained as described above includes the output section 5
00, the secondary winding (T2) to the primary main winding (T2) of the transformer (T) due to an arbitrary fault such as a short circuit and overload.
When the back electromotive current (Ir) is generated by the back electromotive voltage generated in 1a), the switch section (Q) is connected via the resistor (R1b).
Since an excessive voltage and current are applied to the gate of the switch, an excessive current flows between the drain and the source, thereby damaging the switch unit (Q).

FIG. 3 is a circuit diagram of a power supply device using an overvoltage detection method according to the prior art.

The power supply apparatus based on the overvoltage detection method shown in FIG. 3 is a circuit for solving the problems of the power supply apparatus based on the control integrated circuit of the prior art and the power supply apparatus based on the RCC method shown in FIGS. This circuit has the same configuration as that of the power supply device of the control integrated circuit and the power supply device of the RCC system shown in FIGS.

A power supply device based on an overvoltage detection method according to the prior art includes an input unit 300, a transformer (T), and an output unit 500. The input unit 300 includes a rectifying unit 15 that is a bridge diode that rectifies an input voltage (Vi), and a control unit 21 that receives feedback signals of overcurrent and overvoltage from an output unit 500 and inputs the signals.
And the voltage from the auxiliary winding (T1b) of the primary winding of the transformer (T) is connected to the gate of the switch (Q) via the resistor (R4) and the capacitance (C4), and
1 is connected to the gate and controlled, and the output of the rectifier 15 is distributed by a resistor (R1c) and a Zener diode (Dz). The voltage between the two terminals is further divided by resistors (R2c) and (R3c), and the activation unit 30 is connected to the gate of the switch unit (Q).

In the power supply apparatus according to the prior art overvoltage detection system configured as described above, the secondary winding (T2) of the transformer (T) is output from the output unit 500 due to an arbitrary fault such as a short circuit or overload. ) Generates a back electromotive current (Ir) due to a back electromotive voltage to the primary main winding (T1a), the voltage is limited to a predetermined potential by a Zener diode (Dz) via a resistor (R1c), and the Zener diode (Iz) Overvoltage exceeding the voltage set by Dz) is cut off because it flows to the minus end of the rectifying unit 15 which is the ground end, and the voltage distributed by the resistor (R1c) and the Zener diode (Dz) is further reduced by the resistor (R2c). , R3c) and applied to the gate of the switch section (Q).

The back electromotive current (Ir) generated by the back electromotive voltage (V1) is reduced by the resistors (R1c, R2c) and applied to the gate of the switch section (Q). ), The gate voltage corresponds to the turn-on voltage of the field effect transistor, and the switch section (Q) operates stably.

However, the power supply device of the prior art overvoltage detection system including the starting unit 30 including the resistors (R1c, R2c, R3c) and the zener diode (Dz) is turned on only by the remote control device so as to be remotely controlled. When the power supply unit consumes little power and the load is light, overcurrent and overvoltage due to short circuit at the output unit are fed back to the input unit via the transformer unit, and the back electromotive force due to the back electromotive voltage is applied to the switch unit. Is applied, the switch section, which has already been sensed with the overcurrent and overvoltage and turned off by the control section, is affected by the back electromotive current applied to the gate, so that an excessive voltage is applied to the switch section. However, if a circuit is designed to reduce the influence of the switch section due to the excessive voltage, the circuit becomes complicated. .

Accordingly, the present invention has been devised to solve the above-mentioned conventional problems, and a main object of the present invention is to apply a back electromotive current to a starting unit via a switch unit. As described above, by providing a start unit at the output of the switch unit and the minus end of the transformer unit, the power consumption is reduced at no load or light load, and the device operates stably against overvoltage or overcurrent.
It is to provide an initial startup circuit of a power supply device with increased efficiency.

[0021]

According to a first aspect of the present invention, there is provided an initial start circuit of a power supply device, wherein a rectifying unit for rectifying an input voltage, and an output of the rectifying unit are applied to perform switching. A switching unit configured to induce a voltage switched by the switching unit from the main winding to the auxiliary winding and the secondary winding; and a switching unit configured between the switching unit and the transformation unit. And a drive unit that receives the voltage from the auxiliary winding of the transformer unit and controls the switch unit, by activating the switch unit by a distribution unit that distributes a voltage applied through the main winding of the transformer unit. The output from the secondary winding of the transformer is generated as a rectified and smoothed output voltage, and an output unit from which the output voltage is sensed and a signal sensed by the output unit are input. This allows the switch There is composed of a control unit to be switched, the power consumption is reduced, and characterized in that it is operated stably against overvoltage or overcurrent.

According to a second aspect of the present invention, there is provided an initial startup circuit of the power supply device according to the first aspect of the present invention, wherein the distribution means of the activation section is arranged between the drain and the gate of the switch section. It is characterized by comprising a connected first resistor and a second resistor connected between a source and a gate of the switch unit.

According to a third aspect of the present invention, there is provided the power supply apparatus according to the first aspect, wherein the distribution unit of the activation unit is connected to the minus side of the main winding of the transformer. A third resistor and a fifth resistor connected in series so as to distribute a drain voltage between a drain and a source of the switch unit, and a voltage divided by the third resistor and the fifth resistor; And a fourth resistor applied to the gate of the switch section.

According to a fourth aspect of the present invention, there is provided an initial start circuit of the power supply device according to the first aspect of the present invention, wherein the distribution unit of the start unit is connected to a minus side of a main winding of the transformer. A third resistor, a fourth resistor, and a fifth resistor are connected in series so that the drain voltage is distributed between the drain and the source of the switch unit, and the distributed voltage is applied to the gate. And a Zener diode connected in parallel to the fourth resistor and the fifth resistor so that the voltage distributed by the third resistor and the fourth resistor is maintained at a predetermined potential.

According to a fifth aspect of the present invention, there is provided the power supply apparatus according to the first aspect, wherein the Zener diode is connected in parallel to the fifth resistor. The voltage divided by the fourth resistor and the fifth resistor is maintained at a predetermined potential or less.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an initial starting circuit of a power supply device according to the present invention will be described in detail with reference to the illustrated embodiments.

FIG. 4 is a circuit diagram of a power supply device according to the present invention.

The power supply device according to the present invention includes a rectifying unit 15 which is a bridge diode for rectifying an input voltage (Vi), a switch unit (Q) to which an output of the rectifying unit 15 is applied and switched, and The voltage switched by the switch section (Q) is applied to the main winding of the primary winding (T1a).
From the auxiliary winding of the primary winding (T1b) and the secondary winding (T2)
A transformer section (T) for inducing the pressure, and between the switch section (Q) and the transformer section (T), the transformer section (T)
Activating section 40 for activating the switch section (Q) by the voltage applied through the main winding (T1a) of the primary winding of
And an auxiliary winding (T1b) of the primary winding of the transformer (T).
And a drive section 45 for controlling the switch section (Q), and an output voltage (V0) obtained by rectifying and smoothing the output from the secondary winding (T2) of the transformer section (T). And an output unit 500 that senses the output voltage (V0) and feeds it back to the control unit 21. A control unit that receives a signal from the output unit 500 and thereby switches the switch unit (Q). 21. The rectifying unit 15, the switch unit (Q), the starting unit 40, the driving unit 45, and the control unit 21 constitute an input unit 400.

The starting unit 40 is connected between a drain and a gate connected to the minus side of the main winding (T1a) of the transformer (T) of the switch (Q), which is the field effect transistor. It comprises a first resistor (Rdg) and a second resistor (Rsg) connected between the source and the gate of the switch section (Q).

FIG. 5 is a circuit diagram of a main part of a power supply device according to another embodiment of the present invention.

The starting unit 50 of the other embodiment distributes a voltage between the drain and the source of the switch unit (Q) connected to the minus side of the main winding (T1a) of the transformer unit (T). A third resistor (Rdg1) connected in series to
And the fifth resistor (Rsg), and the third resistor (Rdg).
1) and a fourth resistor (Rdg2) for applying the voltage distributed by the fifth resistor (Rsg) to the gate.

FIG. 6 is a circuit diagram of a main part of a power supply device according to still another embodiment of the present invention.

The starting unit 60 of the other embodiment distributes a voltage between the drain and the source of the switch unit (Q) connected to the minus side of the main winding (T1a) of the transformer unit (T). The third resistor (Rdg1) and the fourth resistor (Rdg) connected in series so that the applied voltage is applied to the gate.
2) and the fifth resistor (Rsg), and the third resistor (Rd
g1) and the fourth resistor (Rdg2) are connected in parallel to the fourth resistor (Rdg2) and the fifth resistor (Rsg) connected in series such that the voltage distributed by the fourth resistor (Rdg2) is maintained at a predetermined potential or higher. And a Zener diode (Dz1).

Next, an operation state of the present invention thus configured will be described.

In the power supply device of the present invention shown in FIG. 4, when the power is turned on and the input voltage (Vi) is applied through the rectifying unit 15, the power is supplied through the first resistor (Rdg) of the starting unit 40. Since a voltage via the first winding (T1a) of the transformer (T) is applied to the gate of the switch (Q), the switch (Q) is turned on and the transformer (T)
A voltage is applied to the output unit 500 via the.

Therefore, the output voltage (V0) required for the device is generated and output by the applied voltage via the rectifying unit and the smoothing unit constituting the output unit 500. When an overvoltage and an overcurrent are generated due to the power supply device and an external condition, the output voltage (V0) of the output unit 500 is sensed by an unillustrated sensing circuit to detect the overvoltage and the overcurrent. Since the current is fed back to the switch (Q) via the control unit 21 to control the switch (Q) to be turned off, the power supply device is protected from overvoltage and overcurrent.

When an overvoltage and an overcurrent occur in the output unit 500 due to the power supply device and external conditions, the output unit 500 outputs a secondary winding (T2) and a primary winding (T2) of the transformer (T). Back-induced voltage is generated through the main winding (T1a) of the second device, and the back-induced current (I
r) is applied to the source end of the switch section (Q). At this time, since the switch section (Q) has already been turned off by the control section 21, the back induced current (Ir) due to the back induced voltage is cut off by the turned off switch section (Q).

Accordingly, the negative end of the main winding (T1a) of the primary winding of the transformer (T) and the drain end of the switch (Q), which is turned off and the back induced current (Ir) is cut off, is thus obtained. The first resistor (Rd
g), the influence of the back induced current (Ir) is cut off, and the field effect transistor of the switch section (Q) is protected. This has the effect of also providing a protection circuit function.

On the other hand, when the power supply device is initially started,
The drain voltage of the switch section (Q) is substantially the same as the output voltage of the rectifier section 15 applied via the primary winding (T1a) of the transformer section (T), and the field effect transistor of the switch section (Q) An activation voltage (Vgth: Gate Threshold Voltage) applied to the gate to be activated is a threshold voltage (Threshold) of the field effect transistor of the switch unit (Q).
Voltage), the field effect transistor of the switch unit (Q) is turned on, and the input voltage (Vi) is applied to the transformer (T). Once the field effect transistor is activated, the impedance of the activation part 40 composed of the first resistance (Rdg) and the second resistance (Rsg) becomes much larger than the impedance of the transformer (T), so that the switch part During the switching of (Q), the DC voltage charged in the capacitance (C1) connected in parallel to the output terminal of the rectifier 15 cannot reach the starter 40. That is,
The impedance of the activation unit 40 is further increased by connecting the activation unit 40 to the drain of the switch unit (Q), which is a field-effect transistor, away from the capacitance (C1). Thereby, the influence of the back induced current (Ir) due to the back induced voltage from the transformer (T) is also reduced.

In order to improve the protection function, efficiency and stability of the power supply device, in another embodiment shown in FIG. 5, the switch section (Q) is connected via the main winding (T1a) of the transformer section (T). ) Is applied to the third resistor (Rd
g1) and the fifth resistor (Rsg), and the distributed voltage is applied to the gate of the switch unit (Q) via the fourth resistor (Rdg2), and the starting unit 50 is operated so as to be initially started. You. The activation unit 50 further includes a fourth resistor (Rdg2) connected to the gate of the switch unit (Q) between the third resistor (Rdg1) and the fifth resistor (Rsg). Is applied to the gate of the switch unit (Q) via the fifth resistor (Rsg) and the fourth resistor (Rdg2) even if the back induced voltage due to the overcurrent and overvoltage generated from is applied, and the amount of the overvoltage is adjusted. Therefore, the power supply device operates stably.

Further, in order to improve the protection function, efficiency and stabilization of the power supply device, in still another embodiment different from FIG. 5, the switch unit is connected via the main winding (T1a) of the transformer (T). The voltage applied to the drain of (Q) is a third resistor (Rdg1) and a fourth resistor (Rdg) connected in series.
2), and the distributed voltage becomes lower than a predetermined potential by a zener diode (Dz1) connected in parallel. Then, the voltage that has fallen below the predetermined potential is
Further, the voltage is distributed by the fourth resistor (Rdg2) and the fifth resistor (Rsg) and is stably applied to the gate of the switch section (Q).

Further, the voltage divided by the fourth resistor (Rdg2) and the fifth resistor (Rsg) is connected to a Zener diode (D) configured in parallel with the fifth resistor (Rsg).
Even if it is configured to be equal to or lower than the predetermined potential by z1),
Since the voltage is distributed by the fourth resistor (Rdg2) and the fifth resistor (Rsg), the voltage is reduced below a predetermined potential by the Zener diode (Dz1), and a stable voltage is applied to the gate of the switch section (Q). , The switch section (Q), which is a field-effect transistor, is protected.

Therefore, even if the back induced voltage due to the overcurrent and overvoltage generated in the output section 500 is applied, the third resistor (Rdg1), the fourth resistor (Rdg2), and the fifth resistor (Rs
g) and the starter 60 composed of the Zener diode (Dz1) suppress the reverse induced voltage from being applied to the gate of the switch (Q), so that the power supply device operates stably.

[0044]

As described in detail above, in the initial starting circuit of the power supply device according to the present invention, under no load and light load, the influence on the gate of the switch section is very small and the switch section is protected. Therefore, there is an effect that the characteristics of the power supply device are improved, efficiency is increased, power consumption is reduced, and the power supply device operates as a prevention circuit for preventing overcurrent and overvoltage.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a power supply device using a control integrated circuit according to the related art.

FIG. 2 is a circuit diagram of a power supply device using an RCC method according to the related art.

FIG. 3 is a circuit diagram of a power supply device using an overvoltage detection method according to the related art.

FIG. 4 is a circuit diagram of a power supply device according to the present invention.

FIG. 5 is a circuit diagram of a main part of a power supply device according to another embodiment of the present invention.

FIG. 6 is a circuit diagram of a main part of a power supply device according to still another embodiment of the present invention.

[Explanation of symbols]

 15 rectifier, 21 controller, 40, 50, 60 starter, 45 driver, 500 output, C1 capacitance, Q switch, Rdg first resistor, Rsg second resistor, Rdg1: third resistor, Rdg2: fourth resistor, Rsg: fifth resistor, T: transformer, T1a: primary winding of primary winding, T1b: auxiliary winding of primary winding, T2: secondary winding Line, Dz1 ... Zener diode.

Claims (5)

[Claims] A rectifying unit for rectifying an input voltage; a switch unit to which an output of the rectifying unit is applied to switch the voltage; A transformer configured to induce a winding, and a switch configured by the distribution unit configured to distribute a voltage applied through a main winding of the transformer and configured between the switch and the transformer; A starting unit, a driving unit for receiving a voltage from the auxiliary winding of the transformer, and controlling the switch unit; and an output voltage obtained by rectifying and smoothing the output from the secondary winding of the transformer. An output unit that is generated and senses the output voltage, and a control unit that receives the signal sensed at the output unit and switches the switch unit, thereby reducing power consumption and reducing overvoltage. Also Is an initial startup circuit of the power supply device, which is operated stably against overcurrent. 2. The initial startup circuit of a power supply device according to claim 1, wherein the distribution unit of the startup unit includes a first resistor connected between a drain and a gate of the switch unit, and the switch unit. And a second resistor connected between the source and the gate of the power supply device. 3. The initial startup circuit of the power supply device according to claim 1, wherein the distribution unit of the startup unit includes a drain and a source of the switch unit connected to a minus side of a main winding of the transformer unit. A third resistor and a fifth resistor connected in series so as to distribute a drain voltage between the third resistor and the fifth resistor; and a fourth resistor for applying a voltage distributed by the third resistor and the fifth resistor to a gate of the switch unit. An initial startup circuit for a power supply device, comprising: 4. The initial startup circuit of the power supply device according to claim 1, wherein the distribution unit of the startup unit includes a drain and a source of the switch unit connected to a negative side of a main winding of the transformer. And a third resistor, a fourth resistor, and a fifth resistor provided in series so that the divided voltage is applied to the gate, and the third resistor and the fourth resistor. And a Zener diode connected in parallel to the fourth resistor and the fifth resistor so that the applied voltage is maintained at a predetermined potential. 5. The initial startup circuit of a power supply device according to claim 4, wherein the Zener diode is connected in parallel with the fifth resistor, so that the Zener diode is divided by the fourth resistor and the fifth resistor. An initial startup circuit for the power supply device, wherein the voltage supplied is maintained at a predetermined potential or lower.