Background of the Invention High voltage dc generators for low currents usually comprise a controllable voltage supply S, a rectifying diode D, a filter capacitor C, and optionally a resistor R over which the voltage is measured. Fig. 1 shows a circuit diagram over one embodiment of such a generator circuit 10.

Throughout this application high voltage refers to voltages above 50V and preferably above 800V, and low current refers to currents below 1A and preferably below 10mA.

In most applications such generators 10 are used in circuits that draw a very small current or no current at all. Examples of such applications comprise all types of electron optics, electron spectroscopy for chemical analysis (ESCA), mass spectrometry, etc. , wherein the generator is used to generate of high voltage fields between two separated electrodes. In such cases it is difficult to obtain a quick response time when lowering the voltage, particularly when lowering the voltage from the kV range to a few volts. The prolonged response time is due to the fact that the filter capacitor C must be discharged to achieve a lowered voltage. During high voltage conditions the capacitor C is charged, and when the voltage from the voltage supply S is lowered, the capacitor C will preserve the voltage at a raised level during the discharge process of the same.

In the circuit 10 of fig. 1 the discharging of the capacitor C takes place through the resistor R.

But as the resistor R generally is given a high resistance to keep the power consumption of the circuit low at high voltages, the discharging process is long. The discharge process follows the discharge curve of a corresponding RC-circuit (fig. 2). As is shown in fig. 2 the discharge process is fast in the initial high voltage phase, but it is extremely slow in the low voltage region.

Fig. 3 shows one attempt to solve this problem, wherein a discharge device 15, comprising three fet-transistors FT in series, is connected in parallel with the capacitor. The transistors are

then set in a conducting state during the discharge process. However, conventional fet- transistors FT rarely withstand voltages of 1kV or more, therefore two or more transistors FT have to be connected in series when voltages above this level is needed, and such arrangements tends to be unnecessarily complicated and fragile.

Summary of the Invention The object of the invention is to provide a new high voltage dc circuit, which circuit overcomes one or more drawbacks of the prior art. This is achieved by the high voltage generator circuit as defined in claim 1, and by the method of claim 10 One advantage with such a high voltage generator circuit is that the discharge process for high voltage circuits may be lowered in a simple and reliable manner.

Another advantage is that the control signals are electrically separated from the high voltage circuit.

Embodiments of the invention are defined in the dependent claims.

Brief Description of the Drawings The invention will be described in detail below with reference to the drawings, in which: Fig. 1 shows a circuit diagram over a conventional high voltage generator circuit.

Fig. 2 shows a discharge curve for the circuit of fig 1.

Fig. 3 shows a prior art high voltage generator circuit.

Fig. 4 shows a first embodiment of the high voltage generator circuit according to the present invention.

Fig. 5 shows a second embodiment of the high voltage generator circuit according to the present invention.

Fig. 6 shows a discharge curve for the circuits in figs 4 and 5.

Detailed Description of Preferred Embodiments In the present invention it has surprisingly been found that the photoelectric effect in a conventional high voltage diode can be utilized to enhance the discharging process for a high voltage generator circuit. More in detail, the invention is based on the phenomenon that certain semiconductor diodes produces a current in the non conducting direction when irradiated with electromagnetic radiation of suitable wavelength, provided that the diode has a housing that is transparent to said radiation.

In a first embodiment of the present invention it refers to a high voltage dc generator circuit capable of providing a high voltage dc output signal, which is highly controllable. The high voltage dc generator circuit 10 comprising a controllable voltage supply S, a rectifying diode D, a filter capacitor C, and optionally a resistor R, wherein the generator circuit 10 further comprises a source of electromagnetic radiation, and a radiation responsive component that produces a discharge current in response to said electromagnetic radiation. Preferably, the radiation responsive component is a high voltage diode (HVD) with a housing that is transparent to light in the visible or infra red range. Hence, the source of electromagnetic radiation preferably is a light source that emits light in said range.

In one embodiment the discharging device is comprised of a high voltage diode with a glass housing, e. g. a BY 8414, Philips Semiconductors, and at least one light emitting diode arranged to emit light onto the diode. Preferably, the light emitting diode preferably emits light in the IR-range. To increase the efficiency, a number of diodes may further be arranged around the diode. The invention is hereafter described with reference to a particular embodiment of this type, comprising a high voltage diode (HVD) and a light emitting diode (LED).

Figs. 4 and 5 show two embodiments of the present invention, wherein the HVD is connected in series and in parallel with the high voltage generator circuit 10 of fig 1, respectively.

In fig. 4 the rectifying diode D in the circuit 10 according to fig. 1 is substituted by a discharging device 20 according to the present invention. In this embodiment, the HVD of the discharging device 20 functions as rectifying diode during non discharging operation. During a discharge process, the current produced by the HVD, when irradiated by the LED, will lead to a reduction of the discharge time. In the circuit according to fig. 4 it is assumed that the voltage supply S allows flow of the adverse discharge current from the diode during the discharge process.

In fig. 5 the discharging device 20 according to the present invention is arranged in parallel with the outputs from the voltage source S and the capacitor C. The circuit of fig. 5 works in the same manner as the circuit of fig. 4 with the exception that no current has to flow through the voltage supply. One advantage with the parallel arrangement of fig. 5 is that the discharge device 20 may be formed as a separate circuit that can be connected to an existing high voltage generator or the like, to achieve improved discharge performance.

As is shown by the discharge curve for the conventional circuit (fig 2), it takes a very long time before the output voltage from the circuit will reach zero volt. In the ideal case the output voltage actually never reaches zero volts. Fig. 6 shows a schematic example of a resulting discharge curve for a circuit comprising the discharge device 20 according to the present invention, wherein the solid line shows the normal discharge process with no contribution from the discharge device, and the dashed line shows the discharging process when the HVD is irradiated by the LED. As is illustrated by fig. 6 the contribution from the discharge device 20 is negligible at high voltages, but as the voltage gets lower the contribution increases, and at low voltages the contribution from the discharge device 20 is dominant, i. e. the whole discharge process is due to the current generated by the HVD. As can be seen in fig. 6 the HVD can further be used to achieve a small negative voltage on the outputs.

When using a source of electromagnetic radiation with low power consumption (e. g. LED), the source of electromagnetic radiation can be held in a constant emitting state whenever the generator is in use, as long as the radiation intensity from the arrangement is kept at a constant

level, so that voltage fluctuations are avoided. The total power consumption for the high voltage circuit will then rise, but only to a small extent. In order to lower the power consumption of the device 20, the source of electromagnetic radiation may be a controllable source of electromagnetic radiation that is controlled to radiate only during the discharge process.

Furthermore, by controlling the light emission from the LED, a possibility of actively controlling the output voltage in the region close to zero volts is achieved, which is very advantageous in many applications. Thus, the discharge device 20 provides a simple but surprisingly simple solution for such applications.

The LED may either be controlled directly by a main control system used to control the voltage of the high voltage dc circuit, or by an independent control device. To control the LED, the main control system is arranged to provide a suitable drive signal for the LED at times when the discharge device 20 shall be active. In one embodiment the main control system provides a simple on/off signal, but in other embodiments, where active control of the voltage around zero volts is desired, the signal from the main control may have different levels, each corresponding to a specific output voltage from the discharge device 20.

An independent control device may either be arranged to control the LED in response to a "discharge signal"from e. g. a main control system, or it may be arranged to register the difference between the desired circuit output voltage Udes and the actual circuit output voltage U, and if the desired Udes output is essentially lower than the actual output U the LED is set in an emitting state. According to one embodiment, the control circuit has means for registering the difference between the voltage over the voltage supply Usup and the circuit output voltage U respectively, and if the voltage supply voltage Usup is considerably lower than the circuit output voltage U, then it controls the controllable source of electromagnetic radiation to emit electromagnetic radiation. One embodiment of a control device simply consists of a difference amplifier that is arranged to turn on the LED when the desired circuit output voltage Udes is less than the actual circuit output voltage U.

There is further provided a method of discharging a high voltage dc circuit comprising the step of emitting electromagnetic radiation onto a radiation responsive component that

produces a discharge current in response to the electromagnetic radiation. The method may also comprise the step of controlling a source of electromagnetic radiation to emit said electromagnetic radiation only during a discharge process, as is discussed above.

EXAMPLE: A discharge device 20 for a high voltage generator circuit was designed using a HVD in the form of a BY 8414 from Philips Semiconductors, and an arrangement of five IR-LEDs (TSFF 5200 from Telefunken) placed around the HVD. Using this arrangement a voltage of about 7 volts was registered over the HVD. The discharge device 20 was arranged in parallel with the outputs from a conventional high voltage generator, EMCO C60 from EMCO High Voltage corp (representing a circuit equivalent to that of fig. 1). The resistor in the circuit had a resistance of 60 MQ, and the total capacitance in the circuit comprising the filter capacitor was lOnF. At a voltage of 6kV the resulting load-current was 100, uA, and the effect 0.6 W.

When the circuit was discharged to 6V (1% o of the initial voltage) in conventional manner without using the discharge device 20 the discharge time was 4.1 sec. When the same discharge process was performed while illuminating the HDV in the discharge device such that it produced a discharge current of 201lA, the discharge time was 1.1 sec, i. e. the discharge time was approximately reduced with a factor 4.