EP0506178B1

EP0506178B1 - Power supply apparatus

Info

Publication number: EP0506178B1
Application number: EP19920200760
Authority: EP
Inventors: Henricus Johannes Maria Van Der Laar
Original assignee: Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1991-03-25
Filing date: 1992-03-17
Publication date: 1996-10-16
Anticipated expiration: 2012-03-17
Also published as: EP0506178A2; EP0506178A3

Description

The invention relates to a power supply apparatus for supplying a device with electric energy, comprising at least one test input for receiving a test signal which is dependent on a variable which itself is dependent on the power applied to the device, which test input is connected to a first input of a comparator circuit, a second input of which is connected to a generator which generates a reference signal which is a measure of a desired value of said variable, an output of the comparator circuit being connected to a control member which controls the power applied to the device by the power supply apparatus so that said variable is essentially equal to the desired value.
An example of such a power supply apparatus is described in Philips Tecnical Review 39 (1980), No. 2, pp. 37-47, notably with reference to Fig. 14. The known power supply apparatus is intended to power a semiconductor laser, a photodiode which is accommodated in the same envelope as the laser generating a photocurrent which is proportional to the light flux of the laser and which constitutes the test signal. The power applied to the laser in the known power supply apparatus can be controlled so that the current produced by the photodiode (monitor) remains constant at a desired value. The control of only one variable, however, involves the risk that the value of another laser variable is no longer within the desired range or, even worse, no longer within the safe range. Driving a semiconductor laser diode beyond the safe working range can readily damage the laser. For safe operation of a laser, therefore, it would be desirable to control the power applied to the laser so that more than one of the laser variables is maintained at or near a desired value. In addition to said monitor current, such variables are, for example the laser current and the laser voltage and the radiant power of the laser. However, in practice this is not very well possible because the various variables are interrelated in a rather complex manner.
It is an object of the invention to provide a power supply apparatus of the kind set forth which enables one variable to be maintained at a desired value while maintaining the other variables at least within limits which are considered to be safe.
To achieve this, the power supply apparatus in accordance with the invention is characterized in that the power supply apparatus comprises at least two test inputs with associated comparator circuits, the generator generating a number of reference signals which corresponds to the number of test inputs, the control member controlling the power applied to the device by the power supply apparatus so that at least one of the variables corresponding to the test signals is essentially equal to the value desired for the relevant variable, the other variables corresponding to the test signals deviating from the associated desired values in a predetermined sense only.
Using the power supply apparatus in accordance with the invention, a variable which can in principle be chosen at random can be maintained at the desired value, the other variables, for example all remaining below the desired value so that exceeding of said value and of a higher, dangerous value is precluded. If a deviation of a variable to a value below a given value is deemed risky, the control member should, of course, be adapted so that the relevant variable always remains above an adjusted value which is higher than the "risky" value.
The control member may comprise, for example a suitably programmed microprocessor which decides which variable is to be maintained at the desired value in order to keep the other variables below (or above) the desired value. This microprocessor can also control the adjustment of the chosen variable and the monitoring of the other variables.
An embodiment in which the control member can be constructed without including a microprocessor is characterized in that the control member comprises a number of semiconductor diodes which corresponds to the number of test inputs, each semiconductor diode comprising a first and a second connection, the first connections being connected to one another and to a current source circuit, each second connection being connected to the output of one of the comparator circuits. A control member thus constructed satisfies the requirements imposed without requiring further control. When it is specified that the variables which are not maintained at the desired value should remain below the desired value, the first connection of each of the semiconductor diodes must be an anode connection.
An embodiment of the power supply apparatus in accordance with the invention which is suitable for a variety of applications is characterized in that the variables represented by the test signals include an electric voltage applied to the device and an electric current taken up by the device.
An embodiment which is particularly suitable for supplying a semiconductor laser with electric energy is also characterized in that the variables represented by the test signals also include the radiant power of the laser and a signal produced by a monitor connected to the laser.
The invention will be described in detail hereinafter with reference to the drawing.

Fig. 1 shows a block diagram of an embodiment of a power supply apparatus in accordance with the invention,
Fig. 2 shows a circuit diagram of a control member for the power supply apparatus shown in Fig. 1,
Fig. 3 shows a graph illustrating the operation of the control member shown in Fig. 2,
Fig. 4 shows a circuit diagram of a reference signal generator for use in the power supply apparatus shown in Fig. 1,
Fig. 5 shows a circuit diagram of a test circuit for use in the power supply apparatus shown in Fig. 1,
Fig. 6 shows a circuit diagram of a comparator circuit for use in the power supply apparatus shown in Fig. 1,
Fig. 7 shows a circuit diagram of an output stage for use in the power supply apparatus shown in Fig. 1, and
Fig. 8 shows a graph with characteristics of a semiconductor laser in order to illustrate the operation of the power supply apparatus in accordance with the invention.

The power supply apparatus shown in the form of a block diagram in Fig. 1 serves to supply a device 1 with electric energy. The device 1 may be, for example a semiconductor laser. The power supply apparatus comprises a test circuit 3 which, in the present embodiment, comprises four test inputs 5a, 5b, 5c, 5d, which can receive test signals from the device 1. The value of each test signal is dependent on a variable which itself is dependent on the power applied to the device 1. The test circuit 3 consists of four sections 3a to 3d, each of which is connected to one of the four test inputs 5a to 5d. The output of each section 3a ... 3d is connected to a first input 7a ... 7d of a comparator circuit 9a ... 9d, a second input 11a ... 11d of which is connected to a generator 13 which is adapted to generate a reference signal which is a measure of a desired value of the relevant variable. The output of each comparator circuit 9a ... 9d is connected to an input 15a ... 15d of a control member 17 which controls, via an output stage 19, the power applied to the device 1 so that at least one of the variables corresponding to the test signals is essentially equal to the value desired for the relevant variable, the other variables corresponding to the test signals not being greater than the relevant desired value.
Fig. 2 shows an elementary circuit diagram of an embodiment of the control member 17. "Hard" voltages U₁ ... U₄ are applied to the inputs 15a ... 15d, i.e. voltages originating from voltage sources without internal impedance. This is symbolically represented by unit amplifiers 21a ... 21d preceding the inputs 15a ... 15d. A unit amplifier 25 is also shown to be connected to the output 23 of the control member 17 so as to indicate that the circuit is not loaded by the impedance at the output.
The control member 17 comprises four semiconductor diodes 27a ... 27d, each of which comprises a first and a second connection. In the embodiment shown, the first connection is the anode connection and the second connection is the cathode connection. The first connections are connected to one another and to a current source circuit 29. Each of the second connections is connected to one of the inputs 15a ... 15d.
In order to simplify the explanation of the operation of the circuit, in first instance the restriction is imposed that there are only two input voltages U₁ and U₂. The current source 29 applies a constant current I_cc to the diodes 27a and 27b. Depending on the voltages U₁ and U₂ presented, the current will be distributed between the two diodes so that a current I₁ flows through the diode 27a and a current I₂ flows through the diode 27b. The output voltage U_o is thus defined. When the diodes 27a and 27b are assumed to be ideal and fully identical, the following relations hold: $I_{1} = I_{sat} [exp (\frac{q}{kT} (U_{o} - U_{1}))-1]$
$I_{2} = I_{sat} [exp (\frac{q}{kT} (U_{o} - U_{2}))-1]$
$I_{cc} = I_{1} + I_{2}$
Therein, I_sat represents the saturation current of the diodes, q is the charge of the electron, k is Boltzmann's constant, and T is the absolute temperature. The output voltage U_o can be determined therefrom: $U_{O} = U_{1} + U_{2} - \frac{kT}{q} ln [exp (\frac{q}{kT} U_{1})+exp (\frac{q}{kT} U_{2})]+ \frac{kT}{q} ln[\frac{I_{cc} +2 I_{sat}}{I_{sat}}]$
Fig. 3 graphically shows the transfer of the control member. For the sake of clarity, only one input voltage, being the input voltage U₁ in the present case, is varied. The other input voltage U₂ is maintained constant at an arbitrary value. Depending on the relative position of the input voltages, three regions can be distinguished in the transfer function. ${1.U}_{1} {<U}_{2}$
In formule (4) the exponential term with U₁ can be ignored relative to that with U₂. The constant term with I_cc very well approximates the voltage across the diode U_D if the diode carries the full current I_cc. This is because for silicon diodes the extra term with I_sat in the numerator can be completely ignored relative to the term with I_cc. As a result, the output voltage U_o varies linearly as a function of the input voltage U₁ and is independent of the input voltage U₂: $U_{o} {=U}_{1} {+ U}_{D}$
${2.U}_{1} {≈ U}_{2}$
This voltage region constitutes a transition region. In this case the formule (4) cannot be simplified and the value for the output voltage must be determined by calculation. All individual terms are continuous and can be differentiated, so that the transition is smooth: $U_{O} = U_{1} + U_{2} - \frac{kT}{q} ln [exp (\frac{q}{kT} U_{1})+exp (\frac{q}{kT} U_{2})]+ U_{D}$
If the transition region is defined as the region in which the diode currents do not deviate by more than a factor one hundred, the total transition region for silicon diodes amounts to approximately 2(kT/q)ln0.01 ≈ 230 mV. ${3.U}_{1} {> U}_{2}$
Because the formule (4) is symmetrical in the input voltages, it follows from the interchanging of the indices that the output voltage U_o varies linearly as a function of the input voltage U₂ and is independent of the input voltage U₁. Because U₂ is assumed to be constant, U_o will be constant: $U_{o} {=U}_{2} {+U}_{D}$
If both input voltages vary, the output voltage U_o will follow the lowest input voltage at a voltage distance equal to U_D. The described variation of the output voltage U_o as a function of the input voltages is graphically shown in Fig. 3. It will be evident that the output voltage is substantially always equal to the smaller one of the two input voltages, except for the diode voltage U_D which, however, is constant and known and for which, therefore, correction can be readily made. It is only in the transition region that the output voltage is not exactly equal to one of the two input voltages, but it is never greater than the smaller one of these input voltages. Thus, the device 1 is not endangered and a major advantage of the transition region consists in that no voltage peaks occur upon transition, as would be the case in response to abrupt switching over.
The transfer function has been described above for two input variables. However, it can be readily demonstrated that the described calculation method can be applied to an arbitrary number of input variables. The general formule for the output voltage can thus be written as:
Except for the constant diode voltage U_D, therefore, outside the transition regions where the output voltage gradually changes from one to the other input voltage, the output voltage U_o will be given by the minimum of the input voltages presented: $U_{o} {=min(U}_{1} {,U}_{2} {,.....,U}_{N} {)+U}_{D}$
The effect of the constant term U_D can be eliminated by reducing, for example the input voltages by an amount U_D before presentation to the inputs of the control member. Another possibility consists in the reduction of the output voltage U₀ by this amount. However, because the control member 17 itself forms part of a closed feedback loop (see Fig. 1), the effect of U_D will be reduced by division by the loop gain of the feedback loop.
Fig. 4 shows a circuit diagram of an embodiment of the reference signal generator 13. Using a zener diode 31 and an operational amplifier 33, a stabilized reference voltage U_REF is formed from a supply voltage U_B. Four reference signals I_s, U_s, M_s and L_s can be formed from U_REF by means of four accurate potentiometers 35a, 35b, 35c and 35d. If the device 1 is a semiconductor laser, I_s and U_s may represent desired values of the current I through and the voltage U across the laser, respectively. M_s and L_s then represent desired values of the output signals M and L of a photodiode which serves as a monitor and which is accommodated within the envelope of the laser, and a sensor measuring the light current of the laser, respectively. Parallel to the zener diode 31 there is connected a capacitor 32 and a resistor 34 is connected between the supply voltage U_B and said parallel connection. The time constant of the combination formed by the capacitor 32 and the resistor 34 enables the reference voltage U_REF and the reference signals derived therefrom, to be controlled at a predetermined rate from the value zero to the working point. The parallel connection of the zener diode 31 and the capacitor 32 is connected to the positive input of the operational amplifier 33. When an external signal is superposed on this positive input, the reference signals can be modulated, if desired. The reference signals may in principle have any arbitrary shape; they may also be alternating voltages.
Figs. 5A and B show a circuit diagram of an embodiment of a test circuit 3 for obtaining test signals I_m, U_m, M_m and L_m which represent the variables I, U, M and L. This test circuit comprises four sections 3a ... 3d. For the sake of clarity, the sections 3a and 3b are shown, together with the semiconductor laser, in Fig. 5A, the sections 3c and 3d being shown in Fig. 5B, together with the semiconductor laser. The semiconductor laser is denoted by the reference numeral 37 in both Figures.
The first section 3a comprises a measuring resistor 39 which is connected in series with the laser 37. The voltage across this resistor, being proportional to the laser current I, is converted into the test signal U_m by means of an operational amplifier 41.
The second section 3b comprises two connections 43 and 45 which are connected to the anode and to the cathode, respectively, of the laser 37. The laser voltage U can thus be measured in a currentless manner, so that the voltage drop across the supply leads of the laser is eliminated (four-point measurement). Using an operational amplifier 47, the diode voltage U is converted into the test signal U_m.
As has already been described in the cited article in Philips Technical Review 39 (1980), No. 2, pp.37-47, the semiconductor laser 37 is accommodated, together with a photodiode 49 serving as a monitor, in a common envelope 51 (see Fig. 5B). This photodiode forms part of the third section 3c and detects a light current M emerging at the rear of the laser 37. The current thus delivered by the photodiode 49 is converted into the test signal M_m by means of an operational amplifier 53.
The fourth section 3d of the test circuit 3 comprises a photodiode 55 which is arranged outside the envelope 51 and which detects the light current L produced by the laser 37. The current generated by the photodiode 55 is converted into the test signal L_m by means of an operational amplifier 57.
Fig. 6 shows a circuit diagram of an embodiment of one of the comparator circuits 9a ... 9d. Only the first comparator circuit 9a is shown, because the other comparator circuits 9b ... 9d are identical thereto. The comparator circuit 9a shown comprises two inputs 11a and 7a which receive the current reference signal I_s and the current test signal I_m, respectively. These inputs are connected to the positive and the negative input, respectively, of a differential amplifier 59 whose output produces an error signal U₁ which represents the difference I_s-I_m. The other comparator circuits 9b ... 9d produce output signals U₂ ... U₄ which represent the differences U_s-U_m, M_s-M_m and L_s-L_m, respectively. The output signals U₁ ... U₄ form the input signals for the control member 17 which supplies the control voltage U_o for the semiconductor laser 37. The output signals U₁ ... U₄ of the differential amplifiers 59 are "hard" voltages, so that the unit amplifiers 21a ... 21d shown in Fig. 2 can actually be dispensed with.
The control voltage U_o is applied to the input of the output stage 19, a circuit diagram of an embodiment of which is shown in Fig. 7. The output stage 19 is necessary to ensure that the control memory 17 (Fig. 2) is not loaded by the current to be applied to the semiconductor laser 37. Therefore, the output stage 19 comprises an output transistor 61 which is capable of supplying adequate current so that the unit amplifier 25 shown in Fig. 2 actually can also be dispensed with. The output transistor 61 is controlled by an operational amplifier 63 where to the control voltage U₀ is applied and which does not load the output 23 of the control member 17. The output transistor 61 and the measuring resistor 39 (see also Fig. 5A), across which the laser current is measured, are included in the feedback loop of the operational amplifier 63 so that voltage drops across these components do not affect the laser control itself. The voltage across the laser 37 is measured by way of a four-point measurement as described, so that the voltage drop due to the resistance of the supply leads is again eliminated.
Fig. 8 shows an example of the characteristics of a semiconductor laser diode. The curves 65, 67 and 69 represent the variation of the laser voltage U, the radiant power L and the monitor signal M, respectively, as a function of the laser current I. The reference values I_s, U_s, L_s and M_s are also shown. Using the described power supply apparatus, the laser current I is adjusted to a value I_m for which none of said four variables is greater than the relevant reference value, one of said variables, in this case L, actually being equal to the reference value (L_m=L_s). If the reference value L_s is increased by changing the setting of the potentiometer 35d (Fig. 4), the laser current I will increase until one of the other variables is substantially equal to the reference value, for example M_m=M_s. In the transition region L as well as M is approximately equal to the associated reference value and in any case none of the four variables exceeds the reference value.
As has already been described, the power supply apparatus in accordance with the invention is particularly suitable for the supply of energy to a semiconductor laser, notably in measuring and life test set-ups. However, it will be evident that the apparatus can be used whenever two or more process variables are to be measured and controlled. It is to be noted that the invention is not restricted to the adjustment of a component, apparatus or process to a smallest value, given the values of a number of variables. The function of the control member 17 is transformed to the highest setting, given the value of a number of variables, simply by reversing the polarity of the diodes 27a ... 27d (Fig. 2) and the direction of the current I_cc. Thus, by combination of the highest and the lowest setting within the control member 17 it is even possible to control a process in a given range, given the lowest and highest setting of a number of variables. A suitable field of application is the field of electric supply equipment in which generally the electric voltage and current are variables. By a combination of the positive lowest and negative highest setting within the control member, it is thus even possible to realise a so-called four-quadrant power supply. A four-quadrant power supply is a power supply capable of delivering as well as dissipating power. The nature of the device being powered is irrelevant in this respect. Notably capacitive, inductive or negative impedances can be driven without giving rise to stability problems, because the invention utilizes real, non-complex measured values of current and voltage. The power supply apparatus can thus also be used as an adjustable load for other power supplies or other equipment.

Claims

A power supply apparatus for supplying a device (1) with electric energy, comprising at least one test input (5a, ..., 5d) for receiving a test signal which is dependent on a variable which itself is dependent on the power applied to the device (1), which test input (5a, ..., 5d) is connected to a first input (7a, ..., 7d) of a comparator circuit (9a, ..., 9d), a second input (11a, ..., 11d) of which is connected to a generator (13) which generates a reference signal which is a measure of a desired value of said variable, an output of the comparator circuit (9a, ..., 9d) being connected to a control member (17) which controls the power applied to the device (1) by the power supply apparatus so that said variable is essentially equal to the desired value, characterized in that the power supply apparatus comprises at least two test inputs (5a, ..., 5d) with associated comparator circuits (9a, ..., 9d), the generator (13) generating a number of reference signals which corresponds to the number of test inputs, the control member (17) controlling the power applied to the device (1) by the power supply apparatus so that at least one of the variables corresponding to the test signals is essentially equal to the value desired for the relevant variable, the other variables corresponding to the test signals deviating from the associated desired values in a predetermined sense only.
A power supply apparatus as claimed in Claim 1, characterized in that the control member (17) comprises a number of semiconductor diodes (27a, ..., 27d) which corresponds to the number of test inputs (5a, ..., 5d), each semiconductor diode comprising a first and a second connection, the first connections being connected to one another and to a current source circuit (29), each second connection being connected to the output of one of the comparator circuits (9a, ..., 9d).
A power supply apparatus as claimed in any one of the preceding Claims, characterized in that the variables represented by the test signals include an electric voltage applied to the device (1) and an electric current taken up by the device.
A power supply apparatus as claimed in Claim 3 for supplying a semiconductor laser (37) with electric energy, characterized in that the variables represented by the test signals also include the radiant power of the laser (37) and a signal produced by a monitor (49) connected to the laser.