NL8103265A - AC=DC power converter using microprocessor - controlling firing of thyristor bridge circuit supplying power to output rectifier circuit via HF transformer - Google Patents

Abstract

This industrial converter uses a h.f. thyristor bridge coupled by a transformer to a rectifying and filtering output circuit. The firing timing of the thyristors is controlled by a microprocessor fed with signals representing the following data: input voltage characteristics, output voltage, polarity and the currents drawn b the diodes which are in anti-parallel with the thyristors. The circuit has a high conversion efficiency and a power facto approaching unity. The circuit between the balance points of the thyristor bridge consists of a series-resonant circuit in series with the primary of the trasforme. The microprocessor memory holds a tble of input voltages versus delay tifiringes, used in timing the thyristor pulses.

Description

v 'Λ \ * Energy converter

The invention relates to an energy converter, in which energy from a source, using a series resonant circuit, at least two thyristors, diodes connected antiparallel to it and a bridge circuit provided with a high-frequency output transformer, and an energy buffer a load is applied, further comprising a control circuit which delivers the fire pulses to bring the respective thyristors into the conducting state.

Such energy converters have long been known to serve the energy of a single or multiphase a.c. or from a d.c. convert source into energy with a single or multiphase a.c. voltage of a different amplitude and / or frequency, with a pulse-shaped voltage or with a d.c. tension. If the des-15 concerning thyristors located in the bridge circuit are in the conducting state, energy is supplied from the source via the series resonant circuit to the energy buffer and energy is extracted from this buffer by the load. In the time between the respective thyristors being in the conducting state, the energy supplied to the buffer can be returned to the source via the series resonant circuit, or can be decreased by the load. The setting of the energy balance is determined by the times when the thyristors located in the bridge circuit are brought into the conductive state.

An energy converter as described in the preamble is known, for example, from the U.S. U.S. Patent 3,953,779. The energy converter described in this patent is provided with an analog control circuit which, in response to the current in the series-30 resonant circuit and to a voltage derived from the output voltage of the output transformer, delivers control pulses, with which the respective thyristors in the conducting be brought to condition.

Control circuits in an energy converter, as known so far, will be designed differently with a pulsating load than with a (semi) continuous load. Likewise, 8103265 ^ * \ - 2 - such control circuits will differ depending on the application of the power converter for voltage or current stabilization. The delays built into the driver circuits also depend on the frequency of the series-5 resonant circuit. When the control circuits are used in differently dimensioned bridge circuits, the built-in delay must therefore be adjusted every time.

The object of the invention is now to provide a control circuit which has considerably greater flexibility compared to the known control circuits, in that it can be adapted in a simple manner to the specific application of the energy converter and the specific dimensioning of the bridge circuit herein.

According to the invention, the control circuit is therefore provided with an output voltage measuring circuit, diode current detectors for supplying the presence of current through said diodes indicating signals, a polarity detector for determining the polarity of the voltage in the series resonant circuit. thyristor ignition circuits for generating said fire pulses and a microprocessor, which, with the aid of a programmatically built-in and depending on the measured output voltage and on the detected current passage through said diodes, determines the times at which control signals for the by the polarity detector designated thyristor-25 ignition circuits are output.

Outside of the interface circuits, the control circuitry therefore comprises a digital control circuit realized by a microprocessor, so that the great flexibility of the control circuit can be obtained in a programmatic manner.

The invention will now be further elucidated with reference to the annexed figures, of which:

Fig. 1 shows an exemplary embodiment of the energy converter according to the invention

Fig. 2A, 2B, 2C and 2D diagrams, by means of which the operation of the energy converter shown in Fig. 1 will be illustrated; 8103265 '* 1 -3-

Fig. 3 shows a block diagram of the construction of a microprocessor to be used for the realization of the invention;

Fig. 4- shows a diagram by which the operation of the microprocessor will be explained; while in FIG. 5 a flow chart is presented with reference to the operation of the control circuit.

The energy converter shown in Fig. 1 comprises a bridge circuit 1, an energy buffer 2 and a control circuit 3.

Via the bridge circuit 1 and the buffer 2, energy from a source 4 is supplied to a load 5. The bridge circuit is provided with a series resonant circuit formed by a coil 6 and a capacitor 7, four thyristors 8, 9 , 10 and 11, diodes 12, 13, 14 and 15 connected in parallel, a high-frequency output transformer 16 and four rectifier diodes 17, 18, 19 and 20. The operation of such a series resonant bridge circuit is known. and described in IEEE Transactions on Industrial Electronics and Control Instrumentation, vol. IECI-17, No. 3, May 1970, p. 209-221 and vol. IECI-23, No. 2,

May 1976, p. 2/14/14/09 and in said U.S. U.S. Pat. No. 3,953,779.

In FIGS. 2A and 2B, the course of the current i ^ through the coil 6, respectively. the voltage E ^ across points P and Q is shown.

In phase A (thyristor phase), thyristors 8 and 11 are in the conductive state. The current i ^ initially increases and the voltage E ^ increases, after which the current i ^ decreases again to zero; the voltage E ^ thereby goes to its peak value.

As soon as the current i ^ has decreased to zero, the thyristors 8 and 11 stop conducting and an opposite current flows through diodes 12 and 15. In the phase B (diode phase) 30 then entered, the voltage E1 decreases slightly as a result of this diode current.

When the diode current finally becomes zero, the voltage E ^ remains. constant until thyristors 9 and 10 are brought to the conductive state. In the thyristor phase C and the subsequent diode phase D, the current is equal, but opposite to that in phase A, respectively. B and also the voltage E ^ has a course which is equal but opposite to that in phases A and B.

8103265 - - 4 -> *

I

In Fig. 2C and 2D, the course of the current through the coil 6, respectively. the voltage E ^ across points P and Q shown in case the thyristors are already brought into the conductive state in the diode phases; one speaks in this case of the "cut-in mode". The situation shown in FIGS. 2A and 2B is hereinafter referred to as "trigger mode".

In phases A and C, energy is supplied from source 4 to energy buffer 2 and extracted from this buffer by load 5; in phases B and D, part of the energy supplied to the buffer 2 is returned to the source via the series resonant circuit and part is taken from the load 5.

By correct adjustment of the ignition moments of thyristors 8, 11 and 9, 10, an equilibrium situation can be created, depending on the energy supplied to the series resonant circuit and the energy extracted from the buffer. on the buffer 2 is kept constant. The magnitude of the voltage across the primary winding of the output transformer 16 is therefore constant and the resonant frequency of the circuit is determined by the product LC, where L is the inductance 20 of the coil 6 and C the capacitance of the capacitor 7 proposes.

The setting of the energy balance is realized in the control circuit 3. This control circuit comprises an input voltage measuring circuit 21, an output voltage measuring circuit 22, two diode current detectors 23 and 24, a polarity detector 25, 25 a pair of thyristor ignition circuits 26 and 27, a short circuit circuit 28 and a microprocessor 29. The circuits 21 to 28 together form the interface for the microprocessor 29.

The measuring circuits 21 and 22 give an analog voltage value, which is proportional to the input and resp. the output voltage of the energy converter.

The diode current detectors 23 and 24 output signals indicating the presence of current through the diodes 12, 15 and 12, respectively. 13, 14. In order to prevent a short circuit in the bridge circuit, the fire impulses for thyristors 9 and 10 may only be emitted if it is established that thyristors 8 and 11 are blocked. Likewise, conversely, the fire impulses for thyristors 8 and 11 may first be delivered 8103265 5 ♦ - 5 - if it is established that thyristors 9 and 10 are disabled.

The thyristors are cut off when the diodes connected in parallel to them come on. The conducting state of the diodes is detected by means of the diode current detectors, which emit the digital signal "I" if the respective diodes are in the conducting state and the digital signal "0" if the respective diodes are cut off.

The polarity detector 25 outputs the digital signal "I" if the voltage overς across the capacitor 7 is greater than zero 10 and the digital signal "0" if this voltage is less than zero. In the first case, the polarity becomes positive and negative in the second case After the thyristor pair 8, 11 has entered the conductive state, the voltage £ ¢, as shown in Figures 2B and 2D, acquires a positive polarity. positive, then the thyristor pair 9, 10 is eligible to be brought into the conducting state, ie the polarity of the voltage across the capacitor 7 determines for which thyristor pair the fire pulses are to be generated.

The thyristor ignition circuits 26 and 27 convert the digital control pulses of the microprocessor, which determine the ignition time of the respective thyristors, into analog fire pulses.

The short-circuit circuit 28 ensures that the voltage across the primary winding of the output transformer 16 is made zero for a time interval determined by the microprocessor. In the present embodiment, this has been achieved by shorting the rectifying circuit behind the secondary winding of the output transformer 16. Diode 30 prevents the output voltage from being short-circuited at the same time. If the output voltage is too high, in the cut-in mode and with a pulsating load, the energy converter must be turned off until, after energy has been pulsed in time or not, the output voltage has fallen below its set value.

When restarting the energy converter it is desirable that a certain residual voltage is present across the capacitor 7, so that the voltage Ej, already after the first thyristor phase, takes on its original amplitude. This is achieved by applying the above-mentioned short circuit during the last cut thyristor phase and the subsequent complete diode phase, after which the energy converter has been switched off. Namely, there is then a simple relationship between the residual voltage across the capacitor 7 and the ignition time of the respective thyristor pair in the last incised thyristor phase. This time and the duration of the short circuit is determined by means of the microprocessor, which provides a digital signal to short-circuit circuit 26 at the appropriate time.

The heart of the digital control is formed by the microprocessor 29. Fig. 3 shows a possible embodiment of this microprocessor. The microprocessor is composed of an "Ambit 2901 B" bit slice 31 of "Advanced Micro Devices", a programmable memory 32, a program selection circuit 33, a memory address counter 34, a condition switch 35, an analog multiplexer 3d, a analog-to-digital converter 20 37, digital multiplexers 38 and 39 and a readout register 40.

Multiple programs can be stored in memory 32; this makes the control circuit suitable for various applications of the energy converter, as already mentioned in the description introduction. The program is selected using the program selection circuit 33. The program in the present embodiment is composed of 32 bit words, of which - bits 1-8 in an "address mode" indicate the jump addresses ADR ^ g for the program, - the bits 1-4 in a "direct mode" information DA to be supplied via the input channel to the 30 bit slice - bits 9-25, the information required for the logic operations taking place in the bit slice, - bit 26 the control signal WR for the readout register 40, bits 27-29 control signals ISj ^ for multiplexers 36, 38, 35 and 39, and - bits 30-32 control signals CSj for condition circuit 35.

8103265 'm t- - 7 -

The format of the words is shown in Fig. 4.

The information to be supplied to the bit slice via the input channel consists of: - the voltage values of the input and output voltage measuring-5 circuit; these values are supplied through the analog multiplexer 36 after they have been digitized in the analog-digital converter 37 and through the digital multiplexer 39; - the digital signals from the diode current detectors, from the polarity detector and from the information determined by the bits DA1; TO this information is supplied through digital multiplexers 38 and 39.

The information supplied via the read register 40 is formed by the control signals to be supplied to the ignition circuits and to the short-circuit.

In Fig. 5 the flow chart is given for the operations taking place in the microprocessor in case the control circuit is used in a pulsed load power converter.

In program step 1, the measured polarity in the 20 bit slice 31 is determined. Depending on the observed value of this polarity, the bit slice in program step 2 or 2 'delivers the control pulse for one of the two ignition circuits 26, 27. Suppose now that as a result of this the thyristors 8 and 11 are brought into the conducting state, ie that the thyristor phase A has arrived. Program step 3 then introduces a delay after which most of the thyristor phase A has elapsed. In program step 4, a counter in the bit slice with a certain value is set and started.

In program step 5, the measured output voltage in the bit slice 30 is compared with the set value. If this output voltage turns out to be too low, it is then checked in program step 6 whether the diode phase B has already started. If the latter is the case, the following thyristor pair can be ignited after a delay introduced in program step 7. The program then starts again at program step 1. If no diode current has been detected in program step 6, it is checked in program step 8 whether the counter 8103265 - 8 -

*> J

has already counted down to zero in the bit slice. If this is the case, now that a time interval corresponding to the set counter position has elapsed, the thyristor phase A is apparently not followed by the diode phase B and is restarted with program-5 step 1. As long as the counter position in the bit slice is not yet is zero, it is assumed that thyristor phase A has not yet expired and diode phase B can still occur. If it is established in program step 5 that the output voltage is too high, then the moment the thyristors 9 and 10 are brought into the conducting state 10, the short circuit must be applied simultaneously to the primary winding of the output transformer Ιό. First, however, it is checked in program step 9 whether the diode phase B has already started. If the latter is the case, then after a delay introduced in program step 10, which now depends on the desired residual voltage on the capacitor 7, the thyristor pair 9, 10 can be ignited and the short circuit applied. If no diode current has been detected in program step 9, it is checked in program step 11 whether the counter in the bit slice has counted down to zero. If this is the case, now that a time interval corresponding to the set counter position has elapsed, the thyristor phase A is apparently not followed by the diode phase B. As long as the counter position in the bit slice is not yet zero, it is assumed that the thyristor phase A is not yet has passed and the diode phase B can still start. In program step 12, the polarity of the voltage Ec is determined, and in program step 13 or 13 'the control pulse for the thyristor pair to be ignited is then applied and the short circuit is applied. After the then-running thyristor phase C, a complete diode phase D follows.

The short circuit remains until after this diode phase. For this purpose, the required delay is applied in program step 1Λ, after which the short circuit is removed in program step 15. The energy converter has now stopped supplying energy to the buffer 2. After the next energy pulse has been extracted from the buffer, it can be checked whether the output voltage 35 has fallen below the set value. If this is the case, the energy converter can be restarted and the program starts

N

8103265 * »- - 9 - again at step 1. If the output voltage is still too high, wait again until the next energy pulse has been drawn from the buffer. The comparison of the output voltage every time after energy extraction from the buffer is realized in program step 16.

The delays introduced in program steps 7 and 10 correspond to the duration of the respective diode phases. In the foregoing consideration, it has been assumed that these delays are permanently programmed. However, an improvement is obtained by making these delays dependent on the value of the input voltage; in this way, variations in the input voltage are discounted. A table must then be included in the microprocessor, which contains the required delay at various values of the input voltage.

8103265

Claims (3)

1. Energy converter, wherein energy is supplied from a source, using a series resonant circuit, at least two thyristors, antiparallel connected diodes and a bridge circuit provided with a high-frequency output transformer and an energy buffer, wherein there is furthermore a control circuit which supplies the fire pulses to bring the respective thyristors into the conducting state, characterized in that this control circuit is provided with: - an output voltage measuring circuit; diode current detectors to output signals indicating the presence of current through said diodes; - a polarity detector to determine the polarity of the voltage in the series resonant circuit; - thyristor ignition switches for generating said fire pulses and - a microprocessor, which, with the aid of a delay built-in and depending on the measured output voltage and of the current passage through said diodes, determines the times at which control signals for the polarity detector indicated thyristor triggering circuits are delivered. 2. Energy converter according to claim 1, characterized in that the control circuit is provided with a short-circuit, which, in case the energy converter is loaded in pulse form, and as soon as it has been determined in the microprocessor that the measured output voltage is greater than the value at which the energy converter is set, the primary winding of the output transformer 30 short-circuits for a time determined in the microprocessor, after which the microprocessor outputs the next control signal for the respective thyristor ignition circuit once the measured output voltage is determined to be less than the value at which the power converter is set. 8103265% - η - 3. Energy converter according to claim 1 or 2, characterized in that the control circuit is provided with an input voltage measuring circuit and the microprocessor contains memory means, in which a table is included, which for each measured input voltage contains the specific delay time which determines the fixed -set the ignition time of the thyristors. 8103265