WO1989005057A1

WO1989005057A1 - High frequency rectifier with sinusshaped primary current

Info

Publication number: WO1989005057A1
Application number: PCT/SE1988/000612
Authority: WO
Inventors: Alfred Lyne; Tadeus Wolpert
Original assignee: Telefonaktiebolaget L M Ericsson
Priority date: 1987-11-20
Filing date: 1988-11-15
Publication date: 1989-06-01
Also published as: SE8704605L; SE460007B; JPH02502240A; GB8915037D0; FI893386A; FI893386A0; GB2219447A; SE8704605D0

Abstract

A high-frequency rectifying apparatus, which rectifies an incoming AC voltage (Un) from a mains supply to an outgoing DC voltage (Uo) at a given level. The apparatus contains primary a power circuit with a rectifier bridge (DB) across its input, and a power transistor (TR) as switching element. In accordance with the invention, the primary power circuit comprises only one magnetically coupled inductor (TF) with two windings (k1 and k2), where the primary winding (k1) is connected in the primary power circuit to the power transistor (TR) and the secondary winding (k2) is connected to a secondary power circuit. The power transistor (TR) is then controlled as that the rectifier works as a so-called fly-back rectifier, i.e. during the interval when the transistor (TR) conducts, magnetic energy is stored by means of the primary current i1? and during the time interval when the transistor (TR) is blocked the inductor (TF) will be discharged across the secondary power circuit. The rectifier apparatus furthermore contains two regulation means R1 and R2 forming a regulation system for converting the rectified input voltage to the desired output direct voltage (Uo).

Description

High frequency rectifier with sinusshaped primary current .

TECHNICAL FIELD

The present invention relates to high-frequency rectifying apparatus according to the preamble to claim 1, for rectifying an incoming AC voltage from a mains, e.g. 220V AC, to a desired DC voltage, e.g. 50V DC. The apparatus in accordance with the invention is primarily for use in supplying power to telephone exchanges.

BACKGROUND ART

Switch-mode rectifers with high-frequency rectifying are obtaining wider and wider use in the power supply of telecommunication plant and other appli¬ cations. In the present context, high-frequency rectifying signifies that the rectifier includes a chopping rectifier circuit operating at a substantially higher frequency than that of the mains supply. The salient advantages of this rectifier type, compared with low-frequency rectifiers, e.g. of the transistor type, are small, light components, noiseless operation and rapid regulation.

A usual solution for this rectifier type is given in D. 3. Becker, "A 3000 W High- Frequency, Single Phase, Switch-Mode Type, Telecommunications Battery Charger", Proceedings of the INTELEC 1986, p. 81. The supply voltage is rectified in a diode bridge and smoothed by a capacitor. The rectified mains voltage is converted to an output DC voltage in a DC converter at high switching frequency (usually 20-100kHz) by the use of rapid semiconductor switches and magnet components adapted to this frequency.

A disadvantage with the described solution is that the rectifier takes heaviiy distorted AC power from the mains. The effect factor will be poor, and the rectifier current will contain some overtones that create troublesome disturbances in the supply. Rectifiers with heavily distorted primary current also cause difficulties in supply from standby powerpiants (diesel engines and generators), e.g. instability and voltage distortion. The drawbacks described above have led to certain configurations now containing means which result in sinusoidal primary current. The technique for such active conversion of the current wave shape is known from such as F. E. Spooner, "Switch Mode Power Supplies with Idealised Performance", Proceedings of the INTELEC, 1986, p. 89. The idea here is that a special regulation system operating at high frequency affects the instant value of the current many times during a half period of the mains supply.

In this case two rectifying steps, each with its own semiconductor switch and regulating circuit are used. A pre-regulator affects the current wave form, while a converter converts its input voltage to an output voltage isolated from the mains voltage. The converter's regulation circuit keep the output voltage or current constant, depending on the operation mode.

The switching frequency in the pre-regulator and in the converter can be different, but both steps operate at high frequency.

Such a rectifier takes up sinusoidal current from the mains in phase with the mains voltage. Seen from the mains input, the rectifier can be regarded as a resistive load. The rectifier constitutes a perfect solution to the demands placed on it, but its implementation is relatively expensive, complicated and voluminous.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide high-frequency rectifying apparatus giving a solution to all the above-mentioned problems, i.e. a) conversion of energy at high frequency to a suitable output voltage isolated from the mains voltage; b) constant voltage or current regulation on the output; c) active influence on the primary current wave to give it sinus shape in phase with the mains voltage.

The rectifier in accordance with the invention is therefore given the distinguishing features disclosed in the characterizing portion of claim 1. BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in more detail and with reference to the accompanying drawings, where

Figure 1 is a block diagram of the inventive rectifying apparatus. Figure 2 is a time chart over control pulses supplied to a switching element in the inventive rectifier.

Figure 3 is a time chart over the primary current in the inventive rectifier; Figure 4 is a time chart over the secondary current in the inventive rectifier; and

Figure 5 is a time chart of the primary current and the secondary currents of the inventive rectifier according to the invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The high-frequency rectifying apparatus in accordance with the invention and Figure 1 contains a rectifier bridge DB, which full-wave rectifies an incoming mains voltage Un, e.g. a sinusoidal AC voltage at 220V and frequency 50 Hz. The mains voltage does not need to be completely sinusoidal, and can be distorted as well as containing overtones, according to what has been described above.

The rectifier bridge DB is connected to a primary power circuit containing the primary winding k, of a inductor TF, a controllable switching element comprising a MOSFET transistor TR and a low ohmic resistor R as current- sensing means.

Some other kind of current sensing can also be chosen, e.g. a current trans¬ former.

The apparatus in Figure 1 further comprises a secondary power circuit con¬ taining the secondary winding k„ of the inductor TF, a diode D2 and a smoothing filter comprising a capacitor C2 and inductance LI. A further capacitor C3 can be optionally connected across the power circuit output, which forms the output of the apparatus, where the desired DC voltage Uo occurs. In accordance with the invention, the rectifying apparatus thus contains- .a primary power circuit comprising only one magnetically coupled inductor having two windings kl and k2, where the primary winding kl is connected in the primary power circuit to the controllable switching element TR and the secondary winding k2 is connected to the secondary power circuit. The switching element TR is then controlled so that the rectifier works as a so called fly-back rectifier, i.e. during the interval when the transistor TR conducts, magnetic energy is stored by means of the primary current i-, and during the time interval when the switching element is blocked the inductor TF will be discharged across the secondary power circuit. This will be described in more detail in connection with Figures 2-5.

The rectifier apparatus furthermore contains two regulation means Rl and R2 forming a regulation system for converting the rectified input voltage to the desired output direct voltage Uo.

The regulation means Rl comprises a comparator JFl for comparing the output voltage Uo with a reference voltage Ur, and a conversion circuit G for galvanlcally isolating the output voltage Ulr from the comparator JFl to the other regulation means R2. The conversion circuit G can be an optoswitch of a known kind, for example.

The other regulation means R2 comprises a multiplier M, a comparator JF2 and a controllable pulse oscillator PO, which sends control pulses to the transistor TR. These control pulses have a variable duty cycle with a fixed frequency fh, which is much greater than the mains frequency, e.g. in the order of magnitude 20 kHz. The pulse oscillator PO may comprise an oscillator sending a fixed frequency, a multivibrator circuit clocked by the oscillator and zeroed by the output signal from the comparator JF2 and a drive circuit. It is also possible to have a so-called VCO, i.e. a controllable oscillator with its frequency controlled by the output signal from the comparator JF2. In the case described below, the frequency fh of sent pulses is constant, while the pulse width varies in response to the output signal from the comparator JF2.

The function of the apparatus according to Figure 1 is as follows. The comparator JFl in the first regulation means Rl sends a signal Ulr giving the difference between the output voltage Uo and the reference voltage Ur, where U2r is principally the same as Ulr, but galvanically separated from the output circuit.

The multiplier M receives both the full wave rectified mains voltage and the voltage U2r, which indicates the difference between Uo and Ur. The multiplier thus sends a magnitude Urs, which is the same as Us, but where the amplitude varies in response to the variation in U2r.

The full wave rectified mains voltage Us is used in the embodiment of the regulation means R2 illustrated here. It is also possible to form Us and then Urs with the aid of a digital sinus generator, which is synchronized with the mains voltage, e.g. with its zeros.

The comparator JF2 forms the difference between the sensed value of the primary 'current i, represented by the value UR = R x i , and the magnitude Urs. In the embodiment described here, the pulse oscillator PO sends control pulses to the transistor TR, which has constant frequency fr, but whose duty-cycle is responsive to the value of the output magnitude U from the comparator JF2. The magnitude U^ can either be an analogue or a digital voltage. In Figure 2 the control pulses from the pulse oscillator PO are shown. At t = 0 it is assumed that Urs > UR, and the transistor TR is turned on. When i, has increased to a given value at t = t-, , Urs = UR and the control pulse is at zero, resulting in that the transistor TR ceases to conduct and i_j = 0. The voltage drop UR will be equal to zero during the time t-, -T until the transistor TR once again conducts, since the pulse oscillator sends the control pulses at a fixed frequency. The primary current i-, will thus be "switched on" with a given frequency fh, and "switched off" at a varying phase angle of the control pulses, see Figure 3. The primary current i-, will thus follow the shape of Urs. However, the magnitude Urs is the full wave rectified sinusoidal voltage Us multiplied by a magnitude U2r denoting the deviation of the output voltage Uo from the reference voltage Ur. The pulse pattern according to Figure 3 is symmetrical from the crest of the sinus curve towards the zeros. Let it be now assumed that the output voltage Uo falls, e.g. due to the load increasing. The actual current i, cannot build up to the right Uo, and that magnitudes Ulr and U2r increase. This change causes the magnitude Urs to have an increased value. In turn, after the comparison with U_R, this gives an increased value to U_f for each amplitude value at the times T, 2T, 3T, ... . Since the frequency fh is much greater than the mains frequency (f. > 400 times the mains freuqency) there is a very rapid regulation of the primary current i-, so that it always follows the wave form of the voltage U simultaneously as a constant value of U is maintained.

The appearance of the secondary current i₂ is shown in Figure 4. As above mentioned, the conversion stage operates as a fly-back rectifier, i.e. when the transistor TR conducts and the primary current i-, flows, there is a magnetizing of the inductor TF via the primary winding k-, (Figure 3). When the transistor TR is blocked, the inductor TF is demagnetized via the secondary winding k_z, and a secondary current i₂ flows through the output circuit, see Figure 4. As in Figure 3, the pulse pattern is symmetrical from the crest of the sinus wave towards the zero passes. Regulation of the output voltage Uo thus takes place by the primary current i- being controlled to the right value (= reference value Urs), which is the so-called current mode control.

Fig 5 shows a time chart common to both voltages appearing across the windings kl and k2 during the time intervals when these are conducting current and during a semiperiαd of the feeding sinusoidal alternating voltage U .

At t = 0, t = T, t = 2T and so on the transistor is controlled to conducting state by the control pulses according to Figure 2 and at the times t-. , t_2> — the transistor TR will be blocked. The function of the primary side and the secondary side will then be as follows during the different time intervals:

0 - t- . The winding k-, conducts current, starting a voltage built up across the winding k, . The voltage drop across the winding k₂ is of such polarity that the diode D2 is blocked and no current flows through this diode (i = 0). The winding k, will be magnetized. t-. - T. The primary current i, is interrupted by the transistor TR. This means that the voltage drop across the secondary winding k₂ is reversed and momentarily grows to a certain (negative) value so that the diode D2 will be conducting. The current i₂ charges the capacitor C2 to a certain voltage. This is mainly the output voltage U as the resistance of the inductance LI is small.

______-> The transistor TR conducts again, which means that the voltage drop across the winding k_? is reversed and the diode D2 is blocked meaning that i„ =

0. At the same time the voltage is built up over the winding kl but now from a higher starting position depending on the surpressed sinusoidal voltage U s .

During the following time intervals during the half-wave of U there is an alternating reversion of the voltage drop of the windings kl k2 according to the above. The voltage drop over the winding kl is kept on a constant (negative) value during the intervals (t, -T, t₂ -2T and so on) when this winding is conducting. The output voltage U is mainly the voltage of the charged capacitor C2, being mainly constant.

The voltage-time-area of the secondary winding voltage depends on the time t, , t₂ i.e. on the duty-cycle. As the voltage-time areas of the two voltages of the windings are equal also the secondary voltage depends on the duty cycle. The inductor TF thus gives a voltage ratio varying timely with the variations of the duty cycle of the control pulses according to Fig 2. The function of the transformer is a sinus modulated fly-back and is consequently not the same as a transformer function of a converter.

The inner feedback loop in the other regulation means R2, which regulates the value of the primary current i, , operates with great rapidity adapted to the frequency f, , while the outer feedback loop in the first regulation means R-, n which switches the output voltage Uo, operates more slowly, although sufficiently rapid to ensure good regulation of the output voltage.

Claims

C L A I M S

High-frequency rectifying apparatus for rectifying an incoming AC voltage (Un) to an outgoing DC voltage (Uo) at a given level, comprising a) rectifying devices (DB) for the incoming AC (Un) to generate a rectified DC voltage (Us), b) an incoming power circuit comprising a controllable switching element (TR) as well as a sensing means (R) for the current through the circuit, c) an outgoing power circuit over the output of which said DC voltage (Uo) is obtained, comprising a rectifier (D2) and a capacitor (C2), d) means for magnetic coupling between said incoming and outgoing power circuit, e) a first and a second regulation means (Rl and R2 respectively) to form a control quantity for controlling said switching element (TR) in dependence on the sensed output DC voltage (Uo) when comparing this with a reference voltage (Ur) and in dependence on the sensed current when comparing this with a value (Urs) proportional to the instantaneous value of the DC current (Us), and f) a controllable pulse oscillator (PO), which to the controllable switching element (TR) emits pulses with a determined, in comparison to the mains frequency high pulse frequency (fh) with a variable duty cycle depending on said control quantity (Uf), characterized in that said magnetic coupling means comprises an inductor (TF) having two magnetically coupled windings (kl, k2) where one of the windings (kl) is part of the incoming and the other winding (k2) is part of the outgoing power circuit, one winding being in series with the switching element (TR) in the incoming power circuit and the other winding being in series with said rectifier (D2) and capacitor (C2), one of the windings (kl) being magnetized so that a pulse shaped voltage modulated with said rectified AC current (Us) is generated during the time (0 - t, , T - t₂ and so on) the switching element (TR) is conducting and the other winding (k2) is magnetized so that a pulse-shaped voltage with a constant amplitude is generated during the time (t, - T, t„ - 2T, and so on) when the switching element (TR) is blocked.