MXPA01005380A - A pulse width modulation power converter - Google Patents

Abstract

This invention concerns electrical power conversion by utilising pulse width modulation with several carriers (22). The invention can be used with either analog or digital input (21). Numerous advantages can be obtained by intelligent control of the phase shift&THgr;p between the carriers. Hereby a reduction of the high frequency content from the carriers is obtained. Besides of this a common mode free output will be present for special cases (e.g. N=4 and&THgr;p=&pgr;). Finally a reduction of the current in the switching devices increases the power handling capabilities.

Description

ENERGY CONVERTER BY MODULATION OF PULSE WIDTH DESCRIPTION OF THE INVENTION This invention relates to the conversion of electric power using pulse width modulation (PWM). The power converters of the switching mode are frequently implemented by means of a normal type H bridge. This can be controlled either by a class AD modulator or BD class, for example as described in the Journal of the Audio Engineering Society, July / August 1997, pp. 554-570. The AD class is characterized by simple implementation and as an advantage has a common mode free output. But, on the other hand, it has some disadvantages: • The first harmonic of the carrier and the intermodulation components are present. • At high values of energy, the current in the individual switching device becomes too high and the implementation of the H-type bridge is not attractive for long. The BD class solves the problem with the first harmonic of the carrier and the products of intermodulation, since only high frequency components are present above the second harmonic. On the other hand, a signal is now present at the output Ref: 129029 common. The energy management capabilities for the BD class have the same limitations as the class AD, since this also uses a type H bridge. It is also known to convert the power by means of PWM and known improvements in the literature are known as PWM of multi-level, such as G. Carrara, S. Gardella, M. Marchesoni, R. Salutari & G. Sciutto, "A New Multilevel PWM Method - a Theoretical Analysis", IEEE Transactions of Power Electronics, Vol 7, No. 3 (July 1992). The base in general is a switching power stage with multiple voltage levels of power supply and often a complex control circuitry. However, each switch is still capable of handling the full load current, and additionally, the general disadvantage of these methods is the severe distortion introduced by the diodes in the signal path. This is detrimental, for example, to electric motor units because the distortion causes audible magnetostriction noise and a residual DC component can cause saturation. In audio amplifiers, distortion is obviously not tolerated. Therefore, there is a need for a different approach, which avoids the above disadvantages. More particularly, it is the purpose of the present invention: • To reduce the high frequency content for the differential output, • Obtain a common mode free output, • Reduce the current in the individual switching device. This is obtained in the method according to the invention, which is particular since N / 2 PWM generators are connected in parallel on each side of a load, using a total of N PWM generators. The modulation of each PWM generator is obtained by N / 2 versions shifted in phase of the carrier. The N / 2 carriers are equally distributed over 180 ° or 360 °. The conversion is performed in a scalable structure of two or more half bridges, each controlled by individually generated PWM signals. The PWM signals are generated from a comparison between a reference signal, which can be either analogue or digital and the inverted reference signal with several versions shifted in phase of a carrier. The generated signals corresponding to the reference signal are used on one side of the load and the PWM signals corresponding to the inverted reference signal are used on the other side of the load. The outputs of the half-bridges are added together for each side of the load. Several advantages can be obtained by intelligently controlling the phase shift between the carriers. The invention can be used in many applications, such as audio amplifiers, motor control, DC-DC converters, etc. The invention will be designated Pulse Width Modulation with Carrier Displaced in Phase, Balanced (BPSCPWM). The mathematical expression for the phase shift between the carriers is given by: 2p 1)? N = -, according to claim 2. "N 4tt 2)?" = -, according to claim 3. "N The N / 2 PWM signals are created by a comparison between the carriers and the reference signal.

These are added together on one side of the load. On the other side of the load, the carriers are compared to the inverted reference signal and added together. The advantage of using N / 2 carriers and N half-bridges are: • The current in the load is evenly divided between the half-bridges on one side of the load. This will lead to a reduced effort of the individual switching device. • The high frequency content will be reduced for both phase shift choices,? P. 2p • For? = -, the high frequency content p N for differential mode it will start around N '• / -. , with the corresponding intermodulation products. fs is the carrier frequency. The common mode high frequency content will start around the first harmonic of the carrier frequency. Ap • For? = -, the high frequency content "N for differential mode will start around N - fs for N - f N / 2 odd, and around - for N / 2 par. odd, the common mode spectrum will start around N - f -, and for N / 2 pair there will be no common mode in the output.

A possible implementation of the invention is explained in the following. N PWM signals are created as described above. The comparators carry out the comparison between the carriers and the reference signal. The simple half-bridges make the amplification of the PWM signals from the comparators. The sums of the amplified versions of the PWM signals are carried out by means of inductors, such that each PWM generator is connected to the output via an inductor. The inductors offer a filtering, but an additional filtering is possible and advisable. In the case where N is equal to 4 and? P = p, a version implemented in an easy way is obtained, by using comparators with inverted and non-inverted outputs.

In this case it is possible to use only one carrier and two comparators. The outputs of the two comparators, in this case should not be connected as explained in the general description, but rather an inverted and non-inverted output is used on both sides of the load, and the two outputs should be taken from each comparator . Ap For the case where N / 2 is an even number and? = -, there is an especially simplified implementation for addition inductors. The two inductors that come from each side of the load share an inductor core. The PWM signal on one side shares the inductor core with the inverted version of the PWM signal on the other side of the load. In an implementation of a BPSCPWM system, the decision of how many media bridges to use and the decision of which one to use depends on the actual requirements. Several factors can 'affect the decision: • The desired high frequency content in the common mode spectrum. The high frequency content will be reduced with increasing N. • The high frequency content in the differential mode spectrum. The high frequency content will be reduced with increasing N. • The current in the switching devices. The current in the switching device individual will be reduced with increasing N. • The complexity of the modulator is increased by N. As the carrier, signals are possible both on the individual side and on the double side or a combination of the two classes. The invention requires that carriers be DC-free for optimal performance, and therefore, it is advisable to conduct a pass-through filtering of the carriers before the comparators. The PWM signals can be generated digitally for example in a DSP. A control system can be implemented to improve the linearity of the system. Using global feedback could do it. You can also implement local feedback around each half bridge. The two proposed control systems can be combined. The topology of the local feedback is an obvious possibility for a system that uses digital input, since the global, normal feedback requires an A / D converter in the feedback path. The principle of the invention is independent of the use of the PWM signal and is independent of the contribution of the amplifier stage or stages of power or power.

BRIEF DESCRIPTION OF THE. INVENTION OF THE FIGURES The invention will be described in the following with reference to the figures: Figure 1 shows the general structure of the modulator for the invention. • Figure 2 shows the general modulator structure together with a possible embodiment of the energy stage. • Figure 3 shows a particular Ap mode of the modulator in the case where N / 2 is even and? = - with a Simplified energy stage. • Figure 4 shows a particular mode of the modulator in the case where N = 4 and? P = p with a simplified energy stage. • Figure 5 shows possible carrier signals that are useful in the invention. • Figure 6 shows an example of a control system with both global feedback and feedback before filtering, applied to a system with analog input. • Figure 7 shows a system with local feedback for each half bridge. • Figures 8-11 show the characteristics of the time domain for the invention in the case of N = 4.

DETAILED DESCRIPTION OF THE FIGURES Figure 1 shows the general structure of the modulator for a balanced embodiment of the invention.

The input Vr (t) to the system (11) is either analog or digital. An inversion of the reference signal is performed (12) The carrier Vc.? (J) (13) is the basis for generating the other displaced versions in phase of the carrier, since these are displaced in phase by the angle? P (14) between one carrier and the next. The reference signal is compared (15) with the generated carriers, and generates N / 2 PWM signals, which are added together (16). The inverted version of the reference signal is also compared with carriers and the N / 2 generated PWM signals are also added together. The load (17) is placed between the two sums of the PWM signals. Figure 2 shows a possible general embodiment of the balanced solution. The reference signal (21) and the carriers (22) are compared (23) producing PWM signals as described above. The generated PWM signals control the bridge means (24), which cause an amplification of the PWM signals, because the high state voltage of the PWM signal is now equal to the supply voltage Vcc. The sum of the PWM signals is done by means of inductors (25). Inductors also filter the summed PWM signal, resulting, but additional filtering may be necessary. The addition of a capacitor (26) in parallel with the load (27) facilitates this. This implements effective filtering of the second order of the output signal. In the case that more filtering is needed, a filter of differential or common, additional mode could be applied. Figure 3 shows an alternative mode for N / 2 Ap pair and? P -. As shown, only N / 4 is required carriers in this case. The method requires N / 2 comparators with two outputs, not inverted and inverted. The non-inverted PWM signals generated from the reference signal (31) together with the inverted PWM signals generated from the inverted reference signal (32) are used on the load side, while the remainder is used in the other side of the load. The sum inductors that correspond to the amplified PWM signals coming from the same comparator share the same inductor core (33), since the PWM signals are inverted versions. Figure 4 shows a system modality described above. For this case N = 4 and? = p. As shown, only two comparators and one carrier are needed. Additionally, only two inductor cores are used for the four inductors L. Figure 5 shows examples of three classes different carrier signals that can be used in the invention. The first two classes are sawtooth while the bottom one is a triangular wave. In general, any periodic signal is suitable for the carrier. But, it should be emphasized that the nice high-frequency characteristics for the differential mode and the common mode are only obtained for the two-sided carrier. Figure 6 shows a general control for the system. The figure shows the possibility for the feedback from the exit (61) and the feedback before the exit (62), which will be a feedback of the PWM signals. Figure 7 shows a local control system for the individual bridge half. This can be combined with the control system of Figure 6. The non-amplified PWM signal (71) is used as a reference for a comparison with the feedback of the amplified version of the PWM signal (72). This will produce an error signal (73), which is used for the correction. Figures 8-11 show the resulting characteristics of the time domain for the invention for the case for N =. In the 4 figures the upper graph shows the modulation signal and the interleaved carriers, the center graph shows the resulting differential output signal and the background graph shows the signal in mode common, resulting. Figure 8 and Figure 10 show single-sided modulated waveforms, and Figure 9 and Figure 11 show modulated, double-sided waveforms. The latter also show that where the two carriers are equally distributed over 360 ° and the carriers are double-sided, there is no common mode signal at the output. It is noted that in relation to this date, the best method known by the applicant to carry out the present invention is that which is clear from the present description of the invention.

Claims (11)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A device for the conversion of electrical energy of a reference signal, comprising a plurality of pulse width modulators to compare the reference with a carrier signal and generate N modulated signals, modulators that are arranged in two groups, each connected to one side of a load, and a means to amplify and sum the modulated signals of each group to drive the load, characterized in that the signal carrier moves in phase in at least N / 4 carrier signals shifted in phase, each one connected to a first input of a modulator in each group, a second input of each modulator in the first modulator group is connected in parallel to the reference signal, and a second input of each modulator in the second modulator group is connected in parallel to an invert reference signal rtida.
2. 2. A device in accordance with the claim 1, characterized in that the carrier signal moves in phase in N / 2 in carrier signals shifted in phase connected to N / 2 modulators in each group.
3. 3. A device according to claim 2, characterized in that a phase shift (? P) between the carriers shifted in phase is 2p equal to -, so that carriers shifted in phase they are distributed evenly over 180 °.
4. 4. A device according to claim 2, characterized in that a phase shift. { ? ) among the carriers displaced in phase is Ap equal to -, so that the carriers displaced in phase they are evenly distributed over 360 °.
5. 5. A device according to claim 1, characterized in that the carrier signal is shifted in phase in N / 4 carrier signals shifted in phase connected to N / 4 modulators in each group, and where each modulator has an inverted output, additional.
6. 6. A device according to any of the preceding claims, characterized in that each carrier signal is a sawtooth, a triangular wave a combination of a sawtooth and a triangular wave.
7. 7. A device in accordance with any of the previous claims, characterized in that each carrier is digitally generated and the modulators comprise digital comparators.
8. 8. A device according to any of the preceding claims, characterized in that the means for amplifying and adding the modulated signals each comprises a bridge means and an inductor.
9. 9. A device according to claim 8, characterized in that the inverted and non-inverted amplified outputs are connected to inductors that share the same inductor core.
10. 10. A device according to claim 8, characterized in that a control system is applied in each bridge means.
11. 11. A method for converting electrical energy using a plurality of pulse width modulators on each side of a load, characterized in that each modulation signal is prod by comparing carrier signals shifted in phase with a reference signal on one side of the load and with the reference signal inverted on the opposite side of the load. P l hJ 20 \\ S 2 16 SUMMARY OF THE INVENTION This invention relates to the conversion of electrical energy by using pulse width modulation with several carriers (22). The invention can be used with an analog or digital input (21). Numerous advantages can be obtained by intelligent control of the phase shift? P between the carriers. From this mode, a reduction of the high frequency content of the carriers is obtained. In addition to this, a common mode free output will be present for special cases (for example N = 4 and? P = p). Finally, a reduction in the current in the switching devices increases the power management capabilities.