ES2536153B1 - Control of the average value of the current flowing through a coil, a transistor and a diode in electronic power converters modulated by pulse width - Google Patents

Abstract

A control technique is presented that regulates the average value of the current flowing through a coil, a transistor and a diode belonging to the power stage of a switched, non-resonant converter, modulated by pulse width. It allows to regulate the average value of any of the currents indicated by measuring the current flowing through the coil, the transistor or the diode. The time it takes to correct a disturbance is the minimum possible in several cases. The correction of the disturbances does not show excess. The proposed control technique is applicable to DC / DC, AC / DC converters, inverters, power factor correction circuits, motor control, etc.

Description

Control of the average value of the current flowing through a coil, a transistor and a diode in electronic power converters modulated by pulse width.

DESCRIPTION

 5

 Control of the average value of the current by a coil, by a transistor and by a diode in electronic power converters modulated by pulse width.

Technical sector

 10

 The present invention falls within the field of electronic pulse width modulated power converters ("PWM converters: pulse width modulation converters") and more specifically with the regulation of the average value of the linear current into pieces that circulates through coils, by transistors and diodes.

State of the art 15

 The control of the average value of the current flowing through a coil, a transistor or a diode belonging to the power stage of a switched converter, modulated by pulse width, has multiple applications in the field of Power Electronics as , for example, in DC / DC converters, AC / DC converters, power factor correction circuits, inverters, motor control, etc. twenty

 Since they began using switched converters until today, various control techniques have been published to regulate this type of currents. Among the many techniques proposed, the following should be noted:

 _ Peak current mode control

 25

 _ Control in average current mode (“Average current mode control”)

 _ Hysteretic band control (“Hysteretic current mode control: U.S. Patent 4456872”)

 _ Constant off-time current mode control (30)

 _ Control with constant on-time current mode control

 The basic characteristics of the previous techniques can be consulted, among others, in the following references: 35

Richard Redl, Nathan O. Sokal, “Current-mode control, five different types, used with the three basic classes of power converters: small-signal ac and large-signal dc characterization, stability requirements, and implementation of practical circuits”. PESC’85 Record (IEEE Catalog no. 85CH2117-0), pp. 771-785

 40

Richard Redl, Istvan Novak, "Instabilities in current-mode controlled switching voltage regulators". PESC’81 Record (IEEE Publication 81CH1652-7), pp. 17-28, 1981 IEEE.

Lloyd Dixon, Average current mode control of switching power supplies. Application Note U-140, Unitrode Integrated Circuits.

 All the techniques published so far have one or several disadvantages in relation to the speed with which a disturbance is corrected, the error in permanent regime, the complexity of the design of the regulator (control stage 5), the number of components required by the implementation of the technique, its cost, etc.

 The control technique proposed in the present invention stands out against the techniques published so far in the following:

 10

_ Minimizes the time it takes to correct a disturbance. In several cases, this time is the shortest possible.

_ Presents a null error in permanent regime.

 fifteen

_ The response of the controlled current to a step in the reference signal does not exceed.

_ It is not necessary to have any knowledge of Automatic Regulation to design the proposed control system, although it is a non-linear control.

 twenty

_ The implementation of the proposed technique requires very few components, all of them low cost.

_ It can be used to regulate the average value of the current flowing through a coil, a transistor or a diode belonging to the power stage of a switched converter, which operates in continuous driving mode. It can be used in converters with synchronous rectification. 25

_ It is not necessary to measure the same current that is regulated. With this technique, the average value of the current flowing through a coil, a transistor and a diode can be regulated by measuring a current proportional to the current flowing through any of these components. Thus, for example, the average value of the current flowing through a coil can be controlled by measuring only the current flowing through a diode through which the same current flowing through the coil (or a proportional value) circulates during the intervals of time in which the coil yields part of the energy it has stored.

Description of the invention

 35

 The present invention applies to electronic power converters characterized by the following:

_ The transistor or transistors of the power stage function as switches. That is, they are saturated or in court, constantly changing from one state to another at high frequency. During the time intervals in which a transistor is saturated, the current flowing through it varies linearly over time, said current being equal to or proportional to the current flowing through the coil.

_ The current flowing through the coil is linear in pieces and current always flows through the coil. That is, the converter operates in continuous driving mode. The invention is applicable to converters with synchronous rectification.

_ Both the input voltage and the output voltage of the converter are either constant or vary 5 with a frequency much lower than the switching frequency of the transistor (or transistors) of the power stage.

_ The voltage drop in the capacitors belonging to the power stage of the converter varies with a frequency much less than the switching frequency of the transistor (or transistors) of the power stage.

 In order to facilitate the understanding of the proposed control system and without limiting its scope to non-isolated DC / DC converters, the following paragraphs will refer to a reducing DC / DC converter (“buck”) Non-insulated represented in Figure 1. The currents that circulate in permanent regime 15 through the coil, through the transistor and through the diode of this converter are represented in Figures 2, 3 and 4 respectively.

 In relation to Figures 1-4 and a pulse width modulated converter in general, the following terms will be used in this patent:

Id: average value of the current flowing through the diode in permanent regime.

IL: average value of the current flowing through the coil in permanent regime.

 25

iLpico: peak value of the current flowing through the coil in permanent mode.

iLvalle: valley value of the current flowing through the coil in permanent regime.

IT: average value of the current flowing through the transistor in permanent regime. 30

: relationship between the reference voltage of the internal control system (RsIL) and the reference voltage of the external control (vref). sLrefkRIv

L: coil inductance 35

Q: transistor trigger signal. A high voltage level in this signal indicates that the transistor is saturated (the energy stored by the coil grows linearly over time), while a low voltage level indicates that the transistor is cut (the energy stored by the coil decreases linearly with time).

 40

R: Zero reset input of the RS flip-flop used to implement the internal control logic.

Rs: current sensor gain (its value is assumed to be constant).

Rs · id (t): voltage provided by the sensor that measures the current flowing through the diode

Rs · iL (t): voltage provided by the sensor that measures the current flowing through the coil 5

Rs · iT (t): voltage provided by the sensor that measures the current flowing through the transistor

RsIL: reference voltage of the internal control.

 10

S: one-set input of the RS flip-flop that is used to implement the internal control logic.

toff: time during which the transistor is cut in each switching cycle. Depending on the internal control used, this time may be constant or variable with the operating conditions of the converter. fifteen

TOFF: constant time during which the transistor is cut in each switching cycle. In this case it is true that toff = TOFF = cte.

toff min: minimum time during which the transistor is cut in each switching cycle, when the current sensor 20 measures the current flowing through the diode.

ton: time during which the transistor is saturated in each switching cycle. Depending on the internal control used, this time may be constant or variable with the operating conditions of the converter. 25

Ton: constant time during which the transistor is saturated in each switching cycle. In this case it is true that ton = Ton = cte.

ton min: minimum time during which the transistor is saturated in each switching cycle when the current sensor 30 measures the current flowing through the transistor.

ts = ton + toff ≡ duration of 1 transistor switching cycle. If an internal control is used that causes the transistor to remain saturated for a constant time in each switching cycle, then it is true that: ts = Ton + toff .. If the internal control used causes the transistor to remain in cut for a constant time in every 35 switching cycle, then it is true that: ts = ton + Toff.

vc: voltage generated by the internal control. In each switching cycle, this voltage is compared with the voltage provided by the current sensor to determine the moment at which the transistor becomes saturated (remaining in that state for a time ton = Ton = cte) or it is cut ( remaining in said state 40 a time toff = Toff = cte).

vref: external control reference voltage. It establishes the average value, in permanent regime, of the current to be regulated. Its value can be equal to RsIL, RsIT or RsId.

vL (t): voltage drop in the coil (see Figure 1). VL (t) is considered positive when the coil increases the energy it stores. 5

vL-off: value of the voltage drop in the coil [vL (t)] during the time intervals in which the coil yields part of the energy it has stored (vL-off <0). In the converter of Figure 1 corresponds to the case in which the transistor is cut.

 10

vL-on: value of the voltage drop in the coil [vL (t)] during the time intervals in which the coil increases the energy it stores (vL-on> 0). In the converter of Figure 1 it corresponds to the case in which the transistor is saturated.

α: signal present at the output of the redisparable monostable that timing the constant time that transistor 15 is saturated (Ton) or that is cut (Toff), in each switching cycle.

β: signal present at the monostable output that timing the minimum conduction time (ton min) or the minimum cut-off time (toff min) of the transistor, depending on whether the current sensor measures the current flowing through the transistor or measure the current flowing through the diode. twenty

 The control technique proposed in this patent can be divided into two parts, corresponding to an internal control and an external control nested.

 Internal control: 25

 The task of internal control is to make the average value of the current flowing through the coil follow the reference voltage (RsIL) generated by the external control. To achieve this there are 2 options:

 1st option: Use an internal control that causes the transistor to remain saturated for a constant time (ton = 30 Ton = cte) in each switching cycle (see Figure 1). The start of said time interval (Ton) is determined by the moment at which the voltage generated by the current sensor equals a control voltage (vc) defined as follows (see Figure 5):

                                                                                                                         (1) 35 2soncsLLonRTvRIvL

 The calculation of vc is very simple if during the toff interval of each switching period the value (vL-on) of the voltage drop in the coil can be measured or predicted during the following interval ton = Ton = cte (see Figure 5 ). Since the value of the term Rs · Ton / 2L is constant and known in advance and RsIL is the reference voltage of the internal control. 40

 It is necessary to mention that there are converters (topologies) in which the value (vL-on) of the voltage drop in the coil during the ton intervals is known at all times (in Figure 1, vL-on = vi  vo) , while there are other converters (topologies) in which said value is only known during the ton intervals of each switching period. In these cases, what can be done is to measure its value during the ton interval of each switching period and save it to generate the vc value during the next toff interval. 5

 At each switching cycle, at the end of the interval ton = Ton = cte the transistor is cut and maintained in this state while the voltage provided by the sensor that measures the current flowing through the coil [RsiL (t)] or by the diode [Rsid (t)] is greater than the control voltage vc (see Figures 5, 8, 14). At the moment when both voltages are equalized, the transistor is saturated and a new timing of Ton seconds is started. At the end of said timing, the transistor is cut again, thus initiating a new switching cycle.

 In the event that the sensor measures the current flowing through the diode, it is necessary to ensure that the transistor remains in cutoff for a minimum time (toff-min) in each switching cycle (see Figure 14). Since in this way, regardless of the disturbance that occurs in the operating conditions of the converter, the internal control will always know the value of the current flowing through the coil every Ton seconds. This allows you to regulate the value of the current flowing through the coil during the transient states. The duration of the minimum time that the transistor is in cut (toff-min) must be sufficient so that during it the value of the current flowing through the diode [Rsid (t)] can be measured and compared with the value of vc . twenty

 2nd option: Use an internal control that causes the transistor to remain in cut for a constant time (toff = Toff = cte) in each switching cycle. The start of this time interval (Toff) is determined by the moment at which the voltage generated by the current sensor equals the control voltage (vc) defined as follows (see Figure 6): 25

                                                                             (2) 22soffsoffcsLLoffsLLoffRTRTvRIvRIvLL

 The calculation of vc is very simple if during the ton interval of each switching period the value vL-off  of the voltage drop in the coil can be measured or predicted during the next toff interval. Since the value of the term Rs · Toff / 2L is constant and known in advance and RsIL is the reference voltage of the internal loop.

 There are converters (topologies) in which the value of the voltage drop in the coil during the toff intervals is known at all times (in Figure 1, vL-off = vo), while there are other converters (topologies) in which said voltage drop is only known during the toff intervals of each switching period. In these cases, what can be done is to measure its value during the toff interval of each switching period and save it to generate the value of vc during the next ton interval.

 At each switching cycle, at the end of the interval toff = Toff = cte the transistor is saturated and maintained in this state while the voltage provided by the sensor that measures the current flowing through the coil [RsiL (t)] or 40 times the transistor [RsiT (t)] is less than vc (see Figures 6, 10 and 12). At the moment in which both are equalized

voltages the transistor is cut and a timing of Toff seconds starts. When said timing ends, the transistor is saturated, thus initiating a new switching cycle.

 In the event that the current flowing through the transistor is measured, it is necessary to ensure that the transistor remains saturated for a minimum time (ton-min) in each switching period (see Figure 12). Since in this way, regardless of the changes that occur in the operating conditions of the converter, the internal control will always know the value of the current flowing through the coil every Toff seconds. This allows you to regulate the value of the current flowing through the coil during the transient states. The duration of the minimum time that the transistor is saturated (ton-min) must be sufficient so that during it the value of the current flowing through the transistor [RsiT (t)] can be measured and compared with the value of vc. 10

 The internal control system is not continuous. During constant time intervals (Toff or Ton, as the case may be), the internal control does not perceive changes in the reference voltage (RsIL) or in the voltage drop in the coil (vL-on or vL-off according to be the case), having to wait for the next non-constant time interval (ton or toff, as the case may be) to perceive these changes and modify the value of the control voltage (vc).

 fifteen

 The use of an internal control that causes the transistor or transistors of the power stage to switch either with a saturation time or with a constant cut-off time in each switching period is decided by the converter designer. And for this you must take into account the following:

_ For the converter to operate with a constant conduction time (ton = Ton = cte), 20 must be measured in each switching cycle, the current flowing through the coil or the current flowing through the diode (see Figures 8 and 14)

_ For the converter to operate with a constant cut-off time (toff = Toff = cte), in each switching cycle, the current flowing through the coil or the current flowing through the transistor must be measured (see Figures 10 and 12)

 Among the most important properties of the internal control loop are the following:

_ A mathematical analysis of its dynamics, as well as various simulations and empirical results confirm that the internal control system is stable for [tON / (tON + tOFF)]  [0, 1).

_ The error in permanent regime is null.

_ The internal control can be implemented in its entirety analogically, regardless of the case considered. This makes it suitable for converters that operate at both low and high switching frequencies.

_ The control logic can be easily modified so that the converters can operate both in continuous driving mode and in critical driving mode, according to the power demanded by the load.

_ In converters with synchronous rectification, the internal control can regulate the average value of the current flowing through the coil in both directions if the voltage vc and the voltage generated by the current sensor are allowed to take both positive and negative values.

External control: 5

 It is only necessary to implement an external control loop in case you want to regulate the average value of the current flowing through the transistor or through the diode. In case you want to regulate the average value of the current flowing through the coil, it is enough to implement the internal control described in the previous paragraphs, regardless of the topology considered. 10

 The task of external control is to determine the permanent regime value of the reference voltage of the internal control (RsIL) from its own reference voltage (vref) and the operating conditions of the converter at all times. The relationship (k) between these reference voltages is convenient to express according to the values vL-on and vL-off of the voltage drops in the coil. In general, this relationship depends on the topology of the power stage of the converter and the type of current to be regulated [iT (t) or id (t)]. In 15 cases, the following cases may occur:

_ If the average value of the current flowing through the transistor is regulated (vref = RsIT), the following is true:

                                                       (3) 20 1 LonisLrefsTsTsTLoffovvRIkvkRIRIRIvv

Reducer

_ If the average value of the current flowing through the diode is regulated (vref = RsId), the following is true:

                                              (4) 1 LoffisLrefsdsdsdLoniovvRIkvkRIRIRIvvv

Reducer

 25

_ If the average value of the current flowing through the coil is regulated (vref = RsIL), it is true that:

              (k = 1 for all topologies) (5) sLrefrefRIkvv

 There are topologies in which vL-on and vL-off are not known at all times. In such cases, it is necessary to sample and save its value so that it is available (known) for at least the next switching period. In practice, it is only necessary to sample these voltage drops with the frequency necessary for the control system to perceive the significant changes that occur in them.

 The control system proposed in this patent is applicable to converters with synchronous rectification, 35 allowing automatic control of the direction of power transfer between the input and output of the converter, according to the value (positive or negative) of the reference voltage (vref).

Brief explanation of the drawings

Figure 1: Topology of the power stage of a dc-type converter (buck)

Figure 2: Current flowing through the coil of the reduction converter represented in Figure 1, in permanent mode 5 and operating in continuous driving mode.

Figure 3: Current flowing through the transistor of the reduction converter represented in Figure 1, in permanent mode and operating in continuous driving mode.

 10

Figure 4: Current flowing through the diode of the reduction converter represented in Figure 1, in permanent mode and operating in continuous driving mode.

Figure 5: The control voltage generated by the internal control (vc), the voltage generated by the sensor that measures the current flowing through the coil [RsiL (t)] and the voltage drop in the coil [vL (t)], corresponding to the case in which the transistor remains saturated for a constant time (Ton) in each switching cycle. The relationship between the value of the voltage vc and the value of the reference voltage of the internal control (RsIL) is indicated.

Figure 6: The control voltage generated by the internal control (vc), the voltage generated by the sensor that measures the current flowing through the coil [RsiL (t)] and the voltage drop is represented in permanent mode in the coil [vL (t)], corresponding to the case in which the transistor remains in cut for a constant time (Toff) in each switching cycle. The relationship between the value of the voltage vc and the value of the reference voltage of the internal control (RsIL) is indicated.

 25

Figure 7: Simplified partial scheme of the control system (internal and external) proposed in this patent, corresponding to the case in which the current sensor measures the current flowing through the coil and the transistor remains saturated for a constant time (Ton) in Each switching cycle.

Figure 8: Relationship between the tensions present in various points of the scheme indicated in Figure 7, both in permanent and transient regime.

Figure 9: Simplified partial scheme of the control system (internal and external) proposed in this patent, corresponding to the case in which the current sensor measures the current flowing through the coil and the transistor remains in constant cut-off time (Toff) in each switching cycle. 35

Figure 10: Relationship between the tensions present in various points of the scheme indicated in Figure 9, both in permanent and transient regime.

Figure 11: Simplified partial scheme of the control system (internal and external) proposed in this patent, 40 corresponding to the case in which the current sensor measures the current flowing through the transistor.

Figure 12: Relationship between the tensions present in various points of the scheme indicated in Figure 11, both in permanent and transient regime.

Figure 13: Simplified partial scheme of the control system (internal and external) proposed in this patent, corresponding to the case in which the current sensor measures the current flowing through the diode. 5

Figure 14: Relationship between the tensions present in various points of the scheme indicated in Figure 13, both in permanent and transient regime.

Figure 15: Diagram of a circuit that supplies 30 light emitting diodes (“LEDs: light emitting diodes”) with a constant current of 1A, from the typical output voltage of a power factor correction circuit (375 ± 25v / 100Hz), using the control technique proposed in this patent. The average value of the current flowing through the coil (and LEDs) is regulated with a null error, measuring the current flowing through the transistor.

 fifteen

Figure 16: Relationship between various voltages in the scheme indicated in Figure 15.

Embodiments of the invention

 The internal control system, regardless of the current measured by the sensor [iL (t), iT (t) or id (t)], can be implemented in its entirety analogically, although it is also possible to implement it with a microcontroller, a digital signal processor (“DSP: Digital Signal Processor”), a reconfigurable door array (“FPGA: Field Programmable Gate Array”) or a specific application integrated circuit (“ASIC: Application Specific Integrated Circuit”).

 25

 The internal control proposed in this patent can be implemented in a variety of ways, with identical or very similar results in terms of the time it takes to correct a disturbance and the magnitude of the overflow presented by the current flowing through the coil during a transient. Figures 7, 9, 11 and 13 show, by way of example, simplified analog schemes of possible embodiments of the internal control system, in accordance with the different cases that can occur in practice. 30

 As for external control, it can be said that although it can be implemented through an analog circuit, it is much easier to implement it using a microcontroller, a DSP, an FPGA or an ASIC. Figures 7, 9, 11 and 13 indicate the calculation to be performed by the external control. The value of the parameter k that appears in these figures is indicated in equations 3, 4 and 5, together with its value for the case of the converter 35 represented in Figure 1.

Claims (7)

1. A method that regulates the average value of the current, linear in pieces, that circulates through a coil belonging to the power stage of a switched electronic converter, which operates in continuous driving mode. To correct the average value of the current flowing through the coil, the proposed method modifies in every 5 switching cycle the relationship between the time that the transistor (or transistors) of the power stage is in cut and the time that is saturated .  With this method of regulation, in each switching cycle, the transistor (or transistors) remains in a state (cut or saturation) for a constant time T and then passes to the other state (saturation or cut) in which a variable time remains, whose duration depends on the operating conditions of the converter. The duration of the variable time is determined, in each switching cycle, as the time it takes for the voltage generated by the current sensor to match a control voltage vc defined as follows:   () 2scsLLRTvRIvtL  When the variable duration state ends, the transistor (or transistors) goes to the other state with a constant duration (T) and so on. 2. In the regulation method of claim 1, if it is desired that the transistor (or transistors) of the power stage remain saturated for a constant time T = Ton in each switching cycle, then to determine the value of vc it is necessary to use the value of the voltage drop in the coil during the time intervals in which it increases the energy it stores [vL (t) = vL-on> 0]. Complying that in this case it is only necessary to measure a current proportional to the current flowing through the coil during the time intervals that the coil yields part of the energy it has stored. 3. In the regulation method of claim 1, if it is desired that the transistor (or transistors) of the power stage remains in constant cutoff time T = Toff in each switching cycle, then to determine the value of vc the value of the voltage drop in the coil must be used during the time intervals in which the coil yields part of the energy stored [vL (t) = vL-off <0]. Complying that in this case it is only necessary to measure a current proportional to the current flowing through the coil during the time intervals in which the coil increases the energy stored. 30 4. Adding an external control loop to the control method of claim 1 regulates the average value (IT) of the current flowing through the transistor (or transistors) of the power stage through which a current proportional to the current flows. current flowing through the coil when it increases the energy it stores. The task of the external loop is to generate the reference voltage RsIL of the control loop (internal) of claim 1 from the reference voltage of the external control loop (RsIT) and the voltage drops in the coil ( vL-on and vL-off). The value of the reference voltage RsIL to be generated by the external loop complies with the following:   1LonsLsTLoffvRIRIv  40 5. Adding an external control loop to the control method of claim 1 regulates the average value (Id) of the current flowing through the diode of the power stage through which a current proportional to the current flowing through the coil when it yields part of the energy it stores. The task of the external loop is to generate the reference voltage RsIL of the control loop (internal) of claim 1 from the reference voltage of the external loop (RsId) and the voltage drops in the coil (vL-on and vL-off). The 5 value of the reference voltage RsIL to be generated by the external loop complies with the following:         1LoffsLsdLonvRIRIv 6. In the control method of claim 1, if only the value of a current proportional to the current flowing through the transistor is measured, it must be ensured that the transistor remains in conduction for a minimum time (ton min) in each switching cycle 7. In the control method of claim 1, if only the value of a current proportional to the current flowing through the diode is measured, it must be ensured that the transistor remains in cut for a minimum time (toff min) in each cycle switching fifteen