Patent Application
20160298590
This invention relates to a controller for a flyback switching mode power supply, to an integrated circuit including said controller, to a flyback switching mode power supply including any of the said controller or integrated circuit, and to a capacitive discharge ignition system which includes said flyback switching mode power supply. The invention further relates to a method of controlling the flyback switching mode power supply and to a controller and to a computer program product that causes the controller to perform said method.
A Capacitive discharge Ignition System (CDI system) is an electronic ignition system which may be used in spark ignition engines of automotive vehicles. The CDI system uses a capacitor discharge current to fire a spark plug used to ignite a mixture of fuel and air in a combustion chamber of the spark ignition engine.
A typical CDI system consists of a Switch Mode Power Supply (SMPS) that may up convert a DC voltage for example generated in a battery of the automotive vehicle into a high voltage required by an ignition coil connected to the spark plug. The DC voltage may be 12 V while the high voltage supplied to the ignition coil of the spark plug may be typically 150-300 V.
In capacitive discharge ignition systems, the Switch Mode Power Supply (SMPS) is typically a flyback SMPS that uses a transformer and a switch at a primary side of the transformer to perform a so called “flyback” action. The flyback action originates from a reversal of a voltage across a secondary winding of the transformer in consequence to an off state of the switch at the primary side of the transformer. Energy accumulated in the transformer at the primary winding during an on state of the switch, it is released by the transformer at the secondary winding during an off state of the switch. The reversal of the voltage induced across the secondary winding, which is a consequence of a sudden collapse of a magnetic flux in the transformer at the primary winding, is large enough to forward bias a rectifying device arranged between the secondary winding and the capacitor. The rectifying device is used to block currents flowing from the capacitor to the secondary winding during an on state of the switch. By forward biasing the rectifying device, a current may flow from the secondary winding to the capacitor, thereby discharging the transformer at the secondary winding and charging the capacitor. Operation of a flyback SMPS in continuous mode can result in very high currents and heat dissipation. The flyback SMPS is thus typically driven in discontinuous conduction mode to allow a complete discharge of the transformer at the secondary winding during the turn off state of the switch. In the discontinuous current mode the current at the secondary winding falls to zero before the switch is turned on again for another cycle. The discontinuous current mode ensures that no DC current is flowing in the transformer and that all energy stored in the transformer during the turn on state of the switch is transferred to the capacitor during the turn off state of the switch.
In literature many types of flyback switching mode power supplies are disclosed that make use of the discontinuous conduction mode.
For example U.S. Pat. No. 7,719,248B1 discloses a switch-mode converter and a method that uses a sensed current to control the switch-mode converter operating in a discontinuous conduction mode. In one embodiment of the U.S. Pat. No. 7,719,248B1, the switch-mode converter may be a flyback switching mode power supply. The solution provided by U.S. Pat. No. 7,719,248B1 includes a switch-mode converter controller, a comparator and a finite state machine. The comparator receives and compares a sensed current at the primary winding of the transformer with a desired peak current. The finite state machine is configured to operate the switch-mode converter in a discontinuous conduction mode. Responsive to comparisons made by the comparator, the finite state machine turns on the switch and observes an on time duration of the switch until the sensed current reaches the peak current.
Patent application US20080123380 discloses an SMPS and a driving method thereof. The SMPS disclosed in US20080123380 includes a first coil of a primary side of a transformer for transforming an input DC voltage, a second coil and a third coil at a secondary side of the transformer to respectively provide the output voltage of the SMPS and a bias voltage for driving a pulse width modulation signal generator. The pulse width modulation signal generator receives a feedback voltage corresponding to a first voltage generated from the second coil, a sense signal corresponding to the current flowing through the switching transistor, and a third voltage corresponding to a second voltage generated from the third coil. The pulse width modulation signal generator controls an on or off time of the switch so that the SMPS may be driven in a discontinuous conduction mode.
One of the disadvantages of the above mentioned solutions is that a current or a combination of a current and voltages need to be sensed by the SMPS controller in order to drive the SMPS in the discontinuous conduction mode. Since the output of the flyback SMPS is inherently a voltage, extra components are de facto needed in either of the two mentioned solutions. The extra needed components add up to an overall cost of the SMPS. Flyback SMPS used in CDI system are extremely sensitive to cost. A small difference in cost in the order of a few cents may be for example a buying decider of components for increasingly cheaper automotive vehicles containing these CDI systems.
A standard SMPS typically feeds an uncharged capacitive load, which electrically is similar to a short circuit, only at the point of switch on. From thereon the standard SMPS typically delivers a fixed voltage supply to a parallel arrangement of the charged capacitive load and a resistive and/or inductive load. In contrast, in a CDI system the capacitor is discharged up to 200 times a second and thus the standard SMPS spends a significant amount of time operating into a ‘short circuit’. Accordingly, there is a need to control the SMPS in a manner that diverges from traditional means.
The present invention provides a controller for a flyback switching mode power supply as claimed in claim 1, an integrated circuit including the controller, a flyback switching mode power supply including the controller or said integrated circuit and a capacitive discharge ignition system including any of the controller or integrated circuit or flyback switching mode power supply. The present invention further provides a method of controlling a flyback switching mode power supply, a controller to perform the method and a computer program product including instructions that cause the controller to perform the method.
Specific embodiments of the invention are set forth in the dependent claims.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. In the Figures, elements which correspond to elements already described may have the same reference numerals.
When the switch SW1 is turned on, the input voltage Vin generated by the voltage source 15 is applied across the primary winding L1. The input voltage Vin is positive in this example and an increasing magnetizing current I1 flows in the primary winding L1 and the switch SW1. During this phase the magnetic flux is built up only by the magnetizing current I1. The polarity of the secondary winding L2 and of the rectifying device RD is such that no current is flowing at the secondary side of the transformer T. When the switch SW1 is turned off, the magnetizing current I1 at the primary side of the transformer T immediately vanishes. As a consequence the magnetic flux in the transformer T needs also to vanish. For that a demagnetizing current I2 needs to flow at the secondary side of transformer T bringing the rectifying device RD into conduction. This decreasing demagnetizing current I2 flowing through the rectifying device RD charges the capacitor C to an increasing output voltage Vout. A controller 20 is used to sense the output voltage Vout and to turn on the switch SW1 when the first derivative of the output voltage Vout over time becomes smaller than a predetermined threshold dth. The controller 20 may be part of the flyback switching mode power supply 10 or alternatively the controller 20 may be external to the flyback switching mode power supply 10. In
During a transfer of the energy stored in the transformer T in a previous on state of the switch SW1 to the capacitor C, the output voltage Vout at which the capacitor C is charged will flatten out and the first derivative of the output voltage Vout will decrease. When the first derivative of the output voltage Vout will be close to zero, the transfer of said energy to the capacitor C via the demagnetizing current I2 flowing through the rectifying device RD may be nearly completed. In other words, the controller 20 detects the output voltage Vout, uses the output voltage Vout and compares the first derivative of the output voltage Vout with a predetermined threshold dth to turn on the switch SW1 when the first derivative of the output voltage Vout over time becomes smaller than the predetermined threshold dth. When the predetermined threshold dth is not identical to zero, only part of energy stored in the transformer T (i.e. in the core of the transformer T during a previous on cycle of the switch SW1) is transferred to the capacitor C. When the predetermined threshold dth is identical to zero the total amount of energy stored in transformer T has been transferred to the capacitor C. By using the controller 20 to sense and use the output voltage Vout across the capacitor C to turn on the switch SW1, a controlled flyback switching mode power supply that makes use of voltage control is realized. No additional expensive voltage to current converters are needed to detect a feedback signal used to control the switch SW1. Further to that, the output voltage Vout is the only feedback signal used by the controller 20 to turn on the switch SW1. In the prior art documents cited in the background more than one feedback signal consisting of at least a current is needed to control the switching device in the flyback switching mode power supply. The present solution thus saves costs of implementation with respect to the prior art solutions. Besides that and as earlier explained, the controller 20 is used to control the amount of energy transfer from the transformer T to the capacitor C by sensing the output voltage Vout and comparing the first derivative of the output voltage Vout over time against the predetermined threshold dth. If the predetermined threshold dth is zero, the capacitor C is charged until the demagnetizing current I2 falls to zero, i.e. until the energy previously stored in the transformer T during the on state of the switch SW1 falls to zero. If the predetermined threshold dth is above zero then the demagnetizing current I2 remains above zero, resulting in a greater throughput of power at the expense of higher currents and heat dissipation.
In an embodiment of the flyback switching mode power supply 10, the controller 20 may be configured to turn on the switch SW1 for a fixed duration ton. By turning on the switch SW1 for a fixed duration ton, a fixed amount of energy is stored in the transformer T during a turning on of the switch SW1. This fixed amount of energy is later transferred to the capacitor C after a turning off of the switch SW1. A small fixed duration ton may reduce switching losses in the switch SW1 and consequently reduce heat dissipation in the switch SW1. The duration ton may vary according to the input voltage Vin.
A charging rate of the capacitor C depends upon a pre-existing voltage at the capacitor C. If the capacitor C is initially discharged, i.e. the output voltage Vout at time 0 is 0V, a long time may be required to transfer a given amount of energy stored in the transformer T to the capacitor C. If the capacitor C is already charged to a finite output voltage Vout, a shorter time may be required to transfer the same given amount of energy stored in the transformer T to the capacitor C. The energy once stored in the capacitor C has a quadratic relationship with the voltage across the capacitor C:
E(t)=CVout2(t)/2 (1),
Wherein E(t) is the energy stored in the capacitor C at time t and Vout(t) is the output voltage Vout at time t across the capacitor C. By explaining the following we refer to
In another example the controller 20 may be further configured to keep the switch SW1 in an off state when the output voltage Vout across the capacitor C has reached a predetermined target voltage Vtarget. The capacitor C may be charged, as previously described, using a cycle by cycle voltage control in which a given amount of energy is transferred in each off cycle of the switch SW1 from the transformer T to the capacitor C. The given amount of energy transferred in each off cycle of the switch SW1 is added to the energy previously stored in the capacitor C in a previous cycle until the predetermined target voltage Vtarget across the capacitor C is reached. Once the predetermined target voltage Vtarget is reached across the capacitor C, the controller 20 may keep the switch SW1 in an off mode thereby halting the control of the flyback switching mode power supply 10. For example, depending on the application, the predetermined target voltage Vtarget may be fixed in a range between 150-300 V. For the capacitive discharge ignition systems (CDI systems) discussed in the background, a value of the predetermined target voltage Vtarget may depend upon a high voltage necessary for an ignition coil that generates the ignition voltage across the spark plug, thereby igniting a mixture of fuel and air in a combustion chamber of a spark ignition engine. The value of the predetermined target voltage Vtarget may be thus chosen in a way such that the flyback switching mode power supply 10 may be suitable to ignite a specific mixture of a specific fuel and air.
The predetermined threshold dth may be chosen in function of how fast the capacitor C may be charged and in function of the heat dissipation capacity of the switch SW1. In a further embodiment the predetermined threshold dth may be less than 0.5 V in 10 us. A predetermined voltage threshold may be less than 1% of the predetermined target voltage Vtarget. In this embodiment the predetermined voltage threshold is chosen to be 0.5 V. A value of the predetermined voltage threshold depends upon how accurately the controller 20 is calibrated.
In another exemplary embodiment, the controller 20 may be configured to connect a load in parallel with the capacitor C in order to discharge the capacitor C after the output voltage Vout has reached the predetermined target voltage Vtarget. The load in parallel with the capacitor C may be the ignition coil used in the above mentioned capacitive discharge ignition systems to transform the predetermined target voltage Vtarget to the thousands of Volts necessary for a spark plug to ignite the mixture of fuel and air. The capacitor C utilized in the flyback switching mode power supply 10 for capacitive discharge ignition systems may be frequently discharged to create ignition sparks. The capacitor C may thus be re-charged after being discharged at a charge rate of up 200 times a second, i.e. every 5 ms, or even at a faster charge rate for higher revving engines or multiple cylinder engines. The controller 20 may thus trigger a connection with the ignition coil every 5 ms or every time the capacitor C has been charged to the predetermined target voltage Vtarget. The ignition coil may work as a pulse transformer rather than a storage medium.
As already mentioned
If the output voltage Vout is smaller than the predetermined target voltage Vtarget a sequence of actions may be performed: turning on 300 the driver for a fixed ton duration, turning off 400 the driver, reading 500 the output voltage Vout at time N, reading 600 the output voltage Vout at time N+1, comparing 700 a difference between the output voltage Vout at time N+1 and the output voltage Vout at time N with a predetermined threshold voltage; if said difference is larger than the predetermined threshold voltage, the output voltage Vout across the capacitor C may be read in the reading 1000 after which the method may enter in a loop called in
It should be noted that the method of controlling the flyback switching mode power supply according to the embodiments of the invention shown in
A computer program product may be used that includes instructions to cause any of the controllers 20, 21 or 22 described through the
In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. For example, the connections between electrically coupled devices may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise the connections may for example be direct connections or indirect connections. For example in
Because the circuits implementing the present invention are, for the most part, composed of electronic components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. For example the controller 20, 21, 22, the sensing device 30, the pulse-width modulation unit 35, 36 of
Although the invention has been described with respect to specific polarity of potentials, skilled artisans appreciated that polarities of potentials may be reversed. For example the magnetizing current I1 and the magnetizing current I2 as well as the input voltage Vin and the output voltage Vout may have an opposite polarity to the polarity shown in
It is to be understood that
Also for example, in the embodiments of
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an”, as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an”. The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2013/060375 | 11/25/2013 | WO | 00 |
