Abstract:
Disclosed is a method and apparatus that includes a power supply having a primary coil and a secondary coil. The secondary coil generates an output voltage and a feedback voltage related to the output voltage. The feedback voltage is sampled at a time instant that is digitally controllable. The output voltage is determined from the feedback voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to power supplies and more specifically to isolated switching power supplies. 
         [0002]    Efficient and ever smaller size power supplies are in high demand in almost all electronics devices in a wide range of applications. For example, smaller and more efficient power supplies are needed in telecommunication and embedded system applications, Power-over-Ethernet (POE) applications, microprocessors and chipsets requiring precise and robust voltage regulation, personal computers, cellular telephones, personal digital assistants (PDAs), etc. 
         [0003]    Power supply architectures can be classified according to the structure of their power stage and associated properties. One such power supply is a flyback power supply.  FIG. 1  shows a flyback power stage circuit  100  having a transformer consisting of three coils  104 ,  108 , and  112 . Coil  112  is the primary coil, coil  104  is the feedback coil, and coil  108  is the secondary coil. The secondary coil  108  is connected to a diode  116  and a capacitor  120  to rectify and smooth the output voltage signal. The output voltage V out    120  is the voltage between the two endpoints of the capacitor  120 . The input voltage V in    122  is the fixed potential of one end of the primary coil  112  relative to the system ground. The feedback voltage  132  is the voltage of the node  134  relative to the system ground. It is typically proportionally related to the voltage across the feedback coil  104  through a resistive voltage divider. 
         [0004]    A metal oxide semiconductor field effect transistor (MOSFET)  124  is typically connected to one endpoint of the primary coil  112 . The MOSFET  124  switches on and off, thereby transferring electromagnetic energy from the primary coil  112  to the secondary coil  108 . The switching MOSFET control waveform duty cycle sets the output voltage level. It also induces changes in the voltage between the two endpoints of the feedback coil  104  (and, thus, in the feedback voltage  132 ). When the MOSFET  124  switches on and off, the feedback voltage  132  changes. Therefore, the feedback voltage  132  is a pulsed waveform. 
         [0005]    The output voltage V out    120  can be measured indirectly via the feedback coil  104 . Thus, the output voltage V out    120  can be measured indirectly from the feedback voltage  132 . At specific time intervals the feedback voltage  132  is proportional to the output voltage  120 . The feedback voltage  132  is compared to a reference voltage level to compute an error signal used to modify the MOSFET switch control waveform and close a control loop that regulates the output voltage  120  at a wanted level. 
         [0006]    The flyback circuit  100  uses analog components to indirectly measure the output voltage V out    120 . The analog components measure the feedback voltage V out    132  during a specific time period. In particular, when the MOSFET  124  is turned “on”, the primary coil  112  obtains a fixed, non-zero voltage. When the MOSFET  124  is then turned “off”, the feedback coil  104  obtains a voltage (through induction) and a feedback pulse appears. Thus, the feedback voltage is measured during the time intervals that the MOSFET  124  is switched “off”. However, due to leakage inductance effects primarily which cause an initial voltage spike, it takes a finite time until the feedback voltage  132  accurately represents the output voltage  120 . In order to extract the relevant output voltage information from the feedback voltage  132 , a fixed delay is introduced between the MOSFET  124  switch-off command and the enabling of feedback-voltage-measuring circuitry. This fixed delay is set by a choice of resistors and capacitors. 
         [0007]    Because resistors and capacitors are analog components, there are traditionally imperfections associated with each of them. These imperfections can result in imprecision with respect to their listed value. Thus, a resistor may have an actual value that is slightly different than its listed value. These imperfections will likely have an effect on the resulting time. Further, the values of resistors and/or capacitors may fluctuate with temperature changes. As a result, many attempts may be needed to determine which resistors and which capacitors to use in order to accurately measure the feedback voltage. Additionally once component values have been chosen to implement a certain fixed delay, these components would have to be replaced to implement another fixed delay. This can be required if the power supply is used under different load conditions. 
         [0008]    Therefore, there remains a need to more accurately determine output voltage information from the feedback voltage of a flyback circuit. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    In accordance with an aspect of the present invention, a flyback power supply has a primary coil and a secondary coil. A rectification stage connected to the secondary coil generates an output voltage. A feedback voltage is also generated, which is related to the output voltage. The feedback voltage is sampled at a time instant relative to the MOSFET switch-off command instant that is digitally controllable. The output voltage is determined from the feedback voltage. In one embodiment, the flyback power supply also has a feedback coil to facilitate the measuring of the feedback voltage. Alternatively, the feedback voltage can be measured by a transistor, diode, and resistor configuration. 
         [0010]    In one embodiment, the time delay between the MOSFET switch-off instant and the feedback voltage sampling instant is set by programming a register. This programming sets a fixed propagation delay for the digital control signal of a switch (e.g., a metal oxide semiconductor field effect transistor), that also induces the appearance of a feedback voltage pulse, back to a feedback voltage sampling circuit. In one embodiment, the programming of the register determines a number of flip flops to use to delay the sampling of the feedback voltage relative to the switch-off command instant. The induced feedback voltage typically experiences a spike right after the switch-off command instant before leveling off into a plateau. The sampling of the feedback voltage occurs at the start of the plateau. 
         [0011]    In another embodiment, the present invention includes a power supply having a primary coil, a feedback coil, and a secondary coil. The secondary coil generates an output voltage and the feedback coil generates a feedback voltage related to the output voltage. A digital loop control samples the feedback voltage at a first time instant and a digital delay chain delays the first time instant by a programmable number of clock cycles. A software register is used to program the delay. 
         [0012]    In one embodiment, a control and software interface module provides a hardware interface between the software layers and the digital delay chain through control and status registers and hardware connections. The control and software interface module can also include an interface between registers and a control state machine. 
         [0013]    An embodiment of the present invention can also include a sample and hold circuit for receiving the feedback voltage. The sample and hold circuit is connected to an analog to digital converter for converting the feedback voltage into a digital signal (e.g., digital word). 
         [0014]    These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of a flyback power stage circuit having three coils and producing an output voltage and a feedback voltage; 
           [0016]      FIG. 2  is a timing diagram of a feedback voltage waveform associated with the feedback voltage; 
           [0017]      FIG. 3  is a block diagram of a switching voltage regulation circuit that digitally samples the feedback voltage of a flyback power stage; and 
           [0018]      FIG. 4  is a block diagram of an isolated flyback power supply. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 2  is a timing diagram  200  for the feedback voltage  132  of the flyback circuit  100 . The timing diagram  200  includes a waveform  204  representing the feedback voltage  132 . The waveform  204  is plotted with respect to a ground line  208  (i.e., zero volts). The timing diagram  200  also includes a switch state  212  illustrating the state of the MOSFET switch  124 . Although described above and below with a MOSFET switch, any switching element may be used (e.g., transistor(s), logic gates, relays, switch, etc.). The waveform  204  is below an input voltage Vin line  216  during a first period  220 . A first period  220  occurs while the MOSFET switch  124  is in an “on” state, as shown with the “ON” block  224  of the switch state  212 . When the MOSFET switch  124  is turned “off” (shown with the “OFF” block  228  of the switch state  212 ), the feedback voltage  132  initially spikes upward, as shown with spike  232 . The feedback voltage  132  then levels off at a plateau  236  until it collapses by itself or the MOSFET  124  is switched back “on”. Once this occurs, the feedback voltage waveform  204  (and, therefore, the feedback voltage  132 ) falls at point  240  and levels off close to the ground  208  at line  244 . 
         [0020]    The time that the feedback voltage waveform  204  accurately represents the output voltage V out    120  is during the plateau  236 . Thus, if a circuit designer uses analog components such as resistors and capacitors to set a time constant to measure this feedback voltage  132  (and, therefore, to indirectly measure the output voltage V out    120 ), the values of the resistors and capacitor have to be extremely accurate because the time period of the plateau is typically extremely small (e.g., in the few hundreds of nanoseconds). Because of natural imperfections in analog components such as resistors and capacitors, it is often difficult to set their values correctly. Often, a lab technician has to choose resistor and capacitor values through trial and error. Further, resistor and/or capacitor values may be affected by changes in temperature. 
         [0021]      FIG. 3  shows a block diagram of a circuit that digitally samples feedback voltage V feedback    302  of a flyback power supply  304 . In particular, the digital sampling, or measuring, of feedback voltage V feedback    302  is performed by solid state circuit  308 . The feedback voltage V feedback    302  is transmitted to an analog front-end circuit  312 . The analog front-end circuit  312  includes a sample and hold circuit  316 . The sample and hold circuit  316  is used to interface the analog feedback voltage V feedback    302  to an analog-to-digital converter (ADC)  320 . The sample and hold circuit  316  holds the relevant analog value of the feedback voltage V feedback    302  steady until the next sampling instant. During that time, the ADC  320  performs operations to convert the sampled value into a digital word to be used for further digital processing. 
         [0022]    The ADC  320  converts the analog feedback voltage V feedback    302  into a digital word  324  and transmits the digital word  324  to a digital loop control  328  in circuit  329 . The output  330  of the digital loop control  328  is a switching control waveform primarily controlling the state of the MOSFET switch  340  (ON or OFF) through a MOSFET driver  336 . It is also transmitted through a digital delay chain  332  back to the sample and hold circuit  316 . The digital delay chain  332  includes one or more flip flops (e.g., D flip flops) clocked at a digital system clock to cause a known delay that is a multiple of the digital system clock period. For example, if the digital delay chain  332  uses one flip flop, the delay is one clock cycle. Similarly, if the digital delay chain  332  uses two flip flops, the delay is two clock cycles. 
         [0023]    Thus, the output signal  330  of the digital loop control  328  is used for two purposes. First, the output signal  330  is used to determine the state of the MOSFET switch  340 . As a result, the output signal  330  controls when the MOSFET switch  340  is turned “on” and when the switch  340  is turned “off”. This also sets the boundaries, especially the start, of the feedback pulse in the V feedback  waveform. 
         [0024]    Second, the output signal  330  is used to determine when to trigger the sample and hold circuit  316  based on the digital delay chain  332 . The digital delay chain  332  introduces a delay relative to the time when the MOSFET switch  340  is switched “off”. Since the same control waveform determines the MOSFET state inducing the appearance of the feedback pulse and the sampling instant after being precisely delayed, this relative sampling instant is controllable and can be precisely adjusted. This delay sets the sampling instant relative to the instant when the MOSFET is switched off, which is also the start of the feedback pulse in the V feedback  waveform. The digitally controlled delay of the digital delay chain  332  enables the circuit  308  to sample the feedback voltage waveform  204  (and, therefore, the feedback voltage V feedback    302 ) at the time at which it accurately represents output voltage Vout  348  (i.e., at the plateau  236 ). 
         [0025]    The circuit  308  also includes a high level control and software interface  360 . The high level control and software interface  360  includes registers  352  (and their access interface) and control state machines  356 . The registers  352  are software registers that can be programmed to enable the changing of when the feedback voltage V feedback    302  is measured while the MOSFET switch is “off”. The data stored in the registers  352  are transmitted to circuit  329 , as shown with control arrow  364 . 
         [0026]    The control state machines  356  are state machines that transition from one state to another based on the status  368  received from circuit  329 . For example, it can consist of a state machine with four states: IDLE, DISABLED, LOOP_ENABLED and VOLTAGE_STEADY. The state machine starts in IDLE state when reset and moves to the DISABLED state. A wanted voltage control word can be programmed and passed to the digital loop control circuit. The state subsequently moves to LOOP_ENABLED and the digital loop control operation is enabled through the control interface  364 . The loop operation will compare the sampled feedback voltage word to the voltage control word and use the error word to modify the output signal  330  duty cycle so that the error is reduced and thus move the output voltage closer to the wanted voltage. When the error word magnitude goes below a small threshold, the digital loop control indicates through the status interface  368  that a steady state has been reached. The state machine consequently moves to a VOLTAGE_STEADY state which can be reported to software layers. Based on high-level operations the state machine state can be directed to DISABLED again for regulation under new conditions or to IDLE if it is reset. 
         [0027]    As the circuit  308  is digital and can be programmed via the registers  352 , the circuit  308  accurately samples the feedback voltage V feedback    302  and also provides flexibility due to software-controlled adjustments. 
         [0028]    Moreover, change in the sampling point can occur dynamically to allow continued operation over a wide range of load conditions. For example, if the resistance of the load  372  varies, this can change the shape of the feedback voltage V feedback  pulse waveform  302 . In one embodiment, software controls the registers  352  so that the change in feedback voltage V feedback    302  causes an adjustment in the registers  352 . The change in feedback voltage V feedback    302  causes a status signal (shown with arrow  368 ) to be sent to the control and software interface  360 . Software then uses this status signal to adjust the sampling point of circuit  329 . Once the feedback voltage V feedback    302  is correctly measured, the output voltage V out    348  can be accurately determined. 
         [0029]    Although described above with three coils, the isolated flyback power supply can alternatively have two coils.  FIG. 4  shows an isolated flyback power supply  400  that is connected to solid state circuit  308  of  FIG. 3 . The power supply  400  has a primary coil  404  and a secondary coil  408  but does not have a feedback coil. The feedback voltage V feedback   410  is instead measured by using the transistor  412 , diode  416 , and resistors  420 ,  422 . Specifically, the voltage at the drain of the MOSFET  426  minus the voltage between the base and emitter of the transistor  412  is converted to a current by the resistor  422  and then back to a voltage at the resistor  420 . Stated mathematically: 
         [0000]        V   feedback 426=( V   drain MOSFET   −V   BE )(Resistor 420/resistor 422) 
         [0030]    The foregoing Detailed Description is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.