Patent Application
20080049459
The time that the feedback voltage waveform 204 accurately represents the output voltage Vout 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 Vout 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.
The ADC 320 converts the analog feedback voltage Vfeedback 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.
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 Vfeedback waveform.
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 Vfeedback 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 Vfeedback 302) at the time at which it accurately represents output voltage Vout 348 (i.e., at the plateau 236).
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 Vfeedback 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.
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.
As the circuit 308 is digital and can be programmed via the registers 352, the circuit 308 accurately samples the feedback voltage Vfeedback 302 and also provides flexibility due to software-controlled adjustments.
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 Vfeedback pulse waveform 302. In one embodiment, software controls the registers 352 so that the change in feedback voltage Vfeedback 302 causes an adjustment in the registers 352. The change in feedback voltage Vfeedback 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 Vfeedback 302 is correctly measured, the output voltage Vout 348 can be accurately determined.
Although described above with three coils, the isolated flyback power supply can alternatively have two coils.
V
feedback426=(Vdrain MOSFET−VBE)(Resistor 420/resistor 422)
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.
