DC-DC flyback converters using synchronous rectifiers are well known. When isolation between the input and output stage is required, the output voltage can be sensed by various methods for regulation feedback. Some ways to convey the output voltage while maintaining isolation include using an optocoupler or using an auxiliary winding on the primary side of the transformer. However, those ways require additional circuitry, space, power, and cost.
A more elegant way of detecting the output voltage is to sense a voltage at a terminal of the power switch when the power switch is off during the discharge (or flyback) cycle of the converter. The voltage at the terminal of the power switch is generated due to a current flow in the secondary winding. Such a sensed voltage is substantially equal to the input voltage plus N*VOUT, where N is the winding ratio of the primary and secondary windings. (The voltage drop across the synchronous rectifier is ignored for simplicity.) However, such a scheme requires a minimum duty cycle in order for the sensing to be accurate, since current must periodically flow in the secondary winding in order to create the primary side sense voltage. Such a scheme also generally requires a minimum load in the form of a load resistor so as to draw a minimum current during the discharge cycle in the event the actual load is in a standby mode drawing little or no current.
If there were no minimum load resistor and the actual load went into a very light current standby mode or was disconnected, the minimum duty cycle may be greater than that needed to achieve a regulated output voltage, and the output voltage would exceed the desired regulated level. Thus, the minimum load current must be above a threshold current to prevent this. The minimum load resistor reduces the efficiency of the converter.
Although the converter may be controlled to switch at even lower duty cycles to lower the minimum load current that it can generate, such lowering of the duty cycle reduces the converter's ability to react to load current transients. For example, if the load suddenly drew an increased current during a switching cycle, the output voltage may droop below a threshold for proper operation of the load before the output was sensed in the following cycle.
FIG. 1 illustrates one type of flyback converter 10 that uses a minimum load resistor R1 and which detects the output voltage VOUT by detecting the voltage at the primary winding L1 when the synchronous rectifier MOSFET M2 is turned on during the discharge (or flyback) cycle. No optocoupler or auxiliary winding is used to detect VOUT.
A transformer 12 has a primary winding L1 and a secondary winding L2. The MOSFET M1 is controlled by an output regulation and control circuit 14 to connect the winding L1 between the input voltage VIN (e.g., a battery voltage) and ground during a charging cycle.
To achieve a regulated VOUT, the MOSFET M1 is turned off after a controlled time, and the synchronous rectifier MOSFET M2 is turned on. The current through winding L2 is transferred to the load and the smoothing capacitor C1 at the required voltage.
For regulation feedback, the circuit 14 detects the voltage at the drain of MOSFET M1 during the discharge cycle (current flowing through winding L2), where such a voltage is related to VOUT. Sensing an output voltage by a signal at the primary side of the transformer is sometimes referred to as primary side sensing. The user selects the value of a feedback resistor RFB and the value of a reference resistor RREF such that (RFB/RREF)*Vref equals the desired regulated voltage, where Vref is a bandgap reference voltage applied to an error amplifier. Such primary side sensing circuits for detecting VOUT are well known and need not be described in detail. The full data sheets for the Linear Technology LT3573 and LT3748 flyback converters, incorporated herein by reference and available on-line, describe the operation of the feedback circuit. This operation is also described in U.S. Pat. Nos. 7,471,522 and 7,463,497, assigned to the present assignee and incorporated herein by reference. Other known primary side voltage sensing techniques may be used.
The circuit 14 continues to control the duty cycle of MOSFET M1, at a variable frequency or a fixed frequency, to regulate VOUT based on the sensed voltage.
A synchronous switch control circuit 16 may control MOSFET M2 to turn on at the proper times or, alternatively, the circuit 14 may directly control the synchronous rectifier MOSFET M2 to turn on when MOSFET M1 turns off. MOSFETs M1 and M2 are typically never on at the same time. The diode D2 represents the drain-body diode of the MOSFET M2. Many conventional techniques may be used to sense when to turn the MOSFET M2 on. In one embodiment, the synchronous switch control 16 detects a voltage across the MOSFET M2. When the MOSFET M1 switches off, the voltage across MOSFET M2 will become negative (drain voltage lower than ground), and this sensed voltage reversal causes the synchronous switch control circuit 16 to turn on MOSFET M2. When the secondary winding L2 current ramps down to zero, the drain voltage will rise, causing the synchronous switch control circuit 16 to turn off the MOSFET M2. With each cycle of MOSFETs M1 and M2 turning on and off, a current pulse is provided to the output, which is smoothed by the capacitor C1 to generate a DC regulated output voltage VOUT.
Various other conventional schemes may also be used to control the turning on and off of the MOSFET M2 to emulate a diode.
The output regulation and control circuit 14 may use any type of conventional technique to regulate, including current mode, voltage mode, or other modes.
When the load current is above a certain threshold current, conventional operation of the converter 10 is used to accurately regulate VOUT. However, when the actual load current falls below a threshold current, the required minimum duty cycle of the converter 10 generates too much current and causes VOUT to rise above the regulated voltage. Such light load operation still requires a minimum duty cycle to sample the voltage at the primary winding L1. In the event that the actual load is a type that has a standby mode that draws very little power, the converter 10 is provided with a minimum load current resistor R1 to help dissipate the winding L2 current so regulation can be maintained at the minimum duty cycle. Alternatively, or in conjunction, a zener diode D3 is used to ensure VOUT does not rise above a threshold level. Resistor R1 and zener diode D3 are optional, since the minimum current drawn by the actual load may be sufficient to substantially maintain regulation at the lightest load current.
FIG. 2 illustrates the current IL1 through the primary winding L1, the current IL2 through the secondary winding L2, and the voltage VD at the drain of the MOSFET M1 for a relatively low duty cycle operation.
At time T1, the MOSFET M1 turns on to charge the primary winding L1, causing a ramping current to flow in winding L1. MOSFET M2 is off at this time.
After a variable or fixed time, at time T2, MOSFET M1 shuts off and MOSFET M2 turns on. This may be at the minimum duty cycle. This ceases current in the primary winding L1 and causes the current through the secondary winding L2 to ramp down while charging the output capacitor C1 and providing current to the load. During this discharge cycle, the voltage across the MOSFET M1 is related to the output voltage VOUT and is sampled during this time by the circuit 14.
At time T3, the secondary winding L2 current ramps down to zero and the MOSFET M2 turns off to cause a discontinuous mode. The MOSFET M2 may be turned off by a circuit that detects a slight reversal of current through the winding L2 by detecting the voltage across the MOSFET M2.
After time T3, the parasitic capacitance of MOSFET M1 and the inductance of winding L1 create an oscillating tank circuit, and the settled voltage across the MOSFET M1 is VIN.
At time T4, the MOSFET M1 turns on again, and the cycle repeats, which may be the minimum duty cycle.
Additional detail of various converter circuits are described in U.S. Pat. Nos. 5,481,178; 6,127,815; 6,304,066; and 6,307,356, assigned to the present assignee and incorporated herein by reference.
During a medium to high current mode of the converter 10, the converter 10 varies the duty cycle or the peak or average current in winding L1 to regulate the output voltage.
During a light load condition, such as a standby mode, it is important that the converter 10 draw as little current as possible to increase system efficiency or extend battery life. Such standby modes typically occur for relatively long periods. It would be desirable to not require a minimum current load circuit (e.g., resistor R1) or a minimum duty cycle at light load currents to enable the converter 10 to regulate VOUT when the actual load is in its standby mode. By doing away with the minimum current circuit or minimum duty cycle, while still achieving substantial regulation when the actual load is drawing zero or very little current, efficiency would be improved and battery life would be increased. Further, in any converter solution, it would be desirable to retain good transient response.