Abstract:
A MOSFET is used as a synchronous rectifier in a fly-back DC/DC converter and connected in series with a secondary winding of a transformer. The MOSFET is repetitively turned on and off in response to the turning off and on of a primary switch connected in series with a primary winding of the transformer. Circuit elements on the secondary winding side detect voltage transients across the MOSFET caused by reverse currents when it is turned off and shorten at least a next on-period of the MOSFET in response to the detection of such transients.

Description:
TECHNICAL FIELD 
     The invention relates generally to DC/DC converters, and more specifically, to a method and an arrangement for controlling synchronous rectifiers in DC/DC converters. 
     BACKGROUND OF THE INVENTION 
     To rectify high frequency AC voltages in switched DC/DC converters, synchronous rectifiers in the form of MOSFETs are often used instead of diodes. Since the voltage drop across a MOSFET is lower than across a diode, the efficiency of converters with MOSFETs will be higher. However, since a MOSFET cannot be turned off instantaneously, its turn-off will not occur at exactly the same moment as the turn-on of a primary switch but a little later. This will cause reverse currents through the MOSFET, causing voltage transients to appear across the MOSFET. To eliminate these voltage transients, unique control signals are needed for the MOSFET to function properly. It is known to provide these control signals from the primary side of the converter, e.g. via an additional transformer. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to control a MOSFET on the secondary side without any need of transferring control signals from the primary side of the converter. Voltage transients across the MOSFET caused by reverse currents when it is turned off are detected. At least a next on-period of the MOSFET is shortened in response to the detection of such transients. As a result, the MOSFET will be controlled more accurately, and transients are eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described more in detail below with reference to the appended drawing on which FIG. 1 shows a fly-back DC/DC converter with an example embodiment of a control arrangement in accordance with the invention, and FIGS. 2A-2D are diagrams illustrating different signals in the example embodiment in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a fly-back DC/DC converter with an example embodiment of a control arrangement in accordance with the invention. The converter comprises a transformer TR having a primary winding connected in series with a primary switch T 1  to output terminals of a schematically illustrated DC voltage source V 1 , and a secondary winding connected in series with a MOSFET T 2  to an output capacitor C 1  for generating an output DC voltage. The MOSFET T 2  is used as a synchronous rectifier and is controlled in accordance with the invention to improve the efficiency of the converter. 
     In a manner known per se, the MOSFET T 2  comprises a source S, a drain D, and a gate G as well as a body diode D 1  connected with its anode to the source S and with its cathode to the drain D of the MOSFET T 2 . In the example embodiment shown in FIG. 1, the drain D of the MOSFET T 2  is connected to the source S of the MOSFET T 2  via a resistor R 1  in series with a diode D 2  and a capacitor C 2 . Also, the drain D is connected to the source S via a diode D 3  in series with a capacitor C 3 . 
     The interconnection point between the diode D 2  and the capacitor C 2  is connected to the collector of a transistor T 3  whose emitter is connected to the gate G of the MOSFET T 2  and to the emitter of a transistor T 4  whose collector is connected to the source S of the MOSFET T 2 . The bases of the transistors T 3  and T 4  are interconnected and connected via a resistor R 2  to the collector of the transistor T 3 . The interconnected bases of the transistors T 3  and T 4  are also connected to the collector of a transistor T 5  whose emitter is connected to the source S of the MOSFET T 2 . 
     The base of the transistor T 5  is connected via resistor R 3  in series with a capacitor C 4  to the drain D of the MOSFET T 2 . The interconnection point between the resistor R 3  and the capacitor C 4  is connected via a resistor R 4  to the collector of a transistor T 6  whose emitter is connected to the interconnection point between the diode D 3  and the capacitor C 3  via a resistor R 5  and whose base is connected to the interconnection point between the diode D 2  and the capacitor C 2 . 
     As mentioned above, the switch T 1  is the so-called primary switch of the fly-back converter. When the switch T 1  is on, magnetic energy is stored in the transformer TR. The voltage U across the secondary winding of the transformer TR is negative. When the primary switch T 1  goes off, the voltage U will be positive and energy will be transferred via the MOSFET T 2  to the output capacitor C 1 . 
     The transistors T 3  and T 4  are two emitter-followers that quickly can charge/discharge the gate G of the MOSFET T 2 . The transistor T 5  turns the MOSFET T 2  on and off via the emitter-followers T 3  and T 4 . The transistor T 5  is turned on and off by the capacitor C 4 . When the primary switch T 1  turns on, the MOSFET T 2  is still conducting, but the current through the MOSFET T 2  reverses its direction when the voltage U starts to fall. The transistor T 5  senses this change via the capacitor C 4  and turns off the MOSFET T 2 . By charging the capacitor C 4  via the resistor R 4 , the time when the transistor T 5  turns on, i.e., when the MOSFET T 2  turns off, can be varied. 
     The transistor T 6  senses the voltage difference between the voltage across the capacitor C 3  and the voltage across the capacitor C 2 . The capacitor C 3  is charged to the peak voltage of the voltage U including any transient emanating from a non-desired turn-off current from the MOSFET T 2 , while the capacitor C 2  is charged to the voltage U excluding any transients since the resistor R 1  filters out any transients. 
     When the voltage difference between the capacitors C 3  and C 2  exceeds the base-emitter voltage of the transistor T 6 , the transistor T 6  will conduct and via the resistor R 4  set the off-time of the transistor T 5 , i.e. the on-time of the MOSFET T 2 , so that the voltage difference between the capacitors C 3  and C 2  will be smaller than a few volts after a number of switch cycles 
     With reference to FIGS. 2A-2D, the operation of the converter illustrated in FIG. 1 will now be described in more detail. 
     FIG. 2A illustrates a couple of cycles of the voltage UT 1  across the primary switch T 1 . The primary switch T 1  is supposed to be on from the beginning and is supposed to be turned off at times t 1  and t 1 ′ and turned on at times t 2  and t 2 ′. When the primary switch T 1  is on, the capacitor C 2  is charged via the resistor R 1  in series with the diode D 2 , and the capacitor C 3  is charged via the diode D 3 . At time t 1  when the primary switch T 1  turns off, the body diode D 1  of the MOSFET T 2  begins to conduct to charge the output capacitor C 1 . 
     If FIG. 2B, the source-drain voltage US-DT 2  of the MOSFET T 2  is illustrated. Base current will be supplied to the transistor T 3  via the resistor R 2 , and the gate G of the MOSFET T 2  will be charged causing the MOSFET T 2  to become saturated. 
     FIG. 2C illustrates the gate-source voltage UG-ST 2  of the MOSFET T 2 . At time t 2 , the primary switch T 1  turns on again. When the voltage across the MOSFET T 2  changes polarity at time t 2  as illustrated in FIG. 2B, the transistor T 5  becomes saturated via the capacitor C 4  and the resistor R 3 , and the MOSFET T 2  goes off. However, due to the fact that it takes some time to discharge the gate of the MOSFET T 2  after that the transistor T 5  is turned on, the MOSFET T 2  will turn off a little late causing a reverse current to flow from the output capacitor C 1  back into the transformer TR. This reverse current causes a voltage transient across the MOSFET T 2  when the MOSFET T 2  is off. 
     That transient, which is illustrated in FIG. 2B at time t 2 , charges the capacitor C 3  via the diode D 3  to a voltage that is higher than the voltage across the capacitor C 2 , causing the transistor T 6  to start conducting. The collector current from the transistor T 6  then sets the time when the transistor T 5  starts to draw gate charge from the gate G of the MOSFET T 2 . Towards the end of the next off-period of the primary switch T 1 , i.e. towards time t 2 ′, almost all gate charge has been drawn from the gate G of the MOSFET T 2  before the primary switch T 1  goes on again at time t 2 ′ as illustrated in FIG.  2 C. 
     When the primary switch T 1  turns on at time t 2 ′, the MOSFET T 2  is prepared, i.e., most of the gate charge has been drawn off and the MOSFET T 2  is not fully conducting. Thus, no reverse current spike will appear across the MOSFET T 2  at time t 2 ′ as apparent from FIG.  2 B. 
     FIG. 2D illustrates the base-emitter voltage UB-ET 5  of the transistor T 5 . At time t 1 , the voltage change across the secondary winding of the transformer TR turns off the transistor T 5  via the capacitor C 4  and the resistor R 3 . Hereby, the gate G of the MOSFET T 2  is charged via the resistor R 2  and the transistor T 3 . Between times t 1  and t 2 , the base-emitter voltage UB-ET 5  of the transistor T 5  is reversed and, consequently, the transistor T 5  is not conducting. 
     When the primary switch T 1  is turned on at time t 2 , the reverse voltage across the MOSFET T 2  turns on the transistor T 5  and turns off the MOSFET T 2 . Between times t 2  and t 1 ′, the base of the transistor T 5  is forward biased. Therefore, the gate G of the MOSFET T 2  is low, and the MOSFET T 2  is not conducting as is apparent from FIG.  2 C. 
     For the next period, i.e., the time between t 1 ′ and t 2 ′, the voltage difference between the capacitors C 2  and C 3 , caused by the transient appearing at time t 2 , turns on the transistor T 6  causing the capacitor C 4  to be discharged via the resistor R 4 . As a result, the transistor T 5  will start conducting at a time tx before time t 2 ′ as indicated in FIG.  2 D. 
     Also at time tx, via the transistor T 4 , the transistor T 5  will begin to draw gate charge out of the gate G of the MOSFET T 2  to turn off the MOSFET T 2  earlier than time t 2 ′ as illustrated in FIG.  2 C. 
     Thus, due to the earlier turn-off of the MOSFET T 2 , the transient or current spike that appeared at time t 2  will not appear at time t 2 ′ as illustrated in FIG.  2 B. However, should a current spike be present also at time t 2 ′, the MOSFET T 2  will be turned off earlier towards the end of its next on-period. As should be apparent from the above, no control signals have to be transferred from the primary side of the converter to control the MOSFET on the secondary side.