Modern integrated logic circuits and microprocessors require high power at low voltage, and this presents considerable challenges to power supply engineers. High power at low voltage is usually associated with high current. In most power converter, there is at least one diode rectifier connected in series with the converter output. The forward voltage drop of rectifier diodes becomes significant when the output voltage is low. The output voltage of modern power converters for computers can be 2.2V or less while the forward voltage drop of a silicon diode is 0.7V. High output current produces a lot of losses in these output rectifier diodes. Such losses reduce efficiency of low voltage converters significantly.
A common way to reduce losses in an output rectifier is to replace it by a transistor, which is known as a synchronous rectifier. This makes use of the low forward voltage drop of a transistor, like a MOSFET, which can reduce losses significantly. However, a transistor is an active device that needs to be driven. It has to be given a driving signal in synchronism with the time at which it is required to turn on. This becomes an important issue in synchronous rectifier.
In a forward converter, a popular prior art synchronous rectifier makes use of the secondary winding of the main transformer as a means for driving the synchronous rectifier. The gates of the synchronous MOSFETs are connected to two terminals of the main transformer secondary winding. Alternating voltage at the secondary winding drives the MOSFET in synchronism with the converter main switch. This method is simple but vulnerable to input line voltage variation. At high input voltage the secondary transformer voltage may be too high for the MOSFET gates while at low input voltage the secondary transformer voltage may not be sufficient to drive the MOSFET gates. This limits the input voltage range of such converters. On the other hand, leakage inductance of the main transformer brings about a time period in which the body diodes of both synchronous MOSFETs conduct simultaneously while there are no gate drive voltages. MOSFET body diodes has high forward voltage drop and dissipation in this time period becomes significant especially at high converter switching frequency. Therefore this type of self-driven synchronous rectifier has limitations in its input voltage range, converter switching frequency and efficiency.
A lot of researchers attempted to tackle the problem of driving synchronous rectifier. In U.S. Pat. No. 5,179,512, Fisher invented a gate drive circuit for synchronous rectifier but it can only be used in resonant converters. In U.S. Pat. Nos. 5,126,615 and 5,457,624, Gauen and Hastings respectively invented drive circuits but they can be applied to non-isolated buck converters only. In U.S. Pat. No. 5,303,138 Rozman invented gate drive circuits but did not solve the problem of wide input voltage range. In U.S. Pat. No. 5,097,403 Smith used current sense rectifiers and electronic circuitry to detect current but it is applicable to MOSFET devices with current sense facility only. In U.S. Pat. No. 4,922,404 Ludwig went into the complexity of using a microprocessor to drive synchronous rectifiers.
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