©2002 Fairchild Semiconductor Corporation

Application Note 7510 Rev. A1

and compared to the measured response. Switching wave-

forms of the power MOSFET are also modelled and com-

pared to the measured results.

SYNCHRONOUS RECTIFIER

The schematic of a synchronous rectifier circuit is shown in

Figure 2. The rectifier power MOSFETs are a pair of cross

coupled RFH75N05 megafet devices. Conduction is offered

by a forward gate bias with negative drain current (third

quadrant mosfet operation) and voltage blocking is assured

by a slightly negative gate bias for first quadrant MOSFET

operation.

VM1 to VM4 are voltage sources of zero potential and are

used to permit a recording of branch currents. The trans-

former secondary normally used in a supply of this sort is

represented by voltage source V1 and leakage inductance

L2. Filter inductor L1 and capacitor C1 provide energy stor-

age and smoothing for the 100KHz square wave of V1. Rise

and fall times of the square wave are not critical, but were set

at 40ns. Gate coupling resistors R1 and R2 are somewhat

critical, in that too high a value will restrict the conduction

transition time of the MOSFET. Alternatively, a value too low

will permit a high voltage drain spike to appear on the gate of

the MOSFET.

The calculated output voltage turn on transient is shown in

Figure 3. Of course this represents a feed forward circuit

response only. In practice, the modelled drive circuit with

pulse width modulation and feedback would provide a much

faster response which would be slew rate limited. The ripple

voltage is 5mV RMS.

The efficiency for this portion of the synchronous rectifier

circuit is plotted in Figure 4 as a function of temperature from

PROBE (PSPICE's waveform plotter routine) is included.

This equation yields a solution rapidly.

Transition voltage waveforms of the input voltage, one drain

voltage, and one gate voltage are plotted in Figure 5. The

value of drain voltage during third quadrant conduction is

approximately -0.2 volts. Other waveforms are readily

available by use of the PSPICE system.

L2

10

9

2

4

5

V1

L1

20

µh

2m

Ω

C1

100

µF

R4

0.25

Ω

+

-

VM3

VM4

VM1

VM2

XM1

RFH75N05

XM2

RFH75N05

3

1

R3

-

+

-

+

-

+

-

+

8

40nh

R2

5

Ω

R1

5

Ω

6

7

11

8

FIGURE 2. SYNCHRONOUS RECTIFIER CIRCUIT

5

3

1

100

200

300

TIME (

µs)

POWER OUT = 100W

FREQUENCY = 100kHz

FIGURE 3. RECTIFIER OUTPUT VOLTAGE

050

100

98

96

94

92

POWER OUT = 100W

FREQUENCY = 100kHz

EFF = (1/(1 + AVG(AVG(I(VM4) * V(8)

+ I(VM3) * V(7) + I(R3) * V(4.5)

))/AVG(AVG(I(R4) * V(5))))

FIGURE 4. RECTIFIER EFFICIENCY

20

10

0

-10

100

300

5100

5300

TIME (ns)

V(9.2)

V(7)

V(3)

INPUT V(9.2)

GATE V(3)

DRAIN V(7)

FIGURE 5.

TRANSITION VOLTAGE WAVEFORMS

Application Note 7510