Microsemi

Colorado Division

800 Hoyt Street, Broomfield, CO. 80020, 303-469-2161, Fax: 303-466-3775

Page 6

Copyright

2000

2003-02-05 Rev. IR

SRM4010

40A Synchronous Rectifier Module

CO L O RA D O DI V I S I O N

Discontinuous/Light Load Operation

The SRM4010 is well behaved in light mode operation, which is

also called discontinuous conduction mode. In this mode of

operation, the inductor current goes to zero. During this mode,

there are times when both MOSFETS in the module are OFF.

Discontinuous operation can be at the normal switching

frequency or in the event the converter operates in a skipped

cycle mode.

The module senses a valid turn-on of the primary MOSFET by

the voltage rise on the Catch MOSFET Drain voltage (Pin1). In

the case of light mode operation, there is an extended period

where the primary-side power switch is OFF. During this period

of time, the voltage on the Drain can be ringing. The SRM4010

power module can distinguish between a voltage rise due to the

primary MOSFET being turned ON and ringing. The

distinguishing characteristic between the two cases is the speed of

the voltage rise. A voltage rise driven by the turn on of the

primary MOSFET is fast compared to ringing. The dV/dT

threshold for sensing a valid turn on can be adjusted by changing

the bias on the SPD pin (pin9). Biasing this pin to ground through

a resistor will make the normal turn on respond to a slower

transition. This could be necessary if a turn on snubber is used to

slow the normal turn on speed. Biasing this pin to the REGOUT

through a resistor makes the controller less sensitive. In some

cases where there is large high speed ringing on the transformer

waveform, it is necessary to add a R-C damper to change the

resonant frequency of the ringing so that there is more difference

between the normal turn on waveform and the ringing.

Current Mode Control and High Bandwidth

voltage mode control. The module can tolerate large step

changes in pulse width that can occur in high bandwidth control

loops without having cross conduction of the rectifying

MOSFETs.

For best efficiency, the current mode control should have

adequate ramp compensation to prevent cycle by cycle

instabilities where the pulse width is changing between

alternating cycles. This stabilization is accomplished in the

current mode control by adding in a compensating ramp to the

current feedback signal.

In the case of a sudden increase in load or drop in input voltage,

current mode control allows rapid increases in pulse width to

correct for the resulting output change. The Catch MOSFET

drive is momentarily reduced before the end of the next OFF time

when there is a sudden increase in ON time of the primary

Switch. The ON time then recovers to the normal prediction time

over a number of cycles. This technique prevents problems of the

Catch MOSFET being ON when it needs to be OFF, yet

maintains good efficiency in the steady state condition.

Internal 5V Regulator

The internal regulator is a linear regulator that requires at least

least 6.5 volts at the lowest point of the ripple. The voltage at pin

4 is normally generated from the internal diodes and a peak

holding capacitor of 0.47

µF. To achieve the required 6.5 volts

of input to the regulator, the peak transformer voltage during

either reset or the forward mode needs to be at least 1 volt above

this. This accounts for 0.5 volts of capacitor voltage drop and 0.5

volts of drop in the rectifier diodes. If an external load is added to

sure that a minimum voltage of 6.5 volts is maintained.

Externally injected voltage must be diode isolated to prevent

loading of the internal diodes.

In applications where the peak transformer voltage is below the

minimum 7.5 volts needed to provide proper regulator input

voltage, power can be supplied to the regulator input from an

additional secondary winding through a diode. The peak voltage

voltage of 20 volts. Voltages this high are undesirable however

since there is a power loss associated with the voltage drop in the

regulator.