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參數(shù)資料
| 型號(hào)： | SRM4010 |
| 英文描述： | DC:DC Converter |
| 中文描述： | 直流：DC轉(zhuǎn)換器 |
| 文件頁(yè)數(shù)： | 6/11頁(yè) |
| 文件大小： | 224K |
| 代理商： | SRM4010 |
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
W
M
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SRM4010
40A Synchronous Rectifier Module
COLOR ADO DIVISION
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
Converters
The SRM4010 accommodates current mode or
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
1.5 volts of headroom. The voltage on pin 4 REG
IN
should be at
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
μ
F. 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
the REG
IN
pin or the REG
OUT
pin, care must be exercised to make
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
fed to the REG
IN
pin should be below the absolute maximum
voltage of 20 volts. Voltages this high are undesirable however
since there is a power loss associated with the voltage drop in the
regulator.
T
H
P
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