QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 888A-B
36V-72VIN, ISOLATED SYNCHRONOUS FORWARD
3
ground directly across the +Vout and –Vout (or +Vin
and –Vin) terminals. See 0 for proper scope probe
technique.
1.
The optional input LC filter stage (C2/L5) lowers ac
input rms current. A power supply’s complete in-
put filter must have output impedance that is less
than the converter input impedance to assure sta-
bility. This may require a damping impedance. (See
Linear Technology Application Note AN19 for a
discussion of input filter stability.) A source with a
50mOhm or higher ESR at the filter resonant fre-
quency is one way of providing damping for the fil-
ter elements provided on the DC888A. For bench
testing, adding an 82uF electrolytic capacitor such
as a Sanyo 100MV82AX to the input terminals will
provide suitable damping and ripple current capa-
bility. The values selected have a filter resonant
frequency that is below the converter switching
frequency, thus avoiding high circulating currents
in the filter.
2.
Set an input power supply to a voltage of 36V.
Make sure that it is capable of 36V to 72V at a cur-
rent supplying capability of at least 8A per number
of phases being tested. Then, turn off the supply.
3.
With power off, connect the supply to the input
terVin and –Vin.
a.
Input voltages lower than 36V can keep the con-
verter from turning on due to the undervoltage
lockout feature of the LTC3725.
b.
If efficiency measurements are desired, an am-
meter capable of measuring at least 8Adc per
phase can be put in series with the input supply
in order to measure the DC888A-B’s input cur-
rent.
c.
A voltmeter with a capability of measuring • 72V
can be placed across the input to get an accurate
input voltage measurement.
4.
Turn on the power at the input.
NOTE:
Make sure that the input voltage •72V.
5.
Check for the proper output voltage of 5V.
6.
Turn off the power at the input.
7.
Once the proper output voltages are established,
connect a variable load capable of sinking 40A per
phase at 5V to the output terminals +Vout and –
Vout. Set current to 0A.
a.
If efficiency measurements are desired, an am-
meter or a resistor current shunt that is capable
of handling at least 40Adc per phase can be put
in series with the output load in order to meas-
ure the DC888A-B’s output current.
b.
A voltmeter with a capability of measuring at
least 5V can be placed across the output termi-
nals in order to get an accurate output voltage
measurement.
8.
Turn on the power at the input.
NOTE:
If there is no output, disconnect the load to
verify that the load is not set too high.
9.
Once the proper output voltage is established, ad-
just the load within the operating range and ob-
serve the output voltage regulation, ripple voltage,
efficiency and other desired parameters.