DRC1746_15 Datasheet, PDF (4/8 Pages) Analog Devices – HIGH POWER OUTPUT HYBRID DIGITAL TO SYNCHRO RESOLVER CONVERTERS

- ANALOGDEVICES FAX-ON-DEMAND HOTLINE
Page 13
DRC1745/DRC1746
POWER DISSIPATION, PULSATING POWER SUPPLIES
AND 8EA T SINKING
The DRC1745 and the DRCI746 can be used with conventional
de power supplies or a pulsating power supply on the output
stage (see Figure 3). The laner gives significant reductions in
power dissipation within the hybrid package without any attendant
loss of accuracy.
= ^{WAVEFORM MUST BE IN PHASE WITH CONVERTER REFERENCE}
ev""" A SIN ",tl CONSISTENT WITH MAINTAINING A POWER
SUPPLY EXCESS OVER THE OUTPUT WAVEFORM GR£ATER
-mAN V...
When using a pulsating power supply, full advantage can be
taken of the special design which allows the power supply to
have a very low de pedestal voltage. This results in minimized
v..
power dissipation. The pedestal voltage can in fact be as low as
3 volts. The combined pedestal plus peak supply voltage must
not exceed the absolute maximum rating.
Full accuracy is retained during operation on pulsating power
supplies because the output stage employing these supplies is
OBSOLETE ^{only used to provide current gain. OveralJ operational loop gain}
is independently powered. There are no special switch-on/switch.
off power supply sequencing requirements, and fulJ internal
protection is provided.
The section below demonstrates the power dissipation differences
for different load conditions when using dc supplies and pulsating
power supplies.
DC Power Supplies:
With inductive loads, the dc resistance is low compared with ac
impedance; therefore care should be taken to ensure that no dc
offset occurs at the sin and coo outputs. Note that under external
current limit conditions asymmetry of the power supplies could
occur, forcing a large dc offset to be present at the sin and cos
outputs causing heavy power dissipation in the device. Case
temperature must be maintained below 125°G.
As the reference input, ARI>is directly coupled, outPUt offset
will occur if any dc component is present at this input.
When using de power supplies, the expression for additional
IV Be USUAllY EOUALS v 0)
Figure 3. Pulsating Power Supply Format
Examples of Power Dissipation:
Many factors influence the power dissipation within the hybrid.
The folJowing two examples, using typical load values and worst
case digital angle conditions (45 degrees), illustrate the saving in
power dissipation which can be achieved by using Iipulsating
power supply employing a low pedestal voltage.
Note that in the following examples we have chosen:
V de = :t IS volts
Vp = 3volts
Vo = 9.6volts(6.8voltsrms)
VO£ = 9.6volts(shouldbe chosento equalYo)
II = 292mA(equivaJenttoa1.4VAmeanload)
load dependent power dissipation is:
1) DC power supply, e= 45° resistive load.
-;:-- ^{I I} p
=
2V.u)I',
Ir
(!Sme+ ,,",ooe-)
VoliCosa
2
(1)
Where V0 is the peak output voltage.
II is the peak valne of the output load current.
e is the digjtal angle.
exis the load phase angle.
Vde is the dc power supply voltage (usually :t 15 volts).
Pulsating Power Supplies:
When using a pulsating power supply, the expression for additional
load dependent power dissipation within the hybrid is:
I I I I p
=
2₌V_-'I:T_TLI_.!.(Sine
+ Cos6) +
-Va,,!1
11' ^(Sina-aCosa)
(2)
Where
Vlie is the peak ac component of the pulsating power
supply assumed equal to the peak output voltage, Vo.
I, is the peak value of the output load current.
e is the digital angle.
ex is the load phase angle.
Vp is the dc pedestal vol rage of the pulsating power
supply.
Vo
= Note that II = jZfwhere Vo Peak output voltage
= 2xVREF
Izj = output load
p
=
2x15xO.292(Sin4+5C° os45°-)
'IT
9.6xO.292xl
2
= 3.943 - 1.402
= 2.54 Watts
2) As example (I) but with a 3 volt pedestal pulsating power
supply.
From equation (2):
p = 2 x 3 x 0.292(Sin45° +Cos45°) + 9.6 x 0.292 x 0
'IT
1T
= 0.79 Watts
Thus the pulsating power supply has cut down the internal
dissipation by 1.75 watts, a ratio of 3.2:1.
A similar calculation using an inductive load shows a reduction
from 3.94 Watts, using a de power supply, to 1.68 Watts, when
a 3 volt pedestal pulsating power supply is used. Thus the pulsating
power supply has cut down the internal dissipation by 2.26
Watts, a ratio of 2.3:1.
The graph shown in Figure 4 shows the temperature at the
hottest part of the base of the hybrid (in the middle of the base
between" + 15V(P)" and the opposite uN/C" pin) for resistive
loads up to 2VA using de supplies and pulsating supplies with
pedestals of 3 volts and 5 volts.
Figure 5 shows a similar graph for inductive loads up to IVA.
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REV. A