A several kilowatt power level, fast-variable linear resistor was synthesized by using analog components to control the conductance of power MOSFETs. Risetimes observed have been as short as 500 ns with respect to the control signal and 1 to 2 microseconds with respect to the power source voltage. A variant configuration of this load that dissipates a constant power set by a control signal is indicated. Replacement of the MOSFETs by static induction transistors (SITs) to increase power handling, speed and radiation hardness is discussed

Niedra, Janis M.

English

NASA Technical Reports Server

NASA Contractor Report 187155
9dz; _Z_
Analog Synthesized Fast-Variable
Linear Load
Janis M. Niedra
Sverdrup Technology, Inc.
Lewis Research Center Group
Brook Park, Ohio
('JA_w-L._"-IbTl-_b) A_,IALL, C: _YNIH_TSI_L r)
i-AST-VAOI_L:,LL LI[_FA# LtJAL_ Fin,_l r_,rtpor+
(_,v_r_ru_; [echnolo:,y) 7 r) CSCL O'yC
July 1991
(;_/J3
N91-2 74,"
Prepared for
Lewis Research Center
Under Contract NAS3-25266
RI/ A
National Aeronautics and
Space Administration
https://ntrs.nasa.gov/search.jsp?R=19910018132 2020-03-19T16:36:52+00:00Z

ANALOG SYNTHESIZED FAST-VARIABLE LINEAR LOAD
Janis M. Niedra
Sverdrup Technology, Inc.
NASA Lewis Research Center Group
Cleveland,Ohio 44135
ABSTRACT
A several kilowatt power level, fast-variable linear
resistor has been synthesized by using analog
components to control the conductance of power
MOSFETs. Risetimes observed have been as short as
500 ns with respect to the control signal and 1 to 2/_s
with respect to the power source voltage. A variant
configuration of this load that dissipates a constant
power set by a control signal is indicated. Replacement
of the MOSFETs by SITs to increase power handling,
speed and radiation hardness is discussed.
INTRODUCTION
Fast-variablelectronicallysynthesizedelectricalloads
are thought to be desirableforcontrolofcertainquick
responding space power generatingsystems that work
best deliveringa constant power, as for example the
free-piston Stirling engine coupled to a linear
alternator[II.Such loadsare alsousefulfortestingthe
transient response, or stability,of various power
generating and conditioningsystems to sudden load
variations.
This paper describes a several kilowatt power level
linear resistor of variable value that has been
synthesized electronically by using a fast analog
multiplier and operational amplifier to control the
conductance of one or more power MOSFETs. Unlike
loads synthesized by digital switching of discrete
resistors [21, here the control of resistance is very
smooth. Beacause of feedback, this control follows a
precise, device-independent rule. Moreover, the
observed 500 ns risetime with respect to the control
signal is considerably faster than the response of a
known pulse-width modulator controlled load {3]. The
potential for even higher speed and power based on the
use of alternative transistors is pointed out. And this
circuit may be reconfigured to dissipate a constant
power, as set by a control signal, regardless of
variations of the source voltage within some range.
PRINCIPLE OF OPERATION
Figure 1 shows the principle of operation to be that of
a controlled current regulator, which, apart from the
multiplier, is similar to the scheme used in a known
current pulser [41. The instantaneous load current {I}
produces a voltage drop IR s across the (fractional
ohm) current sensing resistor (Rs). Within the range
of normal operation, where the current control
MOSFET shown is not saturated, the gate driving
operational amplifier enforces the equality
IRs = k Vc V, (1)
where V C isa controlvoltageand k isthe product of
the multiplierconstant and any attenuation factor
appliedtothe sourcevoltage(V).Thereforethiscircuit
appears to the power sourceas a resistorofvalue
R S
R-V/I- kV¢ (2)
Note that in thissimple dc analysisthe value of the
seriesload/current limiting resistor(RL) does not
enter,exceptas the dominant part ofthe leastpossible
R, given by
Rmi n -- R L + RDS,o n ÷ R s ; (3)
the on-state resistance (RDS,o) of a high power
MOSFET isusuallyfractlonalo_m. No higher order
analysis is attempted here. Note too that V C may
readilybe made composite,such as V C _ (VC + V0) ,
sincemodern analog multipliersoften have summing
inputsto acceptsome additionalvoltageV 0.
Power Ratings
The restrictions that heat sinking and dissipation
ratings of R L and the power transistor impose on the
maximum permissible continuous source voltage
(Vmax) are easily derived. Assuming controlled case
temperatures, the dissipation limits PT,m for thea_.
MOSFET and PRL,max for R L axe then specified; the
absence of secondary breakdown in a MOSFET is
clearly an advantage here. Neglecting R s and RDS,on,
transistor operation will be within its Safe Operating
Area, provided
2 (4)1/4 Vmax/RL -_ PT,max '
and similarly heat damage to R L will be avoided if
2
Vm_/RL -< PaL,max ' (5)
It follows that the minimum power rating of R L must
be 4 times that of the MOSFET to insure against heat
damage at minimum R.
Increased Power
Paralleling resistors and transistors in order to increase
load power is quite practicable with this circuit. One
such arrangement that replaces the RL-MOSFET
combination of Figure 1 is illustrated in Figure 2.
Although paralleling of resistors is easy, the paralleling
of MOSFETs, which are hardly ever perfectly matched,
requires care if performance is not to be compromised
by unequal division of current. If the mismatch is not
great, then the standard remedy shown of using low
value source lead resistors suffices to enforce current
sharing, but otherwise one may need more involved
methods [5]. The small ferrite toroid shown in the gate
leads chokes off any tendency of the transistors to
oscillate in a circular mode. Two or more of the load
units defined by Figure 2 may in turn be parallel
connected to the same driver if the burden of driving
the increased gate capacitance and the likely loss of
speed are acceptable.
TEST CIRCUIT
The circuit outlined in Figure 1 was implemented
using a Burr-Brown type MPY634BM wide bandwidth
analog multiplier and a type 3554BM fast-settling
operational amplifier for the driver. This multiplier has
a l0 MHz bandwidth and a :i=0.5% maximum
4-quadrant error, and the operational amplifier is rated
to slew at 1000 V/#s and to settle to within :i=0.05%
in 150 ns. However, this multiplier has a 1% settling
time of 1 or 2 _s and this driver is limited to 0.1 A
output current. Two 350 V load units, each as shown
in Figure 2, were constructed on water cooled copper
base plates and parallel connected to the same driver
through 2 ft long coaxial cables. The type MTM15N40
is a 15 A, 400 V, 250 W, N-channel, enhancement
mode power MOSFET - each load has two of these in
parallel. Each of the 3 standard, aluminum housed,
noninductive, 250 W rated resistors can dissipate at
least 450 W to the water cooled plate, which is
somewhat short in total power of the desired 4 times
the combined MOSFET power. Circuit details not
shown include a provision to control the driver output
to just below the MOSFET gate threshold voltage for
the state of zero source voltage V. This obviates the
need to move unwanted gate charge and thereby
improves risetime in the case of a V-pulse rising from
zero.
Perform ance
From the applications point of view, the aspects of
most interest were taken to be risetimes with respect to
both source voltage and control signal, linearity of the
load current with respect to each of these voltages, and
power dissipation capability. These interrelated
properties are presented for the test circuit only to
show what can be achieved with readily available
components, but are otherwise very much dependent
on circuit layout, component type, and power and
current levels in complicated ways.
With source fixed at 350 V, the load current rose at
about 16 A/#s maximum rate in response to a step in
V C. Output current limiting of the gate driver was
found to be a large contribution to this limiting slew
rate. The load current response to a 350 V source step
rising in 1 /zs, with V C l_Lxed to give R = 76 12, was a
waveform of similar rise rate, but modulated by
oscillatory transients. Thus the onset of these
instabilities limited the usable source rise time to
about 1 to 2 gs. Load current traces produced by the
above control and source step inputs are compared in
Figure 3.
Several experiments were performed to test the
bilinearity of the load current in V c and V, _s
predicted by Eq. 1, since this property is important for
use of the load in a control loop. These tests were
based on large amplitude triangular waveforms ranging
in frequency from 1 to 250 kHz for V C and from 1 to
20 kHz for V; V-source cutoff was responsible for the
latter 20 kHz limit. Figures 4A and 4B show that good
linearity of load current with V C was obtained up to
250 kHz, a 0.4 /zs time delay and increased corner
rounding being the only apparent infidelities at the
high frequency. A 1 kHz triangular source waveform,
with R--67 12, produced a load current trace that
could be brought into indistinguishable overlap with
the source waveform on the oscilloscope, as shown in
Figure 5A. This experiment was repeated at 20 kHz
and higher powers, with R set to 37 12 and 67 fl, and
again practically congruent traces were obtained. The
20 kHz, 67 12 traces are reproduced in Figure 5B.
Each load unit could consistently dissipate power up to
the limits of its components, provided its base plate
temperature was controlled and current sharing among
its directly paralleled MOSFETs was enforced.
Deviation from equal current division among the load
units was less critical because each unit remains
individually protected. Should a MOSFET fail,
experience has shown this circuit to be resistant to
chain-reaction failure of the remaining MOSFETs. An
overheated MOSFET usually short circuits and hence
hogs the current, taking the load off the other
MOSFETs and throwing the burden onto the resistors.
This allows ample time for a safe shutdown or even a
partial recovery through fused isolation of the failed
unit. In my experience, the gate drive amplifier was
never damaged by such a MOSFET failure.
SUMMARY AND DISCUSSION
ACKNOWLEDGMENTS
This work was sponsored by the NASA Lewis Research
Center under contract NAS3-25266, with G.E.
Schwarze as the Project Manager.
1.
5.
A several kilowatt power level linear resistor of
variable value has been synthesized electronically by
using a fast analog multiplier and operational amplifier
to control the conductance of one or more power 3.
MOSFETs. These transistors in series with low
inductance power resistors are all mounted on a water
cooled base plate and comprise a load unit. Two or 4.
more such load units can be operated in parallel from
the same controller in order to increase the power
dissipation capability. Experience has shown that this
circuit is resistant to chain-reaction failure of the 5.
power MOSFETs. The electronic control of the
resistance of this load is smooth and fast: load current
risetimes of 0.5 _s have been observed with respect to
the control signal. Usable risetime with respect to the
power source voltage was a slower, but still 6.
respectable, 1 or 2 _s.
This circuit may have the potential of even higher
speed and specific power if the MOSFETs are replaced
by a vertical channel power JFET called a static
induction transistor, or SIT [6,7}. SITs have V-I
control characteristics resembling those of a triode
vacuum tube. In that sense a SIT is a closer
approximation to a controllable resistor than is a
MOSFET and hence may provide a faster response in
the present circuit. Also, SITs are available in
packages rated up to several kW and are highly
resistant to fast neutron fluences up to at least 101_
n/cm 2 and to gamma irradiation up to at least 1.8
Mrads [8]. High power, speed and radiation hardness
may make the SIT highly desirable in this load
controller for use near space nuclear power reactors.
A potentially useful, although untested, variant of this
load is the following. By reconfiguring the multiplier to
form the quotient (Vc/V) of the control to the source
voltage, this circuit can be made to dissipate a given
power, as set by Vc, regardless of the variations of V
within some bounded range. This mode of control may
be useful in compensatory parasitic loading of constant
power type generating systems.
7.
8.
REFERENCES
J.C. Slaby, "Free-Piston Stirling Technology for
Space Power", NASA TM-101956, June 1989.
W.L. Ross, "Electronically Controlled Resistor
Bank", NASA Tech Briefs, p. 30, July/August
1987. See also "A Digitally Controlled System for
Effecting and Presenting a Selected Electrical
Resistance", NASA invention disclosure case
MFS-29149-1, NASA accession no. N87-29737.
H.T. Jones, "Linear Load Dissipates Constant
Power", EDN 32, 179 (4 March 1987}.
J. Casas and J.-F. Loude, "Fast Current Pulser for
Superconducting Samples", Cryogenics 27, no. 7,
379 (July 1987).
J.M. Niedra, "Paralleling Power MOSFETs in
Their Active Region: Extended Range of Passively
Forced Current Sharing", NASA CR-180902,
NASA Lewis Research Center, July 1989.
S. Teszner and R. Gicquel, "Gridistor - A New
Field-Effect Device", Proc. IEEE 52, 1502 (1964).
J. Nishizawa et al., "Recent Development of the
Power Static Induction Transistors in Japan", in
Proc. of the 14th International PCI, Long Beach,
CA, 14-17 September 1987, and references therein.
A.J. Frasca and G.E. Schwarze, "Neutron and
Gamma Effectson the Electricaland Switching
Characteristics of Power Static Induction
Transistors (SITs)_, in Proc. of the 7th
Symposium on Space Nuclear Power Systems,
Albuquerque, NM, 7-10 January 1990.
Vc C
V
ATrENUATOR I
RL_ II
D
k Vc _vvi+
/
MULTIPLIER I Rs _Rs
Functionaldiagram of synthesized linearload. Notethat
V > O is required by the N-channelMOSFET.
Figure 1.
D D
FERRITE S
MTM15N40
(15A,400V,250W)
Figure 2. A 1500 W load unit using paralleledcomponents. This unit replaces
the RL-MOSFET combination in Fig. 1.
A-- 12
_- 8
8
4
o
..J
0
I
io
0
0
A. A load currentstepcaused by a step in the control
voltageVc. Source: 350 V. The Vc shownvaries
the current between 10%and 90%of maximum.
Figure 3.
6
4
2
0
4OO
IO0
B,
v
-_12 _.sI-.,I--
A loadcurrent step caused by a 350 V sourcestep.
Vc is fixed to give R = 76 Q.
Load current response to control signal and source voltage step inputs.
Two paralleled load units were driven through 2 ft. long coaxial cables.
-- 12
8
4
0
0
I
_o
8
/\ /\
/ "_ /- \
/ \ / _,
1 \ r/ _/ N
/ \ / \
I \ / \
/ \ /
A. Control signal is a 1 kHz triangularwave.
Figure 4.
12
8
4
0
4
2
0
/"x \ / ,,
j \j \,/
#\ /_.
/\ /\
/ \/ \/
B. Control signal is a 250 kHz triangular wave.
Load current response to triangular wave control signals.
Same load as in Figure 3,
Source: 350 V.
I 1.5
=_ 1.0
0.5
o
0
o 100
> 50
_ o
/\ /\
/ "% / ,
/ _/
/\, /\/ \ /
/ \/
2
\
0
2O0
\ _°i
--_12oo_.9-
A. Source voltageisa 1 kHz triangularwave.
Figure 5.
\ / \ /
\/ \/
\ /\ // \
\ / k j
\/ \/
B. Source voltage is a 20 kHz triangular wave.
Load current response to triangular wave source voltage variation.
Vc is fixed to give R = 67 _. Same load as in Figure 3.
o
N/ A
Report Documentation Page
Adm|r_l_'stion
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA CR- 187155
4. Title and Subtitle
Analog Synthesized Fast-Variable Linear Load
7. Author(s)
Janis M. Niedra
9. Performing Organization Name and Address
Sverdrup Technology, Inc.
Lewis Research Center Group
2001 Aerospace Parkway
Brook Park, Ohio 44142
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135- 3191
5. Report Date
July 1991
6. Performing Organization Code
8. Performing Organization Report No.
None (E- 6377)
t0. Work Unit No.
590-13-11
t 1. Contract or Grant No.
NAS3-25266
13, Type of Report and Period Covered
Contractor Report
Final
14. Sponsoring Agency Code
15. Supl_ementary Notes
Project Manager, Gene E. Schwarze, Power Technology Division, NASA Lewis Research Center, (216) 433-6117.
Prepared for the 26th Intersociety Energy Conversion Engineering Conference cosponsored by ANS, SAE, ACS,
AIAA, ASME, IEEE, and AIChE, Boston, Massachusetts, August 4-9, 1991.
16. Abstract
A several kilowatt power level, fast-variable linear resistor has been synthesized by using analog components to control
the conductance of power MOSFETs. Risetimes observed have been as short as 500 ns with respect to the control
signal and 1 to 2 ps with respect to the power source voltage. A variant configuration of this load that dissipates a
constant power set by a control signal is indicated. Replacement of the MOSFETs by SITs to increase power handling,
speed and radiation hardness is discussed.
17. Key Words (Suggested by Author(s))
Linear load
Load controller
Power circuit
Electronic resistor
!l1g. Security Classif. (ol the report)
Unclassified
18. Distribution Statement
Unclassified - Unlimited
Subject Category 33
20. Security Classif. (of this page) 21. No. of pages
Unclassified 6
NASAFORM162(IOCT86 *For sale bythe NationalTechnicalInformation Service,Springfield,Virginia 22161
22. Price*
A02


Analog synthesized fast-variable linear load

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server