Section 4: Theory of operation
Model 2657A High Power System SourceMeter® Instrument Reference Manual
4-2
2657A-901-01 Rev. B/December 2012
Compliance limit principles
A compliance limit acts as a clamp. If the output reaches the limit value, the High Power System
SourceMeter
®
instrument attempts to prevent the output from exceeding that value. This action
implies that the source will switch from a V-source to an I-source (or from an I-source to a V-source)
when a limit is reached.
As an example, assume the following:
System SourceMeter
®
instrument: V
SRC
= 1000 V; I
LIMIT
= 10 mA
Device under test (DUT) resistance: 10 k
Ω
With a source voltage of 1000 V and a DUT resistance of 10 k
Ω
, the current through the DUT should
be: 1000 V / 10 k
Ω
= 100 mA. However, because the limit is set to 10 mA, the current will not exceed
that value, and the voltage across the resistance is limited to 100 V. In effect, the 1000 V voltage
source is transformed into a 10 mA current source.
In steady-state conditions, the set compliance limit will restrict the Model 2657A output. This holds
true except for the limit conditions, as described in
(on page 2-18) or for fast
transient load conditions.
The Model 2657A can also be set to limit power. This limit can be set in addition to any voltage or
current limits specified. The power limit restricts power by lowering the present limit in effect (voltage
or current) as needed to restrict the SMU from exceeding the specified power limit. The limit operation
of the instrument changes dependent on the source mode (current or voltage), load, and the
configured limits (current, voltage, and power). For additional details on using limits, including load
considerations when specifying both a current (or a voltage) limit and a power limit, see the
(on page 4-4) topic.
For information on implementing limits, see
(on page 2-19).
Overheating protection
Proper ventilation is required to keep the High Power System SourceMeter
®
instrument from
overheating. Even with proper ventilation, the Model 2657A can overheat if the ambient temperature
is too high. The High Power System SourceMeter
®
instrument has an over-temperature protection
circuit that will turn the output off if the instrument overheats. When the over-temperature protection
circuit turns the output off, a message indicating this condition is displayed. You cannot turn the
output on until the instrument cools down.
Power equations to avoid overheating
You do not have to be concerned about overheating if the ambient temperature is
≤
30 °C (86 °F); if
the ambient temperature is > 30 °C (86 °F), the instrument may overheat if operated in a manner that
exceeds the calculated maximum duty cycle, DC
MAX
. Calculate DC
MAX
using the
(on page 4-3). Exceeding the calculated maximum duty cycle may cause the temperature
protection mechanism to engage. When this happens, an error message is displayed and the
instrument output is disabled until the internal temperature of the instrument is reduced to an
acceptable level.
The maximum duty cycle equation is derived from the power equation below by solving for
DC
MAX
.
The general power equation describes how much power an instrument channel can source and sink
before the total power cannot be fully dissipated by the instrument's cooling system. This equation
incorporates all of the factors that can influence the power dissipated by the instrument.
Summary of Contents for 2657A
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