Part One
Test Saturation Voltage to
Achieve High Efficiency
Build a low-cost saturation tester to measure
the saturation voltage of switching transistors
accurately in the presence of high switching
voltages or noise.
E
fficiency is the name of the game in the design
of power supplies. This is a major reason why
switch-mode power supplies (SMPSs) are chosen
for power-supply designs. However, even SMPSs
have their own losses, which are associated with
their switching transistors, be they MOSFETs, bipolar
transistors or insulated-gate bipolar transistors (IGBTs).
Understanding these losses via accurate measurements
can minimize the losses and increase efficiencies even
further.
This is the first part of a two-part article series on understanding saturation losses in SMPSs. Part one discusses
the contribution of saturation losses to power-supply inefficiency, how those losses are a function of a transistor’s
saturation voltage, and techniques for measuring saturation voltage. The second part, to appear in the next issue,
describes in detail how to build a novel low-cost tester for
accurately measuring saturation-voltage losses in the presence of high switching voltages or noise.
Forms of Inefficiencies
These inefficiencies generally take three forms: switching
losses, saturation losses and the power required to drive the
By Richard Dunipace, Principal Technical Marketer,
Standard Products Group, Fairchild Semiconductor,
Irving, Texas
transistors. Let’s examine each in more detail.
Switching losses result from the time it takes the transistor to switch from a high to a low level and back again,
and are affected by the parasitic capacitances of the circuit
and transistor. When switching, the transistor is in a linear
mode and hence highly inefficient. These inefficiencies are
typically controlled by making sure the switching device
transitions quickly and cleanly from one level to the other,
and by keeping the duty cycle of the transitions low versus the switching period. Parasitic capacitances are those
present in the circuit, but do not contribute to the power
output from the power supply. These are controlled by
careful layout and selection of the switching transistor and
power-supply topology.
Saturation losses are the main sources of inefficiency of
the three types of losses mentioned. The saturation voltage is one of the key items that determine the efficiency
of SMPSs. Saturation losses are a function of the voltage
dropped across the transistor due to the on-resistance of
the switching transistor when conducting and the current
flowing through the switching transistor. Saturation losses
are controlled by selecting a switching transistor with the
lowest saturation or on-resistance possible given other
VCC
+VCLAMP
C1
2 mA
Input
amplifier
Input
+400 V
0V
R1
R2
Sampling
diode
Switching
transistor C
C2
B
–VCLAMP
Fig. 1. When an oscilloscope is set to a high input-voltage range of say
200 V to 500 V, its typical input circuit can’t discern the low saturation
voltage of 0.5 V to 3 V for an accurate measurement to be made.
32
Power Electronics
Technology
March 2008
Figure
1
+9 V
E
(+)
(–)
+Clamp
+9 V
D2
D1
UF4007
Q1
Output
amplifier
D3
–Clamp
(+)
10 mA
–9 V
Output
(–)
Fig. 2. In this diagram, diode D1 can withstand up to 1 kV. This enables accurate measurement of low saturation voltages when an
oscilloscope’s input is set to a comparatively much higher voltage.
803PET23_fig2
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constraints such as cost, and by properly
driving the switching device.
Saturation probe
The power required to drive the switch(+) (+)
High-voltage
ing transistor does not contribute to the
High-side
To oscilloscope’s Floating inputs
Input Output
differential
probe
input
overall power output from the power supswitch
(-)
(-)
ply and thus lowers the overall efficiency
Load
of the power supply. These drive losses
Saturation probe
(+)
(+)
vary depending on the specific transisGround-referenced
Low-side
tor used, the operating frequency of the
Input Output To oscilloscope’s input
inputs
switch
(
)
(
)
supply and the type of transistor. Driving
losses associated with MOSFETs and
IGBTs are largely due to the input and
Miller capacitances of these transistors.
Half-bridge
Thus, these losses are a function of the
required switching and drive voltage, Fig. 3. A high-voltage differential probe is needed for accurate measurements of saturation
and power-supply switching frequency. voltage using a saturation probe in high-side switches. Low-side switches do not require
There is no constant bias required to keep such a probe.
MOSFETs or IGBTs conducting.
Figure 3
Bipolar junction transistors (BJTs) are different. They
C-B
forward
require a constant bias and specialized drive waveforms to
C-E
C
C
C
bias
saturation
turn them on and off efficiently. This is the reason MOSFETs
voltage
B
B
B
B-E
BJT
BJT
BJT
and IGBTs have largely displaced BJTs in recent years. This
C-E
forward
trend has slowed recently and, in some cases, reversed itself
reverse
voltage
E
E
E
voltage
B-E
as competition has forced many manufacturers to reduce
reverse
the cost of their power supplies.
voltage
(a)
(b)
(c)
BJTs are the lowest-cost switching transistors on the
market. They also offer better saturation performance
Fig. 4. A saturation probe can be used for accurate saturation-voltage
than MOSFETs, especially at high voltages, assuming that
measurements for the collector-to-base (a), the base-to-emitter (b) and
comparable transistors are evaluated. BJTs thus offer good
the collector-to-emitter junctions of bipolar junction transistor (BJT).
value but with increased drive losses and complexity.
Interestingly, drive losses often can be fully offset by
so large that it damages the input attenuator and that the
the power savings realized from the improvement in
clamp adequately
803PET23_fig4 protects against continuous overload. A
saturation performance and by using drive circuits that
saturation probe provides a way to ignore the high-voltage
maximize drive efficiency (e.g., proportional drive). Note
switching waveform, while producing an accurate look at
that the increased complexity is generally due to the lack
the saturation voltage (Fig. 2).
of power-supply chips specifically designed to drive BJTs.
As shown in Fig. 2, the voltage across the collector to
Some newer power-supply chips offer high drive efficiency
emitter, or drain to source (MOSFET) of the switching
with minimum complexity.
transistor, is sampled by diode D1, a UF4007, which has a
75-ns maximum switching time and will withstand 1 kV.
Measuring Voltage
It also has relatively low capacitance (17 pF at 4V). D1 is
An SMPS’s switch can have a high voltage across it in
forward biased by a 2-mA current source when the input
comparison to the saturation voltage, which makes it difvoltage is below 5.2 V. The diode disconnects the circuit
ficult to get an accurate saturation-voltage measurement.
from the switching transistor when the voltage across the
This is especially true in off-line supplies. If you use an
switching transistor is greater than 5.2 V, protecting the
oscilloscope and set it to a range that does not overload the
probe circuit.
oscilloscope’s input, then the 0.5-V to 3-V saturation voltage
Diodes D2 and D3 clamp any leakage currents or tranis lost in comparison to the 200-V to 500-V (or more) signal
sients. Note that the 5.2-V input limit was chosen to be
across the switching transistors in off-line supplies. If the
high enough to capture most reasonable saturation voltages,
oscilloscope is set to a low range to get the needed resoluwhile low enough to limit the maximum output swing to
tion, then the input is overloaded and may be damaged, or
allow 0.5-V/div input scaling on the oscilloscope without
will simply produce an inaccurate answer (Fig. 1).
overloading the oscilloscope’s input.
Fig. 1 shows a typical input circuit for an oscilloscope.
When D1 conducts, the voltage at the base of Q1 is apIf a large voltage is applied, causing the clamp diodes to
proximately one diode junction above the actual sampled
conduct, the input attenuator (all-pass network) will take
voltage. Output amplifier transistor Q1 (emitter-follower)
a charge and produce an input offset voltage on the wavedrops the voltage at its base-one junction to restore the
form. Of course, this assumes that the input voltage is not
sampled voltage at the output. Both the sampling diode
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33
Power Electronics Technology March 2008
smps efficiencies
Fig. 5. An oscilloscope image of the reverse base-emitter voltage
signal for a lamp-ballast switching BJT. This measurement is
important because many manufacturers do not properly limit
this voltage as per most application notes that specify no more
than -6 V or so (n-p-n devices).
and the output amplifier transistor
are biased with current sources so that
the current and offset for both devices
remain constant as the sampled voltage changes. This gives good X1 dc
Fig. 6. An oscilloscope image of the reverse collector-emitter
voltage of a lamp-ballast BJT plus the saturation voltage.
The saturation voltage waveform is not a flat slope as would
normally be expected because the lamp-ballast is semiresonant.
coupled performance from -7 V to
5.2 V. Each device also thermally compensates the other to give good thermal
tracking over laboratory temperature
ranges. Note that the 2-mA sampling
current flows through the switching
transistor. This is well below the current that normally flows through the
switching transistor in most applications and can be ignored. The current
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Power Electronics Technology March 2008
www.powerelectronics.com
smps efficiencies
was chosen to give moderate speed
while conserving battery power.
Notice that this circuit is intended
for use with ground-referenced powersupply switch transistors and directly
interfaces the input of an oscilloscope.
It is run by two 9-V batteries for safety
and isolation. If measurement of the
high-side transistor is required, the
output from the saturation probe can
be connected to the input of a highvoltage differential probe. The differential probe output is then connected
to the oscilloscope (Fig. 3). The top of
this figure shows how the saturation
probe can be used with a high-voltage
differential probe to make high-side
measurements.
A few of the possible measurements
that can be made with the saturation
probe are shown in Fig. 4. Remember,
the measurements can be performed
on MOSFETs, IGBTs, BJTs or diodes.
Figs. 5 and 6 are some pictures taken
from a lamp ballast evaluation.
Fig. 4a illustrates how the probe can
be used to make collector-base voltage
measurements on BJTs. Fig. 4b shows
forward- and reverse-voltage measurements on the base-emitter junction,
and Fig. 4c illustrates how the probe
might be used to measure both saturation voltage and base-emitter (and for
FETs, drain-source) reverse voltage.
Fig. 5 shows both the forward and
reverse base-emitter voltage for a
lamp-ballast switching BJT transistor.
The reverse-voltage measurement is
important with BJTs because many
manufacturers do not properly limit
this voltage as per most application
notes that specify no more than -6 V
or so (n-p-n devices). This allows some
margin for overshoot.
In fact, the reverse voltage can be
limited to -0.7 V with excellent results.
Beyond -6 V, there is evidence that the
safe operating area (SOA) of the BJT
starts to degrade. The reason for the
reverse voltage is to rapidly turn off
the transistor and to be able to use the
reverse-bias SOA breakdown voltage
(VCBO) rather than the forward-bias
SOA (VCEO).
If the emitter-base is zenered, the
transistor can avalanche, producing
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very high current flow and catastrophic failure in most applications.
This is especially true for half-bridge
applications.
Fig. 6 illustrates the reverse collectoremitter voltage of a lamp-ballast BJT
plus the saturation voltage. As noted,
the saturation voltage is one of the
main inefficiencies in an SMPS. The
lower the saturation voltage, the better the power-supply efficiency. Note
that the saturation-voltage waveform
is not a flat slope as would normally
be expected, because the lamp ballast
is semi-resonant.
Measurement of the reverse collector-emitter (or drain-source) voltage
is important in BJTs or FETs. With
BJTs, the reverse voltage indicates the
possibility that the collector base could
be forward biased during switching.
If this happens, the forward current
can extend the time it takes to turn
off the transistor (extend storage time)
and could potentially cause transistor
failure if the discharge time constant
is large enough to allow the transistor that’s off to still conduct when the
alternate transistor turns on.
Under these conditions, the full
power-supply voltage would be shorted
through the half-bridge transistors,
resulting in very high instantaneous
power and possible catastrophic failure
in microseconds. This issue could be
of particular interest if a lamp-ballast
output is opened momentarily due to a
bad connection or simply when lamps
are changed out without turning off the
power, as can occur in normal lampfixture maintenance. This is the reason
that freewheeling diodes are placed
across the collector-emitter junction
in most applications.
With both MOSFETS and BJTs,
freewheeling or reversal diodes provide
a path for an inductive or resonant
tank current during transistor turnoff. These diodes provide a discharge
path to avoid forward biasing the
collector-base junction of BJT switching transistors (reverse current protection), or gate breakdown and possible
catastrophic failure with MOSFETs.
The diodes should be very fast-acting
types. PETech
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Power Electronics
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2008
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