Converters Application Report
SLVA085 - March 2000
1
Off-Line SMPS Failure Modes PWM Switchers and
DC-DC Converters
Dan N. Bennett
Analog & Mixed Signal Solutions
ABSTRACT
Todays voltage regulator modules (VRMs) employ off-line SMPS techniques. Traditionally,
they are usually discarded when they fail because they are so challenging to debug and so
inexpensive to replace. Investigation of these failed modules reveals some common failure
modes. Some failures are caused by other systems passive components that fail after
repeated electrical stress. Recognizing these common failure modes brings new life to a
switch-mode power supply.
Contents
Off-Line SMPS Operation
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SolutionCommon Power Supply Failures
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Testing and Repair
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Negative Voltage Output Too High
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Output Capacitor Replacements
6
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Output Diode Replacements
6
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Substitute Diodes
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Conclusion
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List of Figures
1 Typical Line Switch-Mode Power Converter
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2 The Push-Pull Circuit
3
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3 Typical Waveform at the Input and Output of the Driver Transistors in Continuous Mode
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4 Discontinuous-Mode Waveform
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Off-Line SMPS Failure Modes PWM Switchers and DC-DC Converters
Off-Line SMPS Operation
PC power blocks and other dedicated power supplies are known as
switch-mode power
supplies, or SMPS. They convert the line ac voltage to low-voltage high-current dc. A typical
SMPS can be simplified as shown in the block diagram of Figure 1. Figure 2 illustrates a typical
push-pull switcher circuit.
In most switching supplies, the 120-Vac or 240-Vac input first passes through a fuse and a line
filter. The ac input is then rectified by a full-wave bridge rectifier and filtered by a pair of
high-voltage capacitors. This creates two high-voltage sources from either side of the full-wave
bridge, one positive and the other negative. For 120 Vac at 60 Hz the dc voltage is
Vrms
×
1.414
×
2 = 338 V (
±
169 V). The ripple is now 120 Hz, which is easily filtered by the
high-frequency dc-to-dc power stage.
A pair of transistors is then used to switch these high voltage supplies across the primary
winding of a transformer. This switching action is very fast and typically switches at speeds
around 50300 kHz. An integrated circuit such as the TL5001 or TL494 is commonly used to
control the transistors at this switching rate. This IC not only controls the switching speed of the
transistors, but also controls the conduction time of each transistor. This
pulse-width modulation
(PWM) sets the
on and off duty cycle of the devices. The output voltage of the power supply is
determined by this timing.
Other features designed into the PWM controller help regulate, stabilize, and provide the
requirements for higher load current and instantaneous current. These power transistors charge
and discharge the transformer primary and thus induce power to the secondary winding.
Isolation between the line and the dc supply voltage is implemented using this transformer
mutual coupling.
The output of the transformer (which is now a pulse-width-modulated voltage at the switching
frequency) is then rectified by special high-speed diodes to change it back to dc. For the 5-Vdc
output, there are usually two diodes housed in a single package. This package is usually a
TO-220 or TO-218 three-leaded package. The 12-Vdc and 5-Vdc outputs each have their own
pair of output diodes. These outputs are not pure dc, and require extensive filtering to remove
the high-frequency component generated by the switching action of the transistors. Filtering is
accomplished using a combination of inductors and capacitors in a low-pass configuration.
The output voltage of the power supply is regulated by feeding some of the output back to the
integrated circuit that controls the switching transistors (see Figure 2).
Failure of the control IC can induce various power supply failure modes, from a reduction in
regulation and/or response, to complete output failure. Protection within the IC invokes
shutdown of all control signals, turning the module off.
Figure 1 shows a simple diagram of a typical line switch-mode power converter. This particular
one accepts 240 Vac, and incorporates line isolation using a transformers primary and
secondary windings, and an optoisolator to decouple the feedback line from the dc regulated
output. This block diagram symbolizes both single and synchronous transistor switch elements
and the associated control IC typically used. SLVA085
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Off-Line SMPS Failure Modes PWM Switchers and DC-DC Converters
Filter
240 V AC
Switch
Fuse
Rectifier
+
Reservoir
Capacitor
Switching
Regulating
Element
Control
Circuit
Transformer
Isolation
Barrier
Rectifier
Filter
Output
Fuse
Feedback
Figure 1. Typical Line Switch-Mode Power Converter
Figure 2 shows the ac and dc power paths with synchronous transistor switching elements. T1 typifies the line
isolation and the output buck elements of D2, D3, L1, and C3.
N1
N1
N2
N2
Q2
PWM Control
IC
C1
C2
+ VI
D1
D2
T1
n:1
LL
C3
RL
VO
Q1
Figure 2. The Push-Pull Circuit
The more common failure modes result from the power switching devices and from passive
components associated with the power supplies. Typically, failure modes do not reside in these
small control signalsthey are low voltage devices. These switching signals should still be
checked to assure proper operation. A check of the PWM control signal at the base of the power
transistor using a scope probe is a good way to verify the operation of the PWM controller.
WARNING:
Care should be taken when probing the primary-side components in an off-line
power supply, since lethal voltages are present at these nodes. Isolation of
ac-powered test equipment should also be required.
Care should also be taken not to load these nodes during test: using a low-capacitance scope
probe is best. Even low capacitance probes can affect the waveform. Figure 3 displays the
typical waveform at the input and output of these driver transistors in continuous mode. Figure 4
displays the discontinuous-mode waveform, and describes the contributions due to various
components.
In Figure 3, channel 2 is the switching voltage at the collector of one of the power devices
driving current into the transformer. The lower waveform of channel 1 is the actual gate signal
applied by the control IC. In this case, the dc offset of this signal is 146 V. SLVA085
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Off-Line SMPS Failure Modes PWM Switchers and DC-DC Converters
Figure 3. Typical Waveform at the Input and Output of the Driver
Transistors in Continuous Mode
In Figure 4, the drain-to-source voltage drops to Vsat when FET is on and the inductor current
increases. When the FET is turned off, the inductor current must continue to flow somewhere; as
a result, the drain-to-source voltage rises to forward bias the output diode (Vo + Vdiode) as the
inductor current discharges to zero. When the inductor is fully discharged, the drain node is
floating and the voltage rings due to parasitic currents, eventually returning to the input voltage.
Q1 on
(IL Increasing)
Inductor Flyback
(IL Decreasing)
IL = Zero
Note That Voltage Is
Approaching VI (5 V).
Power Switch
Drain Source and
Gate Source
Figure 4.
Discontinuous-Mode Waveform
SolutionCommon Power Supply Failures
Typically, only a small number of components fail in switching-regulator power supplies. The
most common failure is the switching transistors themselves. The transistors short-circuit,
causing massive amounts of current to be drawn across the transformer, blowing the input line
fuse. SLVA085
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Off-Line SMPS Failure Modes PWM Switchers and DC-DC Converters
This is the common failure mode for a bipolar transistor P-N junction during a short-load event.
The high current seen at the device junction displays a typical failure mechanism. During this
overcurrent, the junction depletion regions grow so large that the majority carriers occupy both
doped regions, creating a very low resistive region. The heat generated can cause a thermal
runaway condition. This first failure
state is thus the short circuit of the P-N, base-collector
junction.
This transistor failure is often caused by bad capacitors. It is extremely common to find output
filter capacitors that are swollen or leaking electrolytic material. Any capaci