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Texas Instruments Incorporated
Power Management
Save power with a soft Zener clamp
By John Betten
Applications Engineer and
Senior Member, Technical Staff
Figure 1. Zener diode with resistor provides soft clamp for
no-load output voltage
Flyback converters are wildly popular due to
their low cost, their isolation, and the ease
with which additional output voltages can be
implemented. For multiple-output flybacks,
one output voltage—typically the highest-
power output—is tightly regulated by means
of feedback to the control circuit. Additional
outputs are usually added by tightly coupling
transformer windings to the main regulated
winding. Linear regulators or DC/DC switch-
ers may be added, or the outputs can be left
unregulated. This last option is the most
efficient, but many times voltage regulation
suffers when the outputs are heavily or
lightly loaded while the main output voltage
has the opposite load level. This cross-
regulation problem is highly dependent on
the transformer leakage and winding struc-
ture, as well as on other parasitic circuit
components. One of the worst scenarios is
when the main output is heavily loaded and the unregu-
lated winding is completely unloaded. Any voltage ringing
present on the transformer’s secondary winding is often
peak-detected by the output rectifier, causing the unregu-
lated output voltage to greatly increase. It is not uncom-
mon for the output voltage to rise to twice its nominal
voltage in this situation. This can be catastrophic to any
downstream load that cannot tolerate a higher voltage or
that does not present minimal loading at all times to
dissipate the leakage energy.
Several solutions can remedy this no-load overvoltage
condition. The simplest solution would be to add a preload
to the unregulated output in the form of a resistor. This
will load the output enough to dissipate the leakage energy
and to lower the output voltage to an acceptable level.
Unfortunately, this load will always be present and causes
a loss of efficiency that is often considered unacceptable.
A second option is to simply add a Zener diode to the
unregulated output. The diode’s voltage rating must be
set higher than the nominal output voltage after the typi-
cal 5% or 10% part tolerance is included. This means the
diode won’t conduct or dissipate power until the output
voltage rises high enough. While this may seem like an
ideal solution, several potential problems exist. Once the
Zener diode conducts, its impedance drops significantly
and provides little resistance to current flow. The current
flow into the diode, and hence the power dissipated in it,
is determined by parasitic circuit components and thus is
9.5
9.0
8.5
8.0
No Clamp
115- Resistor
and 6.8-V Zener
7.5
7.0
6.8-V Zener
6.5
6.0
0.00
0.01
0.02
0.03
Output Current (A)
hard to control. Higher-power converters can potentially
source a large current and easily destroy a Zener diode.
For this reason, it is risky to add a small Zener diode and
difficult to calculate the power dissipated.
Another option is to use a snubber to dissipate the leak-
age energy. This generally dissipates more power than
using a preload resistor and does not always provide as
much no-load voltage reduction on the output.
A soft Zener clamp, which consists of a resistor in series
with a Zener diode, can provide a good compromise. It
can clamp the unregulated output voltage to a level that is
lower than that of the unclamped output voltage but higher
than that of a Zener diode alone. To determine the resis-
tor’s value, the output can be loaded with just enough
current to reduce the high output voltage to the desired
safe level. Figure 1 shows an example where the desired
no-load output voltage is 7.4 V. The series resistor’s value
can be calculated by subtracting the Zener diode’s nominal
voltage from this voltage and dividing the result by the
preload current. The benefit of this circuit is that it does
not dissipate power at loads that would typically be seen
in operation. Under extreme cross-load conditions, this
circuit clamps the “runaway” output voltage to a much
more predictable level.
Related Web site
19
Analog Applications Journal
4Q 2010
High-Performance Analog Products
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SLYT392
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