Power Electronics Technology

“Time ago, when amplifiers or servomechanisms were driven by
vacuum tubes-based circuits, warm-up times during the power-on
sequence could induce large loop gain variations.”
point as well as the phase margin. We know by experience
that the elements constitutive of the converter will
exhibit variations along the product life cycle. These
variations can be linked to natural production spreads
(for instance, resistors or capacitors affected by lot-tolot
tolerance). The converter environmental operating
conditions also have an impact on components. Among
these variables, temperature plays an important role
and affects passive or active component parameters. It
can be capacitors or inductors equivalent series resistors
(ESR), the optocoupler current transfer ratio (CTR),
or the beta of bipolar transistors for instance. These
variations impact the loop gain by shifting it up or down
depending on the affected parameters.
If the gain curve undergoes a shift, the 0-dB cross-
over frequency will transition to a new value imposing a
different bandwidth to the converter. How can the converter
stability be affected under these changes? Well, if
the new crossover tales place at a point where the phase
margin is weak, you may degrade the transient response
so that the overshoot is no longer acceptable. It is thus
your responsibility, as a designer, to ensure that these
dispersions do not suddenly increase the gain at a frequency
where you approach the -180° limit. You need
sufficient gain margin as defined by
www.powerelectronics.com June 2013 | Power Electronics Technology 17