a voltage-loop crossover frequency given by (32) of
Hz. Fig. 15 shows the experimental closed-loop output-voltage
transient response for a step load change from 50 to 133 W.
As discussed in Sections III and IV, the THD of the inputline
current is affected by the approximate exponential carrier
and, depending on the load, by operation in the DCM. Another
cause of distortion is related to the fact that the emulated
resistance is directly proportional to the output voltage
, as given by (7). This implies that the open-loop ac input
current is affected by the output voltage ripple at 120-Hz
and higher line-frequency harmonics. Given a constant openloop
control input and the output voltage ,
the input current for small ripple is given by
(33)
where is the nominal emulated resistance.
Because of the product of and , the second harmonic
in will result in the third harmonic in . In closedloop
operation, the output voltage ripple also affects the input
current waveform through the control input . As a result
of the output voltage ripple, additional distortion of the input
current can therefore be expected.
Fig. 16(a) shows the full-power open-loop line-frequency
waveforms at the nominal line voltage (120 Vrms). Fig. 16(b)
shows the corresponding harmonic spectrum of the ac inputline
current, with the open-loop spectrum (dark pen) overlaying
the closed-loop spectrum (light pen).
The largest open-loop fifth harmonic is only 1.86% of the
fundamental, and the largest closed-loop third harmonic is
4.19%. The THD of the ac input-line voltage was measured at
approximately 1%. It can be seen that open- and closed-loop
distortion only differ in the third and fifth harmonics, with the
resulting THD for both well below 10%.
Fig. 17(a) and (b) shows the open-loop line-frequency
waveforms at the nominal line voltage (120 Vrms) and 50
W of output power, together with the overlayed harmonic
spectrums of the open- and closed-loop ac input-line current.
Distortion can be observed in the input ac-line current, which
is the result of operation in the CCM around zero crossings
of the ac-line voltage. This waveshape corresponds to the
theoretical prediction of Fig. 3. The total current harmonic
distortion is still very small for both open- and closed-loop
operation, with the largest harmonics equal to 3.05% (seventh)
and 5.39% (third), respectively