Page 306 - Rashid, Power Electronics Handbook
P. 306
296 S. Hui and H. Chung
R L Integral pulse-density modulation has to be used. Many
well-established PWM techniques cannot be employed.
I L +
V V I Despite these advantages, this resonant converter concept has
S C X paved the way for other soft-switched converters to develop.
-
S
15.14.2 Active-Clamped Resonant dc Link
FIGURE 15.48 Equivalent circuit of resonant link inverter.
Inverter
In order to solve the high-voltage requirement in the basic
and inductor current i is
L
pulsating dc link inverters, active clamping techniques (Fig.
V s 15.49) have been proposed. The active clamp can reduce the
ÿat
i ðtÞ I þ e I cosðotÞþ sinðotÞ ð15:15Þ
L
x
M
oL per-unit peak voltage from 2.0 to 1:3ÿ1:5 [41]. It has been
reported that an operating frequency in the range of 60 to
where 100 kHz has been achieved [45] with an energy ef®ciency of
97% for a 50-kVA drive system.
a ¼ R=2L ð15:16Þ The design equations for an active-clamped resonant link
inverter are
ÿ0:5
o ¼ðLCÞ ð15:17Þ
o
p !
1 p kð2 ÿ kÞ
2 2 0:5 ÿ1
o ¼ðo ÿ a Þ ð15:18Þ T ¼ ¼ 2 L C r cos ð1 ÿ kÞþ ð15:20Þ
r
L
o
f L k ÿ 1
and
where T is the minimum link period, f is the maximum link
I M ¼ I ÿ I x ð15:19Þ L L
Lo
frequency, and k is the clamping ratio. For the active-clamped
resonant inverter, k is typically 1.3 to 1.4 p.u.
The resistance in the inductor could affect resonant beha-
The rate of rise of the current in the clamping device is
vior because it dissipates some energy. In practice, (i ÿ I )
L
x
has to be monitored when S is conducting. In addition, S can
be turned on when (i ÿ I ) is equal to a desired value. The di ðk ÿ 1ÞV s
x
L
objective is to ensure that the dc link voltage can be resonated dt ¼ L r ð15:21Þ
to zero voltage level in the next cycle.
The pulsating dc link inverter has the following advantages:
The peak clamping current required to ensure that the dc bus
reduction of switching loss; return to zero voltage is
snubberless operation;
high switching frequency (>18 kHz) operation becomes s
possible, leading to the reduction of acoustic noise in kð2 ÿ kÞC r
I ¼ V s ð15:22Þ
sp
inverter equipment; and L r
reduction of heat sink requirements and thus improve-
ment of power density.
This approach has the following limitations:
Vcc
The peak dc pulsating link voltage (2.0 per unit) is higher
than the nominal dc voltage value of a conventional Lr
inverter. This implies that power devices and circuit
components of higher voltage ratings must be used.
This could be a serious drawback because power compo-
Vdc
nents of higher voltage ratings are not only more Cr
expensive, but usually have inferior switching perfor-
mance than their low-voltage counterparts.
Although voltage clamp can be used to reduce the peak
dc link voltage, the peak voltage value is still higher than
normal and the additional clamping circuit makes the
control more complicated. FIGURE 15.49 Active-clamp resonant link inverter.
