Page 136 - Rashid, Power Electronics Handbook

8 MOS Controlled Thyristors (MCTs) 123

Generation-1 MCTs did not turn on rapidly in the vicinity of
zero anode-cathode voltage and this posed a problem in soft-
switching applications of an MCT. However, Generation-2
MCTs have enhanced dynamic characteristics under zero
voltage soft switching [16]. In an MCT, the PMOS On-FET
together with the pnp transistor constitute a p-IGBT. An
increase in the number of turn-on cells (decrease in the on-
resistance of the p-IGBT) and an enhancement of their
distribution across the MCT active area enable the MCT to
turn on at a very low transient voltage allowing zero voltage
switching (ZVS). During zero voltage turn-on, a bipolar device
such as the MCT takes more time to establish conductivity
modulation. Before the device begins to conduct fully, a
voltage spike appears, thus causing a modest switching loss
[12]. Reducing the tail-current amplitude and duration by
proper circuit design can minimize the turn-off losses in soft-
switching cases.

8.11.2 Resonant Converters
Resonant and quasi-resonant converters are known for their
reduced switching loss [17]. Resonant converters with zero
current switching are built using MCTs and the circuit of one
such, a buck-converter, is shown in Fig. 8.9. The resonant
commutating network consisting of L , C , auxiliary switch T ,
r
r
r
and diode D enables the MCT to turn off under zero current.
r
The MCT must be turned off during the conduction period of
D . Commutating switch T must be turned off when the
r
Z
resonant current reaches zero.
A resonant dc link circuit with twelve parallel MCTs has
been reported [18]. In this circuit, the MCTs switch at zero-
voltage instants. The elimination of the switching loss allows
operation at higher switching frequencies, which in turn
increases the power density and offers better control of the
spectral content. The use of MCTs with the same forward drop
provides good current sharing.

FIGURE 8.8 The MCT turn-off and turn-on waveforms under hard Dz
switching.

L1
Soft-switching converters are being designed primarily to
MCT
enable operation at higher switching frequencies. In these Tr
Dr
converters, the power devices switch at zero voltage or zero Vin
current, thereby eliminating the need for a large safe operating
Cr Lr Load
area (SOA) and at the same time eliminating the switching
losses entirely. The MCT converters will outperform IGBT and
power MOSFET converters in such applications by giving the Do
highest possible ef®ciency. In soft-switching applications, the
MCT will have only conduction loss, which is low and is close
to that in a power diode with similar power ratings [12]. The FIGURE 8.9 Power circuit of MCT resonant buck-converter.