Page 165 - Rashid, Power Electronics Handbook

154 Y.-S. Lee and M. Chow

10.5.2.1 Inrush Current time can be signi®cantly higher than the normal on-state
The resistor R inrush in Fig. 10.26 is used to limit the inrush voltage drop. This may cause voltage spikes in the circuit.
current imposed on the diodes during the instant the recti®er It should be interesting to note that, as far as circuit
is being connected to the supply. The inrush current can be operation is concerned, a diode with a long reverse recovery
very large because capacitor C initially has zero charge. The time is similar to a diode with a large parasitic capacitance. A
worst case occurs when the recti®er is connected to the supply diode with a long forward recovery time is similar to a diode
at its maximum voltage. The worst-case inrush current can be with a large parasitic inductance. (Spikes caused by the slow
estimated from forward recovery of diodes are often wrongly thought to be
caused by leakage inductance.) Comparatively, the adverse
V m effect of a long reverse recovery time is much worse than
I inrush ¼ ð10:81Þ
R sec þ R ESR that of a long forward recovery time.
Among commonly used diodes, the Schottky diode has the
where R sec is the equivalent resistance looking from the shortest forward and reverse recovery times. Schottky diodes
transformer secondary and equivalent series resistance (ESR) are therefore most suitable for high-frequency applications.
of the ®ltering capacitor is R . Hence, the employed diode However, Schottky diodes have relatively low reverse break-
ESR
should be able to withstand the inrush current for a half-cycle down voltage (normally lower than 200 V) and large leakage
of the input voltage. In other words, the Maximum Allowable current. If, due to these limitations, Schottky diodes cannot be
Surge Current (I ) rating of the employed diodes must be used, ultra-fast diodes should be used in high-frequency
FSM
higher than the inrush current. The equivalent resistance converter circuits.
associated with the transformer windings and the ®ltering Using the example of a forward converter, the operations of
capacitor is usually suf®cient to limit the inrush current to an a forward recti®er diode, a ¯ywheel diode, and a clamping
acceptable level. However, in cases where the transformer is diode will be studied in Section 10.6.1. Because of the
omitted, for example, the recti®er of an off-line switch-mode dif®culties encountered in full analyses when taking into
supply, resistor R inrush must be added for controlling inrush account parasitic=stray=leakage components, PSpice simula-
current. tions are extensively used here to study the following [3–5]:
Consider as an example a single-phase bridge recti®er,
The idealized operation of the converter;
which is to be connected to a 120-V 60-Hz source (without
the adverse effects of relatively slow recti®ers (e.g., the
transformer). Assume that the Maximum Allowable Surge
so-called ultra-fast diodes, which are actually much
Current (I FSM ) rating of the diodes is 150 A for an interval slower than Schottky diodes);
of 8.3 ms. If the ESR of the ®ltering capacitor is zero, we the improvement achievable by using high-speed recti-
estimate from Eq. (10.81) that the value of the resistor for ®ers (Schottky diodes);
limiting inrush current resistance is 1:13 O.
the effects of leakage inductance of the transformer;
the use of snubber circuits to reduce ringing; and
10.6 High-Frequency Diode Rectifier the operation of a practical converter with snubber
Circuits circuits.
Using the example of a ¯yback converter, the operations of a
In high-frequency converters, diodes perform various func- ¯yback recti®er diode and a clamping diode also will be
tions, such as rectifying, ¯ywheeling, and clamping. One studied in Section 10.6.2.
special quality a high-frequency diode must possess is a fast The design considerations for high-frequency diode recti®er
switching speed. In technical terms, it must have a short circuits will be discussed in Section 10.6.3. The precautions
reverse recovery time and a short forward recovery time. that must be taken in the interpretation of computer simula-
The reverse recovery time of a diode may be understood as tion results are brie¯y discussed in Section 10.6.4.
the time a forwardly conducting diode takes to recover to a
blocking state when the voltage across it is suddenly reversed 10.6.1 Forward Rectifier Diode, Flywheel Diode,
(which is known as forced turn-off). The temporary short and Magnetic-Reset Clamping Diode in a
circuit during the reverse recovery period may result in large Forward Converter
reverse current, excessive ringing, and large power dissipation,
all of which are highly undesirable. 10.6.1.1 Ideal Circuit
The forward recovery time of a diode may be understood as Fig. 10.28 shows the basic circuit of a forward converter. Fig.
the time a nonconducting diode takes to change to the fully on 10.29 shows the idealized steady-state waveforms for contin-
state when a forward current is suddenly forced into it (which uous-mode operation (the current in L is continuous). These
1
is known as forced turn-on). Before the diode reaches the fully waveforms are obtained from PSpice simulations, based on the
on state, the forward voltage drop during the forward recovery following assumptions: