Flyback Rectifier Diode and Clamping
10.6.2 Flyback Rectifier Diode and Clamping
• D R and D F are MBR2540 Schottky diodes. D M is an
Diode in a Flyback Converter
MUR460 ultra-fast diode. • M 1 is an IRF640 MOS transistor.
10.6.2.1 Ideal Circuit
• Transformer T 1 has a practical coupling coefficient of Figure 10.38 shows the basic circuit of a flyback converter. Due 0.996.
to its simple circuit, this type of converter is widely used in low- • The effective winding resistance of L P
cost low-power applications. Discontinuous-mode operation tive winding resistance of L M
(meaning that the magnetizing current in the transformer falls winding resistance of L S
to zero before the end of each switching cycle) is often used • The effective series resistance of the output filtering because it offers the advantages of easy control and low diode reverse-recovery loss. Figure 10.39 shows the idealized steady- • The switching operation of the converter has reached a state waveforms for discontinuous-mode operation. These steady state.
waveforms are obtained from PSpice simulations, based on
172 Y. S. Lee and M. H. L. Chow
0V V1(VPULSE)
DT
500mA 0A –500mA
I(DM) 5.0A
0A –5.0A
ID(M1) 100V
0V –100V
V(100) 200V
0V –200V
V(3) 20V
0V –20V
V(6) 20V
0V –20V
V(9) 20V
0V –20V
V(6,9) 20A
0A –20A
I(DR) 20A
0A –20A
I(DF) 20A
15A 10A
I(L1) 4.9V
4.8V 4.7V
0s
20us V(99)
Time FIGURE 10.31 Waveforms of forward converter using Schottky (fast-speed) diodes as output rectifiers.
10 Diode Rectifiers 173
0V V1(VPULSE)
DT
4.0A 0A –4.0A
I(DM) 4.0A
0A –4.0A
ID(M1) 400V
0V –400V
V(100) 400V
0V –400V
V(3) 100V
0V –100V
V(6) 40V
0V –40V
V(9) 100V
0V –100V
V(6,9) 20A
0A –20A
I(DR) 20A
0A –20A
I(DF) 15.0A
12.5A 10.0A
I(L1) 4.2V
4.1V 0s
20us V(99)
FIGURE 10.32 Waveforms of forward converter with practical transformer and output filtering capacitor having non-zero series effective resistance.
174 Y. S. Lee and M. H. L. Chow
D M 100
V IN = 50 V, D M = MUR460
D R = MBR2540, D F = MBR2540
9 L 1 99 M = IRF640, R = 24 W
C 1 = 3000 pF, C = 3500 mF
ESR of C L = 0.05 W, L 1 =8 mH
1 M IN = 0.576 mH
L P = 0.576 mH, L
3 L S = 0.036 mH, N
0 P :N M :N S =4:4:1
R L = 0.35 W
Pulse
Effective winding resistance of L P =0.1 W Effective winding resistance of L M =0.4 W Effective winding resistance of L S = 0.01 W 0 Coupling coefficient K = 0.996
FIGURE 10.33 Forward converter with snubber circuit (R 1 C 1 ) across transformer.
the following assumptions: t = DT to the energy stored in the secondary-winding current I(LS) just after t
• D R is an idealized rectifier diode with infinitely fast = DT :
switching speed. • M 1 is an idealized MOS switch with infinitely fast
L P [I (LP)] 2 = L S [I (LS)] 2 (10.100) switching speed and
On-state resistance [I (LS)] (10.101)
IN
DT
Off-state resistance L P V IN
DT (10.102) L S L P • Transformer T 1 has a coupling coefficient of 0.99999999.
I (LS) =
• The switching operation of the converter has reached a N P V IN
DT (10.103) steady state.
I (LS)
N S L P Referring to the circuit shown in Fig. 10.38 and the wave-
forms shown in Fig. 10.39, the operation of the converter can The amplitude of I(LS) falls at the rate of
be explained as follows:
1. For 0 < t < DT
dI (LS)
−V o
The switch M 1 is turned on at t = 0.
dt
V (3) = 0 for 0 < t < DT and I(LS) falls to zero at t = (D + D 2 )T. Since D 2 V o =
V IN (N S /N P )D The current in M 1 , denoted as ID(M1), increases at
the rate of
V IN N S
D (10.105)
d ID(M 1)
D 2 is effectively the duty cycle of the output rectifier
3. For (D + D )T < t < T
The output rectifier D R is reversely biased.
2 The output rectifier D
R is off. The output capacitor C L provides the output current
2. For DT < t < (D + D 2 )T
The switching M 1 is turned off at t = DT . to the load R L .
The collapse of magnetic flux induces a back emf The switching cycle restarts when the switch M 1 is in L S to turn-on the output rectifier D R . The initial
turned on again at t
=T.
amplitude of the rectifier current I(DR), which is also
denoted as I(LS), can be found by equating the energy From the waveforms shown in Fig. 10.39, the fol- stored in the primary-winding current I(LP) just before lowing information (for discontinuous-mode operation)
10 Diode Rectifiers 175
0V V1(VPULSE)
DT
1.0A 0A –1.0A
I(DM) 4.0A
0A –4.0A
ID(M1) 100V
0V –100V
V(100) 200V
0V –200V
V(3) 50V
0V –50V
V(6) 40V
0V –40V
V(9) 40V
0V –40V
V(6,9) 20A
0A –20A
I(DR) 20A
0A –20A
I(DF) 15.0A
12.5A 10.0A
I(L1) 4.2V 4.1V
0s
20us V(99)
FIGURE 10.34 Waveforms of forward converter with snubber circuit across the transformer.
176 Y. S. Lee and M. H. L. Chow
V IN = 50 V, D M = MUR460 D M 100 R 2 C D R = MBR2540, D 2 = MBR2540 F
M 1 = IRF640, R = 24 W
9 L 1 99 R 2 = 10 W, R 3 = 10 W V = 3000 pF, C = 10 nF, C
= 10 nF
C L = 3500 mF, ESR of C L = 0.05 W
90 L
L 1 =8 mH, L P = 0.576 mH
V IN C 1 3 C 3 L
M = 0.576 mH, L S = 0.036 mH 5 M 1 L M 0 N P :N M :N S = 4: 4 : 1
Pulse
Effective winding resistance of L P = 0.1 W Effective winding resistance of L M = 0.4 W Effective winding resistance of L S = 0.01 W 0 Coupling coefficient K = 0.996
FIGURE 10.35 Practical forward converter with snubber circuits across the transformer and rectifiers.
can be obtained: damp the ringing voltage across the output rectifier D R , and
a resistor–capacitor-diode clamping (R 1 C 1 D S ) is used to clamp
• The maximum value of the current in the switch M 1 is
the ringing voltage across the switch M 1 . What the diode D S
does here is to allow the energy stored by the current in the ID(M 1) max =
leakage inductance to be converted to the form of a dc voltage across the clamping capacitor C 1 . The energy transferred to • The maximum value of the current in the output rectifier C 1 is then dissipated slowly in the parallel resistor R 1 , without
D R is
ringing problems.
The simulated waveforms of the flyback converter (circuit
N P V IN
I (DR) max =
given in Fig. 10.40) for discontinuous-mode operation are
DT
shown in Fig. 10.41, where the following assumptions are made:
• The output voltage V o can be found by equating the input energy to the output energy within a switching cycle.
• D R and D S are MUR460 ultra-fast diodes.
×[Charge taken from V • Transformer T 1 has a practical coupling coefficient of = R L T
V IN
IN in a switching cycle]
• M 1 is an IRF640 MOS transistor.
2 • The effective winding resistance of L DT P V
effective winding resistance of L S
• The effective series resistance of the output filtering
capacitor C L
DV IN
• The switching operation of the converter has reached a
2L P
steady state. The waveforms shown in Fig. 10.40 are considered to be
• The maximum reverse voltage of D R , V(6,9) (which is the
acceptable.
voltage at node 6 with respect to node 9), is
V (DR) max = V (6, 9) max =V IN
+V o