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185Transformer-isolated Circuits
The effective switching frequency is double the oscillator frequency. Each switch
pro
vides the equivalent of a single switch forward converter. The bipolar drive doubles
the effective duty cycle and the operating frequency of the output fi lter is double the
switching frequency.
The control IC must provide two-phase output pulses to alternately drive the switches.
Additionally, the circuit will behave badly if both switches conduct at the same time. The
transformer will allow very large switch currents to fl ow if both switches conduct at the
same time. A push-pull control IC must provide the ability to set a proper amount of dead
time between the alternate phases. This will ensure that one switch is off before the other
switch begins conducting.
6.11 Practical Push-Pull Circuit Design
Typical steps for designing a push-pull converter are listed below:
1. Choose a controller IC based on power level and bill of material constraints.
2. Choose the switching frequency.
3. Use the input voltage range goal to select the maximum duty cycle goal.
4. Pick the output diodes.
5. Calculate the output inductor value.
6. Design the transformer winding ratios.
7. Determine the maximum power and pick the switches.
8. Choose the output capacitor based on ripple requirements.
9. Design the auxiliary supply, if needed.
10. Design the ancillary IC components, including the feedback circuit.
Our push-pull example is a telecom supply that converts 48 V to an isolated 5 V/20 A
supply with 100 mV of ripple. Figure 6.19 shows the circuit we are designing.
A search for control ICs designed specifi cally for push-pull or bridge operation yields
very few parts. Most of the fi rst- and second-generation current mode controllers (such
as the 1846) provide the necessary functions, but they need a large number of external
components for a working supply. There are not many modern control ICs designed
for push-pull and bridge operation. Some manufacturers have only one or two parts
for this application and many have no modern products at all for this market. This is
U1
LM5030
V
in
Vcc
SW A
Comp
Rt
FB
SS
GND
SW B
Csense
IRF7451
IRF7451
T1
6.5:1
32TCQ030
C1
56µF/6.3 WV
R1
0.130
U2
TL431
U3
4N27
R2
1K
R3
2K
R4
470
R5
1K
C2
330 nF
100 nF
D3
D
C4
10µF
5 V Return
5 V/20 A
48 V Common
48 V
C5
150 nF
R6
26 K
R7
10
R8
10
2.1:1
7.6µH
C3
470µF/63 WV
R9
300
C6
100 pF
Figure 6.19 : An isolated push-pull design
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