Application Note 1612
ISL6844 Reference Design: ISL6844EVAL3Z
Where:
Introduction
This document focuses on Intersil’s solution for the flyback
converter. An inexpensive approach with discrete circuitry has
been adopted instead of an integrated solution. Low cost and
optimal performance are the prime objectives.
η = Converter’s efficiency, assuming 75%
P
F
= Total output power
OUT
= Switching frequency
sw
As a result, the peak magnitizing current = 1.06A
The transformer’s turn ratio can be determined from:
Intersil’s superior industry-standard ISL684x family of PWM
controllers would best serve the needs of this design. Some
key features of this family of parts include:
(V
+ V ) ⋅ (1 – d )
F 2
out
V
-------------------------------------------------------
n =
• 40ns peak current sensing
• 1A MOSFET driver
⋅ d
MAX
(EQ. 2)
IN, MIN
d
+ d < 1
2
MAX
ISL6844 was selected for its large UVLO hysteresis, UVLO start
threshold, and the fact that the converter has been designed
for a maximum operational duty cycle of 50%, thus protecting
the IC by limiting the duty cycle in case of extreme fault
conditions.
Where:
n = Turns ratio between the primary and the secondary
windings
V = Forward drop across the diode, assuming 0.6V
F
Specifications
• Operating Input Voltage: 24V DC ±10%
• Output Voltage: ±15V
d = Duty cycle of diode conduction time
2
d T is the magnitizing current reset time. Setting d to 0.5,
2 sw
2
Equation 2 yields the transformer’s turn ratio of 1.
• Output Current: 100mA
D1
1
n
+15V
VIN
• Ripple: 50mV
P-P
CIN
CS
COUT1
• Switching Frequency: 300kHz
• Topology: DCM Flyback
RS
DS
n
COUT2
D2
OUT
CS
-15V
Design Procedure
RSENSE
VIN
Figure 1 shows a simplified circuit of the solution. It is
assumed that loads are balanced for both positive and
negative outputs. The turn ratio of the auxiliary winding is
chosen to be the same as the secondary winding. Figure 2
shows typical operational waveforms of a flyback converter in
discontinuous conduction mode.
ISL6844
VREG
n
VDD
FB
CAUX
GND
Determine the Maximum Duty Cycle and
Transformer Turn Ratio
FIGURE 1. SIMPLIFIED CIRCUIT
ISL6844 clamps the duty cycle to 50%. However, in this
converter design, it is assumed that the operating maximum
duty cycle, d
21.6V.
, will be 35% at the minimum input voltage of
max
Given the power level, the flyblack converter is designed to
operate in discontinuous conduction mode. The magnitizing
inductance can be calculated using Equation 1:
2
2
max
⋅ F
sw
V
⋅ d
IN, MIN
-----------------------------------------
L
= η ⋅
M
2 ⋅ P
(EQ. 1)
out
2
2
21.6 ⋅ 0.35
-------------------------------------
2 ⋅ 3 ⋅ 300×10
= 0.75 ⋅
= 23.8μH
3
November 28, 2011
AN1612.1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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