DN43

Design Note 43
Issue1 January 2000
High Efficiency DC-DC Isolated Converter
Systems peripheral functions often
require separate 3.3V or 5V supplies.
This function may require an isolated
power supply, as the logic supply cannot
sink or source enough current. Isolated
DC-DC converters are readily available
in the market place, however if there are
some spare inverters available in the
hardware design then a low cost
solution can be configured.
The circuit in Figure 1 is a replacement
for the MAX253 driver circuit. It uses the
same transformer as recommended in
the Maxim application circuit (part
number 76253/55 for 5V-5V isolated
conversion, and 76253/35 for 3.3V-5V
isolated conversion).
C2
0u1F
C1
150pF
L1
47uH
Q1
FMMT617
1
2
3
U1
7404
4
U2
7404
5
6
U3
7404
D1
Vout
ZHSC750
Vcc
R3
3k3
TR1
+
R2
10k
+
R1
22k
The inverters produce an RC generated
complimentary square wave output
which in turn is used to drive Q1 & Q2
(FMMT617 SuperSOT transistors).
These power the ‘off the shelf’
transformers specified above. The
operation of the inverters and the
transformer imitates the operation of
the MAX253 IC and provides an
oscillator frequency of 240KHz. The
centre tap of the primary winding of the
transformer is connected to the power
supply (3.3 or 5V) and is de-coupled to
ground via C4.
C5
15uF
C6
15uF
+
GND
C4
15uF
D2
C3
0u1F
ZHSC750
Q2
FMMT617
9
8
U4
7404
R4
3k3
Figure 1
DN 43 - 1
Design Note 43
Issue1 January 2000
Two schottky rectifier diodes ( Zetex
ZHCS750) are then employed to
reproduce the DC supply. The LC
network acts as a filter to ensure a steady
output signal.
Maximum efficiency of the transformer
is approx. 85% so ideally the design
needs to get as close to this as possible.
This is achieved by careful selection of
Q1, Q2, D1 and D2, and the driving
frequency.
For Q1 & Q2 FMMT617 (surface mount)
or ZTX618 (through hole) transistors
have been chosen. These medium
power devices require little base drive
due to their high gain. Also they exhibit
very low VCE(sat) characteristics making
them ideal for this switching application.
For D1 & D2 ZHCS750 Schottky diodes
have been selected. They have very low
Vf (forward voltage drop) rating and fast
switching speed. Consequently they
consume very little power enhancing
the overall efficiency of the circuit.
Setting R1 to 22K, R2 to 10K and C1 to
150pF the circuit will oscillate at 240KHz.
This is the optimum frequency and gives
the highest efficiency result.
Figure 2
A circuit efficiency in excess of 80% was
achieved. Also the circuit was able to
drive a load at the maximum power
rating of the transformer ( 1W for the /55,
500mA for the /35).
Typical applications include RS-485 and
RS-232 data interface (transceiver)
applications.
Both transformers are manufactured by
Newport Components,Tel:+44(0)1908
615232.
Figure 2 shows the waveform as the
transformer drives into D1( Trace 2), the
upper waveform (Trace1) shows the
output from the circuit driving into a
500mA load. As can be seen the
frequency of operation is 240kHz, and
the resultant output waveform is ripple
free.
DN 43 - 2
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