MC34167, MC33167
http://onsemi.com
15
1000
T1
+ +
MC34167
Step−Down
Converter
0.001
0.001
Output 1
MBR20100CT
1000
+ +
MC34167
Step−Down
Converter
0.001
0.001
Output 2
MBR20100CT
1000
+ +
MC34167
Step−Down
Converter
0.001
0.001
Output 3
MBR20100CT
0.01
RFI
Filter
100
3.3
1N4003
MJE13005
220
0.047
1N4937
50
100k
T
2
1N5404
115 VAC
T
1
= Core and Bobbin − Coilcraft PT3595
T
1
= Primary − 104 turns #26 AWG
T
1
= Base Drive − 3 turns #26 AWG
T
1
= Secondaries − 16 turns #16 AWG
T
1
= Total Gap − 0.002,
T
2
= Core − TDK T6 x 1.5 x 3 H5C2
T
2
= 14 turns center tapped #30 AWG
T
2
= Heatsink = AAVID Engineering Inc.
T
2
= MC34167 and MJE13005 − 5903B
T
2
= MBR20100CT − 5925B
+
+
The MC34167 can be used cost effectively in off−line applications even though it is limited to a maximum input voltage of 40 V. Figure 28 shows
a simple and efficient method for converting the AC line voltage down to 24 V. This preconverter has a total power rating of 125 W with a
conversion efficiency of 90%. Transformer T
1
provides output isolation from the AC line and isolation between each of the secondaries. The
circuit self−oscillates at 50 kHz and is controlled by the saturation characteristics of T
2
. Multiple MC34167 post regulators can be used to provide
accurate independently regulated outputs for a distributed power system.
Figure 28. Off−Line Preconverter
R , THERMAL RESISTANCE
JAθ
JUNCTION-TO-AIR ( C/W)°
30
40
50
60
70
80
1.0
1.5
2.0
2.5
3.0
3.5
010203025155.0
L, LENGTH OF COPPER (mm)
P
D(max)
for T
A
= +50°C
Minimum
Size Pad
2.0 oz. Copper
L
L
Free Air
Mounted
Vertically
P
D
, MAXIMUM POWER DISSIPATION (W)
R
q
JA
Figure 29. D
2
PAK Thermal Resistance and Maximum
Power Dissipation versus P.C.B. Copper Length