Design Idea DI-123
TinySwitch-III
Application
Device
®
9.65 W, Dual Output Power Supply for Residential
Heating Control
Power Output Input Voltage
9.65 W
185-265 VAC
Output Voltage
5V / 0.25 A, 24 V / 0.35 A
Topology
Flyback
Residential Heating Control TNY275P
Design Highlights
•
•
•
•
•
Simple, low cost design only requires 38 parts
<150 mW no-load consumption at 230 VAC input
Meets CEC / ENERGY STAR active-mode efficiency
and no-load input power requirements
Both outputs exhibit good cross regulation
Meets CISPR-22 Class B/EN55022 B without input
X or Y capacitors or an input common mode choke
MOSFET is turned off. Then the energy stored in T1 transfers
to the secondary where it is rectified and filtered by D1 and
C2 and D4 and C5. The primary side RCD clamp (D8, C4
R1 and R8) limits the peak DRAIN voltage spike caused by
transformer leakage inductance.
Power Integrations
E-Shield™
transformer construction
techniques, the RCD clamp, a snubber (R11 and C14), a
simple π filter (C7, C8, L1 and L5) and the frequency jitter
function of the
TinySwitch-III
family, provide good EMI
margin, even with the output return connected to safety earth
ground (see Figure 3).
Operation
The flyback converter shown in Figure 1 uses a TNY275
(U1) to provide two output voltages: 5 V at 250 mA and
24 V at 350 mA. Typical applications are residential heating
controllers (furnace) or any application where 2 outputs are
required. The 9.65 W of output power is delivered by using
the increased current limit of U1, which is selected by the
value of C11.
The MOSFET integrated within U1 switches the primary of
transformer T1. Each time it turns on, the primary current
ramps until it reaches an internal current limit and the
L2
330
µH
R8
75 kΩ
Key Design Points
•
•
The turns ratio of the two secondary winding was
optimized for output voltage centering.
The use of a fast instead of an ultrafast diode for
D8 improves efficiency by recovering some leakage
inductance energy.
R10
10
Ω
C13
1/2 W
470 pF
L4
FB
C4
1 nF
1 kV
1
T1 10
24 V, 350 mA
C12
47
µF
35 V
C6
100
µF
16 V
5 V, 250 mA
D1 UF4003
6
D4 1N5819
C2
220
µF
35 V
L3
FB
D2
1N4937
L
F1
1A
D3
1N4007
3
R1
100
Ω
185-265
VAC
C7
10
µF
400 V
C8
10
µF
400 V
D
D8
FR106
8, 9
EF20
C5
330
µF
10 V
RTN
R2
47
Ω
EN/UV
BP/M
TinySwitch-III
U1
TNY275P
N
R11
100
Ω
S
R3
15 kΩ
1%
R9
261 kΩ
1%
D5
1N4937
D7
1N4007
R12
30
Ω
U2
PC817A
R4
1 kΩ
C10
R5
100 nF
3.3 kΩ
C14
33 pF
1 kV
L5
FB
C15
0.1
µF
50 V
C11
10
µF
50 V
L3, L4 and L5 are
3.5 mm × 10 mm
Ferrite beads.
U3
LM431
2%
R6
10 kΩ
1%
PI-4495-090706
Figure 1. TinySwitch-III 9.65 W Residential Heating Controller Power Supply.
DI-123
September 2006
POWERINT [ POWER INTEGRATIONS, INC. ]
