Design Idea DI-123
TinySwitch®-III
9.65 W, Dual Output Power Supply for Residential
Heating Control
Application
Device Power Output Input Voltage
Output Voltage
Topology
Residential Heating Control TNY275P
9.65 W
185-265 VAC 5V / 0.25 A, 24 V / 0.35 A Flyback
Design Highlights
MOSFETisturnedoff. ThentheenergystoredinT1transfers
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.
•
•
•
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
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.
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.
•
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
R10
C13
470 pF
10 Ω
1/2 W
C4
L2
330 µH
L4
FB
1 nF
24 V, 350 mA
T1
1 kV
1
3
10
6
C12
47 µF
35 V
C2
220 µF
35 V
D1 UF4003
D4 1N5819
R8
75 kΩ
L3
FB
5 V, 250 mA
RTN
D2
D3
R1
C6
C5
1N4937
1N4007
100 Ω
100 µF
330 µF
16 V
L
8, 9
10 V
D8
EF20
FR106
F1
1 A
C7
10 µF
400 V
C8
10 µF
400 V
TinySwitch-III
185-265
VAC
R2
U1
TNY275P
47 Ω
D
S
R3
15 kΩ
1%
R9
261 kΩ
1%
EN/UV
BP/M
R11
100 Ω
N
R4
U2
1 kΩ
PC817A
C10
100 nF
R12
30 Ω
R5
3.3 kΩ
D5
1N4937
D7
1N4007
C14
33 pF
1 kV
C15
0.1 µF
50 V
C11
10 µF
50 V
U3
LM431
2%
R6
10 kΩ
1%
L3, L4 and L5 are
3.5 mm × 10 mm
Ferrite beads.
L5
FB
PI-4495-090706
Figure 1. TinySwitch-III 9.65 W Residential Heating Controller Power Supply.
September 2006
DI-123