DI-123 Design Idea

®
TinySwitch-III

9.65 W, Dual Output Power Supply for Residential

Heating Control

Application Device Power Output Input Voltage Output Voltage Topology

Residential Heating TNY275PN 9.65 W 185 - 265 VAC 5 V / 0.25 A, 24 V / Flyback
Control 0.35 A

Design Highlights off. Then the energy stored in T1 transfers to the secondary
• Simple, low cost design only requires 38 parts where it is rectified and filtered by D1 and C2 and D4 and C5. The
• <150 mW no-load consumption at 230 VAC input primary side RCD clamp (D8, C4 R1 and R8) limits the peak
• Meets CEC / ENERGY STAR active-mode efficiency and no- DRAIN voltage spike caused by transformer leakage inductance.
load input power requirements Power Integrations E-Shield™ transformer construction tech-
• Both outputs exhibit good cross regulation niques, the RCD clamp, a snubber (R11 and C14), a simple π filter
• Meets CISPR-22 Class B/EN55022 B without input (C7, C8, L1 and L5) and the frequency jitter function of the
X or Y capacitors or an input common mode choke 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 Key Design Points
provide two output voltages: 5 V at 250 mA and 24 V at 350 mA. • The turns ratio of the two secondary winding was optimized for
Typical applications are residential heating controllers (furnace) or output voltage centering.
any application where 2 outputs are required. The 9.65 W of • The use of a fast instead of an ultrafast diode for D8 improves
output power is delivered by using the increased current limit of efficiency by recovering some leakage inductance energy.
U1, which is selected by the value of C11. • Maximize the value of R8 and minimize the value of C4 for the
lowest no-load consumption. However, verify that the peak
The MOSFET integrated within U1 switches the primary of drain voltage is <650 V at high line, under maximum overload
transformer T1. Each time it turns on, the primary current ramps conditions.
until it reaches an internal current limit and the MOSFET is turned

R10
10 Ω C13
1/2 W 470 pF
L2 C4
1 nF L4
330 μH FB 24 V, 350 mA
1 kV 1 T1 10

R8 D1 UF4003 C2 C12
75 kΩ 220 μF L3 47 μF
D4 1N5819 35 V FB 35 V
6
3
D2 D3 R1 5 V, 250 mA
C5 C6
1N4937 1N4007 100 Ω 100 μF
330 μF
L 8, 9 10 V 16 V
D8
FR106 EF20 RTN
F1 C7 C8
1A 10 μF 10 μF TinySwitch-III
185-265 400 V 400 V R2
VAC U1
D TNY275PN 47 Ω R3 R9
EN/UV 15 kΩ 261 kΩ
R11 BP/M
1% 1%
N 100 Ω
R4
S U2 1 kΩ
R12 PC817A C10 R5
D5 D7 30 Ω 100 nF 3.3 kΩ
1N4937 1N4007 C14 C15 C11
33 pF 0.1 μF 10 μF
1 kV 50 V 50 V U3
LM431
2% R6
L5 L3, L4 and L5 are 10 kΩ
FB 3.5 mm × 10 mm 1%
Ferrite beads.
PI-4495-122007

Figure 1. TinySwitch-III 9.65 W Residential Heating Controller Power Supply.

www.powerint.com December 2007

• Select transformer wire gauge sizes so that each winding layer 80

PI-4496-090706
occupies the full bobbin width. This lowers leakage inductance EN55022B Limits
70
and improves output cross regulation.
• Use option to add E-Shield windings in PI Transformer Designer QP
60
software to reduce conducted EMI noise generation. AV
50
• To lower the no-load power consumption even further, use an
optional bias supply circuit to feed the rated data sheet current 40 QP

dBμV
(through a series resistor) to the
30
BP/M pin of U1.
AV
20

Output Load Regulation (%) 10

Range
(V) (mA) -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 0
5 50-250
-10
24 88-350
-20
0.15 1.0 10.0 100.0
Table 1. Worst Case Output Cross Regulation at 185 VAC Output Loads Varied as
Shown and Maximum Deviation Recorded. MHz
Figure 3. Worst Case Conducted EMI at 230 VAC, Full Load Output Return
Grounded.
80

PI-4497-090506
79 Transformer Parameters
Efficiency (%)

EF20 TDK PC40, or equivalent, gapped for ALG

Core Material
78 of 323 nH/t²

EF20, 10 pin (5+5), horizontal pin shine P-2015

Bobbin
or equivalent
77
Shield: 15T, 2 × 31 AWG
Primary: 36T + 35T, 33 AWG
76 Shield: 4T, 5 × 29 AWG
Winding Details
5 V: 3T, 4 × 26 AWG T.I.W.
24 V: 11T, 26 AWG T.I.W.
75 (4 × 26 AWG = quadfilar 26 AWG)
185 205 225 245 265
Apply 2 mm tape margin to both sides of bobbin
Winding Order
AC Input Voltage (V) Shield (1-NC), tape, Primary (3-1), Shield (NC-1),
(pin numbers)
5 V (6-8), 24 V (10-6)
Figure 2. Full Load Efficiency vs Input Voltage.
Primary: 1.62 mH, ±10%
Inductance
Leakage: 50 μH (maximum)
Primary Resonant 800 kHz (minimum)
Frequency
Table 2. Transformer Parameters. (AWG = American Wire Gauge, TIW = Triple
Insulated Wire, NC = No Connection)

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12/07 DI-123