TNY255

TinySwitch is an ideal solution for low cost, high efficiency standby power supplies used in consumer electronic products such as TVs. Figure9 shows a 7.5 V, 1.3 W flyback circuit that uses TNY253 for implementing a TV standby supply. The circuit operates from the DC high voltage already available from the main power supply. This input voltage can range from 120 to 375VDC depending on the input AC voltage range that the TV is rated for. Capacitor C1 filters the high voltage DC supply, and is necessary only if there is a long trace length from the source of the DC supply to the inputs of the TV standby circuit. The high voltage DC bus is applied to the series combination of the primary winding of T1 and the integrated high voltage MOSFET inside the TNY253. The low operating frequency of the TNY253 (44kHz), allows a low cost snubber circuit C2 and R1 to be used in place of a primary clamp circuit. In addition to limiting the DRAIN turn off voltage spike to a safe value, the RC snubber also reduces radiated video noise by lowering the dv/dt of the DRAIN waveform, which is critical for video applications such as TV and VCR. On fixed frequency PWM and RCC circuits, use of a snubber will result in an undesirable fixed AC switching loss that is independent of load. The ON/OFF control on the TinySwitch eliminates this problem by scaling the effective switching frequency and therefore, switching loss linearly with load. Thus the efficiency of the supply stays relatively constant down to a fraction of a watt of output loading.

The secondary winding is rectified and filtered by D1 and C4 to create the 7.5V output. L1 and C5 provide additional filtering. The output voltage is determined by the sum of the optocoupler U2 LED forward drop (~ 1 V) and Zener diode VR1 voltage. The resistor R2, maintains a bias current through the Zener to improve its voltage tolerance.

DRAIN Voltage ................................... .. - 0.3 V to 700 V
Peak DRAIN Current (TNY253/4) ........................400 mA
Peak DRAIN Current (TNY255) ...........................530 mA
ENABLE Voltage ........................................ - 0.3 V to 9 V
ENABLE Current..................................................100 mA
BYPASS Voltage ......................................... -0.3 V to 9 V
Storage Temperature ............................. -65 to 150 °C
Operating Junction Temperature(2) ....... -40 to 150 °C
Lead Temperature(3) ........................................ 260 °C
Thermal Impedance (_JA) ....... 45 °C/W⁽⁴⁾, 35 °C/W⁽⁵⁾
Thermal Impedance (_JC) ............................... 11 °C/W
1. All voltages referenced to SOURCE, T_A = 25 °C.
2. Normally limited by internal circuitry.
3. 1/16" from case for 5 seconds.
4. Soldered to 0.36 sq. inch (232 mm²), 2 oz. (610 gm/m²) copper clad.
5. Soldered to 1 sq. inch (645 mm²), 2 oz. (610 gm/m²) copper clad.

The TinySwitch TNY253 and TNY254  family uses a breakthrough design to provide the lowest cost, high efficiency, off-line switcher solution in the 0 to 10 W range. These  TNY253 and TNY254  devices integrate a 700 V power MOSFET, oscillator, high voltage switched current source, current limit and thermal shutdown circuitry. They start-up and run on power derived from the DRAIN voltage, eliminating the need for a transformer bias winding and the associated circuitry. And yet, they consume only about 80 mW at no load, from 265VAC input. A simple ON/OFF control scheme also eliminates the need for loop compensation.

The TNY253 and TNY254 switch at 44 kHz to minimize EMI and to allow a simple snubber clamp to limit DRAIN spike voltage. At the same time, they allow use of low cost EE16 core transformers to deliver up to 5 W. The TNY253 is identical to TNY254 except for its lower current limit, which reduces output short circuit current for applications under 2.5W. TNY255 uses higher switching rate of 130kHz to deliver up to 10 W from the same low cost EE16 core for applications such as PC standby supply. An EE13 or EF13 core with safety spaced bobbin can be used for applications under 2.5W. Absence of a bias winding eliminates the need for taping/ margins in most applications, when triple insulated wire is used for the secondary. This simplifies the transformer construction and reduces cost.