TinySwitchⅡ and its application in standby power supply

1 Introduction

In recent years, monolithic switching power supplies have become increasingly popular among power supply designers for their low cost, high reliability and design flexibility in power supply systems. TinySwitch-Ⅱ series is the latest second-generation enhanced high-efficiency low-power isolated switching power supply integrated circuit launched by American Power Integrations after TinySwitch. This series of products includes TNY264P/G, TNY266P/G, TNY267P/G, TNY268P/ G, 8 models in total. TinySwitch-Ⅱ is an improved product of TinySwitch. When used to form a power system, the cost is lower than discrete component PWM and other integrated/hybrid power solutions. It is small in size and has high efficiency and reliability. It is especially suitable for applications that require low cost and high efficiency. Applications, such as mobile phone chargers, PC and TV standby power supplies, AC adapters, equipment controls and network terminals, etc.

2. Performance characteristics of TinySwitch-Ⅱ

TinySwitch-Ⅱ has the same simple topology circuit as TinySwitch, but the maximum output power is increased from 10W to 23W. TinySwitch-Ⅱ not only integrates the 700V power MOSFET, oscillator, high-voltage switching current source, current limit and thermal shutdown circuit into a single chip like TinySwitch, it provides startup and operating energy by the drain voltage, without the need for a transformer bias winding and related circuits, and also incorporates automatic restart, input undervoltage detection and frequency jitter functions within the device. This design effectively eliminates audio noise. The fully integrated automatic restart circuit limits the output power to a safe range under fault conditions such as output short circuit or control loop open circuit, which not only limits the short-circuit output current but also protects the load, while reducing the number of components and the secondary feedback circuit. the cost of. The line undervoltage threshold can be set externally with a sense resistor, thus eliminating power brownout failures caused by slow discharge of the input storage capacitor in applications such as standby power supplies. The switching frequency is increased from 44kHz to 132kHz, allowing the use of low-price, small-sized EF12.6 or EE13 cores, thereby reducing the size of the high-frequency transformer and improving power efficiency. At the same time, its 132kHz operating frequency is jittery , can significantly reduce quasi-peak and average EMI, making filtering cost the lowest. In addition, it is extremely energy efficient and reliable.

3. Pin function description

As shown in Figure 1, the TinySwitch-Ⅱ series single-chip switching power supply adopts an 8-pin dual in-line plug-in (DIP-8) or surface-mounted (SMD-8) package. The function of each pin is described as follows:

Drain (D) pin is the drain output pin of the power MOSFET, which provides internal operating current for startup and steady-state operation;

A 0.1μF bypass capacitor needs to be connected between this end of the bypass (BP) pin and the ground (S pole);

Enable/undervoltage (EN/UV) pin This pin has two functions: input enable and input undervoltage detection. During normal operation, this pin can control the on and off of the power MOEFET (when the current of this pin is greater than The power MOSFET is turned off at 240μA). This pin also detects undervoltage conditions through an external resistor connected to the input DC high voltage. If this pin is not connected to an external resistor, there is no input undervoltage function;

The common point of the source (S) pin control circuit is connected to the source of the internal MOSFET. The four sources are connected internally. They are divided into two groups. Two of the S terminals must be connected to the common point of the control circuit. terminal, and the other two S (HVRTN) terminals are connected to the high-voltage return terminal. The typical application circuit is shown in Figure 2.

4. Working principle

Figure 3 is the functional block diagram of TinySwitch-Ⅱ. It integrates a power MOSFET with a withstand voltage of 700V and an on/off controller. Unlike traditional PWM controllers, it uses a simple on/off controller to stabilize the output voltage. The frequency of the oscillator is 132kHz, and a frequency jitter circuit is added to the oscillator with a jitter amount of ±4kHz. This function makes the average and quasi-peak noise of EMI lower.

TinySwitch-Ⅱ usually works in the limiting current mode. When starting up, it samples the EN/UV pin signal at the beginning of each clock cycle, and then decides whether to skip a cycle or how many cycles to skip based on the sampling results, and determines the appropriate limit current threshold. When the drain current ID gradually increases and reaches the ILIMIT value or the duty cycle reaches the maximum value Dmaxx, the power MOSFET is turned off. At full load, TinySwitch-II is turned on during most cycles. At medium load, it will skip a part of the cycle and start to reduce the ILIMIT value to maintain the stability of the output voltage. At light load or no load, it will skip almost all cycles and further Lower the ILIMIT value so that the power MOSFET only conducts for a short time to maintain the energy necessary for the power supply to operate properly. This essentially introduces the principle of PFM modulation. In addition, under light load conditions, when the switching frequency may enter the audio frequency range, the current limit state regulator reduces the current limit in a discontinuous manner. The lower current limit value keeps the switching frequency above the audio frequency, reducing the voltage of the transformer. Magnetic flux density thereby mitigates audio noise.

The enable circuit of the EN/UV pin includes a low-impedance source follower circuit with an output set to 1.0V. The current flowing through this circuit is limited to 240μA. When the current flowing out of this pin exceeds 240μA, the enable circuit The output will produce a logic low (disable). The external resistor connected between the DC power supply and the EN/UV pin can be used to monitor the DC input voltage. When the voltage is lower than the set value, the undervoltage detection circuit reduces the bypass terminal voltage UBP from the normal value of 5.8V to 4.8V. , forcing the power MOSFET to turn off for protection; when the output MOSFET turns off, the 5.8V regulator draws current through the drain voltage to charge the bypass capacitor connected to the bypass pin to 5.8V. When the MOSFET turns on When the current is injected into the bypass pin through the external resistor, the voltage regulator will The pin voltage is clamped at 6.3V, which makes it easy to externally power TinySwitch-II through the bias winding, reducing the no-load power consumption to about 50mW.

When an output overload, output short circuit or open-loop fault occurs, TinySwitch-Ⅱ can automatically restart until the fault is eliminated and transition to normal operating status; the limit current detection circuit is used to detect whether the drain current of the power MOSFET reaches the limit value. In each switching cycle, when the current reaches the limit current ILIMIT, the power MOSFET is turned off during the remaining time of this cycle; the bypass pin of TinySwitch-Ⅱ only needs a 0.1μF capacitor. Due to the small capacitance, the charging time is extremely short. Generally it is 0.6ms, so the power-on process is fast and the power output has no overshoot. When an external resistor (2MΩ) is connected between the EN/UV pin and the positive terminal of the DC input, the switching of the power MOSFET during power-up will be delayed until the DC voltage exceeds the threshold (100V). When power is off, if an external resistor is used, the power MOSFET will continue to switch for 50ms after the output is offset. This feature makes the standby power supply have a slow turn-off characteristic when used as a standby power supply.

5. Application of TinySwitch-Ⅱ in standby power supply

Figure 4 is a PC standby power supply circuit based on TNY267P with a power of 15W. The circuit provides two outputs: main output 5V, 3A and primary auxiliary output 12V, 20mA. The input voltage range is DC140~375V, and the operating frequency of 132kHz allows the transformer core to be used in EE22 type. The line detection resistors R2 and R3 detect whether there is undervoltage in the DC input voltage. The combined resistance of R2 and R3 is 4MΩ, and the undervoltage threshold when powered on is set to DC 200V. Once the power is turned on, as long as the rectified DC input voltage is higher than 140V, it will continue to work until it drops below 140V before shutting down. , this design can provide the required hold-up time for the standby power supply. The auxiliary primary winding generates a 12V output voltage for use by the power supply primary controller. Additionally, this voltage is used to power the TinySwitch-II through R4. Powering the TinySwitch-II externally (although not required) reduces the device's static power consumption by no longer using the internal drain drive current to charge the bypass pin capacitor C3. The resistance of R4 is 10kΩ, providing 600μA current to the bypass pin, which is slightly larger than the current consumption of TinySwitch-II. The excess current is safely clamped at 6.3V by the on-chip voltage regulator tube. The secondary winding is rectified and filtered by D3 and C6. Since the auto-restart function limits the short-circuit output current, there is no need to use larger D3, L1 and C8 for switching noise filtering. Optocoupler U2 and VR1 are used for 5V output detection, and R5 is used to provide bias current to the voltage regulator tube. Since TinySwitch-Ⅱ limits the dynamic range of the optocoupler LED current, the voltage regulator tube can operate at a nearly constant bias current, thus providing the generally required accuracy (approximately ±3%). The power supply efficiency is higher than 78%.

6. Conclusion

This article discusses the working principle and main performance characteristics of TinySwitch-Ⅱ, showing that in low-power switching power supply applications, it has the advantages of fewer components, low cost, small size, high reliability and efficiency, and has broad application prospects.