Efficient, Reliable Power Solutions for Industrial and Consumer Applications


Silvestro Fimiani, Senior Product Marketing Manager at Power Integrations

Over a decade ago, Power Integrations launched its LinkSwitch™-TN IC product family which has proved highly successful. Now the company has introduced LinkSwitch-TN2, a new generation of the family which offers improved efficiency, voltage regulation and additional safety features, extending the range of applications where a non-isolated supply can be used.

The LinkSwitch-TN has been an extraordinary successful AC-DC switching regulator. Designers have benefited from the controller’s reliability, efficiency, design flexibility, small size, and reduced bill of materials requirement. However, modern microcontrollers and displays demand even more accurate supply voltage regulation, plus there has been a considerable shift in industrial and consumer product legislation which requires very low standby power to improve overall efficiency. Power Integrations took a long look at the new and emerging market requirements and the result is the LinkSwitch-TN2 family of AC-DC switching regulators. These feature a wide input voltage range, 85 VAC – 265 VAC, four output current level options 80 mA, 170 mA, 270 mA and 360 mA, output voltage regulation of +/- 3 %, class leading efficiency of over 80 % across a wide current range and an array of safety features including line (input) overvoltage protection, output overvoltage protection, thermal shutdown and short-circuit protection.

Technical Features

The LinkSwitch-TN2 is fabricated using a proprietary BCDMOS process that combines a high voltage power MOSFET switch with a low power switching controller capable of supporting a number of topologies including high-side and low-side buck, buck-boost and flyback. The MOSFET is rated at 725 V to withstand input voltage surges reducing the requirement for external protection circuitry. Figure 1 shows a functional block diagram of the LinkSwitch-TN2. We can use this to illustrate the key features of the part and its operating modes.

In addition to the power MOSFET the device consists of an oscillator, feedback (sense and logic) circuits, 5.0 V regulator, BYPASS (BP/M) Pin undervoltage circuit, over-temperature protection, line and output overvoltage protection, frequency jittering, current limit circuit, leading edge blanking and additional circuitry for auto-restart. Central to the operation of the device is the 66 kHz (nominal frequency) oscillator chosen to allow the use of standard low cost inductors. This generates two internal signals, the maximum duty cycle signal and a timing clock used to indicate the beginning of each cycle. Frequency jitter is applied to the clock, typically 4 kHz peak-to-peak to minimize EMI emissions allowing the use of low cost double sided printed circuit boards. The modulation rate for the frequency jitter is 1 kHz.

Figure 1:  LinkSwitch-TN2 Block Diagram

During normal operation the switching of the MOSFET is controlled by the FEEDBACK (FB) Pin. Switching is terminated when a current greater than 49 µA is delivered into this pin causing the internal feedback node to go low. This signal is sampled at the beginning of each cycle on the rising edge of the clock signal. If it is high, the power MOSFET is turned on (enabled) for that cycle, otherwise the power MOSFET remains off (disabled). Sampling is done only at the beginning of each cycle and changes to the FEEDBACK pin current during the remainder of that cycle does not impact the state of the MOSFET during that cycle. If a current greater than 670 µA is delivered into this pin for two consecutive switching cycles, a fault situation is detected and the part will stop switching and enter into an Auto-Restart time out. This feature can be used to monitor the line (input) voltage when the device is used in a Flyback configuration and provide overvoltage protection on the line voltage. Figure 2 below illustrates one possible circuit implementation.

Figure 2: Line Overvoltage Sensing using FEEDBACK Pin

In addition to the FEEDBACK pin fault detection, Auto-Restart is initiated when a variety of fault conditions are detected such as, output overvoltage, output overload, output short or an open loop condition. During Auto-Restart an internal counter, clocked by the oscillator, gets reset every time the FEEDBACK pin is pulled high. If the feedback pin is not pulled high for 50 mS the power MOSFET switching is disabled for a time equal to the auto-restart off time. The first time a fault is asserted the off time is 150 mS. If the fault condition persists the off time is increased to 1500 mS. The Auto-Restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed.

Output Overvoltage Protection (OVP) is triggered by delivering a current in excess of 6 mA into the BYPASS pin. The BYPASS pin capacitor forms a low pass filter providing noise immunity from inadvertent triggering. During a fault condition resulting from loss of feedback, the output voltage will rapidly rise above the nominal voltage. A voltage at the output that exceeds the sum of the voltage rating of the Zener diode connected from the output to the BYPASS pin and bypass voltage, will cause a current in excess of 6 mA to be injected into the BYPASS pin, which will trigger the auto-restart and protect the power supply from overvoltage. Figure 3 illustrates a typical non isolated buck convertor circuit configuration. Output voltage set via the feedback resistor potential divider R1 and R2 and output overvoltage detection provided by D4 and R3.

Figure 3: Non Isolated 12V, 120mA Continuous Output Buck Convertor

Current through the power MOSFET is internally sensed by the device. When this current exceeds the internal threshold Ilimit (dependant on the chosen device) the power MOSFET is turned off for the remainder of the cycle.  The leading edge blanking circuit inhibits the current limit comparator for a short time after the power MOSFET is turned on so that the current spike caused by the capacitance and ultra fast rectifier diode reverse recovery time will not cause premature termination of the switching cycle. Capacitor C3 connected between the SOURCE (S) pin and the BYPASS pin is used to set the upper and lower current limits for the chosen device. (0.1uF for normal current limit and 1uf for reduced current limit).

Die temperature is sensed internally and thermal shutdown is initiated when the die temperature rises above the threshold 142 °C typical. The power MOSFET is disabled and a 75 °C hysteresis is applied to the control logic so that switching will not be re-enabled until the die temperature has fallen below 67 °C typical.

LinkSwitch-TN2 ICs use a simple ON/OFF control method to regulate the output voltage. The decision to switch or not switch is made on a cycle by cycle basis resulting in excellent transient response and removes the requirement for external control loop compensation networks. At the beginning of each cycle the FEEDBACK Pin is sampled, if Ifb is less than 49 µA the next cycle is initiated. If Ifb is greater than 49 µA the next cycle is skipped. Thus, as the output load is reduced, more cycles will be skipped and if the load increases, fewer cycles are skipped. To provide overload protection if no cycles are skipped during a 50 ms period, LinkSwitch-TN2 will enter auto-restart, limiting the average output power to approximately 3 % of the maximum overload power. Due to tracking errors between the output voltage and the voltage across C3 at light load or no-load, a small pre-load may be required (R4). Table 1 below illustrates the cycle by cycle control method during normal operation and auto restart mode.

Table 1: LinkSwitch TN2 ON/OFF control scheme

For maximum efficiency it is better to operate the LinkSwitch-TN2 in Mostly Discontinuous Conduction Mode (MDCM) as opposed to Continuous Conduction Mode (CCM). Table 2 below explains the difference between the two modes of operation and the design trade-offs associated with each operating mode.

Table 2: Comparison of CCM and MDCM Operating Modes

The phrase “mostly discontinuous” is used since a few switching cycles may exhibit continuous inductor current, however the majority of the switching cycles will be in the discontinuous conduction mode. A design can be made fully discontinuous but that will limit the available output current, making the design less cost effective.  Additional differences between CCM and MDCM include better transient response for DCM and lower output ripple (for same capacitor ESR) for CCM. However, these differences, at the low output current levels the LinkSwitch-TN2 is designed for are normally not significant. MDCM is generally preferred as it provides maximum efficiency and gives the lowest overall solution cost but CCM can be used where maximum output current is the prime concern.

LinkSwitch-TN2 can be used in all common topologies, with or without an optocoupler and reference to improve output voltage tolerance and regulation. Table 3 below provides a summary of these configurations and key features. For more information, see the Application Note – LinkSwitch-TN2 Design Guide.

Table 3: Common Circuit Topologies using LinkSwitch-TN2

Design Analysis

Using the circuit in Figure 3, we can analyse the prime design considerations used this LinkSwitch-TN2 Buck Convertor power supply. The device is self-starting from the DRAIN (D) pin with local supply decoupling provided by a small 100 nF capacitor C3 connected to the BYPASS (BP/M) pin when AC is first applied. During normal operation the device is powered from output via a current limiting resistor R3. Here, the device LNK3204D is used in a buck converter configuration. The supply is designed for MDCM operation with the peak L1 inductor current set by the LNK3204D internal current limit. The on-time for each switching cycle is set by the inductance value of L3, LinkSwitch-TN2 current limit and the high voltage DC input bus across C2. Output regulation is accomplished by skipping switching cycles in response to an ON/OFF feedback signal applied to the FEEDBACK (FB) pin. This differs significantly from traditional PWM schemes that control the on time (duty cycle) of the switching cycle. During the ON time current ramps in L2 and is simultaneously delivered to the load. During the OFF time the inductor current ramps down via free-wheeling diode D3 into C5 and is delivered to the load. Diode D3 should be selected as an ultrafast diode (tRR of 35 ns or better is recommended. Capacitor C5 should be selected to have an adequate ripple current rating (low ESR type)). Efficiency of this design is in excess of 80 % across the majority of the converters load range. See Figure 4

Figure 4: Efficiency vs. Output Load (Room Temperature)

The voltage across L2 is rectified and smoothed by D4 and C4 during the off-time of U1. To a first order, the forward voltage drops of D3 and D4 are identical and therefore, the voltage across C3 tracks the output voltage. To provide a feedback signal, the voltage developed across C3 is divided by R1 and R2 and connected to U1’s FB pin. The values of R1 and R2 are selected such that at the nominal output voltage, the voltage on the FB pin is 2 V. This allows this simple feedback to meet the required overall output tolerance of ±3 % at rated output current. Figure 5 illustrates output voltage against load.

Figure 5: Output Voltage vs Output Current (room temperature)

To improve manufacturability, the LinkSwitch-TN2 family are available in a range of package options, 8 Pin DIP, 8 Pin SMD and 8 Pin SO, the SO being the smallest of the packages with a MSL rating of 1 (can be exposed to ambient room conditions 30 °C / 85 % RH indefinitely).

Conclusion

LinkSwitch-TN2 ICs address the requirements of global energy efficiency regulations with a no-load consumption of <30 mW in high-side buck converter topology and <10 mW in flyback topology with external bias. Applications include metering, home and office building automation, industrial controls, consumer appliances and LED lighting, where a LinkSwitch-TN2 IC-based design can replace simple, low efficiency, unreliable cap dropper supplies as a reliable highly efficient alternative, which - because of its inherent tight voltage regulation - requires no post regulation. Designs using the LinkSwitch-TN2 device benefit from improved reliability, improved performance, higher efficiency and lower cost, even when compared against cap droppers which require large and expensive X-capacitors.

Manufacturers of all types want to improve the reliability of their products and reduce the total cost of ownership. Power supplies are an area where significant improvements can be made by taking advantage of the latest technology. With Power Integrations’ long history in high-voltage power semiconductor design and production provide a range of AC-DC converters with reliability designed in. The company subjects 100% of its LinkSwitch-TN2 parts to a high-voltage stress test to ensure maximum quality and reliability.

 

 

More information: Power Integrations    Source: Bodo's Power Systems, January 2017