DN05037/D Design Note – DN05037/D Extremely Low Power Off-line Supply Device Application Input Voltage Output Power Topology I/O Isolation NCP1050 Sensors 90 - 300 Vac 15 mW Flyback None Other Specifications Output Voltage Nominal Current Input Consumption 5 Vdc 3 mA <0.5 mArms Introduction There are many power solutions that comply with the Energy Star directives for stand-by / no load consumption. Most of them rely on converters that switch in bursts or get into skipping mode. The NCP1050 is a good candidate for such applications and this design note describes a simple off-line power supply that operates at a very low power consumption. It is used to power up a motion / infrared sensor and the logic circuit associated with it. This supply provides 15 mW output power while consuming only 0.5 mA from the AC line in order not to trip the GFCI protection. Circuit Description The NCP105X family of gated oscillator type converters is very suitable for this application. In order to minimize losses the NCP1050 is chosen because it has the lowest peak current level in the family and provides 44 kHz operation. The circuit is a typical flyback and since no isolation is required it is pretty simple. The secondary has basically two windings: one provides the main output (5 V) and the other one provides the bias (10 V) for the NCP1050. Although the NCP1050 can be self biased, using an extra winding for bias results in even lower power consumption. The input filtering capacitor (C2) needs to have a very low value in order to minimize the rms input current. A 2.2 uF value works very well. To reduce even more the current spikes due to the input capacitor being charged each cycle, a current limiter (Q1 – Q2) is also added. With all these precautions the converter produces 5 V / 3 mA output while consuming 0.49 mA at 110 Vac while maintaining the inrush current below 2 mA. Input power at 110 Vac input was 51 mW (measured with Yokogawa WT210 power meter and averaged over a 2 minute period). Figure 1 - Schematic August 2012, rev. 0 www.onsemi.com 1 DN05037/D August 2012, rev.0 www.onsemi.com 2 DN05037/D Due to the fact that power is delivered in bursts, the output ripple is significant. Figure 2 (top) shows almost 0.8 V peak voltage ripple across capacitor C8. Beefing up C8 is not a viable option because, in this case, the input current limiter (Q1 – Q2) would prevent it from being charged and the converter will enter a hiccup mode, and will never be able to fully start. Therefore a series transistor (Q3) is added after C8 to act like a soft start and slowly charge a larger capacitor C11. This way, voltage ripple across C11 is reduced bellow 50 mV peak-to-peak - figure 2 (bottom) Figure 2 – Output Ripple Drain Voltage Ripple across C8 Drain Voltage Ripple across C11 August 2012, rev.0 www.onsemi.com 3 DN05037/D Resistors R1 – R2, in front of the bridge, also help reduce current spikes and, in conjunction with capacitor C1, act like an input EMI filter. The EMI scans in figure 3 show the circuit passes Class B requirements. Figure 3 – EMI Scans August 2012, rev.0 www.onsemi.com 4 DN05037/D Figure 4 – Input current at 110 Vac input Figure 5 - Maximum drain voltage at 300 Vac input August 2012, rev.0 www.onsemi.com 5 DN05037/D Board Key Features Simple topology Low part count Extremely low consumption Wide input range References Data sheet NCP1050: Monolithic High Voltage Gated Oscillator Power Switching Regulator 1 1 © 2012 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Michael Borza, e-mail: [email protected] August 2012, rev.0 www.onsemi.com 6