NCP1050: Extremely Low AC-DC Power Supply Used with GFCI

Design Note – DN05037/D
Extremely Low Power Off-line Supply
Input Voltage
Output Power
I/O Isolation
90 - 300 Vac
15 mW
Other Specifications
Output Voltage
Nominal Current
Input Consumption
5 Vdc
3 mA
<0.5 mArms
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
August 2012, rev.0
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
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
Figure 3 – EMI Scans
August 2012, rev.0
Figure 4 – Input current at 110 Vac input
Figure 5 - Maximum drain voltage at 300 Vac input
August 2012, rev.0
Key Features
Simple topology
Low part count
Extremely low consumption
Wide input range
Data sheet NCP1050: Monolithic High Voltage Gated Oscillator Power Switching Regulator
© 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