Transformerless AC power supplies

Capacitively-coupled AC/DC

This is one of the simplest AC power supply designs. A dropper resistor would dissipate too much power and get very hot, but the AC reactance of a series capacitor can be used to drop the input voltage without dissipating too much energy.



The reactance of the dropper capacitor is:



So for a 115VAC input and the 680nF capacitor shown above, Xc works out to be about 3.9 kOhms for 60Hz mains and 4.6 kOhms for 50Hz mains. This is much less than 470k so the parallel resistor can be ignored for the remaining calculations (it is required to discharge the capacitor when the power is switched off and to act as a primitive EMI filter to reduce the conducted harmonics).

The dropper capacitor current can be derived from:



So for an 115VAC/60Hz input, I_rms = 43354 C ≈ 40mA/μF and for a 230V/50Hz input, I_rms = 72257 C ≈ 70mA/μF.

The above circuit will have a maximum output current of 47mA for 230VAC supply or 27mA for a 115VAC supply, so the Zener diode needs to have a 1W rating to survive a no-load condition (12V x 0.047A = 564mW)

The MOV is required to avoid AC surge voltages from exceeding the rating of the dropper capacitor. The fusible resistor has two functions: to act as a fuse if the circuit malfunctions or the output is short-circuited and to act as a resistor to limit both the inrush current and MOV current.

Practical tip:The output voltage rise is not instantaneous. With each peak half-cycle of the mains input, current fl ows into the output capacitor, but during the cross-over period, very little charging current fl ows. Thus the output rises in small steps until the Zener diode voltage limit is reached. Increasing the load slows down the rise time so any rise-time sensitive circuitry operated from such power supplies should have either an under voltage lockout or a long start-up time delay.



Typical transformerless applications include passive IR movement detectors or relay time delay circuits (the relay output contacts provide the necessary output isolation to the mains supply and a 555 timer is not too fussy about the input voltage regulation.



Non-Isolated Buck Regulator

The disadvantages of a limited output current and the slow stabilization time for the capacitively coupled transformerless power supply can be overcome using a high voltage non-isolated buck converter. The AC input is rectified and smoothed to provide a high voltage DC bus which can then be efficiently down-converted to a low voltage output.

The output current restriction is lifted so that higher power designs can be achieved and the very wide input voltage range of the buck converter means that a much smaller bulk capacitors can be used compared with equivalent isolated designs (see input stage below, where a 2.2μF bulk capacitor is all that is needed for a 2.5W design.



The controller IC is a standard buck converter controller that operates from a 5V supply (it is not necessary to have a high voltage part, even though this design is non-isolated.

The rectified 115V or 230VAC mains input (160VDC or 325VDC is used to supply the startup supply voltage via the 300k dropper resistor and the 5.1V Zener. Once the power supply is operational, the 5V output is fed back via the Schottky diode (D2 to bootstrap the supply voltage from the output.

Transistor Q1 acts as a constant current sink. The base is tied to 5V, forcing the emitter to be one junction drop lower at 5V - 0.7V = 4.3V. When the controller output, SW, is high, then no current flows through Q1 as it is reverse biased. When the controller output is low, then the current flowing out of the emitter is 4.3V/50R = 86mA. The current flowing in to the collector must be the same as the current flowing out of the emitter, so the current through the 200R resistor is also 86mA, irrespective of the voltage of the DC bus. The volt drop across the 200R resistor will be equal to 200R x 0.086A = 17.2V. The duty cycle of the controller will be very short when regulating, so this relatively high current will only flow for 1.5% of the time and a ¼ W resistor will suffice.

The push-pull transistor pair Q2 and Q3 level shift the controller’s PWM control signal to a high-side gate drive for the P-channel FET, Q4, which switches between the high voltage DC supply and a voltage that is 12V lower than that, as set by the 12V Zener diode in series with the 100k resistor to ground. The output is then smoothed by the output inductor and capacitor to provide a regulated 5VDC supply.

Q2 and Q3 are general purpose bipolar transistors, only Q1 and Q4 have to withstand the full DC bus voltage. A Spice simulation (simulated without any feedback) shows that the output soon settles to 5.0VDC. In Fig. 9.5 below, the blue trace is the output voltage referenced to ground, the red trace is the gate drive signal and the green trace is the 12V Zener diode voltage (both referenced to the HV bus supply).



There are a number of design limitations to this high voltage buck converter design due to the extremely short on-time. The operating frequency is limited to around 30kHz by the slow response time of the power transistors. This means that a controller IC has to be selected with an adjustable frequency setting as most standard buck converter ICs operate at much higher frequencies. Secondly, the output has a strong saw-tooth ripple even with heavy filtering. The biggest advantage is that the output current is restricted only by the P-channel FET’s power rating and the output inductor power dissipation limits.

High Voltage Linear Regulators

An alternative to the capacitive dropper or discrete buck converter AC/DC is to use a high input voltage linear or switching regulators. With a linear regulator, the output current is limited to 10mA or less, but this is sufficient for many microcontrollers and Internet-of-Things (IoT) applications. One of the characteristics of IoT is that the data links are wireless, so the individual nodes need have only a power supply input without any output connectors and can be permenantly sealed. An isolated internal power supply is therefore not necessary.

Although very inefficient (<3%!), high voltage linear regulators offer tight regulation, low quiescent current and wide AC input range supply from 85VAC to 250VAC or DC supply voltage from around 60V to up to 450V. The output voltage can also be adjusted over a wider range than most other solutions. High voltage linear regulators are also available as SMD components making very compact cost effective solutions possible.



Off-Line Regulator IC

For slightly higher output currents (up to 175mA, a very compact and cost efficient non-isolated off-line AC/DC converter can be constructed using an integrated switcher IC that contains a built-in high voltage switching transistor, a high voltage current source for the internal power supply and a minimum off-time PWM controller with over-current, short-circuit and over-temperature protection, all in one integrated monolithic package. More advanced ICs also include frequency jittering to reduce the EMC signature and cycle skipping to reduce the no-load consumption.



The input is half-wave rectifi ed by diodes D1 and D2 and followed by a simple EMC filter formed by C1, L1, C2 and the ferrite bead. The fusible resistor limits the inrush and input ripple currents and acts as a fuse in the event of a failure. As the controller IC is floating with a direct connection to the high voltage input only, a “trick” is needed to make the circuit work.

The trick is the combination of D3, D4 and C4. If the D3 and D4 are the same part with the same characteristics, then the resulting voltage across C4 is the same as that across C5. In other words, the output voltage is reflected onto C4. The output voltage can thus be regulated and set by the ratio of R1 and R2 connected across C4, even though the common of C4 is not connected to the output. The source-pin referenced voltage across C4 also allows the internal high input voltage shunt regulator to be bypassed and the IC to be powered via R3 once it has started up.

For many IoT applications, it is useful to have a main 5V supply for the 2.4GHz radio or a 12V supply for a relay coil plus a 3.3V supply for the microcontroller and sensors. Adding a cost effective LDO regulator to the 5V or 12V main output adds an additional 3.3V output with very little increase in the BoM cost or overall size of the solution.

The overall efficiencies are high because there are no transformer losses:



For controller ICs with pulse-skipping mode, the no-load power consumption is also very low over the entire input voltage range.





Table 1: Measured no-load power consumption (mW). Dark green fi elds show compliance with 5I no load power consumption limits (≤0.03W). Light green fields show 4* compliance (≤0.15W)

Finally, the PCB can be made very small, making it ideal for building in to IoT applications:



Despite its small size (33 x17 mm) this off -line switcher demonstrator has a universal input voltage range, dual regulated outputs which are short circuit protected and meets EMC regulations without any external components.