Transformerless PSU, round 2

A few months ago, in the article Transformerless PSU, is it worth it? two ways to convert AC mains into usable DC voltage were analyzed in terms of cost, space and performance.

These two types were the traditional step-down transformer approach and a transformerless design that uses the property of reactance of the capacitor to create a voltage divider and effectively step-down the voltage.

In this article, we will analyze a third way to convert AC mains into DC voltage to power small electronics.

Theory

One way to efficiently step-down DC voltage is to use a buck converter, a type of switch mode power supply.

Therefore, if you can rectify AC mains voltage, say 311VAC (RMS value 220VAC), into 220VDC, then you could use a buck converter to step-down 220V to something like 5V.

This operation is possible and it is what the LNK302/304-306 from Power Integrations does.

Although it does not require a transformer, it still uses an inductor. Good news is that these are much cheaper and lighter than transformers.

Transformerless Switching PSU Design

The PSU will have the following design specs:

  • Step-down and rectify 220VAC into 3.7VDC
  • The PSU must be able to source up to 245mA of current to the load

Note: in the last article the design parameters were 12V and 75mA which is 0.9W, we have kept the same power for this design but different voltage and current (3.7V*0.245A=0.9W).

Circuit

The following circuit uses the LNK30x in the high-side Buck – direct feedback topology:

Input Stage

  • RF: this component is a fusible resistor. It provides current limiting and protection in case of a short circuit down the line.
  • DIN1,2: These two diodes form a rectifier to convert the AC into DC with bumps.
  • CIN1,2 and LIN: These 3 components form a pi filter with the purpose of smoothing out the rectifier output and filtering out high frequencies.

LinkSwitch-TN

This IC integrates a power MOSFET as the switching element of the supply and a control circuit with overtemperature protection. For more information check the component datasheet.

Output Stage

The output stage is a mix of classical components that any switch mode power supply has and the ones that interface with the control circuit of the LNK30x

  • Dfw, L, Co: standard components used in a switch mode power supply.
  • Dfb, Rbias, Rfb, Cbp, Cfb: components used in the control circuitry of the LNK30x to keep the supply stable and at the desired value.
  • Rpl: minimum load the LNK30x needs in order to work. If your design draws more than 3mA constantly this is not required.

Choosing components values

Power Integrations have two different resources to aid you with the design of this circuit.

Using these two resources, you can easily determine and calculate the components you need to buy in one afternoon.

Final Circuit

After selecting the components values, the final schematic looked like this:

Cost

Transformerless Switching BOM

All prices are valid for the 11/02/2019 on Digikey:

R3 – RES 8.2 OHM 1W 5% AXIAL – 0.053$/unit

L1 – FIXED IND 1MH 15MA 24 OHM SMD – 0.053$/unit

U1 – IC OFFLINE SWIT OCP 8DIP – 0.77$/unit

C4 – CAP CER 0.1UF 50V X7R 0603 – 0.032$/unit

R2 – RES SMD 2K OHM 1% 1/8W 0805 – 0.005$/unit

R1 – RES 2.37K OHM 1% 1/8W 0805 – 0.005$/unit

C34 – CAP CER 1UF 25V X7R 0805 – 0.055$/unit

C1 – CAP ALUM 10UF 20% 50V RADIAL – 0.052$/unit

D1 – DIODE GEN PURP 600V 1A DO204AL – 0.097$/unit

D5 – DIODE GEN PURP 600V 1A DO204AL – 0.097$/unit

L2 – FIXED IND 820UH 350MA 1.8 OHM TH – 0.17$/unit

C6, C36 – CAP ALUM POLY 220UF 20% 10V T/H – 0.11$/unit

C7, C8 – CAP CER 22UF 10V X5R 0603 – 0.075$/unit

Total = 1.57$/unit

Space

Transformerless Switching

I actually built this PSU so we can realistically see how much space it uses:

This dimension could definitely be optimized by using SMD components for the diodes and making full use of the bottom layer. But for the moment we have 33mm*38mm of space used by the components.

Total area = 1254mm2

Performance

Dissipation and Efficiency

According to the design tool provided by Power Integrations, a overall efficiency of 63% has been calculated.

This means that for our power supply to source the required 0.9W, it will need to consume a total of 1.42W, that’s about 0.79W being dissipated throughout the components.

Other considerations

As this is a transformerless power supply, there is no isolation between AC mains and the low voltage rails (Vcc and GND). For this reason, this power supply should only be considered to be used in applications where the enclosure of the product is totally made out of plastic and the user has no interaction or very limited interaction with the device.

This power supply also has a soft audible noise in the range of 14KHz to 16KHz. Depending on your application, this might not be an issue.

For the application I used it for, there is a microphone present that can definitely sense this noise. Luckily my frequency of interest is at 3KHz and by implementing a low pass filter, this issue was solved.

Also, this power supply is not very good at handling high current inductive loads such as relays or speakers as it reacts a bit slow and losses stability. To sort out this issue, low ESR high capacitance polymer capacitors where used, which increase the cost of the BOM.

Conclusion

Using the LNK306 to implement a transformerless power supply was definitely an improvement compared to using just a series capacitor in terms of efficiency. However, it costs 67 cents more per unit.

Compared to the transformer power supply, it is still almost half the price and lighter. With the drawbacks of less efficiency and added audible noise.

I can see the LNK306 being used for loads of up to 1.5W and for applications that do not require isolation, an audio circuit or inductive loads as a very acceptable replacement for a transformer in order to save costs and the total weight of the product.

Posted in: Product Development

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