Transformerless PSU, round 2

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, 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.

Roberto Weiser

Roberto has experience in electronic embedded systems design and programming from concept to production, including: power electronics, audio electronics, automotive, battery management systems and renewable energies control systems. Roberto is interested in technology and sustainability of which he has practical experience acquired in his University degree, Master's, external courses, placement and also from volunteering in Peru. Furthermore Roberto has skills for business, leadership and teamwork from his time as Chairman of the Latinos and Spanish Society and Resident DJ in Plymouth

This Post Has 9 Comments

  1. sathishraj

    Wow…..Great great Information, For the whole day i was searching for Transformerless power supply for the STM32 low cost product.
    Before reading your blog, i planned to use MP103GN, But it does uses another BJT and having more thermal loss.

    The way you measured the audible noise and told the pros and cons is awesome.

    I have one doubt, the AC input 220v given Phase and Neutral if reverse connected, does the circuit behaves same. Are you tested the Interchanging the Phase and Neutral of AC input the PCBA you made.

    Thank you thank you very much for testing and save my time.

    1. Roberto Weiser

      Hello Sathishraj,

      Glad you found this post useful! For some reason is quite hard to find out there transformerless power supplies design information.

      You can swap Live and Neutral and the circuit will behave the same, however, there is something you really need to be careful with, which unfortunately I found the wrong way… If you are sharing an electrical connection, you must be sure that they both have the same polarities, otherwise, you might short the connection when you connect the two grounds from different components together.

      I was using an oscilloscope which I am assuming it has a non-isolated power supply because when I connected the ground probe to the board ground, a big spike and noise happen in front of my eyes. Luckily I wasn’t touching the conductive part.

  2. sathishraj

    Thanks for the response…

    The DC ground and AC neutral Must not be connected together, that what the Big Spike’s lesson!!!!!
    I too experianced the same, what was happened, it goes upto the EB meter box gone. Big short and burned with viper22A chip.

    In Electronics its all happens….Weiser..

    In the Circuit you made, I have two doubts.

    1. Can we connect a Full bridge rectifier with 4.7uF/400v Electrolytic capacitor as filter and then give the DC power to the D(Drain) Pin of LNK306.
    2. Where is the formula to calculate required output voltage w.r.t Rfb and Rbias.

    Thank you

  3. sathishraj

    In the POWER INTEGRATIONS website, I downloaded PI EXPERT Suite 10.3.In that software lot of options provides me, mostly flyback with a transformer which deviates the purpose I came to the LNK30X chip.

    If possible, could you some send project file using LNK30x, to help me Start the design.

  4. Roberto Weiser

    I am sure it is part of the software as that’s what I used before. Please check again, I do not have any design files. The datasheet and application note has everything you need to design the circuit

  5. sathishraj

    I can’t find in the PI Expert software, only flyback is enabled. The schematic created by the software’s all having transformers. Sorry…I am disturbing you lot.

    If possible, show me the calculation of Rfb and Full bridge rectifier input.

    Thank very much for fast response.

    1. Roberto Weiser

      As I said, download the datasheet and application note from the website and you will find these calculations, explanations and more.

  6. sathishraj

    I am in dilemma between LNK306 and MP174…In datasheet both says same specs. But no body in net have tried and not made any blogs with MP174.
    MP174 promises upto 400mA. MP174 has additional VCC pin for under voltage lockout.

    Could you compare these chips LNK306 and MP174 to make a pros and cons note.

    I am planning to make a mass home automation production product.

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