Capacitive Power Supply

Thread Starter

fastbike

Joined Dec 29, 2020
72
I'm designing a circuit using a MOSFET driver (On Semi FL5150) that draws its power from the 230v mains.
The datasheet shows a diode/resistor circuit to provide power. As this entails significant power dissipation I'm looking to use a capacitive power supply instead. I'm aware of the limitations and risks that this incurs and have these covered off as part of the product design.

The MOSFET driver internally has a zener diode that clamps at 17v. The circuit design requires a max current of 15mA so I have sized the power capacitor to provide that.
Part of the driver design is that the source of the MOSFET connects to the GND of the driver to complete the gate circuit. So the GND will also be at (or a diode drop less than) the supply voltage.
This requires that the capacitive power supply draws its power only in the half cycle where Neutral is more positive than Line so a bridge rectifier is not possible and a simple 1N4007 half wave design is used.

I have drawn up a schematic to model this. The areas I'm looking to check is the placement of the inrush limiting resistor. The way it is placed it will slow the charging current into the power capacitor, but will also affect the charging of the MOSFET gate thus slowing its switch on. I'm guessing not enough to matter.

Any feedback and comments are welcome. In particular have I placed the components in the appropriate place ?

Thanks

1610305020237.png
 

wayneh

Joined Sep 9, 2010
17,152
It used to be against the rules around here to discuss transformer-less power supplies. I was going to link you to that but found only this:
  • Restricted topics. The following topics are considered "out of bounds" and will result in your thread being closed without question:
    • Any kind of over-unity devices and systems
    • Devices designed to electrocute or shock another person
    • Jammers intended for actual use in other than legal academic contexts
    • Amateur/homemade rail guns and other high-energy projectile devices.

    • If You come across a thread or post discussing any of the above topics please report the thread so We can close, and if necessary remove, the offending thread/posts.
So I guess I missed the change. Makes sense, given the demise of the transformer-based power supply.
 

Externet

Joined Nov 29, 2005
1,605
I do not like the single rectifier diode in series with a capacitor. Charges it and nothing else happens even if there is voltage between neutral and ground to power anything.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
I do not like the single rectifier diode in series with a capacitor. Charges it and nothing else happens even if there is voltage between neutral and ground to power anything.
I'm not sure what you mean here. The Mosfet driver is using the voltage supplied by the capacitor and diode.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
As an alternative I could move the R_Inrush to between Neutral and the 1N4007. This slows the inrush to the power capacitor and does not affect the Mosfet gate circuit.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
So after some more research I discovered the circuit as originally drawn would not deliver current to the driver IC, as the diode was blocking the reverse flow of the current so the C_Power capacitor could not charge / discharge. I have added an additional zener, D2, and moved the R_Inrush resistor.

Now to construct a prototype for testing.
1610417893975.png
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
Where did you get your schematic?
I'm looking to replace the resistive power supply with a capacitive power supply. I'm replacing 20x triac based quad channel dimmers, that each are driven with an analog voltage via opto isolator that is buffered on the hot side with an op amp. The op amps requires a separate Vdd per channel, as the FL5150 GND is connected to Line via Q2 (or its diode) in the datasheet schematic.
 

Marley

Joined Apr 4, 2016
411
The Live and Neutral must be completely ISOLATED from Ground. Or you will trip circuit breakers (RCDs) and possibly worse! Both the circuit diagrams above to not achieve this.

Yes, there used to be rules here!
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
The Live and Neutral must be completely ISOLATED from Ground. Or you will trip circuit breakers (RCDs) and possibly worse! Both the circuit diagrams above to not achieve this.
GND is not protective earth. It is the zero voltage reference point for the FL5150 driver chip. I've used the terminology from the datasheet.
There are working circuits in the field using this chip with the resistive power supply as shown in Ron Simpsons schematic.
I'm looking for more efficiency in the power supply.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
I've updated the schematic, removing the mosfet driver and mosfet as this makes it harder to understand. I have added a diode and switch to model the mosfet (when the mosfet is off the switch is open, when on the switch is closed - ignoring R_DSon). I've renamed GND to 0v to improve understanding.

The circuit as shown will charge the smoothing capacitor during the half cycle where line is at lower voltage than neutral. In the half cycle when line is above neutral, the power capacitor can discharge via the zener ZD_power.

1610481662050.png
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
Why do you think it needs to be a Zener?
You are correct, I was thinking I needed to shunt the voltage above 17v (20v allowed for diode drops etc). But the 17v zener (ZD_LOAD) can do this. I've amended the diagram replacing Z_POWER with D2 (1N4007) which allows the circuit to complete when line is higher than neutral.

Many thanks for your help.

1610486600590.png

I will build a test circuit using a transformer to drive the Line / Neutral at 50V AC - resizing the inrush and power capacitor to suit that voltage.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
Those 3 diodes are starting to look uncannily like part of a bridge rectifier. A shame I can't disconnect 0v from LINE (required to complete the Mosfet gate circuit) else I could replace the 1N4007 diodes with a small SMD 1A component such as MDB6S etc.
 

Deleted member 440916

Joined Dec 31, 1969
0
Something I do not like about any of these circuits is they do not have adequate safety measures, for example a single resistor is not able to withstand mains voltages and is a single point of failure, capacitors should be X rated and of course there should be fuses and shunt protection devices on the output able to blow the fuse. These are some of the reasons (there also issues of construction such as creepage & clearance) such activity should be discouraged on DIY fora.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
Something I do not like about any of these circuits is they do not have adequate safety measures, for example a single resistor is not able to withstand mains voltages and is a single point of failure, capacitors should be X rated and of course there should be fuses and shunt protection devices on the output able to blow the fuse. These are some of the reasons (there also issues of construction such as creepage & clearance) such activity should be discouraged on DIY fora.
All good points. I'm showing a simplified schematic just to work through the principles.
The finished product has a fuse on the Line side of the power supply, and all components are appropriately rated for the voltage intended.
Isolation issues including encapsulation of the finished device are part of the design. E.g. there are no user controls, with the device using the power supply being connected to the controlling micro via DIP cased opto isolators.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
I've built a primitive prototype using a 230V to 25V transformer and will run some voltage and current measurements once I have recalculated the values of the components. The transformer provides galvanic isolation (and an element of safety for prototyping purposes) but I'll need to lower the zener voltage to 5v to more accurately emulate the final circuit.
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
I've had time to play around with the simple prototype and take some measurements.
First an updated schematic showing the transformer (230 to 110 VAC) to provide some safety for my experiments and also to lower the voltage so the components on hand could be worked into the prototype. Also a sense resistor was added to the load so the actual voltage drop could be measured to allow the DC current available for the load to be calculated.
1611459822076.png
 

Thread Starter

fastbike

Joined Dec 29, 2020
72
Now for the actual measured results with the load resistor at 1193 ohms and the sense resistor at 11.8 ohms:

Test 1Test 2Test 3
Input Voltage (V AC)55110110
Power Cap (uF)1.0281.0281.028
Inrush Resistor (ohms)999999
Zener Diode Voltage (V)5.15.112
Volts at output (V)55.0811.95
Volts across sense resistor (V)0.0870.1760.173
Observed current (mA)7.414.914.7
Theoretical current (mA)7.11513.9

The actual measurements are very close to the theoretical current. The formula for the theoretical current are from the design notes from Designer Circuits LLC (pdf available on the web)
 
Top