Friday, 8 May 2015

Transformerless Negative Ion Generator - PCB realization (2/3)

PCB realization

The figure below shows the schematics in Eagle CAD of the Negative Ion Generator.
At the bottom left, the AC fan speed controller based on a TRIAC, a DIAC, capacitor, resistor and 500kohm potentiometer. At the middle left, the 220VAC to 75VDC transformerless converter based on 75V Zener and TVS diodes. At the middle-center, the 75VDC to 350VDC step-up (boost) converter based on Linear Technology LT3758A and at the top the 20-stages capacitor-diode voltage multiplier connected to the switching node of the step-up converter. R15 3.3Mohm resistor allows discharging the voltage multipliers capacitors when the circuit is switched off.

C1 4.7nF generates a Soft-Start of the step-up converter that reduces current load at start-up. R2 10.5 kohm is chosen for a 1MHz frequency switching.

R5 must not be installed, otherwise U1 could be damaged!!

Please be aware that R17-R18 resistors are normally not installed, they allow bypassing the 220VAC to 75VDC and the 75VDC to 350VDC in order to connect the voltage multipliers directly to the mains. This is the traditional approach and much simpler negative ion generator that multiplies the 220VAC directly. This option was considered in case the step-up converter approach did not work properly. In this case, all the components between the 220VAC input L-N-GND (including R19) and POT1-2-3 are not installed, R16 is not installed and R17-R18 are installed instead. In this case the switching frequency goes from the 1MHz switching generated by LT3758A to the 50Hz frequency of the mains supply.

Typical diodes used in the voltage multiplier circuits are through-hole mounting devices 1N4007 (1kV). In this case 1kV avalanche SMD diodes AR1PM where used. Capacitor must be at least 1kV rated. Proper rating voltage has to be used in all components in order to withstand the high voltages of the circuit.

Below is the view of the PCB layout (160 x 100 mm). There is no reference ground plane underneath the high voltage area of the voltage multipliers to avoid arcing to GND. The 3.5kV and the 350V areas have been highlighted and labelled with silkscreen. TP1 test point is at 10V when the regulator output is at maximum voltage output of 350V, and at 5.5V when the output voltage is at minimum of 195V:

You can have access to the Bill of Materials on the Digikey website, please note that all components quantities are multiplied by two except for the fan and accessories.

As mentioned in the previous simulation blog entry, C7 2.2uF was not big enough to allow circuit start-up, due to initial peak current loads, so an additional 0.68uF capacitor (CGA9M4X7T2W684K200KA) had to be added in parallel to C7, to increase start-up current;
D2 75V Zener SMA diode had to be replaced by bigger SMB device (SMBJ5374B-TP) and 75V TVS SMA diode had to be replaced by bigger SMC device (SMCJ75A) to allow power dissipation at low loads.

ATTENTION!! Very high voltages are present on this circuit which can be harmful

Verification of the circuit has to be very careful and special safety measures has to be considered. I recommend wearing protective rubber gloves when probing the circuit to avoid getting an electric shock if inadvertently touching a high voltage point. The PCB input has 220VAC, the PCB area in the center, highlighted with silkscreen, from the output of LI and Q1 to R16 reaches 350VDC and the PCB area to the left with no ground reference plane from R15 to JP5, where all the capacitor-diodes voltage multipliers are located, goes from 350VDC to almost 3.5kV. Do not approach these high-voltages areas, the only points that need to be verified are the 75VDC output that can be probed at the right side of R11 or the right pin of JP1 if LED is not installed, and the test point TP1 that divides the output voltage of the DC-DC converter by 35. This test point will be between 5.5V to 10V depending on the position of the 100kohm potentiometer tap connected to POT1-2-3.

There is no risk if touching JP5 or the needle grid connected to it, since there are three 3.3Mohm resistors in series after the 3.5kV output of the voltage multipliers, if the needle grid is touched all the voltage will drop in this 10Mohm resistance.

The capacitor values used on the voltage multipliers are quite low so they do not store a lot of charge. I actually think that to increase the ionizer efficiency these capacitor values should be multiplied by 10, or all of them increased to 100nF, present values range from 10nF to 2.2nF. In this case a 1MHz frequency on the boost converter switching is probably not needed and it could be decreased.

The fan speed controller can actually been improved by increasing R21 from 4.7kohm to a higher value to be manually verified (probably between 100 kohm and 200 kohm), because the fan stops when R20 potentiometer tap is in the middle of its full sweep.

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