This one’s a beauty. It will drive multiple CFLs without the need for heatsinks on the MOSFETs. Fit heatsinks and you can run even more, up to a total of 100W.

Fig.1: an SG 3525 pulse width modulator chip running at 85kHz drives two MOSFETs and a stepup transformer to develop 350V DC to drive compact fluorescent lamps (CFLs) or a small colour TV set.

However, the real bonus is that you don’t have to wind the transformer – a tricky job at the best of times. This kit comes with a pre-wound transformer so all you have to do is to solder it into the PC board.

As pictured, the prototype inverter is running four 11W CFL tubes and the MOSFETs are barely warm. And to do that it is drawing just 3.2A from a 12V battery so it is a pretty efficient device.

It’s compact too, housed on a PC board measuring 83 x 50mm and standing about 58mm high because of the on-board transformer.

The circuit is quite simple, as shown in Fig.1 and is based on an ST SG3525 regulating pulse-width modulator (IC1) which drives a couple of Mosfets and the push-pull stepup transformer.

The on-chip oscillator is set by the 4.7nF (.0047μF) capacitor at pin 5 and the 3.3kΩ resistor at pin 6, giving a frequency of about 85kHz. This is divided by two by an internal flipflop and the flipflop’s complementary outputs drive buffers which can source or sink in excess of 200mA.

It's efficient and it's easy to build, thanks in large part to the pre-assembled transformer. Watch the polarity on the semiconductors and electro.

The buffer outputs at pin 11 & 14 drive two MOSFETs, Q1 and Q2, with the high output current capability being ideal for rapid charge and discharge of the MOSFET gate capacitances, resulting in fast switching times. This results in high efficiency and little power loss in the MOSFETs.

The MOSFETs drive the push-pull step-up transformer. This has four turns on the two primaries and 120 turns on the secondary, giving a stepup ratio of 30 times. The transformer is connected to a bridge rectifier consisting of four BA159 1A high speed diodes and a filter capacitor of a mere 1nF (.001μF). The resulting DC voltage is about 350V.

The output voltage is not regulated unless it exceeds 370V, as set by the resistive feedback network (220kΩ, 270kΩ, 6.8kΩ) connected to pin 1, the inverting input of the internal error amplifier. This samples the output voltage and compares it to a 5.1V internal reference (pin 16) connected to pin 2, the non-inverting input of the same internal error amplifier.

The reason for not regulating the output voltage to a lower value is to avoid reducing the efficiency of the converter. In any case, regulation is not necessary in this application because CFLs can operate over a wide voltage range with a little variation in light output.

The 68nF capacitor at pin 9 provides a "soft start" facility. The capacitor is charged by a 50μA constant current source inside IC1 and this causes the duty cycle of the output signal from pins 11 and 14 to slowly rise from zero to 50%.

The soft start is needed because of the relatively large filter capacitor in an electronic ballast or a CFL.

Under-voltage protection is provided by the SG3525 by virtue of the fact that it will stop operating below 8.5V. This avoids the possibility of inadequate gate drive to the MOSFETS which would lead to over-dissipation. However, note that this does not protect any SLA battery from over-discharge.

Construction

Assembly of the PC board is pretty straightforward as there is only a handful of components. The component layout is shown in Fig.2.

Fig 2: Component overlay for PC board

Mount the resistors and diodes first, making sure that you install the diodes the right way around. Then mount the small capacitors and the electrolytic which must have the correct polarity.

The transformer can be soldered in next. Its leads are already soldered to pins, so it simply drops into the appropriate place on the PC board.

Then install the two MOSFETs and a socket for IC1. When you insert the IC into the socket, make absolutely sure you install it the correct way around (otherwise smoke will escape when you turn it on!).

For up to (say) 50W output (4x 11W CFLs), MOSFET heatsinks aren’t needed. Higher output (such as driving the TV set below) will need small heatsinks on each of the MOSFETs.

Fit four PC pins for the external connections and then you attach the leads for the battery and the output socket (or sockets).

Carefully check your work and then connect one CFL to its socket. Then connect the 12V battery. The CFL tube should light instantly. If not, turn off and carefully re-check your work (which you should have done before!)

Note that while the output of the inverter is polarised, it does not matter which way around a CFL is connected as its electronics contains a bridge rectifier.

Operating a colour TV from the inverter The CFL inverter board mounted on the inside of the TV set case via cable ties (the other board you can see is part of the tube drive). It connects to the TV's main PC board at the same place as the normal mains input connects, taking advantage of the fact that the switch-mode supply immediately rectifies the 240V AC mains (to about 350V DC). NEVER run the TV set from the mains and the inverter at the same time. And note our comments about disconnecting the degaussing circuit. As part of the development of this kit, Oatley Electronics (who own the design copyright) have used this circuit to run a 240VAC 30cm colour TV, as seen below. This "Prima" brand set was purchased at a supermarket for only $99 (truly!), much cheaper than an equivalent 12V TV. Therefore it makes sense to use this circuit with a 240VAC small colour TV, doesn't it? Don’t attempt to do this, however, if you are not experienced with switch-mode supplies and TV set circuitry in general. The only proviso is that the TV’s degaussing coil must be disconnected because it is important that it does not get the 350V DC from the CFL driver circuit. Then from time to time (especially if the TV is moved), the degaussing circuit will have to be reconnected and the set temporarily run from 240VAC to make sure the picture tube is degaussed, otherwise the tube will develop purity problems. Oatley Electronics are currently developing a 12V Degausser which should be ideal for use with this setup. More details when they become available!