Project: Nixie Clock – MC34063 Boost Converter (upgrade)

I really like my Nixie Clock that I built two years ago – but I don’t like to leave it on when I’m not around. The reason for this is that the Nixie tubes are powered directly from the mains which means that touching the Nixie tubes could result in a dangerous electric shock. While the tubes are protected under glass I don’t like the thought of someone accidentally tripping over it and getting electrocuted. So I decided to redesign the clock.

I am going to power the clock from a 9V wall supply and boost that voltage up to 180V to light the Nixie tubes. While this is still quite a high voltage, it is isolated from the mains and is much less dangerous.

My design is largely based on Threeneuron’s Pile o’Poo.

The schematic for the 180V power supply

The design is a boost converter based around the MC34063 which is quite an old chip. While the MC34063 has a built in darlington NPN transistor, it can only support voltages up to 40V which is not enough for us. Therefore we use this transistor to switch a IRF740 N-channel MOSFET (Q31) which can support up to 400V. Because the MC34063 is only able to pull the gate of Q31 high,we need a complimentary pull down. Components Q30, D1 and R7 form an active pull-down circuit which will cause faster switching than using a passive pull-down (ie R7 without D1 or Q30) resulting in a more efficient power supply.

The remainder of the circuit looks like a normal boost converter.

The feedback path takes a voltage divider from the output such that an output of 180V produces a value of 1.25V – by comparing this feedback to an internal reference of 1.25V the chip knows whether it needs to increase the voltage.

Channel 1 shows the voltage on the gate of Q31 and Channel 2 shows the feedback voltage

In the figure above you can see that each time the feedback voltage drops below a given threshold (1.25V), the MC34063 produces a series of pulses to increase the voltage. (Due to the large time scale you cannot see that there are multiple pulses)

Internal diagram of the MC34063 taken from ONSemi’s application note. 

The internal oscillator produces pulses of approximately 25kHz with a duty cycle of about 85%. If the feedback voltage is too low then these pulses will be passed to the transistor via the AND gate and the flip-flop.

Voltage on timing capacitor

These pulses are generated by charging and discharging a capacitor on the Timing Capacitor pin. The capacitor is charged with a constant current of ~35uA and discharged with a constant current of ~200uA resulting in a saw-tooth wave. Due to the difference in currents, the charging time is approximately 6 times longer than the discharging time. The output of the oscillator is high during charging and low during discharging, as visible in the figure below.

Channel 1 shows the voltage on the gate of Q31 and Channel 2 shows the voltage on the timing capacitor.

In reality I found that this circuit was producing approximately 183.5V which is about a 2% error which makes sense given the 2% error in the internal reference of the MC34063, as well as error in components etc. Luckily, the Nixie tubes don’t really care about the exact voltage.